Introduction to Communication Networks
Unit 4
Multiplexing, Framing,
and some solutions...
©EECS 122 SPRING 2007
Spring 2007
Acknowledgements – slides comming from:
•
Data and Computer Communication by Wiliam Stallings (our
supplementary textbook).
•
Data Communications and Networking by B. Forouzan, Mc Graw
Hill, 2004 ( a very nice-to-read book!)
•
Some figures have been used form the earlier issues of the
EECS 122 tought by Prof Jean Walrand.
•
Introduction to Telephones & Telephone Systems by A. Michael
Noll, Artech House, 1986
•
Megabit Data Communication, John T. Powers, Henry H. Stair
II, Prentice Hall
•
Digital Telephony by J. Bellamy: “”, J. Wiley & Sons, 2rd
edition, 2000
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MULTIPLEXING
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Multiplexing
•
•
General Problem: Several - n- different channels (voice, TVchannels) should be supported between a pair of locations.
We would like to avoid usage of n physical links (cables).
Looking at the features of media you will easily see that the
supported bandwidth exceeds by far the bandwidth needed
for each channel...
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Variants of multiplexing
•
The dimensions of multiplexing
–
time (t)
–
frequency (f)
–
code (c)
–
space (si) – sometimes…
•
Care for separation: guard spaces, code orthognonality
•
Multiplexing can be
–
Synchronous (constant allocation)
–
Statistical (variable allocation)
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Frequency Multiplex
•
•
•
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum (in the synchronous
case – for the whole time!)
Note: guard zones in frequency are needed!!
Special case: Wave division Mux
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Schema for FDM
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[Forouzan] mod
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FDM of Three Voiceband Signals
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Example - Community Antenna TV (CATV)
Currently used systems require about 6MHz /TV Channel
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Time Multiplex
•
The whole bandwidth is used all the time, but
– alternatively – by different channels!
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Time Multiplex: Interleaving of data segments
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[Forouzan]
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Time and Frequency Multiplex
[Schiller]
•
Combination of both methods
•
A channel gets a certain frequency band for a certain amount
of time
•
Example GSM cellular telephony: FDM with TDD (8 bidirectional channels per frequency band) is used...
k1
k2
k3
k4
k5
k6
c
f
t
2.18.1
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Time Division Duplex (TDD) and FDD
Similarly a Frequency Division Duplex - FDD with two frequency
Channels: for up-link and down-link respectively, can be defined
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Bursty Data
•
Burstiness of data
–
–
In many data communication applications, data occur in bursts separated
by idle periods
This type of data can often be transmitted more economically by
statistical (or asynchronous) multiplexing...
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Synchronous vs. Statistical TDM
Note: Data slots must be addressed!
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Statistical Multiplexing Gain [mod.from N.Mc Keown, Stanford]
Comment: Synchronous Multiplexing would use 2C bits/s – statistical
uses R<2C. But: how to define R? –
The queue helps „smooth“ the load but there might be losses !!!
Rate
A+B
2C
R < 2C
C
A
R
B
time
Statistical multiplexing gain = 2C/R
Other definitions of SMG: The ratio of rates that give rise to a
particular queue occupancy, or particular loss probability.
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Example of Statistical Multiplexer Performance
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Probability of Overflow and Buffer Size
ρ- is the ratio of the offered load to
the nominal service rate of the
system – see Queuing (later)
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FHSS (Frequency Hopping Spread Spectrum) I
•
Discrete changes of carrier frequency
–
•
•
sequence of frequency changes determined via pseudo random
number sequence
Two versions
–
Fast Hopping: several frequencies per user bit
–
Slow Hopping: several user bits per frequency
Advantages
–
ROBUSTNESS: impact of frequency selective fading and interference
limited to short period !
2.32.1
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FHSS : Schema of the operation
Fast
hopping
Slow
hopping
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FHSS – System overview
user data
modulator
modulator
frequency
synthesizer
transmitter
hopping
sequence
narrowband
signal
received
signal
data
demodulator
hopping
sequence
spread
transmit
signal
narrowband
signal
frequency
synthesizer
demodulator
receiver
2.34.1
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CDMA: Code Division Multiple Access
A Channel: a unique code in the same spectrum at the same time
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Space division multiplexing...wireless...
•
Assume a sectorized antenna
•
Transmisison/receive in one of the sectors does not limit the
usage of other sectors...
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FRAMING
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Framing
•
•
WHY:
–
The physical layer supports bit-synchronization.
–
Data units bigger than a single bit must be recognized...
HOW:
–
–
Time gaps (not good - might be squeezed within the physical layer),
Physical signaling - the physical layer has to support some control
symbols, besides of a 0 and a 1, say a J and K. Example: the
Manchester extension of the IEEE 802.5 - token ring.
–
Field Lenght marker at the beginnig of the field: Whole notion of unit
lost if this lenght marker would get corrupted!
–
Specific symbols: Character oriented, bit oriented variants.
–
Clock based
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Framing - delimiting symbols
•
Delimiting characters in character based transmission, i.e.
the case when the transmitted information is composed of
symbols - c.f.the ASCII code table.
•
SYN SYN - used for the synchronization
•
SOH......STX.......ETX the framing sequence for the
header and for the text.
–
Transmission of binary information:
• DLE
–
STX .......binary information........ DLE ETX
What about a DLE inside the binary information??
• input
binary information: ...................... DLE.........
• transmitted
• extracting
binary information: .... DLE DLE.........
the information: ........... DLE ................
–
A) This scheme is closely tied to an 8 bit character representation.
–
B) A single error can cause a misinterpretation.
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Framing - delimiting symbols (2)
Example
DLE
STX
A
DLE
B
DLE
ETX
DLE
STX
A
DLE
DLE
B
DLE
DLE
ETX
(a)
(b)
ETX
Stuffed DLE
(c)
DLE
STX
A
DLE
B
(a) Data sent by the network layer.
(b) Data after being character stuffed by the data link layer.
(c) Data passed to the network layer on the receiving side.
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Bit oriented transmission-delimiting flag
•
Delimiting flags in bit oriented transmission, i.e. the case
when the transmitted information is represented as a string
of bits (the concept of octets is sometimes used to support
the bit manipulation).
–
•
•
The usual flag pattern:
01111110
Transmission transparency is assured via bit stuffing:
–
The transmitter always stuffs a 0 after 11111
–
The receiver removes a 0 following 11111
User data 01111110 are transmitted as 011111010
Combinations of solutions discussed above are frequently
used to increase the power of framing - e.g. delimiting
flags together with byte (symbol) count.
See additional reading after bit error unit!
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Bit-oriented Transmission
The packet framed by two special bit patterns called flags. Bit stuffing is used
to prevent a flag from occurring in the middle of a packet. The bit-stuffing
procedure is illustrated in (b): a bit 0 is inserted after each pattern 011111 that
appears in the packet. The reverse procedure (bit destuffing) is performed at
the receiver to restore the original packet.
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Clock based
Idea: to have a sequence of markers in predefined
positions – here the sequence 101 in proper distance
all the elements are assumed to have equal length!
Problem: This sequence, with the proper spacing, MIGHT
appear just by chance in the content!!!
Solution: Look several times – the random appearance will
not be repeatable over numerous frame series!
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Examples of transmission systems
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Phone Backbone - FDM Carrier Standard – OLD!
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[Forouzan]
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FDM Carrier Standards - OLD – some numbers
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American Digital Hierarchy
Each channel carries data (voice) digitized at a rate of
8000 samples per second with 8 bit per sample.
A frame contains 24 channels plus one framing bit per
frame. Thus, the required transmission rate for DS-1 is
8000 x (24 x 8 + 1) bits per second = 1.544 Mbit/s.
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American Digital Hierarchy –synchronization
•
[bellamy]
T1/D4 Superframe and D4 Channel Slots.
–
–
A super frame combines 12 frames of 193 bits each.
The framing bits of these frames produce the T1/D4 superframe pattern of
100011011100
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Extended Superframe Format Framing (1)
•
Extension of the super frame from 12 repetitions to 24 repetitions.
•
Framing bit positions take new functions and meanings (24 bits).
•
Framing (6 bits)
•
Error Checking (6 bits)
•
Maintenance Communications (12 bits)
193rd Bit
193rd Bit
125 µs
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Bipolar format
1
0
1
0
1
5,2 µs
1
0
1
Binary code
Basic North American PCM framing and signaling format.
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Some specific ways of using it...
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[Stallings]
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American TDM Carrier Standard
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[Forouzan]
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TDM Carrier Standards
North American and International TDM Carrier Standards
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Just for info – the International Frame...
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Trunks
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Mhmm... Not quite unified...
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Instability of the timing..
• Cyclic changes of the data rate of sending
•
Systematic difference in timing between the sender and
receiver.
•
What can we do?
•
–
cyclic- or fluctuating differences removed by elastic buffer...
–
Systematic difference has to be dealt with
Systematic differences. How?
–
Plesiochronous operation (T-hierarchy)
–
Synchronous operation (SONET)
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Synchronization - Definition of terms:
– Single
discrete signal may be either
• Isochronous
: Constant frequency of signal changes (e.g. 8 kHz of
PCM-coded voice)
• Anisochronous
– Two
discrete signals may be either
• Synchronous
or
• Asynchronous
–
Mesochronous (meso = middle, greek)
· Same
center frequency, limited phase difference
· (e.g. signals from a single oscillator being processed in
different stages)
–
Plesiochronous (plesio = near, greek)
· Nominal
same center frequency (e.g. two independent
oscillators)
–
Heterochronous (hetero = different, greek)
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Changes in Length of Transmission Media
•
•
Path length changes as a result of thermal expansion or contraction of
guided media, or as a result of atmospheric bending of a radio path.
While a path is increasing in length, the effective bit rate at the
receiver is reduced because more and more bits are being ‚stored‘ in
the medium.
Example - 500 km long T2 transmission link using 22-gauge copper
wires which have a velocity of propagation of 189671 km/s:
–
–
•
Thermal expansion coefficient 16.5 ppm/°C
The temperature of the wire increases by 20 °C in one hour:
Change in path length:
•
Change in number of bits:
•
Change of data rate:
•
Relative change in
received data rate:
Prof. Adam Wolisz
Δd = 500 ⋅16.5 ⋅10 −6 ⋅ 20 = 0.165 km
6.312 ⋅106 ⋅ 0.165
ΔB =
= 5.49 bits
189671
5.49
= 1.525 ⋅10 −3 bps
3600
ΔR 1.525 ⋅10 −3
−10
=
=
2
.
41
⋅
10
R
6.312 ⋅10 6
ΔR =
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Synchronization – Elastic Buffer
Can deal with fluctuating differences, introduces a delay...
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Elastic buffer - implementation
Elastic store operation with a one-frame memory.
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Pulse Stuffing Concept (1)
•
Two channel multiplexer showing equal data rates for each
input
No pulse stuffing needed since both
streams have exactly the same rate
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Pulse Stuffing Concept (2)
•
Simplified pulse stuffing example
–
•
Additional information needed to allow adjustments of the
information flow within each sub-channel.
–
•
Assume that stream 1 is slightly faster than 2 – but within legally
(standards!) defined upper bound.
Ci bit specifies if the following Si bit carries tributary data (Ci = 0) or
just stuffing.
The output data rate should be equal to DOUBLE the
maximum LEGALLY Permitted data rate of inputs.
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American Digital Hierarchy
•
Every multiplexing stage adds overhead.
Digital Level
Signal Type
Rate in Mbits/s
Number of
Channels
Channels of
the type
Number of kbits in
overhead
0
DS-0
0.064
1
DS-0
-
1
DS-1
1.544
24
DS-0
8
2
DS-2
6.312
4
DS-1
136
3
DS-3
44.736
7
28
DS-2
DS-1
552
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Pulse Stuffing Concept (3)
•
Example of a higher level multiplexing format for 6.312Mbps
DS2 signal in North American digital hierarchy.
–
A DS2 signal is derived by bit interleaving of four DS1 signals and
adding the appropriate overhead bits.
–
The C1 – C4 bits are repeated 3 times each, in order to allow for
majority voting.
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A real multiplexer…
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Problems of PDH
•
The Plesiochronous Digital Hierarchy had a number of
problems:
–
–
–
–
Each part of the world has its own transmission hierarchy
(expensive interconnection equipment)
Justification (bit stuffing) spreads data over the frame
• add-drop-multiplexers are hard to build
• extract a single voice call -> demultiplex all steps down
• switching of bundles of calls (n * 64 kbit/s) is difficult
• (every switch has to demultiplex down to DS0 level)
The management and monitoring functions were not sufficient
in PDH
PDH did not define a standard format on the transmission link
• Every vendor used its own line coding, optical interfaces etc.
• Very hard to interoperate
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SONET / SDH (1)
•
Synchronous Optical NETwork (SONET) and the Synchronous
Digital Hierarchy (SDH)
–
–
Started by Bellcore in 1985 as standardization effort for the US
telephone carriers (after AT&T was broken up in 1984), later joined by
CCITT, which formed SDH in 1987
Three major goals:
• Avoid
the problems of PDH
• Achieve
higher bit rates (Gbit/s)
• Better
means for Operation, Administration, and Maintenance
(OA&M)
•
SDH is THE standard in telecommunication networks now
•
Originally designed to transport voice - used for everything
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SONET / SDH (2)
•
SONET / SDH - Basic concepts
–
SONET / SDH system consists of switches, multiplexers and repeaters
(and the fiber in between)
–
PATH is the connection between source and destination
–
LINE runs between two multiplexers (possibly through repeaters)
–
SECTION is the connection of any two devices (point-to point)
Source
Multiplexer
Repeater
Section
Multiplexer Repeater
Section
Section
Line
Multiplexer
Section
Line
Path
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SONET / SDH (3)
•
Level
US
Europe,
Japan
Data rate
(gross)
Data rate
(SPE)
Data rate
(user)
1
OC-1
-
51.84
50.112
49.536
2
OC-3
STM-1
155.52
150.336
148.608
3
OC-9
STM-3
466.56
451.008
445.824
4
OC-12
STM-4
622.08
601.344
594.824
5
OC-18
STM-6
933.12
902.016
891.648
6
OC-24
STM-8
1244.16
1202.688
1188.864
8
OC-36
STM-12
1866.24
1804.032
1783.296
9
OC-48
STM-16
2488.32
2405.376
2377.728
10
OC-192
STM-64
9953.28
9621.504
9510.912
No overhead bits needed for justification
–
–
higher speed link is formed by byte-interleaving data from lower
speed links
exact multiples of lower speed data rates
so e.g. OC-12 contains 12 byte interleaved OC-1 frames
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SONET
Clock-based
–
each frame is 125us long
–
e.g., SONET: Synchronous Optical Network
–
STS-n (STS-1 = 51.84 Mbps)
STS
-1
STS
-1
Hdr
Payload
Hdr
Overhead
STS-1 = OC-1
Hdr
•
[PD]
STS
-1
9 rows
Hdr
STS
-3c
90 columns
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SDH - Clocking
•
All network elements are totally synchronous
•
Still, there are delays in the network
•
Hierarchy of clocks, lower levels synchronize to higher levels
Stratum
Min. Accuracy
Min. Stability
Pull-In Range
1
±1 in 10-11
Master Reference
Master Reference
2
±1.6 in 10-8
⇒ ±0.025*
1 in 10-10
Should synchronize with a clock
accurate to ±1.6 in 10-8
2
±4.6 in 10-6
⇒ ±7.0*
±3.5 in 10-9
(some conditions)
Should synchronize with a clock
accurate to ±4.6 in 10-6
3
±32 in 10-6
⇒ ±50*
N/A
Should synchronize with a clock
accurate to ± 32 in 10-6
* = Minimum accuracy relative to 1,544,000 bits/s.
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What about tributary speed differences
[PD]
- Frames appear synchronously, and have always the header of fixed
lenght (9 rows in STS-1) and position (at the begining of the frame!)
- The Payload does NOT have to begin directly after the header – it is
fixed by a pointer (part of th eheader).
- The Payload has always a constant length – thus might „overflow“
into the next frame
- If there are excessive bytes, those are stored in the header – and the
moves to the left. If bytes are missing – the „empty“ bytes are marked
and the pointer move to the right.
87 columns
Frame 0
9 rows
Frame 1
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High Reliability – 60 ms for reconfiguration
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[LUCENT]
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What SONET/SDH does better (conclusions)
•
•
•
SONET (Synchronous Optical NETwork) and SDH (Synchronous
Digital Hierarchy) are almost identical
Interconnection is easy (exists, works)
Justification, if still needed, is performed by pointers
• Data
from each input is placed in a payload container
(Administrative Unit- AU)
– it spans multiple SONET/SDH frames
– a pointer in the header of the SONET/SDH frame signals the
start of the payload container in the frame (in 3-byte increment
for SDH)
– positive and negative justification through this pointer
– slip buffer delay reduces from 193 bit for a T1 signal down to 24
bit
–
Single 64 kbit/s lines (1 byte in the SONET/SDH frame) can be found
and extracted in the frame
–
HIGH RELIABILITY!!!!
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