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
2 of 63
MULTIPLEXING
Prof. Adam Wolisz
3 of 63
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
4 of 63
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)
Prof. Adam Wolisz
5 of 63
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
Prof. Adam Wolisz
6 of 63
Schema for FDM
Prof. Adam Wolisz
[Forouzan] mod
7 of 63
FDM of Three Voiceband Signals
Prof. Adam Wolisz
8 of 63
Example - Community Antenna TV (CATV)
Currently used systems require about 6MHz /TV Channel Prof. Adam Wolisz
9 of 63
Time Multiplex •
The whole bandwidth is used all the time, but – alternatively – by different channels!
Prof. Adam Wolisz
10 of 63
Time Multiplex: Interleaving of data segments
Prof. Adam Wolisz
[Forouzan]
11 of 63
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
12 of 63
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
Prof. Adam Wolisz
13 of 63
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...
Prof. Adam Wolisz
14 of 63
Synchronous vs. Statistical TDM
Note: Data slots must be addressed! Prof. Adam Wolisz
15 of 63
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
Prof. Adam Wolisz
53 of 63
Prof. Adam Wolisz
54 of 63
Prof. Adam Wolisz
55 of 63
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
Prof. Adam Wolisz
56 of 63
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
Prof. Adam Wolisz
57 of 63
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
Prof. Adam Wolisz
58 of 63
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
Prof. Adam Wolisz
59 of 63
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.
Prof. Adam Wolisz
60 of 63
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
Prof. Adam Wolisz
61 of 63
High Reliability – 60 ms for reconfiguration
Prof. Adam Wolisz
[LUCENT]
62 of 63
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!!!!
Prof. Adam Wolisz
63 of 63
