DIGITAL TRANSMISSION
HISTORY – PDH – SONET/SDH
(MINI-LECTURE)
davby@ida.liu.se
IDA/ADIT/IISLAB
©2003–2004 David Byers
David Byers
1
Digital Transmission Systems
§ Asynchronous
„
Signals run at different rates,
independently of each other
Ethernet, modems
§ Plesiochronous
„
Signals are close to each
other in timing, but not
sourced from the same clock
PDH (Tn, En, Jn)
§ Synchronous
Signals run at the same rate;
timing is usually sourced
from the same clock
SDH (SONET)
©2003–2004 David Byers
„
Digital transmission systems can be subdivided into four basic types.
Asynchronous, plesiochronous and synchronous, based on their timing
characteristics. In asynchronous systems, signals in the system, as well as
clocks, run at different rates, and may run entirely out of phase. Timing is
entirely in the signals themselves.
In plesiochronous systems, clocks run close to each other, within a welldefined tolerance. In practice, many systems that are considered
asynchronous are actually plesiochronous.
In synchronous systems all signals run at the same rate and all clocks are
sourced off the same clock. In theory, all clocks should be exactly the same,
but in practice, they will be slightly different, although not so different as to
affect the signals.
2
Voice
65
60
55
50
45
40
35
30
25
100 Hz
1000 Hz
10000 Hz
©2003–2004 David Byers
Idealised voice spectrum
Before data communication came voice, and the development of digital
communications was driven by the need to pack more voice channels into
the same digital telephone trunk. This has subsequently influenced many
communication protocols. In particular, we often see multiples of 64000 bps
and 56000bps. Here’s why.
The frequency spectrum of voice looks something like this. Most of the
energy is in a fairly narrow frequency band from 100 to 5000 Hz. The
information content is even more concentrated; low frequencies don’t carry
a lot of information.
Because of this, and because of the properties of telephone wires,
telephone signals are filtered.
3
Voice Filtering
0
65
60
-10
55
-20
50
-30
45
40
-40
35
-50
30
-60
10 Hz
25
Filter Attenuation
Voice Spectrum
©2003–2004 David Byers
1000 Hz
This diagram shows the voice spectrum on the right and the frequency
response of a filter designed for the telephone system.
In practice one can say that the voice signal is filtered using a bandpass
filter that passes frequencies from 300Hz to 3400Hz, leaving a signal with
approximately a 3100Hz bandwidth. In reality, of course, both higher and
lower frequencies are passed, but they are severely attenuated. Therefore
we might consider the signal to be from 0 to 4kHz. This gives us a
comfortable margin to work with.
Furthermore, digitalisation of the voice signal quantifies it in 255 levels.
4
Voice Digitalisation
§
§
§
§
Frequencies: 0-4kHz
Levels: 0-255
Sample at 8kHz
Encode using PCM
©2003–2004 David Byers
§ 8000 Hz times 8 bits gives
64000 bits per second
§ One sample every 125µs
Nyquist-Shannon Sampling Theorem
If a function s(x) has a Fourier
transform F[s(x)] = S(f) = 0 for |f| >
W, then it is completely determined
by giving the value of the function at
a series of points spaced 1/(2W)
apart. The values sn = s(n/(2W)) are
called the samples of s(x).
Digitizing an analog signal is called sampling, and the Nyquist sampling
theorem tells us that to reconstruct a signal with a frequency of W Hz, we
need to sample it at 2W Hz (and those Ws should have been omegas).
So for digital telephone we have 8000 eight-bit samples per second, giving
us a total bitrate of 64000 bits per second.
That is the source of the ubiquitous 64kbps.
5
DS-x
Designaton Data rate
DS0 multiple
DS0
64 kbps
1
DS1
1.544 Mbps
24
T1 Carrier
DS2
6.312 Mbps
96
4 DS1
DS3
44.736 Mbps
672
T3 Carrier
§ Basic DS0 rate is bundled to form higher signals
§ Bundling is different in Europe and the USA
DS-x signals and T carrier in the USA
E carrier in Europe
DS1 F
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
(24 * 8bit + 1bit) * 8000Hz = 1544000 bits/s
©2003–2004 David Byers
„
„
The basic unit in digital transmission systems is the DS0 signal, which is a
65kbps signal, equivalent to one voice channel.
Higher-level signals can be formed by multiplexing the basic DS0. In the
USA, the basic bundle is called a DS1, and consists of 24 DS0 channels.
This was the basic building block for digital telephone systems. In europe,
bundling starts at 30 channels. The most common DS signals are DS0, DS1
and DS3, although there are a number of others, with different multiplexing
and data rates.
A DS0 transmits eight bits every 125us, but nothing says that it takes 125us
to transmit those bits. A DS1 consists of a single framing bit followed by 24
DS0 channels, eight bits at a time, giving a data rate of about 1544000 bits
per second.
6
DS1 Superframe Framing
F 1 2 3 4 5 6 7 8
F
Frame 1
F
Superframe
Aligns frames
Frame
Channel 24 (DS0)
1 2 3 4 5 6 7 8
Frame 2
F Frame 12
1 0 0 0 1 1 0 1 1 1 0 0
1 0 1 0 1 0
0 0 1 1 1 0
Terminal framing bits
Signaling framing bits
Shaded bits indicate
frames with robbed
bits for signalling
Aligns superframes
©2003–2004 David Byers
Channel 1 (DS0)
The T1 carrier uses framing bits in a repeating pattern to identify where frames start. The
recevicer searches the bit stream for a position where every 193 bit results in the repeating
pattern. Originally the repeating pattern was 1010, but the PCM created by certain tones
(e.g. 1000Hz) also resulted in the same pattern, which could cause the receiver to lock on
to the wrong positions in the stream.
In 1969 the superframe was introduced. Now the framing bits were divided into two parts –
the terminal framing bits, which correspond to the old framing bits, and signaling framing
bits. The new pattern was less likely to appear naturally in the data stream, reducing the
risk of mis-synchronization.
One thing that is missing from this framing format is a suitable channel for OAM signaling.
One mechanism that was used was to set data bits to specific values across the entire
frame to signal e.g. error conditions. The obvious problem with this mechanism is that an
entire 125us frame is lost, and that the error pattern might appear naturally in the data (e.g.
if all channels were silent).
An alternative method is commonly used is called robbe-bit signaling. Here one data bit is
stolen and used for signalling instead of data. In the precursor to DS1, one data bit was
taken from each frame, providing an 8kbps signalling channel, while also reducing the data
channel to 56kbps, since only seven bits per DS0 were used for data. When superframing
was introduced, the signalling scheme could be changed. Instead of robbing a bit in every
frame, a bit was now robbed from every sixth frame. This wastes far less bandwidth. Bit
robbing is fine for voice, where the robbed bit just results in a degradation of the voice
signal. The human receiver adapts. For data communication, however, the robbed bit
results in a bit error.
7
DS1 Extended Superframe
Extended Superframe Format
Frame
1
2
3
Fe
F
5
6
7
0
FDL M
CRC
Frame
LSB
4
M
P
C
M
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0
M
C1
P
C
M
8
M
1
M
C2
P
C
M
P
C
M
P
C
M
a
0
M
M
P
C
M
P
C
M
C3
P
C
M
P
C
M
P
C
M
P
C
M
M
1
M
C4
b
M
1
M
C5
P
C
M
P
C
M
P
C
M
P
C
M
2
3
4
5
M
C6
P
C
M
P
C
M
c
P
C
M
P
C
M
P
C
M
6
7
8
9 10 11 12
d
Superframe Format
Frame
1
Ft
1
F
Fs
3
0
0
P
C
M
4
P
C
M
5
1
0
P
C
M
6
P
C
M
7
0
1
P
C
M
8
a
9 10 11 12 1
1
1
P
C
M
P
C
M
0
1
P
C
M
P
C
M
1
0
P
C
M
b
0
0
P
C
M
P
C
M
1
0
P
C
M
P
C
M
0
1
P
C
M
a
1
1
P
C
M
P
C
M
0
1
P
C
M
P
C
M
0
P
C
M
b
©2003–2004 David Byers
Frame
LSB
2
The superframe concept was further improved, and a new scheme, called Extended
Superframe (ESF) was introduced. In this scheme, 24 frames form an extended
superframe. The framing bit is used for three purposes: framing, signalling information
about the link and a CRC to monitor the performance of the link. The ESF still uses robbed
bit signalling in every sixth frame.
The framing pattern is now only six bits again, but designed to avoid patterns that appear
naturally in voice communicaitons. Reducing the number of framing bits makes it possible
to introduce new features, as does the extension to 24 frames.
The Facility Data Link (FDL) is used for transmitting information related to error conditions
and performance monitoring, and is available for transmission to the customer since it is an
out-of-band channel. Every other F bit is used for the FDL.
The CRC is a six-bit CRC over the previous DS1 frame. The CRC is used to monitor the
performance of the link. It cannot be used to correct errors, just to see how many are
transmitted.
Finally, the LSB of every sixth frame is still robbed for signalling. There are now four bits,
termed a b c and d, giving the same overall bit rate for signalling as in the older superframe
format.
8
DS2 and DS3 Framing
4xDS1
Overhead
4xDS1
Overhead
M0
M1
M2
MX
[48]
[48]
[48]
[48]
C11
C21
C31
C41
[48]
[48]
[48]
[48]
F0
F0
F0
F0
[48]
[48]
[48]
[48]
C12
C22
C32
C42
[48]
[48]
[48]
[48]
C43
C43
C43
C43
[48]
[48]
[48]
[48]
F1
F1
F1
F1
[48]
[48]
[48]
[48]
4xDS1
Overhead
4xDS1
Overhead
4xDS1
Overhead
4xDS1
Overhead
DS3 Overhead
526306 bps
X
X
P
P
M0
M1
M0
[84]
[84]
[84]
[84]
[84]
[84]
[84]
F1
F1
F1
F1
F1
F1
F1
C11
C21
C31
C41
C51
C61
C71
[84] F0
[84] F0
[84] F0
[84] F0
[84] F0
[84] F0
[84] F0
[84]
[84]
[84]
[84]
[84]
[84]
[84]
C12
C22
C32
C42
C52
C62
C72
[84]
[84]
[84]
[84]
[84]
[84]
[84]
F0
F0
F0
F0
F0
F0
F0
[84] C13
[84] C23
[84] C33
[84] C43
[84] C53
[84] C63
[84] C73
[84] F1
[84] F1
[84] F1
[84] F1
[84] F1
[84] F1
[84] F1
[84]
[84]
[84]
[84]
[84]
[84]
[84]
©2003–2004 David Byers
4xDS1
Overhead
The DS3 was, and still is, a very important carrier in backbones in the USA. It provides
about 45Mbps capacity, carrying 28DS1 signals. The way the DS3 is formed follows the
same principles as other plesiochronous signals. Since the DS3 is so well documented, I
prefer talking about it rather than the european E3, which has more practical relevance
here.
The DS3 is formed by multiplexing four DS2 signals, which in turn are formed by
multiplexing seven DS1 signals. At each stage overhead is added and bit stuffing takes
place.
The DS2 signal format is shown in the top box. The M bits are used to find the start of each
multiframe (each row is a multiframe); the C bits are used to control bit stuffing and the F
bits are used to align frames. The [48] parts indicate 48 DS1 information bits.
Bit stuffing is a technique used by multiplexters in asynchronous networks. Bit stuffing may
also be used when embedding a signal from an asynchronous network in a synchronous
link, such as when transmitting a DS3 over SONET. In asynchronous communication,
clocks may run at slightly different rates, so the signals being multiplexed my have slightly
different bit rates. To accommodate this, the multiplexer has an output with capacity enough
to handle all inputs plus extra bits used to bring all inputs up to the same bit rate. In DS2,
the four DS1s are brought to 1545796 bps from 1544000 bps by adding bits. This
compensates for slow clocks in the network. These signals are then multiplexted together
with indicators that allow the receiving end to reverse the bit stuffing. In DS3, the four DS2
signals may also be asynchronous relative to one another, so bit stuffing also takes place at
the DS3 level.
9
DS0's
North
America
Europe
Japan
64 Kbps
1
-
-
-
1.544 Mbit/s
24
T-1
-
J-1
2.048 Mbit/s
32
-
E-1
-
6.312 Mbit/s
96
T-2
-
J-2
7.786 Mbit/s
120
-
-
J-2 (alt)
8.448 Mbit/s
128
-
E-2
-
32.064 Mbit/s
480
-
-
J-3
34.368 Mbit/s
512
-
E-3
-
44.736 Mbit/s
672
T-3
-
-
97.728 Mbit/s
1440
-
-
J-4
139.264 Mbit/s
2016
DS4NA
-
-
139.268 Mbit/s
2048
-
E4
-
274.176 Mbit/s
4032
T-4
-
-
400.352 Mbit/s
5760
T-5
-
-
565.148 Mbit/s
8192
-
E-5
J-5
©2003–2004 David Byers
Speed
This table summarizes the various PDH data rates in the USA (T-carrier),
Europe (E-carrier) and Japan (J-carrier). All are based on the DS0 data rate,
but bundling is slightly different from region to region. Framing also differs,
although the basic principles are the same in all carriers.
Rates above T3/E3/J3 are usually carried optically.
10
SONET/SDH/OC
SONET
OC
§ Synchronous Optical Network
§ Standard in the USA
§ STS framing
§ Optical carrier hierarchy
§ OC-x is x times 51.84 Mbps
SDH
Synchronous Digital Hierarchy
SONET-compatible
ITU standard
Used in Europé
STM framing
51.84 Mbps
155.52 Mbps
622.08 Mbps
2488.32 Mbps
9953.28 Mbps
©2003–2004 David Byers
§
§
§
§
§
OC-1
OC-3
OC-12
OC-48
OC-192
SONET is a standard for a synchronous optical network, designed to
replace the earlier electrical digital networks known as the plesiochronous
digital hierarchy (PDH). SDH is an ITU standard with the same goals,
slightly different yet compatible with SONET. SONET is primarily used in the
USA, whereas in Europe, SDH is used. SONET/SDH equipment tends to be
very expensive, but the technology is probably still the best choice for highperformance, high-reliability links. Unlike cheaper link-layer protocols,
SONET/SDH is designed from the ground up to be fast, scalable,
manageable and reliable.
SONET signals are called STS-N signals. STS stands for Synchronous
Transport Signal. STS-1 is a 51.84Mbps signal, which essentially means
that each byte is a 64kbps signal. The reason for 64kbps is quite simple – a
DS
11
SONET/SDH/OC Data Rates
Optical
Carrier
Data Rate
Payload-SONET
SONET
OC-1
51.84 Mbit/s
50.112 Mbit/s
STS-1
OC-3
155.52 Mbit/s
150.334 Mbit/s
STS-3
SDH
STM-1
466.56 Mbit/s
451.044 Mbit/s
STS-9
STM-3
622.08 Mbit/s
601.344 Mbit/s
STS-12
STM-4
OC-18
933.12 Mbit/s
902.088 Mbit/s
STS-18
STM-6
OC-24
1244.16 Mbit/s
1202.784 Mbit/s
STS-24
STM-8
OC-36
1866.24 Mbit/s
1804.176 Mbit/s
STS-36
STM-12
OC-48
2488.32 Mbit/s
2.4 Gbps
STS-48
STM-16
OC-192
9953.28 Mbit/s
9.6 Gbps
STS-192
STM-64
©2003–2004 David Byers
OC-9
OC-12
This table summarises various OC levels, the corresponding data rates,
SONET payload data rates, SONET carriers and SDH carriers.
12
SONET/SDH Concepts
Path
Line
Repeater
Repeater
Section
Section
Section
Section
Section
PTE
Line
ADM
Repeater
PTE
STS-1 Frame
SOH
9 rows
LOH
POH
90 columns
©2003–2004 David Byers
SPE
SONET/SDH is a complex standard, and beyond what we’re covering here.
Each STS-1 frame, which is the basic signal, contains four parts: section
overhead, line overhead, path overhead and a payload. The overhead
sections carry OAM and signaling information for various parts of the
network. The SOH contains information pertinent to each section, and is
regenerated by section-terminating equipment. LOH contains information
pertinent to lines, and POH contains information pertinent to the entire path.
The payload is also structured, can start anywhere in the frame and may
span two frames. This makes it possible to stuff an asynchronous source,
such as a DS3, into STS-1 frames; the start of the DS3 frames may not
coincide with the STS frames, but the frame structure is retained anyway.
Higher-level STS signals are constructed by interleaving STS-1 signals. The
same nine-row structure is retained, transport overheat (SOH and LOH) go
at the front of the frame, and more than 90 columns are used.
13
SONET/SDH Framing
Frame n
Pointer
SPE
Frame n+1
SPE
©2003–2004 David Byers
Pointer
This picture shows how the payload of a SDH frame (the SPE) can span
more than one frame. This is a common occurrance when an asynchronous
signal needs to be stuffed into a SONET signal. Since the asynchronous
signal is not locked to the SONET clock, frames will not appear at the same
rate as SONET frames do.
14
ATM
PRINCIPLES – ATM PROTOCOLS – IP OVER ATM
TRAFFIC ENGINEERING – SIGNALLING
davby@ida.liu.se
IDA/ADIT/IISLAB
©2003–2004 David Byers
David Byers
15
Standards Organisations and ATM
ITU-T
§ United Nations agency
§ Standards for international
telecommunications
§ Created in 1993
§ Replaced CCITT
ATM Forum
§ Created in 1991
§ Objective to accelerate use of
ATM products and services
§ Three committees
„
„
©2003–2004 David Byers
„
Technical Committee
Market Awareness Committee
User Committee
There are two primary players in the ATM standardisation effort.
The ITU-T is a united nations special agency tasked with the
standardisation of international telecommunications. This agency was
formed in 1993 and replaced the CCITT, which up to that point had been the
main driving force in international telecommunications standardisation.
The ATM forum is a non-profit international organisation formed to
accelerate the use of ATM products and services, and to accelerate
development of ATM. Its technical committee is tasked with working with
other standards bodies to standardise aspects of ATM. It consists of a
number of working groups, each focused on a different area of ATM. There
are working groups for ATM architecture, control signaling, network
management, physical layer issues, wireless and more.
For the most part it appears that the ATM forum standards are further along
than the ITU standards, and that they are more responsive to market needs.
This is perhaps natural, given that the ATM forum is built for and by
stakeholders in the ATM business.
16
Why ATM
§ One International Standard
„
„
ITU-T standard
ATM Forum standard
§ Scalable in Distance
„
„
„
§ Multiple Traffic Types
„
„
§ Scalable in Speed
„
Operates on many different
physical layers
©2003–2004 David Byers
„
Voice (delay sensitive)
Data (loss sensitive)
Video (high bandwidth)
Local Area Networks
Metropolitan Area Networks
Wide Area Networks
ATM was introduced at a time when there were no wide and metropolitan
area networking technologies worth using. Ethernet was limited to 10Mbps,
and is not suitable for wide-area networking, and other technologies were
basically point-to-point links. Technology such as X.25 and Frame Relay
existed, but were limited in their usefulness as they were not easily scalable
and interoperable.
ATM was designed from the ground up to be an international standard,
allowing interoperability between vendors. It was to be scalable in distance
and speed, allowing local to wide area networking over whatever speed the
user was willing to pay for. It was also designed from the start to support
multiple traffic types, including high-quality voice and video.
ATM started out as a CCITT (later ITU) standard. A consequence of this is
that it is a committee product. In the ITU all representatives must be in
agreement for a particular document to be accepted, which slows down
progress considerably. It also implies that ATM has a number of
compromises.
Still, ATM represented a significant step forward, and even today ATM
provides features that other standards do not.
17
ATM Basics
§ Negotiated Service
Connection
„
„
End-to-end connections,
called virtual circuits
Traffic contract
§ Switched Based
„
Dedicated capacity
§ Cell Based
Small, fixed length
©2003–2004 David Byers
„
There are three basic concepts in ATM that the ATM forum like to push.
ATM supports a negotiated service connection, which means that users can
receive the service they are willing to pay for, and they are guaranteed to
receive that service. This enables running real-time delay-sensitive traffic
over an ATM network, something that cannot easily be done on e.g. an
Ethernet.
ATM is switch-based, not router based. Switches perform far better than
routers, as most of the decisions can be made in hardware. Switches
minimise delay. Switches can also provide dedicated capacity to single
connections. Finally, switches, at least historically, are cheaper than routers.
ATM is cell-based. Unlike some other circuit-switched technology, ATM
does not provide for an end-to-end bit stream (although that can be
emulated). It provides packet switching with small, fixed-size packets.
18
ATM Characteristics
§ Connection-oriented
„
No adresses (sometimes)
§ Circuit-based
„
Fast switching
§ Unreliable transport
„
„
Cells can be lost or created
But they are deliverd in-order
§ Minimal error detection
Most error detection left to
end stations
„
Runs on a multitude of
physical media
§ Complete protocol stack
„
From transport protocols to
physical layer protocols
§ Internetworking ready
„
Protocols for internetworking
between organizations and
network types ready
©2003–2004 David Byers
„
§ Medium independent
ATM leaves error detection and correction to the endpoints. The only thing
ATM does is error detection on the header of each cell. This is to simplify
ATM and make the switches cheaper.
ATM is connection-oriented. Prior to communication, a connection must be
negotiated.
Although ATM is packet switched, it is still circuit based. Unlike some circuitswitched technology, ATM does not provide and and-to-end transparent
circuit; it still splits data into cells and switches the cells. It is, however,
circuit based in that prior to communiction, a path through the network must
be negotiated, and all packets beloning to a particular connection will
traverse the same path. Paths may be permanent or set up on demand (or,
in part, both).
Fundamentally, ATM is an unreliable transport in that it can drop cells.
However, a great deal of effort has gone in to ensuring that this does not
happen during normal operation. ATM does, however, guarantee in-order
delivery of cells. It is worth noting that with certain service types, ATM may
actually insert cells into the stream to ensure a steady rate of cell delivery.
19
Negotiated Service Connection
§ Parameters
„
„
„
Traffic characteristics
Peak cell rate
Sustainable cell rate
§ Quality of service
„
QOS B
QOS C
©2003–2004 David Byers
„
Delay characteristics
Cell loss
QOS A
No notes for this page.
20
ATM Protocol Stack
Higher layers
ATM Adaption Layer (AAL)
ATM Layer
Physical Layer
Application
Application
AAL
AAL
Physical Layer
ATM
ATM
Physical Layer
ATM
Physical Layer
©2003–2004 David Byers
ATM
Getting in to the technology of ATM, we’ll start with a look at the ATM
protocol stack.
ATM consists of three layers: the physical layer (PHY), the ATM layer
(ATM) and the ATM Adaptation Layer (AAL). The PHY and ATM layers exist
on all ATM devices, but the AAL only exists on endpoints, not on switches.
ATM diverges from the OSI model; the physical layer performs functions of
the physical and data link layers in ths OSI model. The ATM layer performs
functions of the data link and network layers. The ATM adaptation layer
performs functions of the network and transport layers.
21
ATM Protocol Stack
Q.2931
AAL
Layer
SAAL
Layer
Q.2931
SAAL
Layer
ATM Layer
ATM Layer
Physical Layer
Physical Layer
TE ATM Stack
Switch ATM Stack
©2003–2004 David Byers
Higher
Layers
Control Plane
User Plane
Management Plane
A more detailed look at the protocol stack shows not only layers, but planes.
In addition to the protocol layers, there is a management plane, concerned
with management of the entire protocol stack. There are also control
functions, located in a control plane, concerned with issues such as
signalling. The management and control planes (at least part of them) are
present even on switches in the network, whereas the user plane is
generally not.
22
ATM Cell Structure
§ Small Size
„
„
5 Byte Header
48 Byte Payload
5 bytes
header
48 bytes
payload
©2003–2004 David Byers
§ Fixed Size
§ Header contains virtual
circuit information
§ Payload can be voice,
video or other data types
No notes for this page.
23
ATM Cell Structure
§ Why small cells?
„
„
Small cells reduce
packetization effects for
continuous traffic
Small cells and small buffers
reduce forwarding delay
§ Why fixed cell size?
„
„
Variable cell size reduces percell overhead
Fixed cell size simplifies
equipment
§ Why 53 bytes?
„
©2003–2004 David Byers
„
USA wanted 64 byte payload
for efficiency
Europe wanted 32 byte
payload to eliminate need for
echo cancellation
No notes for this page.
24
ATM Terminology
DTE
Generic term for equipment
connected to the network, e.g. a
modem
DTE
Data General
Data General
DSU Data Service Unit
Equipment that attaches
customer equipment to a public
network
NNI
DTE
Data Terminal Equipment
UNI
DSU
NNI
Network-Network Interface
Interface between ATM switches
UNI
User-Network Interface
Interface between user
equipment and an ATM switch
©2003–2004 David Byers
Switch
Before we move on, lets go one round of the acronym derby.
ATM is full of acronyms, and you need to know them, because ATM people
use them all the time. Here are some that show up a lot in ATM and ISDNrelated stuff.
The UNI is an important function. This is the bit that connects an end user
with the ATM network. A lot of functions, including traffic policing, take place
in the UNI. In many cases UNI is used to refer to the actual piece of
equipment that connects the user to the network; this is more properly
called a DSU.
25
ATM Concepts
§ Virtual Path (VP)
„
„
„
Semi-permanent circuit
Contains up to 65000 VCs
Switched as a whole
§ Virtual Channel (VC)
„
„
On-demand connection
Contained within a VP
VC
©2003–2004 David Byers
§ Switched Virtual Circuit (SVC)
§ Permanent Virtual Circuit (PVC)
VP
In ATM, all cells belong to a connection. These connections are identified by a tuple
<VP,VC>, where VC is the virtual channel a cell belongs to and VP is the virtual path that
the virtual channel belongs to. In other words, circuits in ATM form a two-level hierarchy,
with virtual paths containing virtual channels.
The reason for this construction is that a virtual path containing a large number of virtual
channels can be switched as one. Switches along the path do not need to keep track of
every individual channel, but can simply keep track of a more limited number of paths. This
improves efficiency. Furthermore, it is possible to set up virtual channels within the
parameters of an existing virtual path without involving the entire network.
Virtual paths were originally concieved to be semi-permanent connections, such as the
connection between a branch office and headquarters, or a telecommuting worker and his
workplace. Virtual channels would then be set up dynamically within the preexisting virtual
path.
There are two other basic concepts that tie in to the VP and VC concepts.
A Switched Virtual Circuit (SVC) is a circuit that is set up on demand by the netowork, using
some signalling protocol. A Permanent Virtual Circuit on the other hand is a circuit that is
set up manually and exists over a longer period of time. SVCs offer the highest degree of
flexibility, but PVCs are appropriate when a customer buys a permanent link through a
provider’s ATM network.
26
ATM Cell Header Structure
UNI cell format
GFC
CLP
VPI
VCI
PTI
NNI cell format
VPI
Generic Flow Control
Virtual Path Identifier
Virtual Channel Identifier
CLP
VCI
PTI
CLP
HEC
PTI
HEC
Payload Type Indicator
Cell Loss Priority
Header Error Control
©2003–2004 David Byers
GFC
VPI
VCI
HEC
ATM cells exist in two variants. One variant is used between the end user and the network,
and the other is used within the network itself. The former are called UNI cells and the latter
NNI cells.
The format of the NNI cell header is simple. It starts with the circuit identifier – the virtual
path and channel ID, with 12 bits allocated to the VPI and 16 to the VCI. This implies that
two ATM switches can have 4096 virtual paths between each other, each containing up to
65535 virtual channels. The VCI is followed by a three-bit payload type indicator field, which
indicates what kind of data is in the cell. The CLP is a single-bit field. When set to 1, it
indicates that the cell is a candidate for dropping when a switch must drop cells. When set
to 0, the cell should only be dropped as a last resort. Finally, the HEC is a checksum over
the header, used to detect errors in the header. This is the only form of error control in the
ATM layer (higher layers may add additional error control).
The UNI cell format is identical to the NNI cell format with the exception that the VPI is only
eight bits, and the first four bits of the header are used for generalized flow control instead.
This implies that an end user can have at most 256 virtual paths to the network.
The difference in size of the VPI in the two cell formats is reasonable. A switch may be
connected to several customers, so it may have to aggregate all VPIs from these customers
on the link to another switch. By providing an additional four bits for the VPI in the NNI cell
format allows a switch to aggregate VPIs from 16 UNIs without running out of VPIs.
27
ATM GFC and PTI
§ GFC
„
„
„
Used to control flow of data
from user equipment
Only exists at UNI
Undefined
§ PTI
„
„
„
„
Indicates type of payload
0xx – User data cell
10x – OAM cell
110 – RM cell
000 User data cell, congestion not
experienced, SDU type=0
001 User data cell, congestion not
experienced, SDU type=0
010 User data cell, congestion
experienced, SDU type=1
011 User data cell, congestion
experienced, SDU type=1
100 Segment OAM flow-related cell
101 End-to-end OAM flow-related cell
110 RM cell
©2003–2004 David Byers
111 Reserved
The GFC and PTI fields deserve an extra look.
The GFC field deserves a look, because here are four bits that haven’t been
standardised. When the UNI cell structure was defined, provision was made
for flow control, but standardisation of how flow control would work was left
for later. As a result, the GFC field exists, but interoperable standards for
how to use it do not.
The PTI field indicates the type of data in the cell. Note that it is possible to
indicate congestion in the network by setting the appropriate PTI value.
28
ATM Protocol Stack
Higher layers
ATM Adaption Layer (AAL)
ATM Layer
Physical Layer
Application
Application
AAL
AAL
Physical Layer
ATM
ATM
Physical Layer
ATM
Physical Layer
©2003–2004 David Byers
ATM
Just a picture.
29
©2003–2004 David Byers
Physical Layer
30
Physical Layer
PMD Sublayer
§ Timing Function
§ Encoding/Decoding
TC Sublayer
§ HEC Cell Generation and
Verification
§ Decoupling of Cell Rate
§ Cell Delineation
§ Transmission Frame
Generation and Recovery
Physical Layer
Physical Medium-Dependent (PMD) Sublayer
©2003–2004 David Byers
Transmission Convergence (TC) Sublayer
The physical layer is divided into two sublayers: the physical medium-dependent sublayer and the transmission
convergence sublayer. The PMD deals with low-level issues such as timing and encoding/decoding for
transmission, and is probably, at least in part, implemented in hardware. The TC sublayer, on the other hand, is
concerned with more high-level aspects of transmission. It deals with generating and verifying the HEC,
decoupling cell rate, delineating cells and generating and recovering transmission frames.
The PMD sublayer has two functions: timing and encoding/decoding.
The timing function is responsible for synchronizing the transmitting and receiving PMD sublayers. This may
involve extracting timing information from received signals and tuning the timing of transmitted signals. The
encoding/decoding function generates transmission codes. Although some physical media operate on a bit-by-bit
basis, others encode several bits at once. For example, FDDI, and other media, use a 4B/5B code, where four
data bits are encoded as a five-bit codeword.
The TC sublayer receives complete ATM cells from the ATM layer, but is responsible for filling in the HEC field.
On the receiving end it is also responsible for checking the HEC field and discarding cells that do not pass the
HEC test.
The TC sublayer is also responsible for decoupling the ATM layer from the cell rate required by the PMD. The
PMD may require a continuous stream of cells at a fixed rate, and the TC sublayer is responsible for generating
this stream. That may entail inserting idle frames into the stream when the ATM layer isn’t producing enough
traffic. It is worth noting that the ATM forum recommends that cell rate decoupling be implemented in the ATM
layer; if it is, then the TC sublayer does not have to perform this task.
Cell delination is the process of extracting cells from the transmission stream. There are two fundamental ways of
doing this. One is to exploit framing information in the physical layer protocol; for example, it is possible to map
ATM cells onto an E1 stream using PLCP (documented in IEEE 802.6c and h), and in theory it would be possible
to place ATM cells at specific positions in an E3 or SONET frame (in practice this is not done). The other method
is called HEC cell generation, and is described on the next slide.
Finally, the TC sublayer is responsible for generating transmission frames for framed media, such as SONET. On
the receiving side, the TC sublayer extracts frames from the data stream received from the PMD.
31
Physical Layer Interfaces
SONET/SDH
PDH
Nx64kbps
IMA
TAXI (FDDI)
ATM 25
Fiber Channel
Cell-Based Clear Channel
...
§ ATM can operate over
many physical media
§ At low speeds overhead is
significant
§ Originally defined for highspeed optical links
§ Lower speeds defined to
speed deployment and to
satisfy customer needs
§ Special protocols used at
low speeds
©2003–2004 David Byers
§
§
§
§
§
§
§
§
§
No notes for this page.
32
TC Sublayer
§ HEC Cell Generation and
Verification
„
„
Compute the HEC byte into
ATM cells passed from the
ATM layer
Check HEC byte of received
cells
§ Decoupling of Cell Rate
Pass a continuous stream of
cells to PMD, inserting idle
cells when needed
„
Generate transmission
frames for framed physical
layers (e.g. SONET)
§ Cell Deliniation
„
Decide where cells start and
end in the data stream from
the PMD
©2003–2004 David Byers
„
§ Transmission Frame
Generation and Recovery
The TC sublayer of the PHY has a number of jobs. The first job is
generating and verifying the HEC. The TC sublayer is responsible to ensure
that headers of received cells are correct before further processing them. It
is also responsible for computing the HEC of all outgoing cells.
The second job is cell rate decoupling (this also takes place at the ATM
layer). In ATM, the PHY must generate a continuous stream of cells at the
appropriate speed for the physical medium. The TC sublayer is responsible
for ensuring that this happens. It may have to insert idle cells in the network
if there aren’t enough data cells.
The TC also deals with transmission frame generation and recovery. Some
physical layers used framed transport, and TC is responsible to dealing with
these frames, including recovering them from the bit stream received from
the PMD and computing appropriate framing for transmission to the PMD.
Finally, TC finds cells. It receives a continuous bit stream and must find out
where cells start. There are several ways to do this, including placing cells
at well-defined positions within the frames of a framed transport (such as
SONET) and actually searching for the cells.
33
TC Sublayer: Cell Rate Decoupling
Why?
§ PHY data rate and ATM data
rate may differ
§ PHY data rate must be
maintained
Cells at 100Mbps
(from user)
How?
Cell rate decoupling
§ Insert idle cells in cell stream
©2003–2004 David Byers
Cells at 155Mbps
(e.g. STS-3c)
An example where cell rate decoupling might take place is when a customer
has paid for 100Mbps of capacity, but the actual link supports 155Mbps
(e.g. STS-3c). Here the TC sublayer must insert about 30% worth of idle
cells to make up the slack. Also note that this results in the inter-cell timings
to be somewhat off, which may have to be compensated for at the receiver.
34
TC Sublayer: HEC Cell Delineation
Extract cells from the bit
stream received from PMD
§ Hunt for cell start
„
Correct HEC
for Y cells
Hunt
If a cell starts at bit bi, then
CRC(bi … bi+39) = 0
§ Wait X cells to ensure no
false alarm
Sync
Incorrect HEC
Correct HEC
for X cells
Correct HEC
Presync
©2003–2004 David Byers
§ Assume synchronization
lost on consecutive HEC
failures
One of the jobs of the traffic convergence sublayer is to detect where cells
start.
For some physical media, cells start at predefined positions. This can be the
case in e.g. DS3, where the physical medium includes framing information.
Here ATM cells can start at known positions. In other media, such as DS1
streams and SONET, the TC sublayer only sees a stream of bits from the
PMD sublayer, and must extract the position of cells using some other
mechanism.
It starts in a hunt state, where it is looking for the start of a cell. The
observation is that if a cell starts on a particular bit, then the CRC of that bit
and the following 39 bits will be zero (the HEC cancels out). If the CRC of
those bits is non-zero, then the TC tries to start a cell at the next bit. If all
goes well, and there are no transmission errors, the TC will quickly find a
potential start of a cell.
Once the CRC is correct, the TC waits to see that several cells in a row
arrive correctly. If they do not, then the start it found may have been a false
alarm, so it goes back to hunting. Once several cells arrive OK in a row, the
TC moves to the sync state. Once in the sync state, the HEC state machine
is used to detect and correct errors in cells. If a number of consecutive
errors occur in the sync state, the cell delination state machine moves back
to the hunt state, under the assumption that synchronization has been lost.
35
©2003–2004 David Byers
ATM Layer
36
The ATM Layer
§ Forwarding tables
§ Cell switching
§ Multiplexing/demultiplexing
Multiplexing
AAL
AAL
Switching
Input
port
Input
port
Input
port
Input
port
Switch
table
Switch
table
Switch
table
Switch
table
AAL
Switch Fabric
ATM Layer
Pysical Layer
Output
port
Output
port
Output
port
Output
port
©2003–2004 David Byers
Continuous cell stream
The ATM layer is fundamentally responsible for two tasks: switching and
multiplexing.
Switching takes place mostly on NNIs, and is simply the process of moving
cells between VCCs and taking them from input ports and sending them to
output ports.
Multiplexing is the process of accepting cells from various sources, mainly
the AAL, and multiplexing them onto a continuous stream of cells. The ATM
layer is responsible for creating a continuous stream of cells, and may insert
empty cells when no real cells are being produced.
Multiplexing several sources of cells can introduce timing variations that are
not desirable. Assume one source generates a single cell every 125us.
Assume also that there are lots of other sources, all competing for the same
physical layer. After multiplexing, the cells from the first source may not be
spaced out at exactly 125us; variations may be introduced by the
multiplexing process. This is particularly important for real-time sources and
when such variations cause cells to be sent at a higher rate than the
negotiated rate. More on this later.
37
Switching: Label Translation
§ 4096 VPs, 65535 VCs
§ Not enough for networkwide VP/VC assignment!
§ Therefore VP/VC has
significance only on a
single link
44/214
VPin
VPout
44
68
§ Switches translate labels!
©2003–2004 David Byers
68/214
This demonstrates something very, very basic, which occurs in most label
switching protocols. Cells are associated with virtual paths and channels,
but nothing says that a particular path is identified by the same VPI
throughout the network. This would, in fact, not scale at all since there are
only 4096 VPIs, and coordinating VPI assignments throughout a large
network would be prohibitively difficult.
Therefore VPIs and VCIs only have local significance. When an ATM cell is
switched from input to output interface, the VPI and VIC may be changes.
The cell is still in the same VCC, but that VCC has a different identity on the
new link.
38
Path or Circuit Switching
Forwarding table translates only VPI
Cells stay in the same VC bundle
©2003–2004 David Byers
Forwarding table translates both VPI and VCI
Cells can be switched from VP to VP and VC to VC
At the ATM layer, switching takes place in two basic forms. In one, virtual
paths are effectively terminated at the switch, and switching freely takes
place among all VCCs. This allows a channel to move from one path to
another.
The other basic form is switching VPs as a bundle, ignoring their internals.
39
©2003–2004 David Byers
ATM Adaptation Layer
40
ATM Adaptation Layer
Attributes
Class A
Class B
Class C
Class D
Timing between src and dest
Yes
Yes
No
No
Bit rate
Constant
Variable
Variable
Variable
Connection mode
CO
CO
CO
CL
Example
Circuit
Video
emulation Audio
service
Frame
relay
LAN
emulation
ATM Adaption Layer
AAL1
AAL3/4
AAL3/4
AAL5
©2003–2004 David Byers
AAL2
The ATM adaptation layer is where things start getting real.
ATM defines several classes of service. The four original classes, A, B, C
and D are shown in the table above. Each class corresponds to one AAL,
which contains features to support the needs of that particular class.
41
AAL 1
SAP
AAL1
Convergence Sublayer (CS)
Segmentation and Reassembly (SAR) Sublayer
SAP
Target Networks
§ Constant bit rate
§ Timing information transferred
§ Connection oriented
§ High-rate audio/video
§ Circuit Emulation Service (CES)
©2003–2004 David Byers
Class A Service
AAL1 is used for class A service. Constant bit rate with timing information
transferred. The protocol is connection oriented. The target applications for
this AAL are circuit emulation, where e.g. a T1 is emulated over an ATM
network, and high-rate audio and video.
AAL1 is divided into two sublayers, the convergence sublayer and the SAR
sublayer. It is accessed through service access points.
42
AAL1 SAR Sublayer
47 byes
§ Manages segmentation and
reassembly of AAL1 PDUs
§ Adds one-byte SAR header to
47-byte CS payload
§ Receiver manages reassembly
using sequence count
§ CRC over CSI and SC
§ Parity over CIS, SC and CRC
CS
SAR
SN SNP
Sequence
count
CRC
Parity
©2003–2004 David Byers
CSI
47 byes
The SAR sublayer is responsible to segmentation and reassembly of AAL1
PDUs. It receives 47-byte PDUs from the CS and adds a header consisting
of a sequence number and a checksum. It computes the CRC and assigns
the sequence number.
43
AAL1 CS Sublayer
§
§
§
§
§
Handling of cell delay variation
Processing sequence count
Forward error correction
Performance monitoring
Data transfer modes
„
„
P-Mode
SP
Structured
Unstructured
Data
46 bytes
§ Transfer of timing information
„
„
SRTS
Adaptive method
Non-P-Mode
©2003–2004 David Byers
47 bytes
The AAL1 convergence sublayer is responsible for a number of things.
It is responsible to managing cell delay variations, to ensure that data is sent to the application layer
at a constant rate, even if cells to not arrive at a constant rate. Depending on the application, the CS
sublayer may insert extra bits in the stream when a buffer underrun occurs.
The CS sublayer is responsible for handling the sequence count. It receives the sequence count from
the SAR, and detects misinserted or lost cells. Misinserted cells are discarded, and lost cells may be
compensated for by inserting dummy PDUs.
The CS also provides forward error correction on the data, if required by the application. This feature
may be used by high quality video or audio applications, but may not be required by all applications.
Some applications need to measure performance, such as bit error rate, lost cells, misinserted cells,
buffer overflow and underflows and so forth. The AAL CS is responsible for providing this service.
AAL1 defines two modes of data transfer. In structured mode, the PDU starts with a one-byte pointer
that indicates where in a cell data starts. This field is actually a parity bit and a seven bit pointer. The
SP can be set in cells with an even sequence number. In unstructured mode, each PDU contains 47
bytes of data.
The final function performed by AAL1 is transfer of timing information. For some applications, sender
and receiver need to synchronize their clock frequencies.
SRTS is one mechanism for clock synchronization between senter and receiver. It can be used when
both can be slaved to the same clock. A four-but residual time stamp is transmitted using the CSI bit
in the AAL1 header. The common reference clock must be derived from the network, so this method
does not work on e.g. plesiochronous networks.
In some cases, the AAL may be phase-locked to the network clock, in which case there needs to be
no transfer of information at all.
When no common reference is available, AAL1 uses the adaptive method. Here the buffer fill rate at
the receiver is monitored, and if it is above median, the AAL assumes that it is delivering data too
slowly and speeds up. If it is under median, the AAL slows down.
44
AAL2
SAP
Convergence
Sublayer
AAL2
Service-Specific Convergence Sublayer (SSCS)
Common Part Sublayer (CPS)
SAP
Target Networks
§ Variable bit rate
§ Timing information transferred
§ Connection oriented
§ Cellular networks
§ Private branch exchanges (PBX)
©2003–2004 David Byers
Class B Service
AAL2 is designed for class B service: variable bit rate with timing
information. It is connection oriented.
Target networks of this service include distribution networks for cellular
networks and PBXs. AAL2 consists of a convergence sublayer, which in
turn is split into a service-specific convergence sublayer and a common part
sublayer.
45
AAL2 Multiplexing
SAP
SSCS
SSCS
SSCS
CID A
CID B
CID C
CPS Packets
CPS
CPS PDUs
©2003–2004 David Byers
SAP
One of the features of AAL2 that is not present in e.g. AAL1 is multiplexing.
A single AAL2 session can multiplex data from several sources. Each
source is identified by its own SSCS, but these all communicate with the
same CPS. The SSCS transmits CPS packets to the CPS. The CPS
interleaves these into CPS PDUs, which are sent to the ATM layer.
46
AAL2 Multiplexing
CPS Packet
CID
PPT
CPS Packets
LI
HEC
UUI
CPS PDUs
CPS PDU
P
S
N
OSF
ATM Cells
©2003–2004 David Byers
PAD
This slide shows how CPS packets from several sources, each with a CPS
packet header, are placed into a single AAL2 cell stream. A single CPS
packet may even be split across several CPS PDUs.
CPS Packet
CID – Channel identifier. Used to multiplex streams. Channels are
bidirectional. Channels up to 7 have special meanings or are reserved.
PPT – Packet payload type. PPT other than 3 indicate application data. PPT
3 indicates AAL network management function.
LI – the total number of bytes in the payload of the CPS packet. It is actually
one less than the length, so the maximum length of a CPS packet is 64
bytes.
HEC – Header error control. Used to detect errors in the header.
UUI – User-to-user indicator. Transported transparently by CPS.
The CPS PDU header is called STF (Start field)
P – Parity. Used to detect errors in the STF
SN – Sequence numbers alternates 1-0-1-0 for successive PDUs.
OSF – Offset field. Number of bytes between STF and start of the first
packet in the PDU
47
AAL3/4
Convergence
Sublayer
AAL3/4
SAP
Service-Specific Convergence Sublayer (SSCS)
Common Part Sublayer (CPS)
Segmentation and Reassembly (SAR) Sublayer
Class C/D Service
Features
§ Variable bit rate
§ No timing information
§ Connection oriented
§ Assured/non-assured transfer
§ Message or stream oriented
§ Supports several SSCS variants
©2003–2004 David Byers
SAP
AAL3/4 was origially defined for SMDS (Switched Multi-Megabit Data
Service), but was adapted to ATM. Originally it provided class C service, but
it was found to support class D as well. AAL3/4 is complex with a lot of
overhead, and feels typical of a telecommunications committee product (if
you thought ATM was complext up to now, you’ve got no idea).
We won’t cover AAL3/4 here, because we don’t have enough time. Instead,
we’ll look at AAL5, which is a simpler AAL suitable for e.g. packet switching.
Suffice to say that AAL3/4 provides both packet-oriented and streamoriented service, with both assured and non-assured transfer. In assured
mode, AAL3/4 manages retransmission of lost PDUs. AAL3/4 supports
multiplexing like AAL2.
48
AAL5
SAP
Convergence
Sublayer
AAL5
Service-Specific Convergence Sublayer (SSCS)
Common Part Convergence Sublayer (CPCS)
Segmentation and Reassembly (SAR) Sublayer
Class D Service
Features
§ Variable bit rate
§ No timing information
§ Connectionless
§ Non-assured transfer
§ Message or stream oriented
§ Error detection
©2003–2004 David Byers
SAP
AAL5 is an AAL for class D service. It is simpler than AAL3/4 and suitable
for e.g. IP traffic and other connectionless services. If provides non-assured
transfer of user PDUs, which means that higher-layer protocols must
manage retransmission. Like AAL3/4 it consists of a convergence sublayer,
which is further subdivided into a SSCS and CPCS, and a SAR sublayer.
AAL5 does not provide multiplexing or assured forwarding since these can
be left to higher layers. Therefore, AAL5 is a suitable choice only when
these features can be provided by higher levels. Otherwise AAL3/4 is a
better choice.
49
AAL5 PDU
0-65535
0-47
User PDU
PAD
CPS-UU
CPI
Padding so the entire PDU
becomes a multiple of 48
bits
User-to-user indication
transferred transparently
Common Part Indicator for
future use
Length of PDU in bytes
Error control field
CPS-UU
1
2
CPI
4
LI
CRC-32
Notes
§ SAR segments PDUs into 48byte segments
§ No encapsulation at SAR level
§ SDU part of PTI set to zero for
all segments but the last
©2003–2004 David Byers
LI
CRC-32
PAD
1
AAL5 places user data before a trailer containing fields more commonly
placed in a header.
50
©2003–2004 David Byers
IP Over ATM
51
IP Over ATM
§ LAN Emulation (LANE)
„
„
ATM Network emulates LAN
Allows applications to
operate with no changes
§ IP Switching
„
„
Switch IP packets directly
Proprietary solutions
§ MPLS
„
„
Find next hop towards
destination
§ Classical IP over ATM
„
Treat ATM network as a
point-to-point link
§ Multiprotocol over ATM
„
„
Unified approach for all layer
3 protocols over ATM
Uses NHRP and LANE
©2003–2004 David Byers
„
Successor to IP switching
Independent of ATM
§ Next-Hop Resolution
Protocol
IP and ATM are not a particularly good match. IP has large frames of up to 65k data, with
low overhead, whereas ATM has tiny cells with high overhead. IP does not require QoS.
ATM expends considerable effort to provide QoS. Loss of an ATM cell results in loss of an
IP frame, but it may not be detected by the ATM layer, resulting in unnecessary switchin.
ATM has its own signalling protocols. The ATM philosophy is completely different from the
IP philosophy. And so in.
Still, ATM was the first transport defined for high-performance networks, so IP over ATM is
unavoidable, and there are several methods to run IP over ATM.
LAN Emulation, which exists in several variants, emulates the characteristics of a LAN,
typically an Ethernet, over ATM. Applications that normally run over LAN can run without
modification on top of LAN emulation. This implies that ATM needs to support features of
the LAN such as broadcasting, multicasting and address resolution.
IP Switching is another option, where IP packets are switched directly. It is possible here to
exploit native ATM switching by assigning related IP packets to the same VC.
Another obvious method, perhaps most useful when a permanent or semi-permanent
connection is desired, is to use tunneling. This is probably used more than many people
realize. When purchasing a T3 in the USA or an E3 in Europe, the electrical network may
reach only reach a short distance, before being fed into an ATM network, using CES. When
running PPP over the T3, which is fairly common, this essentially results in tunneling IP
over ATM. It is also possible to run PPP directly on top of ATM.
Finally, MPLS can be integrated into an ATM network. Essentially the signalling plane of
ATM is replaced with IP protocols. The VPI/VCI field in the ATM cells is used to provide
switching of MPLS labels. Then IP can be run on top of MPLS.
52
LAN Emulation
LLC
AAL5
ATM
PHY
Bridge
Bridge
LLC
LLC
IP
LLC
MAC
MAC
LANE Emulates 802.3 and 802.5 MAC/LLC
Can operate as part of the protocol stack
Can operate as part of an Ethernet/ATM bridge
Allows multipls virtual LANs on one ATM network
Preserves contents of 802.3 or 802.5 frames
§
§
§
§
§
LANE
LANE
LANE
AAL5
AAL5
ATM
PHY
MAC
LLC
ATM
ATM
PHY
IP
PHY
MAC
PHY
©2003–2004 David Byers
IP
LANE emulates a 802.3 or 802.5 MAC, allowing higher layers to be
unchanged. There are two important configurations for LANE – one which
allows devices to be directly connected to an ATM network by replacing the
low-level network drivers with LANE, and another which bridges between
legacy networks and an ATM network.
We won’t cover LANE framing in any detail, but it is worth noting that it in no
way alters the original LLC frames; it just adds a header.
53
LAN Emulation
LANE Components
LEC
LECS
LES
BUS
LAN Emulation Client
LAN Emulation
Configuration Server
Lan Emulation Server
Broadcast and Unknown
Server
§ LEC
„
§ LECS
„
Controls which clients belong to
which LAN; handles clients
joining and leaving the LAN;
address resolution
§ LES
„
Handles multicast, some data
forwarding
§ BUS
„
Manages broadcast and
multicast
©2003–2004 David Byers
LUNI LAN emulation User to
Network Interface defines
interaction between LE
clients and servers
Client software in ATM node
LANE is quite complex and requires a number of components.
The LEC is simply the client device. It could be a computer or Ethernet
switch. LEC is really the software in this device, but the distinction isn’t
really that important.
The LECS is a server on the ELAN (emulated LAN), which provides
configuration information. We’ll show the role of the LECS shortly.
The LES is a LAN emulation server, which provides for key functions such
as multicasting.
The BUS is a server that supports broadcasting and communication with
unknown clients (which is done by broadcasting). Since ATM is a nonbroadcast network, the BUS must maintain a point-to-multipoint connection
with all LEC’s.
Finally the acronym LUNI is often seen. It is the protocol between LEC and
servers.
54
LAN Emulation
Configuration
direct VCC
Control direct
VCC
LEC
LECS
LES
Configuration
direct VCC
Control direct
VCC
Control Distribute VCC
Multicast send
BUS
Multicast send
LEC
Multicast Forward VCC
©2003–2004 David Byers
Data Direct VCC
This diagram shows what communication channels typically exist in an
ELAN with two clients.
The control distribute and multicast forward VCCs are point-to-multipont
connections used for broadcasting and multicasting.
55
LANE Operation
§ Initialisation
„
Get address of LES and LECS
§ Configuration
„
Send ATM and MAC address
and frame size to LECS
§ Joining
„
„
Create VCC with LES
Send join request to LES
§ Registration
„
„
Send MAC to LES
Declare if LEC wants ARPs
§ Register with BUS
„
„
Find BUS (ARP for ff:ff:ff:ff:ff:ff)
Register to join multicast VCC
§ Address Resolution
„
„
Send LE-ARP to LES
LES may forward to LECs
§ Broadcast/multicast
„
„
Send frames to BUS
BUS forwards to all LECs
§ Data transfer
„
Get ATM address from LES
Set up SVC for data traffic
©2003–2004 David Byers
„
No notes with this slide.
56
LANE Operation
Address Resolution
Broadcast and multicast
§ Translate MAC address to ATM
address
§ LE-ARP sent to LES
§ LES may forward to LEC (e.g. in
case of bridged LAN, LEC may
need to ARP for the MAC
address)
§ Broadcast and multicast frames
sent to BUS
§ BUS forwards frames over
multicast ATM channel
LE-ARP
LEC
Forward to all LECs
Send
REPLY
LEC
LEC
LEC
LEC
LEC
©2003–2004 David Byers
New SVC
LES
BUS
No notes with this slide.
57
Classical IP Over ATM (CIOA)
Operation
§ Subdivision of the network
§ Contains hosts and routers
§ Served by LIS server
ARMARP for address resolution
Routers between LISes
©2003–2004 David Byers
Logical Independent Subnet
Classical IP over ATM is one of the simplest methods of transporting IP
over an ATM network. Here, IP hosts are classified into logical independent
subnetworks, which roughly correspond to LANs in an Ethernet/IP
environment. The main problem is address resolution – how does an IP host
know how to reach another IP host? This is handled through the use of
ATMARP. ATMARP is a query, sent to a LIS server, asking for the ATM
address corresponding to an IP address. The LIS server, which keeps track
of membership in the LIS much like a LES server does in LANE, responds,
and the IP host can then set up a SVC to the destination.
To communicate from LIS to LIS, regular routers are needed, even if all
LISes are on the same ATM network. These routers belong to both LISes,
and must reassenble IP packets and perform IP processing to forward
them. This is perhaps the most serious drawback of CIOA.
58
Next-Hop Resolution Protocol
Operation
§ Resolve layer 3 address to ATM
address of destination or next
hop towards destination
§ Direct connection set up across
multiple LISes
©2003–2004 David Byers
Operation
NHRP is a protocol for routing layer 3 packets over an ATM network, without involving
intermediary routers.
The concept of a LIS is still in NHRP, but the role of the servers, not called NHRP servers,
is different. When a device wants to send an IP packet to another device, it queries the
NHRP server for the next hop towards the destination. If the destination is on the same LIS
as the source, the NHRP server will respond with the ATM address of the destination, much
like an ATMARP reply.
If the destination is not on the same LIS, the NHRP server forwards the query to another
device that is closer to the destination. This process may be repeated several times until the
NHRP server serving the LIS on which the destination is located can reply with the ATM
address of the destination. This allows the source to set up a connection directly to the
destination, without involving intermediary routers.
If the destination is on a different ATM network, IP packets will be forwarded to an egress
router, which will have to perform IP processing to forward the packet to the destination.
While waiting for the NHRP answer, packets may be forwarded along the routed path. This
reduces overall latency as communication can take place even before a direct connection
can be set up.
A nice feature of NHRP is that is is possible to indicate desired QoS for the connection. I
don’t know if this has been standardised, but it is something that has been under
consideration.
59
MPOA
Very simplified
Principles
§ LANE + NHRP
§ Separate route calculation from
packet forwarding
§ Routers become route servers
Components
§ LANE provides backward
compatibility
§ NHRP extensions provide better
performance for MPOA-aware
clients
©2003–2004 David Byers
§ MPS: MPOA Server
§ MPC: MPOA client
Multiprotocol over ATM is an improvement over LANE, CIOA and NHRP.
The goal is to provide a protocol-agnostic layer 3 transport over ATM, while
still maintaining compatibility with legacy equipment. MPOA basically
accomplishes this by combining NHRP and LANE. It uses LANE for layer 2
forwarding and NHRP to set up direct layer 3 connections.
60
©2003–2004 David Byers
QoS
61
Traffic Characterisation
Additional parameters:
CDVT Cell Delay Variation Tolerance
Characterisation of randomness
in the source that may result in
exceeding the PCR
BT
Burst Tolerance
Computed from MBS to specify
how large bursts an ATM switch
accepts
©2003–2004 David Byers
PCR Peak Cell Rate
The maximum amount of
traffic that can be sent
SCR Sustained Cell Rate
Upper bound on average cell
rate
MBS Maximum Burst Size
Maximum number of cells
submitted at PCR
In ATM, traffic is characterised by a number of parameters, most of which
are listed on this page.
Note that CDVT and BT are not really characteristics of the traffic, but
parameters used to determine if actual traffic conforms to a negotiated level
of service.
PCR is the maximum rate at which cells are ever sent. SCR is an upper
bound of the average cell rate over a specific interval. It is not an average of
the cell rate over the lifetime of a connection. MBS is tha maximum burst
size, or the maximum number of cells that will ever be transmitted back-toback at the peak cell rate.
The CDVT and BT parameters are used in traffic policing. We’ll look at
exactly how they’re used later.
62
QoS Parameters
Cell loss rate
Ratio of lost cells to delivered
cells
CTD Cell transfer delay
Sum of fixed delays a cell
encounters along a path
CDV Cell delay variation
Sum of variable delays a cell
encounters along a path
MCR Minimum cell rate
Minimum cell rate guarantee
for ABR connections
©2003–2004 David Byers
CLR
ATM also specifies a number of QoS parameters. These are signalled
during call setup, and the set is not fixed. The ATM forum and the ITU
specify a different set of parameters, and others may be added later.
Popular parameters include the ones above. Note that the MCR parameter
is only used in available bit rate connections; a concept we haven’t talked
about yet.
63
Service Categories
CBR
rt-VBR
nrt-VBR
Constant bit rate
Real-time variable bitrate
Non-real-time variable
bitrate
ABR
UBR
Available bitrate
Unspecified bitrate
ATM Service Category
CBR
CLR
rt-VBR
nrt-VBR
Specified
MCR
Feedback
Unspecified
Unspecified
Specified
PCR & CDVT
SCR & MBS
UBR
Network
specific
Specified
CTD & CDV
ABR
N/A
Specified
N/A
N/A
Specified
N/A
Unspecified
Specified
N/A
©2003–2004 David Byers
Attribute
This table, which I nicked from somewhere, does a good job of summarising
traffic characterisation and QoS parameters with respect to various service
categories.
ATM supports constand and variable bitrates, which we’ve already talked a
little about. It also supports available bitrate, which is a service category that
requires the sender to adapt to congestion in the network. ABR is a lot like
best-effort with the exception that it is possible to specify a minimum cell
rate.
UBR is a special category. It gives not guarantees at all and has the lowest
precedence of all.
64
Examples
DS1 Circuit Emulation
§
§
§
§
§
CBR
CTD < 100ms
CDV < 5ms
PCR = 8000 cps
CDVT depends . . .
100 Mbps data traffic
§ ABR
§ PCR = 284940 cps
§ MCR = 2850 cps
GSM Voice Connection
rt-VBR
CTD < 100ms
CDV < 10ms
PCR = 1875
CDVT depends . . .
©2003–2004 David Byers
§
§
§
§
§
Here are some purely hypothetical examples of different applications of the
service types. The parameters are probably not currect. The service
categories probably are.
65
The ABR Service
§ Peak and minimum cell rate
§ Feedback-based reactive
congestion control
§ ABR cell rate varies between
MCR and PCR depending on
network load
CBR
Fixed at 1000
©2003–2004 David Byers
ABR
Varies between 2000 and 3000
The ABR service type is likely to be the most popular in the long run as it
provides the highest level of network utilisation. ABR and even VBR
services leave much capacity unused. With minimum cell rate guarantees,
ABR services can even be used for applications that previously would have
used VBR.
In ABR the sender is allowed to increase sending rate up to the PCR when
there is slack in the network, but is also required (or requested) to decrease
the rate when the network is congested. The sender may always send at the
minimum guaranteed rate, the MCR. Note, however, that ABR is not
intended for real-time applications. There are no guarantees of loss or
delay.
ABR is implemented using feedback. When the network starts to become
congested, the network informs the sender of this, and the sender is
required to reduce the rate of transmission. The feedback mechanism is
implemented using resource management cells, which are ATM cells with
the PTI set to 110.
The transmitting end device generates RM cells which are sent through the
network to the receiver, who then returns them to the sender. Along the
way, the network has the opportunity to alter the cells.
66
ABR Congestion Control
Control path
Explicit rate mode
§ Destination returns RM cells
§ Switches set ER in return cells
§ Source adjusts rate accordingly
©2003–2004 David Byers
Binary Mode
§ Switch sets EFCI in forward cells
§ Destination sets CI or NI in
return cells
§ Source adjusts rate accordingly
ABR provides two mechanisms for congestion control.
In binary mode, the sender is told to speed up, hold or slow down (so it’s
really ternary). In explicit rate mode the sending rate is specified by the
network.
This is all accomplished by making every 32nd cell a resource management
(RM) cell. When a switch receives an RM cell, it computes how much
bandwidth is available for the ABR connection, and writes that into the cell.
If may also set a bit called EFCI in the cell header to indicate congestion or
no congestion. The receiver, upon receiving the RM cell, removes the EFCI
bit and sets either ”congestion indication” or ”no increase” in the RM cell and
returns it to the sender. Along the way, switches may further modify the cell.
The sender, upon receiving an RM cell, must obey the indications in the cell.
It is possible for a switch to generate RM cells and send them to the sender.
This may be done if conditions change rapidly and the switch wants to notify
the sender without waiting for the RM cell to traverse the network all the way
to the receiver.
67
ABR Congestion Control
§ Every 32nd cell is a RM cell
„
„
„
„
„
ER set to desired rate
CCR set to current rate
MCR set to minimum rate
ID = 1, DIR = 0, BN = 0, CI = 0,
NI = 0, RA = 0
RM cells are sent through the
network
RM Cell
ID
B C N R
N I I A
ER
CCR
MCR
QL
QL (cont.)
SN
SN (cont.)
©2003–2004 David Byers
§ Switches modify EFCN or ER in
response to network load
§ Destination sets NI or CI and
returns RM cell
D
I
R
The RM cell has several fields.
ER is the rate that the sender may use. It is set by switches along the path.
A switch will never write a higher rate into the ER field than the one already
there. By the time the cell gets to the receiver, the ER field will contain a
rate acceptable to all switches along the path.
CCR is the current sending rate, set by the sender.
MCR is the minimum rate that the sender is willing to accept. This should
have been negotiated when the connection was admitted to the network.
The sender is guaranteed this rate.
ID is fixed to one. DIR indicates the direction of the RM cell. BN indicates if
the cell is a backward notification. CI indicates if there is congestion. NI
indicates that the sender may not increase the transmission rate. I forget
what RA was.
68
ABR Congestion Control
§ Propagation delay issues
„
„
„
125 miles at 622Mbps
means 1462 cells may be in
transit at once
Response to RM cells may
require another 1462 cells
May cause instability
§ Segmented control loops
RM cells to not travel endto-end but are returned by
switches
©2003–2004 David Byers
„
A problem with the RM-based scheme arises when the bandwidth-delay
product is high. Take a network with 125 miles of fiber running at 622Mbps.
In this network, there can be 1462 cells (don’t ask me to re-do the
calculations) sitting in the network at one time. By the time an RM cell has
traversed the network, nearly 3000 cells have been transmitted. Thus,
reaction to changing conditions, although fast in terms of real time, are slow
in terms of the cell rate. This can lead to instabilities, congestion and other
problems.
To counter this, it is possible to create segmented control loops, where RM
cells do not travel all the way to the destination, but are returned by
intermediary switches. A detailed discussion about segmented control loops
is beyond the scope of this lecture.
69
Preventive Congestion Control
Bandwidth Enforcement
§ Strictly enforce negotiated
traffic characteristics
©2003–2004 David Byers
Call Admission Control
§ Only allow connections
whose requirements can be
guaranteed
ABR uses reactive congestion control, but ATM traditionally was based on
preventive congestion control.
This is based on two principles: call admission control (CAC) and bandwidth
enforcement.
70
Call Admission Control (CAC)
Statistical allocation
§ Less than PCR is reserved
§ If all sources send at PCR,
requirements will exceed
available resources
§ Difficult to tune correctly
§ Better utilisation of
resources than nonstatistical allocation
©2003–2004 David Byers
Non-statistical allocation
§ Entire PCR is reserved
§ If senders to not send at
PCR continuously, wastes
resources
Call Admission Control (CAC) is the process of admitting a connection to
the network. The principle is that a connection is only admitted if its QoS
requirements can be met (and other QoS commitments can be kept).
A call begins with a signal from the caller that contains specifications of the
QoS requirements and traffic characteristics. Each switch along the path
from sender to receiver must allocate resources to meet the QoS
requirements; if a switch is unable to do so, an alternate path is attempted
until there are not more paths.
There are two basic principles for resource allocation: statistical and nonstatistical. Non-statistical allocation is appropriate for constant bit rate
applications, but leads to resource waste for VBR connections. Statistical
allocation, on the other hand, can lead to overcommitment. Here a model of
the traffic is used, and enough resources are allocated that statistically, all
QoS commitments can be met. If, for example, all VBR connections would
peak at the same time, however, enough resources to handle the peak
would not be available.
71
Bandwidth Enforcement
Policing
Mechanisms
§ Ensuring that a source does not
exceed the negotiated service
contract
§ Specifically, limit PCR and SCR
to negotiated values
§ Leaky bucket
§ Double leaky bucket
§ GCRA
Tokens
Tokens
K
K
Unbuffered leaky bucket
Buffered leaky bucket
©2003–2004 David Byers
Buffer
Traffic policing involves ensuring that a source does not send outside the
bounds of the traffic contract. The terms that a source must adhere to
typically involve PCR, SCR and MBS.
A simple mechanism for policing is the leaky bucket, which has been
covered in the basic course. The problem with this mechanism is that it is
difficult to tune the parameters so that all nonconforming cells are detected
and all conforming cells are passed transparently. Therefore, this is not the
mechanism preferred in ATM networks.
Instead, the generic cell rate algorithm is commonly used, and it can be
modeled as a double leaky bucket.
72
GCRA Virtual Scheduling
Start
yes
no
Cell not
compliant
yes
§
§
§
§
TAT = ts
TAT >
ts + τ
T = 1/PCR
τ = CDVT
ts – Actual arrival time of cell
TAT – Theoretical arrival time of
the next cell
T
no
Transmission
time
TAT = TAT + T
Arrival time
at the UNI
ts
ts ts
ts
©2003–2004 David Byers
TAT <= ts
Parameters
Consider an example
T=10 tau=15
Arrival times are 0, 12 18 20, 25, 30, 36
Cell 1: TAT=0, ts = 0, so the cell is conformant. TAT = 10
Cell 2: TAT=10, ts = 12, so the cell is conformant. TAT = 20
Cell 3: TAT=20, ts = 18, so the cell is early, but tau+ts=33, so the cell is
conformant
Cell 4: TAT=30, ts = 20, so the cell is early, but tau+ts=35, so the cell is
conformant
Cell 5: TAT=40, ts = 25, so the cell is early, and tau+ts=40, so the cell is not
conformant
Cell 6: TAT=40, ts = 30, so the cell is early, but tau+ts=45, so the cell is
conformant
Cell 7: TAT=50, ts = 36, so the cell is early, but tau+ts=51, so the cell is
conformant
The next cell will have to arrive at 46 or later to be conformant
Note how the value of tau is ”eaten up” by successive early cells.
73
GCRA Continuous Leaky Bucket
Start
Parameters
§
§
§
§
§
X’ = X – (ts – LCT)
yes
X’ <= 0
T = 1/PCR
τ = CDVT
ts – Actual arrival time of cell
X – State of leaky bucket
LCT – Last Compliance Time
no
X’ = 0
Cell not
compliant
yes
X’ > τ
Notes
§ X’ <= 0 – Cell is late
§ X’ > 0 – Cell is early
X = X’ + T
LCT = ts
©2003–2004 David Byers
no
Consider an example
T=10 tau=15
Arrival times are 0, 12 18 20, 25, 30, 36
Cell 1: X=0, LCT=0 ts=0 à X’ = 0: The cell is compliant; LCT=0, X=10
Cell 2: X=10, LCT=0, ts=12 à X’ = -2: The cell is compliant; LCT=12, X=8
Cell 3: X=8, LCT=12, ts=18 à X’ = 2: The cell is early, but X’ is less than
tau so it is compliant; LCT=18, X=12
Cell 4: X=12, LCT=18, ts=20 à X’ = 10: The cell is early but compliant;
LCT=20, X=20
Cell 5: X=20, LCT=20, ts=20 à X’ = 20: The cell is not compliant;
Cell 6: X=20, LCT=20, ts=25 à X’ = 15: The cell is compliant; LCT=25,
X=25
And so on.
74
Policing Both PCR and SCR
§ One GCRA for PCR
„
T = 1/PCR
τ = CDVT
GCRA(1/PCR, CDVT)
§ One GCRA for SCR
„
„
Ts = 1/SCR
τ = Burst tolerance =
(MBS – 1)(Ts – T)
GCRA(1/SCR, BT)
Non-compliant cells
Dropped or marked CLP=1
„
©2003–2004 David Byers
Compliant cells
queued for output
Bandwidth policing is a combination of policing the PCR and the SCR. This
is accmomplished by combining a GCRA for the PCR with a GCRA for the
SCR. First cells are tested for compliance with the PCR, then with the SCR.
So what about cells that are found to be non-compliant? What happens?
The simplistic thing to do would be to just drop them, but if there is slack in
the network, there is no reason not to send them. An option, therefore, is to
simply mark them with CLP=1, so that they can be dropped quickly later in
the network, if need be.
75
©2003–2004 David Byers
ATM Signalling
Signalling is a function mainly needed to manage SVCs. Setting up a PVC
can be a matter of manual configuration, but a SVC needs to be created
and torn down in real time. Therefore, signalling is a crucial part of ATM.
76
Signalling Protocol Stack
Signalling Protocols
Signalling ATM Adaptation Layer (SAAL)
ATM Layer
Physical Layer
§ VPI/VCI = 0/5 – Unassociated signalling for signalling regarding all VCCs;
default circuit
§ VPI/VCI = x/5 – Nonassociated signalling; signalling regarding VCs within
a specific VP
©2003–2004 David Byers
Signalling channel
Signalling protocols are nothing special in ATM. They are just protocols that
run on top of an AAL, as does everything else.
The SAAL, the AAL for signalling, is further subdivided into sublayers
77
SAAL
SAP
Service-Specific Coordination Function (SSCF)
Service-Specific Connection-Oriented Protocol (SSCOP)
Common Part
Convergence
Sublayer (CPCS)
Service-Specific
Convergence
Sublayer (SSCS)
SAAL
AAL5
©2003–2004 David Byers
SAP
The SAAL consists of two sublayers, much like other AALs. Its CPCS is
simply AAL5, which provides most of the transport functionality. The SSCS
is further subdivided into the Service-Specific Coordination Function and
Service-Specific Connection-Oriented Protocol.
The SSCOP provides assured transfer of frames across a network with a
high bandwidth-delay product. It was designed for ATM as earlier protocols,
such as LAP-D in ISDN use a simple ARQ scheme, which is not effective in
ATM networks.
The SSCF maps the services provided by the SSCOP to services required
by the user of the SAAL.
78
Metasignalling
§ Establish
„
„
Establish a signalling channel
Point-to-point, broadcast or
selective broadcast
ASSIGN REQUEST
ASSIGNED/DENIED
REMOVE REQUEST
§ Release
„
Remove a signalling channel
REMOVE
ASSIGNED/DENIED
§ Verify
Verify status of signalling
channel
CHECK REQUEST
CHECK RESPONSE
©2003–2004 David Byers
„
Beyond the normal signalling, there is need for metasignalling, for
establishing and tearing down signalling channels. Metasignalling, since it
deals with only one thing, can be very simple. Metasignalling is not needed
if there is a default signalling channel, which there currently is in ATM.
There are three functions: establish a signalling channel, tear down a
signalling channel and check the status of a signalling channel.
79
Signalling Protocols
Q.2931
Sidebar: SS7
§ User-to-UNI protocol for pointto-point curcuits
SS7,
SS7, Signalling
Signalling System
System 7,
7, also
also
known
known as
as C7
C7 and
and CCIS7,
CCIS7, is
is an
an ITU
ITU
standard
standard for
for out-of-band
out-of-band signalling
signalling
protocol
protocol in
in the
the telephone
telephone network.
network.
It
is
designed
to
It is designed to replace
replace earlier
earlier ininband
band signalling
signalling systems.
systems.
Q.2971
§ User-to-UNI protocol for pointto-multipoint circuits
B-ICI
§ Signalling between networks
§ Based on B-ISUP which is based
on SS7
SS7
SS7 deals
deals with
with issues
issues such
such as
as call
call
setup,
setup, call
call routing,
routing, caller
caller ID,
ID, multimultipart
part calls,
calls, toll-free
toll-free calls,
calls, as
as well
well as
as
OAM
OAM issues.
issues.
PNNI
©2003–2004 David Byers
§ Private NNI-NNI signalling
For communication between the UNI and network, there are two protocols in
use today, although since signalling is noting special in ATM, other
protocols may be used.
Q.2931 is used to set up point-to-point connections. Q.2971, which uses
Q.2931, is used for point-to-multipoint connections. These are both ITU
standards and they are endorsed by the ATM forum (which, in fact, was
instrumental in designing them).
For signalling between network elements, ATM differentiates between
private and public signalling. Private signalling takes place between NNIs
that are under common control, e.g. the network belonging to a particular
carrier. Public signalling takes place between carriers. ATM uses PNNI for
private signalling and B-ICI for public signalling.
PNNI includes all the features necessary to perform routing and set QoS
parameters. B-ICI is more limited. It does not support routing (inter-carrier
links are set up statically) and only supports a subset of the services that
ATM can provide.
B-ICI is based on SS7, the signalling system used in the telephone network.
In addition to these there is AINI and public UNI, both used for public
signalling. AINI may take the place of B-ICI as the design allows better
80
ATM Signalling Protocols
Public UNI or
AINI or
B-ICI
Q.2931
AT&T
Q.2931
MCI
©2003–2004 David Byers
PNNI
81
Q.2931
Notes
Call Establishment
§ Used to establish point-to-point
connection
§ Three groups of message types
§ Each message contains
information elements that can
convey e.g. QoS requirements
§ ALERTING, CALL PROCEEDING,
CONNECT, CONNECT
ACKNOWLEDGMENT, SETUP
Call Clearing
§ RELEASE, RELEASE COMPLETE
Miscellaneous
©2003–2004 David Byers
§ NOTIFY, STATUS, STATUS
ENQUIRY
Q.2931 is the protocol used to communicate with the UNI. It handles
establishment of point-to-point connections. The protocol is very flexible in
that the message format is extensible. Each message consists of a
message type, fixed message parameters and a set of information
elements. These elements can express such things as QoS requirements or
client capabilities.
There are three groups of message types: messages for call establishment,
messages for call clearing and messages for various other things. Call
establishment messages deal with setting up connections. Call clearing
messages deal with tearing them down. The rest deal with getting
information from the network, such as querying the status of a connection.
82
Q.2931
ALERTING
§
Sent by called party to notify
network that it is alerting a human
CALL PROCEEDING
§
Sent by called user to indicate that
the call is being initiated
CONNECT
§
Sent by the called user to indicate
that call is accepted
RELEASE
§
Sent by user to request clear a
connection
RELEASE COMPLETE
§
Sent by user to indicate that clearing
is complete
SETUP
§
Sent by user to initiate establishment
of new call
CONNECT ACKNOWLEDGMENT
Send by network to called user to
indicate that connect is OK
©2003–2004 David Byers
§
This slide lists some of the specifi messages in Q.2931.
83
Adressing in ATM
DCC ATM
AFI
DCC
High Order-DSP
ESI
SEL
High Order-DSP
ESI
SEL
ESI
SEL
ICD ATM
AFI
ICD
E.164 ATM
AFI
DCC
IDC
ESI
SEL
E.164
Authority and Format Identifier
Data Country Code
International Code Designator
End System Identifier
Intra-Endpoint Selector
HO-DSP
Initial Domain Part (IDP)
Initial Domain Identifier (IDI) underlined
Domain-Specific Part
Administered by authority identified by IDI
©2003–2004 David Byers
AFI
ATM devices all have unique ATM adresses. Depending on whether the device is on a
public or private network, it will use different adresses. Devicec on a public network use
E.164 adresses, whereas devices on private networks use OSI NSAP adresses.
E.164 adresses consist of 16 digits, each encoded in BCD format, using four bits. The first
digit determines if the adress is unicast or multicast; the next three indicate a country code
and the remaining digits indicate a city codde, exchange code and end device identifier.
E.164 addresses are more popularly known as ”telephone numbers”. Or almost so anyway.
Private adresses consist of two parts, the Initial Domain Part (IDP) and the Domain-Specific
Part (DSP). The IDP specifies the authority who assigns values for the DSP. It is further
subdivided into Authority and Format Identifier (AFI) and Initial Domain Identidier (IDI). The
AFI indicates the format of the IDI.
The ATM forum has defined three IDIs:
1. DCC (Data Country Code); the DCC consists of a two-byte country code according to
ISO3166 and adresses are adminisered by the ISO’s national member body in each
country.
2. ICD (International Code Designator); The ICD identifies an authority which administers a
coding scheme; the registration authority for the ICD is maintained by the British standards
institute.
3. E.164 Adresses
The AFI identifies what kind of adress it is
The IDI identifies the specific authority within the adressing scheme
The high-order DSP is an adress administered by the coding scheme authority
The ESI is an end-system identifier, within the particular adress identified by the HO-DSP
and IDP
The SEL is to select an endpoint within an end system.
The HO-DSP field may be further subdivided into fields, depending on the coding scheme.
For example, it may include topological information.
84
Q.2931 Call Example
ATM
Network
User
User
SETU
P
PROC
CALL
SETU
P
PROC
CALL
NECT
CON
CON
N AC
K
NECT
CON
Q.2931
PNNI
Q.2931
©2003–2004 David Byers
CON
N AC
K
This shows how a call is set up using Q.2931. Note that Q.2931 is only used
between the user and the UNI. Within the ATM network other protocols,
typically PNNI are used.
85
Q.2931 Release Example
ATM
Network
User
User
RELE
ASE
OMP
REL C
RELE
ASE
OMP
REL C
PNNI
Q.2931
©2003–2004 David Byers
Q.2931
This demonstrates how a call is torn down. Again, Q.2931 is only used
between the user and the UNI. PNNI is used elsewhere.
86
Q.2971
Notes
Message types
§ Use to establish point-tomultipoint connections
§ Starts with point-to-point
§ Can add and drop parties
§ Root: Sender
§ Leaf: Receiver
§ ADD PARTY, ADD PARTY ACK,
PARTY ALERTING, ADD PARTY
REJECT, DROP PARTY, DROP
PARTY ACK
Leaf-Initiated Join
©2003–2004 David Byers
§ A leaf can request to be added
to a multicast tree
ATM supports more features than Q.2931. In particular, Q.2931 only deals
with point-to-point connections. While these are the most common, ATM
supports point-to-multipoint, multipoint-to-point and multipoint-to-multipoint
connections as well. Q.2971 can be seen as an extension to Q.2931, which
supports point-to-multipoint connections. An interesting point of this protocol
is that it not only permits the point to add nodes to a call, but it also permits
leaves to add themselves to an existing point-to-multipoint connection. This
is particularly useful in systems such as MPOA, where a new node might
want to join the existing multicast connection used to simulate LAN
broadcast.
87
PNNI
PNNI Signalling Protocol
§ Discovery of neighbors and link
status
§ Synchronization of topology
databases
§ Summarization of topology state
information
§ Construction of routing
hierarchy
§ Call clearing
§ Call establishment
§ Dynamic call setup
©2003–2004 David Byers
PNNI Routing Protocol
Q.2931 and Q.2971 deal with signalling between the user and the UNI.
Within a private network, other protocols are used. The predominant
protocol is PNNI version 2.0. I will describe some of the features of PNNI,
but mainly stick to features that were already in version 1.0. Version 2.0
offers such features as leaf-initiated join, security and call coordination
(allowing the network to re-establish a failed connection).
PNNI is similar to the user-UNI protocol, but also includes features to deal
with issues such as routing. The PNNI is divided into a routing part and a
signalling part. The routing part is concerned with computing topology and
reachability information, whereas the signalling part is concerned with
establishing and clearing connections, and uses the topology and
reachability information created by the PNNI routing.
The protocols are quite complex, so we will only cover them from a high
altitude. There will be hand-waving. There will be proof by obfuscation. If
you want the details, the specifications are available on-line from the ATM
forum. They’re actually quite readable.
88
©2003–2004 David Byers
Physical Network
PNNI routing is a departure from the routing protocols used in IP. In some
ways it is similar to link-state protocols such as OSPF and IS-IS, but it takes
the concepts much further.
PNNI is a link-state protocol, and as all link-state protocols this implies that
every node has full information about the network topology. In reality, this is
not the case since such a system would not scale very well. A network with
30 nodes would be no problem. One with 300 might work. With 3000 nodes,
forcing every node to have full information (and flooding all changes to all
nodes) would be disasterous. To counter this problem, link-state protocols
create hierarchies. In OSPF, networks are organized in two-level
hierarchies. Specifically, the network is divided into areas, one of which is
designated a backbone area and carries all inter-area communication (well,
not really, since it is possible to fudge direct inter-area links). This improves
scalability, but only to a certain point, since areas cannot be further
subdivided.
PNNI takes the idea to its conclusion. It allows the network administrator to
create arbitrary hierarchies, resulting in a highly scalable protocol. Nodes in
the network end up having complete information about their immediate
surroundings, and summarized information about everything else. The level
of detail decreases with logical distance from a node.
The hierarchy starts with the physical network. Nodes all have addresses
(AESA – ATM End-System Address) and are configured to belong to socalled peer groups.
89
Lowest Hierarchy Level
PG(B.2)
B.2.1
PG(A.2)
B.2.2
A.2.3
B.2.3
PG(B.1)
PG(A.1)
A.2.2
PG(A.3)
A.2.1
A.1.3
B.1.1
B.2.5
B.2.4
B.1.2
A.3.1
A.4.1
A.1.2
A.3.4
A.3.2
A.4.2
A.3.3
A.4.3
A.1.1
A.4.4
A.4.5
B.1.3
C.1
C.2
A.4.6
Peer group
Logical Link
PG(A.4)
Border node
Peer group leader
©2003–2004 David Byers
PG(C)
In a large network, maintaining complete information about the topology is
neither practical or desirable. It requires a large amount of memory, and
changes to the topology, regardless of what parts of the network they affect,
will be flooded throughout the network, wasting capacity. To counter this,
many routing protocols, including OSPF, IS-IS and PNNI create a
hierarchical abstraction of the network.
In PNNI, nodes are grouped into peer groups. The peer group a particular
node belongs to is configured in the node itself – it is not derived from the
network topology. At startup, nodes exchange HELLO messages to
determine their immediate neighbors, and then construct PNNI Topology
State Elements, which describe the immediate surroundings, and flood
these to all nodes in the peer group. By doing this, all nodes will have
complete topology information for their own peer group, and will be able to
route traffic within the peer group. Nodes also flood reachability information
(the addresses each node knows how to reach) through PTSEs. Within
each peer group a peer group leader is elected. This node is responsible for
coordinating actions that involve the entire peer group, such as information
exchange with other peer groups.
Border nodes exchange information with border nodes in other peer groups
regarding the peer group they belong to. This allows neighboring peer
groups to determine if they belong to the same higher-level peer group,
which is necessary to construct the next level of the hierarchy.
90
Second Hierarchy level
PG(A)
B.1
A.1
B.2
PG(B)
A.4
A.2
©2003–2004 David Byers
A.3
Peer groups are represented by logical peer group nodes, implemented by
the peer group leader. Each such node has an end system address, just like
a physical node, so connections can be set up between it and other nodes.
Logical peer group nodes are organized into higher-level peer groups.
As higher level peer groups are performed, three kinds of summarization
takes place. Address summarization abstracts the addresses reachable
within a peer group, decreasing the level of detail in the reachability
information passed to other peer groups. Topology summarization
summarizes the internal topology of peer groups, essentially reducing them
to multi-port nodes. Link summarization summarizes the logical links
between peer groups.
Various information is passed up and down the hierarchy. Higher-level
nodes pass topology information about the higher level down to the peer
groups, allowing them to deduce reachability to other peer groups. Peer
groups, in turn, pass information about local reachability and topology up to
the peer group nodes, giving them information that is passed to other peer
groups.
91
Uplinks
A.2
A.1
A.3
PG(A)
A.4
A.4.1
A.4.2
A.3.1
A.4.3
A.3.2
A.3.4
A.4.4
A.3.3
A.4.5
A.4.6
PG(A.4)
©2003–2004 David Byers
PG(A.3)
Border nodes need to advertise connectivity to other peer groups. This is
accomplished by advertising an uplink; a link connecting the border node to
the peer group node for the neighboring peer group. The uplinks are used
by peer group leaders to construct the higher-level hierarchy – the
connections between peer groups.
92
©2003–2004 David Byers
Complete PNNI Hierarchy
This picture shows the entire PNNI hierarchy for the example network.
93
Single Node Perspective
A
B
C
A.2
A.1
A.3
A.4
A.3.1
A.3.3
©2003–2004 David Byers
A.3.2
A.3.4
As mentioned earlier, nodes have increasingly less detailed information
about the network, the farther away from the node the network is. Viewing
the information held by a single node, this is evident.
Nodes know everything about their peer group as topology and reachability
information is flooded through the peer group. This is the most detailed
level.
Nodes know about their parent peer group, but only have summary
information about the other peer groups within the parent peer group. This
information is flooded from the parent peer group (the peer group leader)
into the network.
Similarly, nodes know about higher-level ancestor peer groups, but only
have summary information about nodes in these peer groups.
94
PNNI Signalling
Source routed
Designated Transfer List (DTL)
§ Allows different path selection
algorithms on different nodes
§ Works even if topology
information is inconsistent
§ List of nodes to pass on the way to
the destination
§ A stack of paths
Why not hop-by-hop
Crankback
§ Rolling back a call partway to try a
different route
©2003–2004 David Byers
§ Sensitive to inconsistencies in
topology databases
§ Sensitive to inconsistencies in
routing algorithms
Call Admission Control (CAC)
§ Procedure for deciding whether to
admit a requested call
PNNI, unlike IP, uses source routing. Although it would have been possible to use hop-byhop routing, the ATM forum has identified problems with hop-by-hop routing that make it
unsuitable to ATM.
Hop-by-hop routing may result in loops since routers do not have complete information
about the entire path taken by a connection. Loop prevention is a huge part of routing
protocols for hop-by-hop routing. Loops are generally caused by one of two things:
inconsistency in routing decisions caused by the use of different routing algorithms; or
inconsistency in routing databases (mainly due to changes in topology information that have
not propagated fully).
This implies that hop-by-hop routing requires the path selection mechanism to be fully
specified and implemented identically on all nodes. Evolution in the path selection
mechanism is not possible. Furthermore, due to inconsistencies it is possible to construct
far-from-optimal paths that are loop free. In a non-connection oriented network this is not a
major problem since the situation will resolve itself. In a connection-oriented network,
however, the cost of bad routing decisions will be paid through the lifetime of the
connection.
Source routing requires nodes to have complete topology information. PNNI accomplishes
this by using a hierarchical model which relieves nodes of knowing the details of the entire
network. Inconsistencies in topology databases are rarely fatal since loops will never be
formed. Similarly, since only one node is involved in the computation of the route, different
nodes can use different path selection algorithms.
PNNI uses source routing with crankback. Nodes create something called a DTL, which is
essentially a stack of paths describing how a connection could be routed. Crankback is the
process of undoing a connection part of the way and trying a new path if it is discovered that
the selected path will not admit the connection.
The algorithms for path selection (construction of the DTL) and call admission are not
specified in the PNNI specification as they do not have to be standardized.
95
PNNI Routing Example
B
A
C
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
In this example, the green box on the left wants to set up a SVC to the
green box on the right. The PNNI hierarchy is illustrated in the slide. Here,
A.1.2 has to decide a route to the destination. Recall that A.1.2 has
complete information about its own peer group and its ancestors. This
implies that as far as A.1.2 is concerned, the destination is B, the top-level
group node containing the destination. Reachability information will have
been summarized to indicate that the destination can be reached through B.
There are a number of different options for A.1.2. One would be A.1.2 to
A.1.1 to A.2 to B. Another is A.1.2 to A.3 to B. A third is A.1.2 to A.1.1 to A.2
to A.3 to B. Which is most appropriate depends on the current conditions in
the network. The shortest path is not always the best path. Looking at the
last of these four paths, note that the actual path through the network will be
far more complex. Again, there are several options. The exact path will be
determined by other nodes along the path – for example, A.1.2 does not
specify routing through peer group B – nodes in B will have to amend the
route to contain these details.
Let’s walk through an example of how path selection works. It all starts with
a SETUP message being sent over the UNI. A.1.2 will then construct a DTL
and issue a SETUP message of its own to the next node in the DTL. This
will then progress until the SETUP message reaches the destination.
96
SETUP Rules (Simplified)
Advance
current elem
X = Current
DTL element
Create DTL
to X
X = Me?
Is X a
neighbor?
Top DTL
Empty?
CAC
Pop top-level
DTL
Create DTL
to destination
Allocate SVC
Send SETUP
CAC
OK?
RELEASE
DTL Stack
Empty?
©2003–2004 David Byers
START
Once the source route has been set up, the calling NNI issues a SETUP call containing a DTL stack.
The DTL stack contains one DTL per level in the PNNI hierarchy. For the path A.1.2 A.1.1 A.2 A.3 B,
three DTLs would be required. One containing A.1.1 A.1.2, one with A.1 A.2 A.3 and one with A B.
Note that the starting node is part of the DTL. Each DTL also has a pointer. In the top-level DTL it
indicates the next node in the list. In the other levels it indicates the current element of the DTL.
When a node receives a SETUP containing a DTL and is not the destination, it checks the top-level
DTL. If its address is at the top, it moves to the next element, if there is one, performs CAC to allocate
a channel to the node represented by the new current element and forwards the SETUP call to the
node indicated by the new current element.
If the top-level DTL is exhausted, the node pops it from the stack and essentially starts over. At this
point, the current element may not be the node itself, but is likely to be a node in a higher level of the
PNNI hierarchy. The node searches for a way to reach the new current node. If there is a direct
connection, it can proceed as before. If there is no direct connection it has to construct a new DTL
and place on the top of the stack, indicating the path to the new current node.
If the entire DTL stack is exhausted, the current node will have to construct a DTL (possibly a stack)
to the destination.
If the node adds a DTL to the stack it is part of the routing process and may have to participate in
crankback. If this is the case, it saves sufficient information to compute an alternate DTL, should
crankback take place.
If the node cannot allocate the required resources for the next step, it will respond with a RELEASE
message, indicating that crankback is taking place. This message is propagated backwards along the
path that has been set up until it reaches a node that has taken part in creating the path. At this point,
that node may attempt to create an alternate path for the DTLs that it added. If it cannot construct an
alternate path, it forwards the RELEASE message back along the path.
97
Now for a real example. The green box on the left wants a SVC to the green box on the
right. The link A.3.1 to B.2.3 cannot support the PCR that is requested. The link from A.3.3
to B.1.2 is down, but A.1.2 doesn’t know that.
A.1.2 selects the following path: A.1.2 A.1.1 A.2 A.3 B resulting in a three-level stack of
DTLs. Each DTL is marked with a level indicating the level in the PNNI hierarchy that the
DTL corresponds to. The top of the stack is the DTL for the path within A.1. It consists of
A.1.2 A.1.1 and is at level 96. The next element is the path through peer group A, and
consists of A.1 A.2 A.3, and it at level 72. The final element is the path within the top-level
peer group, A B, and is at level 64.
A.1.2 sends the SETUP message to A.1.1. This node will pop the top DTL since the target
of that DTL has been reached and send the SETUP message to A.2, which is the current
destination in the next DTL. In this case, A.1.1 knows that its neighbor A.2.2 is in A.2, so it
forwards the message to A.2.2 without altering the DTLs.
A.2.2 sees that the current destination is A.2. Since it is in A.2, it looks at the next element,
A.3. To reach A.3 it adds the DTL A.2.2 A.2.3 with level 96 to the top of the stack and
forwards the SETUP to A.2.3. Since A.2.2 added a DTL, it may be involved in alternate
routing later, so it saves information for this eventuality.
A.2.3 determines that the top DTL is exhausted, pops it and examines the new current
destination, A.3. A.2.3 has a neighbor, A.3.4, that is in A.3, so it forwards the SETUP there.
A.3.4 notes that the target of the top DTL is reached, pops it from the stack and examines
the new current destination, which is B. A.3.4 builds a route to B and places the DTL A.3.4
A.3.2 A.3.3 with level 96 on the top of the stack. Since it added a DTL to the stack, it saves
information so it can participate in alternate routing later.
A.3.2 receives the SETUP and forwards it to A.3.3. A.3.3 removes the top DTLs, noting that
the new current destination is B. At this point setup blocks since A.3.3 cannot create the
connection to B.1.2, and must initiate crankback. It constructs a RELEASE message
indicating the link that could not be used and the reason for failure, and sends the message
back to A.3.2. The release message has crankback level 96 since that is the level of the
current DTL.
Since A.3.2 did not add any DTLs it simply tears down SVCs that have been allocated for
the call and forwards the message to A.3.4. A.3.4 did create DTLs and so attempts
alternate routing. After removing the failed link from consideration, it determines that there
are to paths across A3 to B. One is A.3.2 A.3.3 A.3.1 and the other is directly to A.3.1.
However, the link from A.3.1 to B cannot support the PCR requested, so A.3.4 refrains from
attempting to allocate this path, and sends the RELEASE message back to A.2.2. The
crankback level is set to 72, since that corresponds to the level of the higher-level DTL,
generated by A.1.2.
A.2.2 did create an DTL, but will not attempt alternate routing since the crankback level
indicated by the release message is higher than the level of the DTL it created. Eventually
the RELEASE message is received at A.1.2, which constructs an alternate route to the
destination, this time selecting A.1.2 A.1.1 A.2 B. The troublesome links in A.3 are
eliminated from consideration. The new DTL stack is A.1.2 A.1.1, A.1 A.2 and A B.
The new SETUP is forwarded to A.1.1, which removes the top DTL and forwards the
SETUP to A.2.2. A.2.2 pushes a new DTL A.2.2 A.2.1 onto the stack and forwards to A.2.1.
A.2.1 removes the top DTL and forwards to B.1.1. B.1.1 notes that the destination of the
current DTL has been reached. The current DTL in this case is the DTL A B. Now B.1.1
needs to construct a new source route to reach the destination. It computes the path B.1.1
B.1.3 B.2 B.3, pushing two DTLs onto the stack: B.1.1 B.1.3 and B.1 B.2 B.3. It forwards the
call to B.1.3, which pops the top DTL and forwards to B.2.2. B.2.2 finds a path to B.3 and
pushes B.2.2 B.2.3 onto the DTL stack. The call is forwarded to B.2.3, which pops the top
level and forwards to B.3.4. B.3.4 in turn computes a path to B.3.3: B.3.4 B.3.1 B.3.4. The
call is forwarded to B.3.4, which notes that the destination has been reached.
The overall path is thus A.1.2 A.1.1 A.2.2 A.2.1 B.1.1 B.1.3 B.2.2 B.2.3 B.3.4 B.3.1 B.3.3.
98
PNNI Routing Example
B
A
C
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
99
PNNI Routing Example
DTL Stack
A.1.2 A.1.1
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
100
PNNI Routing Example
DTL Stack
A.1.2 A.1.1
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
101
PNNI Routing Example
DTL Stack
A.2.2 A.2.3
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
102
PNNI Routing Example
DTL Stack
A.2.2 A.2.3
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
103
PNNI Routing Example
DTL Stack
A.3.4 A.3.2 A.3.3
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
104
PNNI Routing Example
DTL Stack
A.3.4 A.3.2 A.3.3
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
105
PNNI Routing Example
DTL Stack
A.3.4 A.3.2 A.3.3
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
106
PNNI Routing Example
DTL Stack
No alternate!
B
A
A.1 A.2 A.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
107
PNNI Routing Example
Wrong crankback
level!
B
A
DTL Stack
C
A.1 A.2 A.3
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
108
PNNI Routing Example
DTL Stack
A.1.2 A.1.1
B
A
A.1 A.2
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
109
PNNI Routing Example
DTL Stack
A.1.2 A.1.1
B
A
A.1 A.2
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
110
PNNI Routing Example
DTL Stack
A.2.2 A.2.1
B
A
A.1 A.2
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
111
PNNI Routing Example
DTL Stack
A.2.2 A.2.1
B
A
A.1 A.2
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
112
PNNI Routing Example
DTL Stack
B.1.1 B.1.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
113
PNNI Routing Example
DTL Stack
B.1.1 B.1.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
114
PNNI Routing Example
DTL Stack
B.2.2 B.2.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
115
PNNI Routing Example
DTL Stack
B.2.2 B.2.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
116
PNNI Routing Example
DTL Stack
B.3.4 B.3.1 B.3.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
117
PNNI Routing Example
DTL Stack
B.3.4 B.3.1 B.3.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
118
PNNI Routing Example
DTL Stack
B.3.4 B.3.1 B.3.3
B
A
B.1 B.2 B.3
C
AB
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
119
PNNI Routing Example
B
A
C
A.2
B.1
A.1
B.2
B.3
A.3
C.1
C.2
B.3.2
A.2.1
B.1.1
A.2.2
B.1.2
B.2.1
B.2.2
B.3.4
B.3.1
B.3.3
A.3.2
A.1.2
B.1.3
A.2.3
B.2.3
B.3.5
A.3.3
A.3.4
A.3.1
©2003–2004 David Byers
A.1.1
120
©2003–2004 David Byers
THE BEGINNING IS THE END
THE END IS THE BEGINNING
121