Data and Computer Communication by Wiliam Stallings (our supplementary
textbook). • Data Communications and Networking by B. Forouzan, Mc Graw.
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 Prof. Adam Wolisz
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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... Prof. Adam Wolisz
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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 Prof. Adam Wolisz
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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 Prof. Adam Wolisz
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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! Prof. Adam Wolisz
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Statistical Multiplexing Gain [mod.from N.Mc Keown, Stanford] Comment: Synchronous Multiplexing would use 2C bits/s – statistical uses R 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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Prof. Adam Wolisz
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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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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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