CET 458/598
Fall 2000
Lecture Notes
Chapter 1 – Introduction
Section 1.1 mostly terminology
Connectivity, scale, link, node, point-to-point, multiple-access, switched network
(circuit vs packet switched, similarities to telephone system and snail-mail), storeand-forward, router, gateway, internetwork, network address, routing, unicast vs
multicast vs broadcast, multiplexing (and demultiplexing), STDM, FDm,
statistical multiplexing (why) packets vs messages, fragmentation, QoS (quality
of service), congestion, client/server communication, streaming vs bursty
communication, bandwidth, throughput, latency, propagation delay,
Classes of network failure: bit errors (bursty or not), packet loss/delay, node/link
failure
Calculation related:
Latency = propagation delay + transmission time + queuing
RTT = 2 × latency ~ 2 × delay × bandwidth
Prop delay = distance/speed of light in the media
Trans Time = size/bandwidth
Delay-bandwidth product importance
Bits/byte accurate bits for KB, Kb, MB, Mb – 8, 8×210, 210, 8×220, 220
Bits/sec accurate bits/sec for KB/s, Kb/s, MB/s, Mb/s – 8×103, 103, 8×106, 106
Section 1.2 some more terminology
Network architecture, layering and levels of abstraction, service interface, peer
interface, protocol, protocol graph, protocol specification, encapsulation, header,
trailer, payload, frame, packet, message, OSI vs Internet particularly their
protocol graphs in Fig 1.17-1.19.
Network Design Requirements
1. generality (flexibility)
2. cost effective
3. fair (access)
4. robust (reliable
5. high-performance
6. scalability (to huge #’s of nodes)
7. adaptable to i) changes in technology, ii) changes in application demands
(new applications the users want)
8. user communication
9. graceful degradation
Section 1.3 skipped about and left out the programming
UDP vs TCP, connectionless vs connection oriented, datagram vs virtual stream
Socket – end-point of a communication (not just the node)
Address -> (host, port) where socket is associated with a port
Implementation model
Process or thread per protocol
Process or thread per message
Procedure calls cheap compared to process/thread context switches
Upcalls
1
CET 458/598
Fall 2000
Lecture Notes
Additional Material (not needed for exam 1)
Critique of OSI
At one time thought to be THE Model. Now it appears in decline but is useful
pedogogically. Reasons for failure are 1) bad timing, 2) bad technology, 3) bad
implementations, and 4) bad politics.
1) Bad timing - the time between peak research activity and expentitures should be
long enough for good standards to be written. This didn't happen. Patially because
of delays in model definition due to politics and partially because TCP/IP was
already around.
2) Bad technology - a) the session and presentation layers existance were driven by
politics and really had no need to exist (holdovers from SNA). b) data link and
network layers were too full and connection oriented because they were defined
from a communications (analog) mentality. Eventually they were split into
sublayers and connectionless technology (proven need from TCP/IP arena) was
added. => extreme over commplication.
3) Bad implementations - due to the complexity initial (proof of concept)
implementation were slow and some have never appeared.
4) Bad politics - "belief . . . [that] a bunch of government bureaucrats [were] trying to
shove a technically inferior standard down the throats of the poor researchers and
programmers down in the trenches" (AST in Computer Networks 3rd ed) caused a
lot of resistance (foot dragging). At least partially true about technology.
Critique of TCP/IP
The model once expounded upon (years late) doesn't clearly define services,
interfaces or protocols nor their interactions. Host-to-network layer not a layer but
a poorly defined interface. No mention of Data Link or Physical layers. IP and
TCP well thought out and implemented but auxillary protocols ad hoc and poorly
constructed. Most need updating or have been superseded.
Chapter 2 – Direct Link Networks/Data Link Layer
What the data link layer does
0. use a physical connection (wire, fiber, space)
1. encode bits
2. frame bit sequences
3. detect errors
4. compensate for errors
5. media access control
Link types (physical media) – twisted pait, coaxial cable, optical fiber, free space (air or
vacuum -> wireless, satellites)
Frequency (in hertz)
Wavelength(λ) = c*df/frequency
where df = degradation factor, dependent on medium
digital -> o’s and 1’s so need to modulate electromagnetic wave to encode bits this is
done by varying frequency, amplitude or phase if nor DC levels
full-duplex and half-duplex circuits
2
CET 458/598
Fall 2000
Lecture Notes
(Chapter 2 continued)
codec – coder/decoder
Shannon’s Theorem – gives an upper bound to capacity of a link (bps) as a function of
signal-to-noise ratio (dB) - this is a practical limit
C = Bw log2(1+S/N)
Note: Nyquist’s Theorem gives a theoretical and absolute limit
Max data rate in bps = 2×Bw×log2 V where V is number of signal levels
(A,H,V)DSL, SONET,
encoding – NRZI, manchester, 4b/5b
framing – imposes structure on bit stream
type – byte oriented, bit oriented, byte counting, bit/byte stuffing, non-data symbols
Error detection
Error detecting vs error correcting codes, CRC, parity (vertical and horizontal)
Reliable Transmission
Above codes help bit error but if can’t correct must retransmit or if lose entire frame must
resend. So use acknowledgement (ack) to say received. Nack says damaged. Leads to
stop-and-wait then on to sliding window process. Need to understand sliding window and
relationship of window size and max-sequence number. Where do timeouts occur? What
happens to duplicate packets? What is a piggybacked ack? When would you expect an
explicit ack? When would an ack cover > 1 frame? What is a selective ack?
Ethernet
Speeds, cable types, repeaters, bridges, spanning tree algorithm (need, high lights), access
control since shared media (CSMA/CD <- carrier sense multiple access with collision
detection), addresses (48 bit unique), type vs length in 802.3, p-persistent, exponential
back-off, jamming signal, when does it work best?
FDDI
Token, monitor, token holding time, token rotation time, priority/real-time support, token
loss
Wireless
802.11, different physical “media”, spread spectrum, direct sequence (chipping code),
hidden nodes and exposed nodes, access control -> RTS & CTS & ACK, distribution
system (stationary part)
3
CET 458/598
Fall 2000
Lecture Notes
Additional Media Access information
So how can the single "broadcast" channel be allocated to the competing users? static or
dynamic allocation
Static allocation
advantage - simple and easy
disadvantage - inefficient: wastes bandwidth on unused subchannels and limits
available bandwidth to users that produce bursty traffic
Dynamic Allocation
analysis model assumptions
1. independent stations (nodes) - each node generates frames with a constant (and
independent) probability 
2. only a single channel is available
3. collisions (frames interfering) are detectable by all stations
• collisions are the only transmission/reception errors
4 frame transmission can begin at any time (continuous) or must begin at the start
of a discrete interval (slot)
5. carrier (transmissions) can/cannot be sensed
1-persistent CSMA
• node listens before sending
if channel busy waits till not busy
when not busy transmits
if collision detected waits random amount of time & tries again
• called 1-persistent because node transmits with probability 1 whenever it finds the
channel idle
• the diameter of the network (and medium, copper or air) effects the longest
propagation delay of the signal (carrier) between the 2 most distant nodes. This is
the length of time for the signal to reach from one node to the most distant node.
If node A and node B are at the extreme ends of the network and node A, sensing
a idle channel, transmits a frame, it must wait until the signal reaches node B and
for that same amount of time again to be positive that node B did not transmit a
frame just as the frame from node A reaches node B (e.g., must wait 2  to be
sure a collision did not occur).
Collision-Free Protocols - sometimes called reservation protocols
• contention period - a non-idle period in which transmissions occur; a non-idle
period which may have collisions
• assumption: N nodes each with a "hard-wired" address in the range 0 to N-1
Limited Contention Protocols
• why this type of protocol, too?
contention oriented protocols
under low load has low delay
under high load efficiency goes down
collision-free protocols
under low load has high delay
4
CET 458/598
Fall 2000
Lecture Notes
under high load channel efficiency increases
 want to combine to get best of both; this is accomplished by using
asymmetric protocols => they react differently at different loads
• optimal probability of successful transmission using a symmetric protocol goes to
1/e by the time the number of nodes reaches 5
=> the way to increase the probability of acquiring the channel is to decrease the
competition (this is common sense, too)
* try dividing the nodes into groups and only the members of a group compete
with each
* alternative - dynamically change the size of the group as a function of load
IEEE 802 LAN Standards
802.2 describes the Logical Link Control sublayer (upper part of data link layer)
802.3/Ethernet
• difference between Ethernet-I/II & 802.3
• 1-persistent CSMA/CD - transmits if idle else waits, on collision waits a random
time
• 10 Mbps (really a family, 1-10 Mbps, Ethernet 10Base5, thin net only 185 m per
segment), 100baseT
• thick net RG8 (500 m), thin net RG58 (200 m), twisted pair - really point-to-point
limited to 100 m), FROL fiber 2KM,
• time domain reflectometer use to detect cable problems, requires knowing the cable
lay out very well
• Manchester encoding, high -> +0.85 v, low. -> -0.85 v
• transceiver - transmit, receive, carrier detection, collision detection
• transceiver cable (AUI) - up to 50 m long, shielded twisted pairs (data in/out,
control in/out, power)
• interface board - assembles data into proper format, generates checksum & transmits
frame; extracts data & does checksum verification on incoming frames; frequently
handles a pool of buffers; some do DMA & interrupt main computer; handles
backoff on collision detection
• repeaters extend length of cable (from 500 m to 2.5 km) & can provide arbitrary
topology without cycles as long as no two nodes are separated by more than 4
repeaters; repeaters are "dumb" devices - they pass "good" packets both ways
• bridges (selective repeaters) only pass packets that need to be on the other side
(destination appears to be on the other side); this is usually accomplished by
keeping track of the source nodes on side A if the packets arriving at side A have
a destination in the list of sources for side A they are not passed through,
otherwise they are passed); This also permits simultaneous packets on either side
of a bridge if they stay on opposite sides)
• the 802.3 protocol
• preamble 7 bytes of 10101010 used to sync receiver's clock
• start of frame 10101011
• addresses 2 or 6 bytes (usually only 48 bit addresses used)
5
CET 458/598
Fall 2000
Lecture Notes
* high bit used when multicasts are to be sent; the rest of the address specifies
the multicast address; an address of all 1's is a broadcast
* next to highest bit - local address (e.g., has no meaning off of the local net)
• data length 2 bytes (Note: on Ethernet this field is a packet type field used to
distinguish upper layer protocols)
• data 0 - 1500 bytes (packet length, addresses to checksum, must be 64-1518
bytes)
• noise burst (jam) sent on collision detection (32 bytes)
• slot time (2) = 512 bit times = 64 bytes
• binary exponential backoff - after i tries with collisions waits 0 to 2i-1 slot times
before retrying, 0 <= i <= 10, when i reaches 16 considered an error and left to
higher layers to recover if possible
• Note: Tokoro & Tamaru acknowledgement scheme may not work through
repeaters & bridges
• channel efficiency for small packets is quite poor (never the less with remote
login packets tend to be small); channel efficiency goes up as the packet size
increases
• problems:
potential for an indefinite wait to send
no priorities, not good in real-time systems
802.4 - token bus
• no collisions, worst case delay is bounded
• physically a linear or tree shaped cable (bus)
• nodes arranged in a logical ring (physical order unimportant)
• nodes know their predecessor's and successor's addresses
• only nodes having THE token (a special control frame) are permitted to transmit
• after transmitting a frame (or more) the node passes the token to it's successor
802.5 - Token ring
• not really a ring but point-to-point links arranged in a circle; links can be [must be
the same on a single ring] twisted pair, coax or fiber
• fair with a "known" upper bound on channel access
• 802.5 is a token ring , other types of rings exist
• physical length of a bit on the ring
* data rate - R Mbps => a bit occurs every 1/R sec
* propagation speed of 200 m/sec => bit occupies 200/R m (at 1 Mhz 1 km
contains 5 bits)
• bits copied into bit buffer, inspected, copied out => 1 bit delay
• the token is removed from the ring when a node wants to transmit; nodes also
remove their own (non token) transmissions
• ring must be "large" enough (propagation delay and bit delays) to contain the entire
token; on short rings artificial delays are inserted to accommodate the token)
• limit on frame size is usually imposed by transmit timer (token holding time)
• network efficiency can approach 100% during heavy load
6
CET 458/598
Fall 2000
Lecture Notes
COMPARISION of 802 LANs
• use roughly similar technology and get roughly similar performance
• 802.3
• advantages
* simple algorithm
* nodes installable "on the fly"
* active components not needed
* delay at low load practically 0
• disadvantages
* minimum valid frame is 64 bytes - a lot of overhead
* nondeterministic
* no priorities
* 2.5 km cable length
* at high load collisions seriously add to overhead & thus negatively impact
throughput
• 802.4
• advantages
* uses highly reliable off the self cable TV equipment
* deterministic (unless repeated loss of token during high load occurs)
* has priorities (and can reserve bandwidth for high pri traffic)
* excellent throughput & efficiency at high load
• disadvantages
* protocol extremely complex
* has substantial delay at low load
• 802.5
• advantages
* has priorities (not as fair as token bus)
* can be built from lots of different media
* only type that can detect and fix cable breaks
* frame size can be short or very long
* throughput and efficiency high at high loads
• disadvantages
* centralized monitor
* significant delay at low loads
•at high loads 802.3 will become overloaded and throughput and efficiency goes to
the dogs; 802.4/5, however, will have an efficiency approaching 100%
Cost of physical plant - Ethernet cheapest, token bus most expensive
Fiber
• advantages
* fiber has high bandwidth
* not affected by electromagnetic interference (from machinery, power surges or
lightning)
* excellent security (hard to tap)
FDDI - Fiber Distributed Data Interface
• 100 Mbps (data rate)
7
CET 458/598
Fall 2000
Lecture Notes
• up to 200 km
• commonly used as a backbone for multiple LANs
• multimode fibers & LEDs
• error rate - specification requires <= 1 error in 2.5*1010 bits (= 250 secs)
• cabling - 2 counter-rotating rings for redundancy
if 1 cable breaks the other is a backup
if both break at (~) same spot the two rings can be joined on either side of the
break to make one large ring
• class A nodes connect to both rings, class B nodes to only one ring
• uses 4 out of 5 encoding (because the technology to support Manchester encoding,
200 megabaud, to costly [then])
* each group of 4 MAC symbols (1,0, & non data) encoded as 5 bits on
medium
* => 32 combinations: 16 data, 3 delimiters, 2 control, 3 hardware signaling, 8
unused
* saves bandwidth
* lacks self-clocking, which Manchester has, for synchronization so uses long
preamble & very stable clocks (frames upto 4500 bytes)
• next page FDDI 4-0ut-of-5 encoding & FDDI frame format
• protocol similar to 802.5
• token passing & sender removes frame on the way back
• due to potential for long delay (1000+ nodes & 200 km fiber), FDDI nodes put
token back on ring as soon as done transmitting frames
• frame structure similar to 802.5
• also has synchronous frames (for switched PCM or ISDN) (FDDI-II)
* generated by a master station every 125 sec
* header, 16 bytes of non circuit switched data, & 96 bytes of circuit switched
data => 96 PCM channels per frame (4 T1 channels or 3 CCITT
channels); each of these multichannels uses
6.144 Mbps; there may be a max of 16 multichannels using 98.3 Mbps for
1536 PCM channels
• frames not reserved for synchronous traffic are allocated on demand by priority
• nodes have token timers if the token is early any node may transmit (when it has
the token) if token is late only high priority may transmit
• FFOL - FDDI Follow on LAN (speed 625 Mbps) backbone for FDDI's
8
CET 458/598
Fall 2000
Lecture Notes
FDDI 4-out-of-5 Encoding and Frame
Data Symbols
4-bit data group
5-bit symbol
Formats
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
11110
01001
10100
10101
01010
01011
01110
01111
10010
10011
10110
10111
11010
11011
11100
11101
Control Symbols
IDLE _ _ _ _ _ _
J
_ _ _ _ _ _
K
_ _ _ _ _ _
T
_ _ _ _ _ _
R
_ _ _ _ _ _
S
_ _ _ _ _ _
QUIET _ _ _ _ _
HALT _ _ _ _ _ _
11111
11000
10001
01101
00111
11001
00000
00100
no more than
two zeros in
a row
plus NRZI used
on all symbols
(transition on a
1 bit and no
transition on 0
bits)
other 5-bit
symbols unused
FDDI Information Frame
PA
SD
FC
DA
SD
PA
SD
FC
DA
SA
FCS
ED
FS
=
=
=
=
=
=
=
=
Information
FCS
ED
FS
4500 bytes
FDDI Token
PA
SA
FC
ED
FCS covers
FC, DA SA,
Information
and FCS
Preamble (16 or more IDLE symbols)
Start delimiter (a J & a K symbol)
Frame control (2 symbols)
Desination address (4 or 12 symbols)
Source address (4 or 12 symbols)
Frame check sequence (8 symbols)
End delimiter (1 or 2 T symbols)
Frame status (an R & an S symbol)
9
CET 458/598
Fall 2000
Lecture Notes
Chapter 3 - Switching/Media Access
Section 3.1
Switch, network scalability and geographic scope, ease of expansion, incremental
expansion without (necessarily) poorer performance, aggregate capacity,
datagram (connectionless), virtual circuit (connection oriented), source routing,
routing table, , permanent vs switched virtual circuit, virtual circuit identifier,
overhead (re: network bandwidth and routing information at nodes), reliability,
delay and scalability for the 3 types of connectivity models, pre-allocation of
resources and effect on QoS ability
Section 3.2
Purpose of a bridge (as opposed to repeater), need for learning bridges, , spanning
tree algorithm: why needed, basics (lowest id selected as root, parallel routes with
highest hop counts “turned off”), bridge limitations (scalability, heterogeneity)
Section 3.3 - ATM
See M. Main’s PowerPoint for depth clues
Leave out Section 3.35 and Section 3.4
Additional ATM Material
B-ISDN (Broadband ISDN) and ATM
Why needed?
high data rate media - ISDN doesn't scale well
multi-media - video & bursty data
intelligent network operations - more services & better reliability
New Architecture
uses cell (small packet) switching technique
designed assuming bursty traffic (even voice is bursty if silences are removed from
the transmitted signal)
48 byte cells plus 5 bytes of header
Why cells so small?
small cells wanted by telephone companies because there would then be a
greater chance that cells would be available when needed & thus avoid
delays; voice delays of > ~10ms results in line echo effects
large cells - data people wanted for less overhead (eg, less fragmentation
overhead [processing time] and less bandwidth wasted on packet/cell
overhead
ended in 2 camps 32 byte or 64 byte cells => 48 byte data + header
ATM - allows for better utilization of bandwidth given variable bit rate and bursty
data
SONET - Synchronous Optical Networks
10
CET 458/598
Fall 2000
Lecture Notes
SONET - developed by Bellcore - becoming widely used in telephone sys
hierarchy of speeds
OC-1
51.84 Mbps
OC-3
155.62 Mbps • initial implementation speed
OC-9
466.56 Mbps
OC-12
622.08 Mbps • initial implementation speed
OC-18
933.12 Mbps
OC-24
1,244.16 Mpbs (1.24416 Gbps)
OC-36
1,866.24 Mbps
OC-48
2,488.32 Mbps
...
OC-240
12,441.60 Mbps (12.4416 Gbps)
STSx - Synchronous Transport Signal level x
(level x corresponds to OC- x level)
built on top of basic optical media, complexity (of agregate-frame structure)
increases as media bandwidth increases
STS is based on the Synchronous Digital Heirarchy where STS-1 forms the
basis for higher data rate media frames
STS-1 frame
125 sec wide (duration)
9 rows of 90 bytes => 810 bytes
27 bytes (of 810) are overhead (9B section o/h & 18B line o/h), this leaves
783 bytes of payload (data)
the frame payload is called synchronous payload envelope, SPE
87 bytes by 9 rows (783 bytes)
9 bytes are path o/h => 774 bytes available for data
fixed-size frames are sent synchronously but data may not be ready at that
time so
data may begin anywhere within the frame
once a payload is formed it is sent as is & thus may span frames
=> thus part of overhead tells where (pointer) within the frame the data
begins
data within the payload may not begin at begining of payload (due to jitter, etc) so
end to end ATM - a particular ATM channel/stream/connection (end-to-end
"thingy") uniquely identified by VCI+VPI
VPI - virtual path identifier
VCI - virtual circuit identifier - id of particular channel within path
long haul carriers look only at VPI & may change at major switching points
switches look at VCI & may alter hop-by-hop
items in cell header format
GFC needed only for subscribers accessing shared media (LANs) for
arbitration, contention resolution, etc
UNI - subscriber interface (user-network interface)
NNI - network interface (network-network interface)
11
CET 458/598
Fall 2000
Lecture Notes
12
CET 458/598
Fall 2000
Lecture Notes
S
e
c
t
i
o
n
Pa y l o a d
O
v
e
r
h
e
a
d
·····
·····
O
v
e
r
h
e
a
d
P
a
t
h
ATM Cel l
·····
·····
Ge n e r a l
Sy n c h r o n o u s Di g i t a l He i r a r c h y
( SDH) F r a me St r u c t u r e
UNI ATM
Cel l Header
bit
8
7
6
5
GFC
4
3
2
1
VPI
bit
1
VPI
NNI ATM
Cel l Header
8 7 6 5 4 3
2
1
VPI
2
VCI
3
PT
2
VCI
ResCLP 4
HEC
5
3
PT
ResCLP 4
HEC
5
octe t
GFC
VPI
VCI
PT
HEC
CLP
Res
=
=
=
=
=
=
=
1
octe t
Gener i c Fl ow Cont r ol
Vi r t ual Pat h I dent i f i er
Vi r t ual Channel I dent i f i er
Payl oad Type
Header Er r or Cont r ol
Cel l Loss Pr i or i t y
Reser ved
13
CET 458/598
Fall 2000
Lecture Notes
AAL very complex
each of the 4 protocols to be supported have implications:
to switch architectures
on congestion control in the network
for error control
in admission control (bandwidth guarantees)
additionally the speed & distances at which these nets are intended to operate
add compexity - it is likely that even very large messages can be entirely
on the "wire" for a lot longer than it takes to put them on the wire; thus,
error control and congestion control have to be proactive (e.g., forward
error control and congestion avoidance)
• this has lead ATM/B-ISDN designers to suggest moving some functionality
usually seen (& defined) at higher layers into the switch hardware
ATM Adaptation Layer - AAL
needed to adapt [older] services to ATM, AAL is an end to end layer
classes of AAL
AAL 1
- circuit emulation: virtual circuit, constant bit rate, connection
oriented
AAL 2
- variable bit rate video: virtual circuit, variable bit rate, connection
oriented
AAL 3/4 -connection-oriented data: virtual circuit, connection
oriented, bursty - X.25, SMDS, DQDB
and/or -connectionless data: variable bit rate data - CNLP, IP,
SMDS, DQDB
AAL 5
- connection oriented variable bit rate: very lean (less error recovery
and no built in retransmissions) for less overhead, reduced complexity for
easier/faster implementation [will probably eventually have
connectionless, too)
Network Layer (sort of) Issues for ATM
basic element is the virtual circuit (officially virtual channel)
normally point to point (end to end) but multicast (point to multipoint channels
are possible
may be permanent (always) or switched (dynamic, like phone call)
unidirectional (use a pair for duplex connections)
doesn't provide acknowledgements (assumed fiber optic as phys layer, highly
reliable, so left error control to higher layers
cells may be discarded due to congestion or damaged but never reordered
2-level connection heirarchy visible to higher layers (virtual circuits may be
agregated into a virtual path (all VC's follow same VP, even when rerouted)
Cell formats
GFC currently unused in UNI
switching at intermediate nodes done by VPI at end node/switch by VCI (allows
for smaller tables
PT now called PTI and now 3 bits
14
CET 458/598
Fall 2000
Lecture Notes
Payload type
000
001
010
011
100
101
110
111
Meaning
User data cell, no congestion, cell type 0
User data cell, no congestion, cell type 1
User data cell, congestion experienced, cell type 0
User data cell, congestion experienced, cell type 1
Maintenance, adjacent switches
Maintenance, end-to-end switches
Resource Management (used by ABR congestion control)
Reserved
CLP - 1 => low priority, 0 => high/normal
Connection Setup uses complex ITU protocol (Q.2931), handled by control plane
ADD PARTY messages can be sent after circuit setup to form multicast group
ATM addresses come in three forms (1st byte tells which)
20 byte OSI style addresses by country
20 byte OSI style addresses by international organization
15 digit decimal ISDN phone numbers (historical)
ATM Service Categories
CBR, VBR, ABR, UBR
CBR - Constant Bit Rate - like a wire (pt-2-pt): no error checking, flow control or
other processing. Useful for voice, video. interactive things without
compression or silences extracted
VBR - Variable Bit Rate - two sublasses RT (real-time) and NRT (non-real-time);
RT has stringent arrival rate requirements but variable bit rate (MPEG with
complete base frame followed by difference frames) => RT requires
minimal jitter
ABR - Available Bit Rate - bursty traffic with a rughly known bandwidth;
network provides rate feedback - asks sender to slow down when
congestion noticed
UBR - Unspecified Bit Rate - makes no promises and gives no feedback about
congestion; perfect for IP
Service characteristic
Bandwidth guarantee
Suitable for real-time
Suitable for bursty
Congestion feedback
CBR
yes
yes
no
no
RT-VBR
yes
yes
no
no
NRT-VBR
yes
no
yes
no
ABR
opt
no
yes
yes
UBR
no
no
yes
no
Quality of Service
15
CET 458/598
Fall 2000
Lecture Notes
when virtual circuit established transport layer and ATM layer agree on QoS
parameters - depends on trafic to be offered, whats available vs whats
desired
some typical parameters - peak cell rate (PCR), sustained cell rate, minimum cell
rate, cell variation delay tolerance (CDVT), cell loss ratio, cell delay
variation, cell error ratio
Congestion control via traffic shaping is part of the Generic Cell Rate Algorithm - which
is the mechanism for enforcing QoS agreements primary parameters of note are
PCR and CDVT - PCR used for admission contro and CDVT needed to adjust for
jitter but sometimes still need to discard packets to control jitter
ATM LANs
• problem - LANs tend to be connectionless and use broadcasts - how can ATM
be used as a LAN when it is connection oriented
* idea 1 - use special server that all hosts have a connection to and it forwards all
packets as appropriate - poor use of bandwidth and single point of failure
or congestion
* idea 2 - use switched (or permanent) virual circuits for each host to all other
hosts - problem how do hosts relate IP to virtual circuit (how to do ARP
without broadcasts)
* idea 3 - use idea 2 add two special servers (LES - LAN Emulation Server, to do
ARP, and BUS - Broadcast/Unknown Server, to take care of bradcast and
muticast)
ATM Forum calls idea 3 a LIS (Logical IP Subnet) and the LES an ATMARP
server (doesn't support broadcasting or multicasting)
ATM Adaptation Layer (AAL)
AAL layer provides different service classes as already discussed. To provide this
flexibility the AAL Layer is divided into 2/3 sublayers. the Convergence
Sublayer (CS) provides the characteristics of the AAL service type being
used. The Segmentation and Reassembly (SAR) sublayer is below that. The
SAR layer adds/removes headers & trailers for the CS layer.The CS
sublayer fragments/defragments arbitrary sized messages into 44 to 48 byte
chunks. It is typically viewed as having a service specific subsublayer and
a common subsublayer.
16
CET 458/598
Fall 2000
Lecture Notes
Some differences in AAL protocols
Item
service class
multiplexing
message delimiting
advance buffer alloc.
user bytes available
CS padding
CS overhead (bytes)
CS checksum
SAR payload (bytes)
SAR overhead (bytes)
SAR checksum
AAL1
A
no
none
no
0
0
0
none
46-47
1-2
none
AAL2
B
no
none
no
0
0
0
none
45
3
none
AAL3/4
C/D
yes
Btag/Etag
yes
0
32 bit word
8
none
44
4
10 bits
AAL5
C/D
no
Bit in PTI
no
1
0-47 bytes
8
32 bits
48
0
none
MidTerm 1 Review
The exam will cover the material discussed in class and in the class text chapters 1
through 3. The above lecture notes provide pointers to relevant sections, terminology, and
concepts in the text and some additional study material. Material may be drawn from the
assigned homework for chapter 1 and 2. Additional study support may be obtained from
the author’s PowerPoint presentations (numbers 2 through 6 on the Web page).
Most questions will fit the true/false, matching, multiple guess, fill in the blank or short
answer (a sentence or four) model. However, you should bring a calculator because there
will be at least 1 calculation type problem.
17