Unit 4 Multiplexing, Framing, and some solutionsinst.eecs.berkeley.edu/~ee122/sp07/Mux_Fram.pdf ·...

Introduction to Communication Networks Spring 2007

©EECS 122 SPRING 2007

Unit 4Multiplexing, Framing,and some solutions...

2 of 63Prof. Adam Wolisz

Acknowledgements – slides comming from:

• Data and Computer Communication by Wiliam Stallings (our supplementary textbook).

• Data Communications and Networking by B. Forouzan, Mc GrawHill, 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, 2rdedition, 2000


MULTIPLEXING


Multiplexing

• General Problem: Several - n- different channels (voice, TV-channels) 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...


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)


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


Schema for FDM [Forouzan] mod


FDM of Three Voiceband Signals


Example - Community Antenna TV (CATV)

Currently used systems require about 6MHz /TV Channel


Time Multiplex• The whole bandwidth is used all the time, but

– alternatively – by different channels!


Time Multiplex: Interleaving of data segments [Forouzan]


f

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 bi-directional channels per frequency band) is used...

t

c

k2 k3 k4 k5 k6k1

2.18.1


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


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 bystatistical (or asynchronous) multiplexing...


Synchronous vs. Statistical TDM

Note: Data slots must be addressed!


Statistical Multiplexing Gain [mod.from N.Mc Keown, Stanford]

A

B

R

2CR < 2C

A+BRate

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.

C

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 !!!


Example of Statistical Multiplexer Performance


Probability of Overflow and Buffer Sizeρ- is the ratio of the offered load to

the nominal service rate of thesystem – see Queuing (later)

19 of 63Prof. Adam Wolisz2.32.1

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 !


FHSS : Schema of the operation

Fast hopping

Slow hopping


FHSS – System overview

modulatoruser data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

2.34.1


CDMA: Code Division Multiple AccessA Channel: a unique code in the same spectrum at the same time


Space division multiplexing...wireless...• Assume a sectorized antenna

• Transmisison/receive in one of the sectors does not limit the usage of other sectors...


FRAMING


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


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 binary information: .... DLE DLE.........

• extracting the information: ........... DLE ................

– A) This scheme is closely tied to an 8 bit character representation.

– B) A single error can cause a misinterpretation.


Framing - delimiting symbols (2)

(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.

Example

(a)

(b)

(c)

DLE STX A DLE B DLE ETX

DLE STX A DLE DLE B DLE ETX

DLE STX A DLE B DLE ETX

Stuffed DLE


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!


Bit-oriented Transmission

The packet framed by two special bit patterns called flags. Bit stuffing is usedto 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.


Clock based

Idea: to have a sequence of markers in predefined positions – here the sequence 101 in proper distanceall 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!


Examples of transmission systems


Phone Backbone - FDM Carrier Standard – OLD! [Forouzan]


FDM Carrier Standards - OLD – some numbers


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.


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


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)

Basic North American PCM framing and signaling format.

1 0 1 0 1 1 0 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

193rd Bit 193rd Bit125 µs

Bipolar format

Binary code5,2 µs


Some specific ways of using it... [Stallings]


American TDM Carrier Standard [Forouzan]


TDM Carrier Standards

North American and International TDM Carrier Standards


Just for info – the International Frame...


Trunks


Mhmm... Not quite unified...


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)


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)


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:

kmd 165.020105.16500 6 =⋅⋅⋅=Δ −

bitsB 49.5189671

165.010312.6 6

=⋅⋅

=Δ

bpsR 10525.13600

49.5 3−⋅==Δ

106

3

1041.210312.610525.1 −

−

⋅=⋅⋅

=ΔRR


Synchronization – Elastic Buffer

Can deal with fluctuating differences, introduces a delay...


Elastic buffer - implementation

Elastic store operation with a one-frame memory.


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


Pulse Stuffing Concept (2)• Simplified pulse stuffing example

– Assume that stream 1 is slightly faster than 2 – but within legally (standards!) defined upper bound.

• Additional information needed to allow adjustments of the information flow within each sub-channel.

– 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.


American Digital Hierarchy

Digital Level Signal Type Rate in Mbits/s Number ofChannels

Channels ofthe type

Number of kbits inoverhead

0 DS-0 0.064 1 DS-0 -1 DS-1 1.544 24 DS-0 82 DS-2 6.312 4 DS-1 1363 DS-3 44.736 7

28DS-2DS-1

552

• Every multiplexing stage adds overhead.


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.


A real multiplexer…


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


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


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)

Section Section Section SectionLine Line

Path

SourceMultiplexer Repeater Multiplexer Repeater Multiplexer


SONET / SDH (3)

• 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

Level US Europe,Japan

Data rate(gross)

Data rate(SPE)

Data rate(user)

1 OC-1 - 51.84 50.112 49.5362 OC-3 STM-1 155.52 150.336 148.6083 OC-9 STM-3 466.56 451.008 445.824

4 OC-12 STM-4 622.08 601.344 594.8245 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 2377.72810 OC-192 STM-64 9953.28 9621.504 9510.912


SONET [PD]

• Clock-based– each frame is 125us long

– e.g., SONET: Synchronous Optical Network

– STS-n (STS-1 = 51.84 Mbps) STS-1 = OC-1

Overhead Payload

90 columns

9 rows

STS-1Hdr STS-1Hdr STS-1Hdr

STS-3cHdr


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 Range1 ±1 in 10-11 Master Reference Master Reference2 ±1.6 in 10-8

⇒ ±0.025*

1 in 10-10 Should synchronize with a clockaccurate 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 clockaccurate to ± 32 in 10-6

* = Minimum accuracy relative to 1,544,000 bits/s.


What about tributary speed differences [PD]

Frame 0

Frame 1

87 columns

9 rows

- 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.


High Reliability – 60 ms for reconfiguration [LUCENT]


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!!!!

Unit 4 Multiplexing, Framing, and some solutionsinst.eecs.berkeley.edu/~ee122/sp07/Mux_Fram.pdf ·...

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