Multiplexing and PSTN

E303 & ISE3.2E

IMPERIAL COLLEGE of SCIENCE, TECHNOLOGY and MEDICINE,DEPARTMENT of ELECTRICAL and ELECTRONIC ENGINEERING.

COMPACT LECTURE NOTES on COMMUNICATION THEORY.Dr Athanassios Manikas, Autumn 2001

Multiplexing &Public Switched Telephone Network

(PSTN)

Outline:

ì PCM: Bandwidth, Bandwidth Expansion Factor, output SNR and ThresholdEffects.

ì CCITT recommendations for PCM (24-channels and 30-channels)ì Plesiochronous digital hierarchies (PDH)ì Synchronous digital hierarchies (SONET/SDH)ì BDSL-type transmission systems

Principles of Communication Theory & Systems Compact Lecture Notes

PSTN 2 Dr A. Manikas� �

1. INTRODUCTION

H( )f

^^^ ^^ ^



ì œ of an analogue signal are transmittedPCM sampled quantized valuesvia a .sequence of codewords

i.e. after sampling & quantization, a is used to map theSource Encoderquantized levels (i.e. o/p of quantizer) to codewords of bits#

i.e. quantized level codeword of bitsÈ #

and, then, a digital modulator is used to trasmit the bits,i.e. PCM system

ì There are three popular PCM source encoders(or, in other words, Quantization-levels Encoders).

� Binary Coded Decimal (BCD) source encoder� Folded BCD source encoder� Gray Code (GC) source encoder



000011010

001010011

010001001

011000000

100100100

101101101

110110111

111111110

m1

m2

m3m4

m5

m6

m7m8 BCD code

Folded BCDGRAY Code

g q (Volts)

g (Volts)

g g(input outputÑ È Ð Ñq

g F q ssamples

sec: occurs at a rate N.B.: F .FÐ # Ñs g

U œ quantizer levels;

#= log#a bU ,3>=level

ì :Note

codeword rate ( ) quant levels rate sampling rate = o/p of source encoder

Å Å Å#-bit

seclevelssec sec

-9./A9<.= =+7:6/=

= = =Þ J #J= 1



ì U œ bit rate: e.g. for 16 levels then r = . r = . b b

bitslevel

levelssec

bitssec#

#Å

J % JÅ Å= =

(e.g. transmitted sequ.=101011001101 ....)Æ Æ

Å

ì

�Å

versions : aof PCM

Differential PCM (DPCM): diff. quantizersDelta Modulation: diff. quants with 2 levels + or

ÚÝÝÝÝÝÝÛÝÝÝÝÝÝÜ

? ?

re encoded using a single binary digit

(DM DPCM)Others

−



2. PCM: BANDWIDTH & "ì we transmit several digits for each quantizer's output level Ê F PCM g� F

where denotes the channel bandwidthrepresents the message bandwidthœFPCM

g

F

ì PCM Bandwidth

baseband bandwidth: F PCMchannel symbol rate

2 Hz

bandpass bandwidth: HzF PCMchannel symbol rate

2 2 ‚

ì Note that, by default, the Lower bound of the 'baseband' bandwidth isassumed and used in this course

ì bandwidth expansion factor œ œ" channel bandwidthmessage bandwidth



ì Example - Binary PCM

F œ Ê FPCM PCM2 2channel symbol rate bit rateœ œ œ J œ J

Å

U

#J

#

#

1 1= # #

log

Hz

F Ê œ Ê œPCM œ J# 1 FJPCM1

# " #



3. NOISE EFFECTS in a binary PCMì It can be proved that the Signal-to-Noise Ratio at the output of a binary

Pulse Code Modulation (PCM) system, which employs a BCDencoder/decoder and operates in the presence of noise, is given by thefollowing expression

SNRout= X

X X

e fe f e f

gn n

!#

! !# #

Ð>ÑÐ>Ñ � Ð>Ñq

= 21+4. .2

#

#

#

#pe

where (type of digital modulator) EUEp = p = .e ef Tœ �È ŸÐ" � Ñ3

e.g. if the digital modulator is a PSK-mod. then EUEp = .e T�È Ÿ#



3.1. THRESHOLD EFFECTS in a binary PCM

ì We have seen that: SNRout=2

1+4. .2#

#

#

#pe

ì Let us examine the following two cases: SNR =high and SNR =low38 38

i) =HIGH ii) =LOWSNR SNRin in

SNR SNRin e in e=high p =small =low p =largeÊ Ê

Ê "� % # ¶ ".p . e##

Ê ¶ # Ê "� % # ¶ % #SNRout e e# # ## # # .p . .p .

Ê ¶ Ê ¶SNR dB SNRout out .p6 #"

% e



ì :THRESHOLD POINT- definition

Threshold point is arbitrarily definedas the SNR at which the SNR (i.e. ) in out p

21+4. .2

#

#

#

#efalls 1dB

below the maximum SNR (i.e. 1dB below the value 2 ).out##

ì By using the above definition it can be shown (...for you ...) that thethreshold point occurs when

p =e ."

"' ###

where is the number of bits per level.#



SNRout

SNRin

1dB γ=8

γ=7

γ=6

γ=5

6dB

6dB

6dB

(dB)

(dB)SNRin,threshold

for γ=8

3.2. COMMENTS on THRESHOLD EFFECTSì Å in PCM will result in a sudden in .The onset of threshold the output noise powerì of Psignal= SNR = SNRÅ Ê Å Ê38 out reaches 6 dB and becomes independent# P=318+6

.. . above threshold: SNRincreasing signal power no further improvement inÊ outì The limiting value of depends only on the number of bits perSNRout #

quantization levels



4. INTRODUCTION to TELEPHONE NETWORKsubscriber-A: 1784-382384 subscriber-B:20759 46266

PSTNTwistedcoperpair

Twistedcoperpair

Junction box (network Termination)

Junction box (network Termination)

Note that, as calls are routed through the PSTN, they will berouted ( ) through a multiplexed hierarchy of switchingcenters



PSTNEnd Office - Class 5

Toll Center - Class 4

Primary Center - Class 3

Sectional Center - Class 2

Regional Center - Class 1

… …

…

…

…

…

…

…

…

…Local Loop Local Loop



ì 1960 British Post Office (BPO) (currently BT) had establisheda with objective the system to be available24-ch PCM systemin 1968. Some of this work become the basis to the formationof a number of CCITT recommendations.

ì In Europe, the original , which were24-ch PCM systemsdesigned mainly for up to 32Km transmission routes, havebeen replaced by .30-ch PCM systems



ì There are CCITT recommendations for PCM.two differentThe main differences between these two recommendations areshown in the following table:

PCM CCITT RECOMMENDATIONS1st Recommentation 2nd Recommentation

Package SizeEncoding Law

=255 (but

24-channels -law

30-channels. E-law. they use =100), =

=7 ; =8 =8 FA-signal is FA-word is placed

.

# # #

E )(Þ'5 bits bits bits6 samples samples samples

Frame-Alignm

distributed ent

Signalling

amongst several frames into a separate slot ( )Signalling information is Signalling information conveyed

TS0

within each for all 30-channels encoded and conveyed in a separate 8-bit TS ( )

Strategies speech-time-slotTS-16



That is,1st CCITT rec. (24-channels PCM)

TS1 TS2 TS24

8 bits

TS3 TS41

bit

T Fs s=1/ =125 secµ

FrameAlignment1/6 bits Signaling Information

1 2 3 4 248 bits 8 bits 8 bits 8 bits

= 193 bitsX=

2nd CCITT rec. (30-channels PCM)

Frame Alignment Signaling Information[4bits k user + 4bits (k+15) user]th th

TS0 TS1 TS31

8 bits

TS1 TS3


1 2 3 308 bits 8 bits 8 bits 8 bits

TS16TS15 TS17

15 168 bits

= 256 bitsX=

1 k 15Ÿ Ÿ



ì Note:

ˆ A-law=better than -law (cheaper to produce and easy equipment.

maintenance, smaller quantization error in particular within themost significant part of the dynamic range).

ˆ in 24-ch PCM the signalling information is conveyed within eachspeech time-slot (technique known as bit stealing). Result: a slightreduction in speech-coding performance.



Single-Channel Path of 2nd CCITT rec. (30-channels PCM)

Message signal

bandwidth=4kHzFg

SamplingFrequency

=8kHzFs

Uniformquantizer

=2Q 8

PAMHIGHWAY

i.e. =64kbits/s rb

HDB3Line Codee

8bits

PCM HIGHWAY

A-lawA=87.6

γ=8Gray Code

bitslevel

Bit rate=γ.Fs

3rduser(say)

Frame Alignment Signaling Information

TS0 TS1 TS31

8 bits

TS1 TS3


1 2 3 308 bits 8 bits 8 bits 8 bits

TS16TS15 TS17

15 168 bits



Implementation of 2nd PCM CCITT RECOMM. (First Level Mltplx )



FDM Hierarchy CCITT recommendations



ì Based on the 24-channels amd 30-channels PCM CCITTrecommendations (primary multiplex groups) the core telephonenetwork evolved from using Frequency Division Multiplex (FDM)technology to digital transmission and switching

ì These two PCM CCITT recommendations have led to two PDH( digital hierarchies) CCITT reccommendations forPlesiochronousassembling the TDM telephony data streams from different calls.

ì Plesiochronous means:" because bits are stuffed into the frames asalmost synchronouspadding and the calls location varies slightly - jitters - from frame toframe"



PDH Hierarchy

Hierarchical American European

0 64 kbits/s 64 kbits/s

1 1,544 kbits/s

Level DS-

DS-0

DS-1

B CEPT-

CEPT-0

CE

B

PT-1

CEPT-2

CEPT-3

2,048 kbits/s

2 6,312 kbits/s 8,448 kbits/s

3 44,736 kbits/s 34,368 kbits/s

4

DS-2

DS-3

DS-4 274,176 kbits/s 139,264 kbits/s

5 565,148 kbits/s

CEPT-4

CEPT-5



ì The 24-channel PDH TDM CCITT recommendation (DS-x)

ì The 30-channel PDH TDM CCITT recommendations (CEPT-x)



Main disadvantage of PDH Networks

ì PDH multiplexing was designed for point-to-point communications andchannels cannot be added to, or extracted from, a higher multiplexing leveldemultiplexing down and then multiplexing up again, throught the entirePDH

ì For instance, to isolate a particular call from DS4, say, it must bedemultiplexed to DS1.

ì i.e. this is and needs very expensive equipmenta very complex procedureat every exchange to demultiplex and multiplex high speed lines

ì American & European Telephone Systems (thereforeare incompatiblevery expensive equipment required to translate one format to the other fortransatlantic traffic )

ì Solution: SONET/SDH Signal Hierarchy



SDH (Synchronous Digital Hierarchy)

ì The tranditional are based on the DS (USA) and CEPTPDH standards(Europe) PCM systems (24-channels and 30-channels PCM CCITTrecommendation)

ì PDH hierarchy is synchronous (extra bits are inserted into thealmostdigital signal stream to bring them to a common rate.

ì In 1988 was adopted by ITU andSDH (Synchronous Digital Hierarchy)ETSI (European Telecommunications Standards Instritute) based onSONET (synchronous optical Networks)

ì SDH signals have a common external timing i.e. SDH is synchronous



ì The used in Europe areSDH standards

which provides Mbits/secSTM-1 155




etc (increments of Mbits/sec )155

ì The most important main standards are , and .STM-1 STM-4 STM-16These are commercially available



SONET/SDH Hierarchy

Hierarchical American European Level SONET SDH

0 3

1 12

2 48

STS- ST -

ST -3 ST -1

ST -12 ST -4

ST -48

B BM

M CEPT-

M CEPT-4

S DS-3

S DS-3

S

œ ‚ œ "‚

œ ‚ œ %‚

œ ‚

%

DS-3 ST -16M CEPT-œ "'‚ %

Key Advantagesì it is channels to meet customer requirementssimple to add and dropì more bandwidth is available for network managementì equipment is smaller and cheaperì network flexibilityì integrate and manage on a single fiber.various types of traffic



PDH NetsSDH Nets

Mobile NetsATM Nets

IP NetsInteligent Networks

etc.Network Gateways

POTSxDSL2G3G

B-ISDNbluetoothethernet

GUIetc.

AccessNetworks

AccessNetworks

CORENetworks

POTSxDSL2G3G

B-ISDNbluetoothethernet

GUIetc.

AccessNetwork

No.1

CORENetwork

No.2

CORENetwork

No.1

CORENetwork

No.3

AccessNetwork

No.3

Gateway Interface



5. DSL-type Transmission Systemsx

INTERNET

LOCALEXCHANGE( )or a street-cabinet

LOCALEXCHANGE

(or a street-cabinet)

SERVICEPROVIDER

MODEMMODEM

POTS Network(Narrowband Network)


To a POTS line Card

To a POTS line Card



Spliterfilter

INTERNET

SERVICEPROVIDER

POTS Network(Narrowband Network)


SpliterfilterxDSL

Broadband Network

E.g. SDH/SONET

xDSL

POTSLine-card


To a POTS line Card



Frequency spectrum

4k 138k 10M-30M

VDSL Spectrum

3.56M 3.94M-10M,-30M -3.94M -3.56M -138k -4k

4k 25k 138k 1.1M-4k-138k -25k-1.1M

ADSL Spectrum

f (Hz)

f

Upstream Upstream

DownstreamDownstream

Downstream Downstream

UpstreamUpstream

POT

SPO

TS

POT

SPO

TS

640kb/s8Mb/s

In practice250kb/s

In practice2Mb/s

3 Mb/s 14 Mb/s



Examples of Bit-rate Requirements for Various Applicationsì In the following table the first number (bit-rate) indicates the minimum

satisfactory rate for users, and the second numbersome RESIDENTIALindicates the rate that will satisfy users.nearly all RESIDENTIAL

Application Downstream Bit Rate (bit/s) Upstream Bit (bits/s)Voice telephony 16k - 64k 16k - 64kInternet, online service access 14k - 3M 14k - 384kemail 9k - 128k 9k - 64kHigh definition TV 12M - 24MBroadcast video 1.5M - 6MMovies on demand 1.5M

--

- 6M 9kMusic on demand 384k - 1.5M 9kVideo phone 128k - 1.5M 128k - 1.5MDistance learning 384k - 3M 128k - 3MShop Home 128k - 1.5M 9k - 64kVideo Games 64k - 1.5M 64k - 1.5M



ì It is clear from the above that for offering a near broadcast quality videoservice, telephony and fast Internet, the network transmission shouldsupport at least 5 Mbit/s.

From POTS to DSLx

ì The POTs coper network (designed and built to support telephony service)provides an extensive infrastructure of twisted pairs which connectsapproximately 30 million residential and business UK customers .

ì Originally, customers wishing to transmit data were restricted to usingmodems which operated in the 4 kHz voice-band. The first voice-bandmodems were introduced in the 1950s and were capable of operating at 300bit/s, but these rapidly developed to 28.8 (and more) kbit/s.



ì As the core network evolved from using Frequency Division Multiplex(FDM) technology to digital transmission and switching, the rollout of the64 kbit/s narrowband digital PSTN network meant that copper transmissionin the access network was no longer restricted to 4 kHz bandwidth.Transmission equipment can now exploit the far greater bandwidthcapabilities of the network infrastructure between the exchange and thecustomers' premises.

ì In 1986 Basic Rate ISDN (ISDN2) was introduced. ISDN2 operates over asingle copper pair and modulates data using one four level pulse torepresent two binary bits (2B1Q). Data is sent simultaneously in bothdirections using echo cancelled hybrid transmission, and adaptiveequalisation is used to automatically compensate for attenuation across thetransmission band. ISDN2 uses 80 kHz bandwidth to deliver 160 kbit/ssymmetric data over access lines up to approximately 5.5 km long (or up to42 dB insertion loss at 100 kHz).



ì The evolution of digital transmission systems continued with thedeployment of HDSL in 1992. HDSL uses the same line code as ISDN(2B1Q) but is capable of delivering 2 Mbit/s services such as ISDN30 andMegaStreamTM over access lines up to 3.7 km in length (or up to 26 dBinsertion loss at 100 kHz). To achieve this the system uses frequencies up to290 kHz, and employs two or three copper pairs each operating at between784 kbit/s and 1 Mbit/s.

The most recent DSL system which is currently available from BT isì ADSL. Unlike ISDN or HDSL, ADSL employs asymmetric datatransmission. and is capable of delivering up to 8 Mbit/s downstream to thecustomer, and 640 kbit/s upstream to the exchange over a single copperpair. ADSL uses frequencies up to 1.1 MHz, but does not use the 4 kHzvoice-band. Customers subscribing to ADSL derived data services cantherefore continue to use basic telephony. Although ADSL is technicallycapable of delivering 8 Mbit/s downstream, the highest bit rate offered byBT's commercial ADSL data products (DataStream and IPStream) is 2Mbit/s downstream and 250 kbit/s upstream.



ì VDSL is the next generation of digital copper transmission system, furtherincreasing the capacity of a metallic twisted pair and using bandwidths ofmore than 10 MHz, and as high as 30 MHz.

ì The standards for VDSL are still under discussion

ADSL (Asymmetric Digital Subscriber Line)ì provides high bit rate transmission to the customer premises for a

combination of services that include video, telephony and data.

ì It is based on the notion that the bit rate requirement for downstream trafficto the customer is much higher than in the upstream direction from thecustomer.

ì For example, the downstream traffic may carry a video channel or highspeed Internet files, while the upstream traffic carries only a narrowbanddata channel for controlling video, or other control signals and low speeddata.



ì ADSL operates over the local access twisted pair network between the localexchange and the customers' premises.

ì It can simultaneously transport the following on a single twisted pair:

ˆ Downstream (towards the customer) bit rates up to 8 Mbit/sˆ Upstream (towards the exchange) bit rates up to 640 kbit/sˆ Voiceband telephony service

ì The operating frequency range of ADSL is limited to between 25 kHz and1.1 MHz. The upstream channel is placed at the lower end of the availablefrequency band where it suffers less attenuation and crosstalk noise and istherefore easier to receive. The downstream channel bandwidth is muchgreater thus enabling the system to achieve higher transmission capacity forconveying services to the customer.



MODULATOR: Discrete Multi-Tone modulation (DMT)

ì The ANSI ADSL standard (T1.413) specifies DMT as the modulationtechnique.

ì DMT is a multi-carrier modulation technique which employs a variant ofOrthogonal Frequency Domain Multiplex modulation. The transmitter andreceiver structures are shown below.

To line

Band PassFilter

D/AEncoder

andbuffer

InversediscreteFourier

transform

Parallelto

serialconverter

TransmitData

Serialto

parallelconverter

DiscreteFourier

transform

Decoderand

bufferReceive

Data

Band PassFilter

From lineA/D

DMT transmitter

DMT receiver



ì The data stream to be transmitted is buffered into blocks of bits.

ì The overall channel spectrum is divided into 256 independent sub-channelsand the blocks of bits are assigned to each frequency sub-channel.

ì The data is encoded by modulating each sub-carrier so that the amplitudeand phase forms a QAM constellation for that sub-channel.

ì Each sub-carrier can encode a variable but pre-determined number of databits, thus forming different constellation sizes (see figure).

Frequency

Sign

al P

ower

QAM sub-channels

Variable constellation sizes per sub-channel



ì The overall data capacity varies with frequency, and those sub-channelswith higher capacity are assigned more bits. The number of bits allocated toeach sub-channel is determined using an algorithm based on each sub-channel's signal to noise ratio. All of these are performed by the first blockof the DMT transmitter diagram.

Frequency

Line

Atte

nuat

ion

Frequency

Bits

per C

hann

el

DMT Data Transmission

Twisted Copper Pair

ì In the second block, the collection of N QAM symbols is then modulated bypassing through the inverse discrete Fourier transform (IDFT) process. Realsignals are then taken from the IDFT output, parallel-to-serial converted,digital-to-analogue converted, and finally band pass filtered beforetransmission.

ì The demodulation process at the receiver is simply the reverse process.

Multiplexing and PSTN

Documents

Transcript of Multiplexing and PSTN