Multiplexing And Pstn
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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 ì ì ì ì
E303 & ISE3.2E
PCM: Bandwidth, Bandwidth Expansion Factor, output SNR and Threshold Effects. CCITT recommendations for PCM (24-channels and 30-channels) Plesiochronous digital hierarchies (PDH) Synchronous digital hierarchies (SONET/SDH) BDSL-type transmission systems
Principles of Communication Theory & Systems
Compact Lecture Notes
1. INTRODUCTION
H(f) ^
^
^
^
^
^
?2?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì PCM œ sampled quantized values of an analogue signal are transmitted via a sequence of codewords. i.e. after sampling & quantization, a Source Encoder is used to map the quantized levels (i.e. o/p of quantizer) to codewords of # bits i.e. quantized level È codeword of # bits 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 PSTN
?3?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
g (inputÑ È gq ÐoutputÑ
gq (Volts) m8 m7 m6 m5
000 011 010
001 010 011
010 001 001
011 000 000
100 100 100
101 101 101
m4
110 110 111
111 111 110
BCD code Folded BCD GRAY Code
gq : occurs at a rate Fs ÐN.B.: Fs #.Fg Ñ
samples sec
U œ quantizer levels;
g (Volts)
m3
,3>= #= log# aUb level
m2 m1
ì Note: codeword rate (o/p of source encoder) = quantÞ levels rate = sampling rate =J= =#J1 Å Å Å #-bit -9./A9<.= sec
levels sec
=+7:6/= sec
?4?
PSTN
Principles of Communication Theory & Systems
ì bit rate: rb = # . J = Å Å
bits levels level sec
Dr A. Manikas
Compact Lecture Notes
e.g. for U œ 16 levels then rb =% .J= Å #
bits sec
Æ Æ (e.g. transmitted sequ.=101011001101 ....) Å
Ú Differential PCM (DPCM): diff. quantizers Ý Ý Ý Ý Delta Modulation: diff. quants with 2 levels +? or ? Ý Ý Å ì versions :Û are encoded using of PCM Ý a single binary digit Ý Ý Ý (DM − DPCM) Ý Ý Ü Others
PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
2. PCM: BANDWIDTH & " ì we transmit several digits for each quantizer's output level Ê FPCM Fg where œ
FPCM Fg
denotes the channel bandwidth represents the message bandwidth
ì PCM Bandwidth baseband bandwidth: FPCM
channel symbol rate 2
bandpass bandwidth: FPCM
channel symbol rate 2
Hz ‚ 2 Hz
ì Note that, by default, the Lower bound of the 'baseband' bandwidth is assumed and used in this course ì bandwidth expansion factor œ " œ
channel bandwidth message bandwidth
?6?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì Example - Binary PCM FPCM œ
channel symbol rate
2
FPCM œ # J1 Ê
PSTN
œ
FPCM J1
bit rate
2
œ
#J= #
œ # J1 Hz Ê FPCM œ # J1 Å log# U
œ#Ê "œ#
?7?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
3. NOISE EFFECTS in a binary PCM ì It can be proved that the Signal-to-Noise Ratio at the output of a binary Pulse Code Modulation (PCM) system, which employs a BCD encoder/decoder and operates in the presence of noise, is given by the following expression X e g! Ð>Ñ# f 2## SNRout = X en Ð>Ñ# fX en Ð>Ñ# f = 1+4.pe .2## ! q!
where pe =f(type of digital modulator) œ pe =TÈÐ" 3Ñ.EUEŸ e.g. if the digital modulator is a PSK-mod. then pe =TÈ#.EUEŸ
?8?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
3.1. THRESHOLD EFFECTS in a binary PCM
2# # ì We have seen that: SNRout = 1+4.pe .2## ì Let us examine the following two cases: SNR38 =high and SNR38 =low i) SNRin =HIGH
ii) SNRin =LOW
SNRin =high Ê pe =small
SNRin =low Ê pe =large
Ê " %.pe .### ¶ " Ê SNRout ¶ ###
Ê " %.pe .### ¶ %.pe .###
Ê SNRout ¶ 6# dB
PSTN
Ê SNRout ¶
?9?
" %.pe
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì THRESHOLD POINT- definition: Threshold point is arbitrarily defined ## as the SNRin at which the SNRout (i.e. 1+4.2pe .2## ) falls 1dB below the maximum SNRout (i.e. 1dB below the value 2## ). ì By using the above definition it can be shown (...for you ...) that the threshold point occurs when
pe = "'."### where # is the number of bits per level.
? 10 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
SNRout (dB) γ=8
1dB
γ=7 γ=6 γ=5 SNRin,threshold for γ=8
6dB 6dB 6dB
SNRin (dB)
3.2. COMMENTS on THRESHOLD EFFECTS ì The onset of threshold in PCM will result in a sudden Å in the output noise power. ì P = Å Ê SNR38 = Å Ê SNRout reaches 6# dB and becomes independent of P ... above threshold: increasing signal power Ê no further improvement in SNRout ì The limiting value of SNRout depends only on the number of bits # per quantization levels
=318+6
signal
PSTN
? 11 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
4. INTRODUCTION to TELEPHONE NETWORK subscriber-A: 1784-382384
subscriber-B:20759 46266
Junction box (network Termination)
Junction box (network Termination)
PSTN
Twisted coper pair
Twisted coper pair
Note that, as calls are routed through the PSTN, they will be routed (multiplexed) through a hierarchy of switching centers ? 12 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Regional Center - Class 1 … … Sectional Center - Class 2 …
…
Primary Center - Class 3 …
… Toll Center - Class 4
… Local Loop
End Office - Class 5 …
PSTN
…
PSTN ? 13 ?
Local Loop
… Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì 1960 British Post Office (BPO) (currently BT) had established a 24-ch PCM system with objective the system to be available in 1968. Some of this work become the basis to the formation of a number of CCITT recommendations. ì In Europe, the original 24-ch PCM systems, which were designed mainly for up to 32Km transmission routes, have been replaced by 30-ch PCM systems.
PSTN
? 14 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì There are two different CCITT recommendations for PCM. The main differences between these two recommendations are shown in the following table: PCM CCITT RECOMMENDATIONS 1st Recommentation 2nd Recommentation Package Size 24-channels 30-channels Encoding Law .-law E-law .=255 (but they use .=100), E=)(Þ' bits bits bits # =7 56 samples ; # =8 samples # =8 samples FrameFA-signal is distributed FA-word is placed Alignment amongst several frames into a separate slot (TS0) Signalling information is Signalling information Signalling conveyed within each for all 30-channels Strategies speech-time-slot encoded and conveyed in a separate 8-bit TS (TS-16) PSTN
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Principles of Communication Theory & Systems
Compact Lecture Notes
That is, 1st CCITT rec. (24-channels PCM) Ts=1/Fs=125µsec TS1 TS2
1
2
TS3 TS4
3
TS24
4
24
8 bits 8 bits 8 bits 8 bits
8 bits
1 193 bits bit = X=
Frame Alignment
1/6 bits Signaling Information 2nd CCITT rec. (30-channels PCM) Ts=1/Fs=125µsec TS0 TS1 8 bits
1
TS1 TS3
2
3
8 bits 8 bits 8 bits
TS15 TS16 TS17
15
8 bits
16
TS31
30 = 256 bits X=
8 bits
Signaling Information
Frame Alignment
[4bits kth user + 4bits (k+15)th user] 1 Ÿ k Ÿ 15 ? 16 ?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì Note: ˆ A-law=better than .-law (cheaper to produce and easy equipment maintenance, smaller quantization error in particular within the most significant part of the dynamic range). ˆ in 24-ch PCM the signalling information is conveyed within each speech time-slot (technique known as bit stealing). Result: a slight reduction in speech-coding performance.
PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Single-Channel Path of 2nd CCITT rec. (30-channels PCM)
Message signal bandwidth Fg=4kHz
3rd user (say)
HDB3 Line Codee
Uniform quantizer Q=28
A-law A=87.6
PCM HIGHWAY bits
γ=8 level Gray Code
8bits
Sampling Frequency Fs=8kHz
PAM HIGHWAY
PSTN
Ts=1/Fs=125µsec TS0 TS1
Bit rate=γ.Fs i.e. rb=64kbits/s
? 18 ?
Principles of Communication Theory & Systems
8 bits
1
TS1 TS3
2
3
8 bits 8 bits 8 bits
Frame Alignment
TS15 TS16 TS17
15
8 bits
TS31
16
30 8 bits
Signaling Information
Dr A. Manikas
Compact Lecture Notes
Implementation of 2nd PCM CCITT RECOMM. (First Level Mltplx )
PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
FDM Hierarchy CCITT recommendations
PSTN
? 20 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì Based on the 24-channels amd 30-channels PCM CCITT recommendations (primary multiplex groups) the core telephone network evolved from using Frequency Division Multiplex (FDM) technology to digital transmission and switching ì These two PCM CCITT recommendations have led to two PDH (Plesiochronous digital hierarchies) CCITT reccommendations for assembling the TDM telephony data streams from different calls. ì Plesiochronous means: "almost synchronous because bits are stuffed into the frames as padding and the calls location varies slightly - jitters - from frame to frame"
PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
PDH Hierarchy American DS-B
European CEPT-B
0
DS-0
64 kbits/s
CEPT-0
64 kbits/s
1
DS-1
1,544 kbits/s
CEPT-1
2,048 kbits/s
2
DS-2
6,312 kbits/s
CEPT-2
8,448 kbits/s
3
DS-3
44,736 kbits/s
CEPT-3 34,368 kbits/s
4
DS-4 274,176 kbits/s
CEPT-4 139,264 kbits/s
Hierarchical Level
5
PSTN
CEPT-5 565,148 kbits/s
? 22 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì The 24-channel PDH TDM CCITT recommendation (DS-x)
ì The 30-channel PDH TDM CCITT recommendations (CEPT-x)
PSTN
? 23 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Main disadvantage of PDH Networks ì PDH multiplexing was designed for point-to-point communications and channels cannot be added to, or extracted from, a higher multiplexing level demultiplexing down and then multiplexing up again, throught the entire PDH ì For instance, to isolate a particular call from DS4, say, it must be demultiplexed to DS1. ì i.e. this is a very complex procedure and needs very expensive equipment at every exchange to demultiplex and multiplex high speed lines ì American & European Telephone Systems are incompatible (therefore very expensive equipment required to translate one format to the other for transatlantic traffic ) ì Solution: SONET/SDH Signal Hierarchy PSTN
? 24 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
SDH (Synchronous Digital Hierarchy) ì The tranditional PDH standards are based on the DS (USA) and CEPT (Europe) PCM systems (24-channels and 30-channels PCM CCITT recommendation) ì PDH hierarchy is almost synchronous (extra bits are inserted into the digital signal stream to bring them to a common rate. ì In 1988 SDH (Synchronous Digital Hierarchy) was adopted by ITU and ETSI (European Telecommunications Standards Instritute) based on SONET (synchronous optical Networks) ì SDH signals have a common external timing i.e. SDH is synchronous
PSTN
? 25 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì The SDH standards used in Europe are STM-1 which provides 155 Mbits/sec STM-2 which provides 310 Mbits/sec STM-3 which provides 465 Mbits/sec STM-4 which provides 620 Mbits/sec etc (increments of 155 Mbits/sec ) ì The most important main standards are STM-1, STM-4 and STM-16. These are commercially available
? 26 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
SONET/SDH Hierarchy Hierarchical
Level
American
SONET STS-B
European
SDH STM-B
0
STS-3 œ 3 ‚ DS-3
STM-1 œ " ‚ CEPT-%
1
STS-12 œ 12 ‚ DS-3
STM-4 œ % ‚ CEPT-4
2
STS-48 œ 48 ‚ DS-3
STM-16 œ "' ‚ CEPT-%
Key Advantages ì it is simple to add and drop channels to meet customer requirements ì more bandwidth is available for network management ì equipment is smaller and cheaper ì network flexibility ì integrate and manage various types of traffic on a single fiber. PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
CORE Networks
Access Networks
PDH Nets SDH Nets Mobile Nets ATM Nets IP Nets Inteligent Networks etc. Network Gateways
POTS xDSL 2G 3G B-ISDN bluetooth ethernet GUI etc.
CORE Network No.2
Access Network No.1
Access Networks POTS xDSL 2G 3G B-ISDN bluetooth ethernet GUI etc.
Gateway Interface
CORE Network No.1
CORE Network No.3
Access Network No.3
? 28 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
5. xDSL-type Transmission Systems INTERNET
SERVICE PROVIDER
MODEM MODEM
To a POTS line Card LOCAL EXCHANGE (or a street-cabinet)
POTS Network (Narrowband Network)
LOCAL EXCHANGE (or a street-cabinet)
LOCAL EXCHANGE (or a street-cabinet) To a POTS line Card PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
INTERNET xDSL Spliter filter
g. E.
SERVICE PROVIDER
/S H SD ET N O
Broadband Network Spliter filter
xDSL
POTS Network (Narrowband Network)
LOCAL EXCHANGE (or a street-cabinet)
POTS Line-card
LOCAL EXCHANGE (or a street-cabinet) To a POTS line Card ? 30 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Frequency spectrum Upstream Downstream
Upstream
ADSL Spectrum
Downstream 640kb/s 8Mb/s In practice
POTS POTS
250kb/s
-1.1M
-138k
Upstream
-25k
-4k
In practice
2Mb/s
f (Hz)
4k
25k
138k
VDSL Spectrum
1.1M
Upstream
Downstream
Downstream
14 Mb/s
-10M,-30M PSTN
-3.94M
-3.56M
-138k
POTS
POTS
3 Mb/s
f 4k
-4k ? 31 ?
138k
3.56M 3.94M
10M-30M Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Examples of Bit-rate Requirements for Various Applications ì In the following table the first number (bit-rate) indicates the minimum satisfactory rate for some RESIDENTIAL users, and the second number indicates the rate that will satisfy nearly all RESIDENTIAL users. Application
Downstream Bit Rate (bit/s)
Upstream Bit (bits/s)
Voice telephony Internet, online service access email High definition TV Broadcast video Movies on demand Music on demand Video phone Distance learning Shop Home Video Games
16k - 64k 14k - 3M 9k - 128k 12M - 24M 1.5M - 6M 1.5M - 6M 384k - 1.5M 128k - 1.5M 384k - 3M 128k - 1.5M 64k - 1.5M
16k - 64k 14k - 384k 9k - 64k 9k 9k 128k - 1.5M 128k - 3M 9k - 64k 64k - 1.5M
PSTN
? 32 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì It is clear from the above that for offering a near broadcast quality video service, telephony and fast Internet, the network transmission should support at least 5 Mbit/s. From POTS to xDSL ì The POTs coper network (designed and built to support telephony service) provides an extensive infrastructure of twisted pairs which connects approximately 30 million residential and business UK customers . ì Originally, customers wishing to transmit data were restricted to using modems which operated in the 4 kHz voice-band. The first voice-band modems were introduced in the 1950s and were capable of operating at 300 bit/s, but these rapidly developed to 28.8 (and more) kbit/s.
PSTN
? 33 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì As the core network evolved from using Frequency Division Multiplex (FDM) technology to digital transmission and switching, the rollout of the 64 kbit/s narrowband digital PSTN network meant that copper transmission in the access network was no longer restricted to 4 kHz bandwidth. Transmission equipment can now exploit the far greater bandwidth capabilities of the network infrastructure between the exchange and the customers' premises. ì In 1986 Basic Rate ISDN (ISDN2) was introduced. ISDN2 operates over a single copper pair and modulates data using one four level pulse to represent two binary bits (2B1Q). Data is sent simultaneously in both directions using echo cancelled hybrid transmission, and adaptive equalisation is used to automatically compensate for attenuation across the transmission band. ISDN2 uses 80 kHz bandwidth to deliver 160 kbit/s symmetric data over access lines up to approximately 5.5 km long (or up to 42 dB insertion loss at 100 kHz).
PSTN
? 34 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì The evolution of digital transmission systems continued with the deployment 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 and MegaStreamTM over access lines up to 3.7 km in length (or up to 26 dB insertion loss at 100 kHz). To achieve this the system uses frequencies up to 290 kHz, and employs two or three copper pairs each operating at between 784 kbit/s and 1 Mbit/s. ì The most recent DSL system which is currently available from BT is ADSL. Unlike ISDN or HDSL, ADSL employs asymmetric data transmission. and is capable of delivering up to 8 Mbit/s downstream to the customer, and 640 kbit/s upstream to the exchange over a single copper pair. ADSL uses frequencies up to 1.1 MHz, but does not use the 4 kHz voice-band. Customers subscribing to ADSL derived data services can therefore continue to use basic telephony. Although ADSL is technically capable of delivering 8 Mbit/s downstream, the highest bit rate offered by BT's commercial ADSL data products (DataStream and IPStream) is 2 Mbit/s downstream and 250 kbit/s upstream. PSTN
? 35 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì VDSL is the next generation of digital copper transmission system, further increasing the capacity of a metallic twisted pair and using bandwidths of more 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 traffic to the customer is much higher than in the upstream direction from the customer. ì For example, the downstream traffic may carry a video channel or high speed Internet files, while the upstream traffic carries only a narrowband data channel for controlling video, or other control signals and low speed data. ? 36 ?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì ADSL operates over the local access twisted pair network between the local exchange 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/s ˆ Upstream (towards the exchange) bit rates up to 640 kbit/s ˆ Voiceband telephony service ì The operating frequency range of ADSL is limited to between 25 kHz and 1.1 MHz. The upstream channel is placed at the lower end of the available frequency band where it suffers less attenuation and crosstalk noise and is therefore easier to receive. The downstream channel bandwidth is much greater thus enabling the system to achieve higher transmission capacity for conveying services to the customer.
PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
MODULATOR: Discrete Multi-Tone modulation (DMT) ì The ANSI ADSL standard (T1.413) specifies DMT as the modulation technique. ì DMT is a multi-carrier modulation technique which employs a variant of Orthogonal Frequency Domain Multiplex modulation. The transmitter and receiver structures are shown below. Transmit Data
Band Pass Filter
Parallel to serial converter
Inverse discrete Fourier transform
Encoder and buffer
D/A To line
DMT transmitter
Band Pass Filter A/D From line
Serial to parallel converter
Discrete Fourier transform
Decoder and buffer
Receive Data
DMT receiver ? 38 ?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì The data stream to be transmitted is buffered into blocks of bits. ì The overall channel spectrum is divided into 256 independent sub-channels and the blocks of bits are assigned to each frequency sub-channel. ì The data is encoded by modulating each sub-carrier so that the amplitude and phase forms a QAM constellation for that sub-channel. ì Each sub-carrier can encode a variable but pre-determined number of data bits, thus forming different constellation sizes (see figure). QAM sub-channels
Signal Power
Variable constellation sizes per sub-channel
Frequency
PSTN
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Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì The overall data capacity varies with frequency, and those sub-channels with higher capacity are assigned more bits. The number of bits allocated to each sub-channel is determined using an algorithm based on each subchannel's signal to noise ratio. All of these are performed by the first block of the DMT transmitter diagram. Twisted Copper Pair Line Attenuation
Bits per Channel
DMT Data Transmission
Frequency
Frequency
ì In the second block, the collection of N QAM symbols is then modulated by passing through the inverse discrete Fourier transform (IDFT) process. Real signals are then taken from the IDFT output, parallel-to-serial converted, digital-to-analogue converted, and finally band pass filtered before transmission. ì The demodulation process at the receiver is simply the reverse process. PSTN
? 40 ?
Dr A. Manikas
Multiplexing & Public Switched Telephone Network
(PSTN)
Outline: ì ì ì ì ì
E303 & ISE3.2E
PCM: Bandwidth, Bandwidth Expansion Factor, output SNR and Threshold Effects. CCITT recommendations for PCM (24-channels and 30-channels) Plesiochronous digital hierarchies (PDH) Synchronous digital hierarchies (SONET/SDH) BDSL-type transmission systems
Principles of Communication Theory & Systems
Compact Lecture Notes
1. INTRODUCTION
H(f) ^
^
^
^
^
^
?2?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì PCM œ sampled quantized values of an analogue signal are transmitted via a sequence of codewords. i.e. after sampling & quantization, a Source Encoder is used to map the quantized levels (i.e. o/p of quantizer) to codewords of # bits i.e. quantized level È codeword of # bits 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 PSTN
?3?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
g (inputÑ È gq ÐoutputÑ
gq (Volts) m8 m7 m6 m5
000 011 010
001 010 011
010 001 001
011 000 000
100 100 100
101 101 101
m4
110 110 111
111 111 110
BCD code Folded BCD GRAY Code
gq : occurs at a rate Fs ÐN.B.: Fs #.Fg Ñ
samples sec
U œ quantizer levels;
g (Volts)
m3
,3>= #= log# aUb level
m2 m1
ì Note: codeword rate (o/p of source encoder) = quantÞ levels rate = sampling rate =J= =#J1 Å Å Å #-bit -9./A9<.= sec
levels sec
=+7:6/= sec
?4?
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Principles of Communication Theory & Systems
ì bit rate: rb = # . J = Å Å
bits levels level sec
Dr A. Manikas
Compact Lecture Notes
e.g. for U œ 16 levels then rb =% .J= Å #
bits sec
Æ Æ (e.g. transmitted sequ.=101011001101 ....) Å
Ú Differential PCM (DPCM): diff. quantizers Ý Ý Ý Ý Delta Modulation: diff. quants with 2 levels +? or ? Ý Ý Å ì versions :Û are encoded using of PCM Ý a single binary digit Ý Ý Ý (DM − DPCM) Ý Ý Ü Others
PSTN
?5?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
2. PCM: BANDWIDTH & " ì we transmit several digits for each quantizer's output level Ê FPCM Fg where œ
FPCM Fg
denotes the channel bandwidth represents the message bandwidth
ì PCM Bandwidth baseband bandwidth: FPCM
channel symbol rate 2
bandpass bandwidth: FPCM
channel symbol rate 2
Hz ‚ 2 Hz
ì Note that, by default, the Lower bound of the 'baseband' bandwidth is assumed and used in this course ì bandwidth expansion factor œ " œ
channel bandwidth message bandwidth
?6?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì Example - Binary PCM FPCM œ
channel symbol rate
2
FPCM œ # J1 Ê
PSTN
œ
FPCM J1
bit rate
2
œ
#J= #
œ # J1 Hz Ê FPCM œ # J1 Å log# U
œ#Ê "œ#
?7?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
3. NOISE EFFECTS in a binary PCM ì It can be proved that the Signal-to-Noise Ratio at the output of a binary Pulse Code Modulation (PCM) system, which employs a BCD encoder/decoder and operates in the presence of noise, is given by the following expression X e g! Ð>Ñ# f 2## SNRout = X en Ð>Ñ# fX en Ð>Ñ# f = 1+4.pe .2## ! q!
where pe =f(type of digital modulator) œ pe =TÈÐ" 3Ñ.EUEŸ e.g. if the digital modulator is a PSK-mod. then pe =TÈ#.EUEŸ
?8?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
3.1. THRESHOLD EFFECTS in a binary PCM
2# # ì We have seen that: SNRout = 1+4.pe .2## ì Let us examine the following two cases: SNR38 =high and SNR38 =low i) SNRin =HIGH
ii) SNRin =LOW
SNRin =high Ê pe =small
SNRin =low Ê pe =large
Ê " %.pe .### ¶ " Ê SNRout ¶ ###
Ê " %.pe .### ¶ %.pe .###
Ê SNRout ¶ 6# dB
PSTN
Ê SNRout ¶
?9?
" %.pe
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì THRESHOLD POINT- definition: Threshold point is arbitrarily defined ## as the SNRin at which the SNRout (i.e. 1+4.2pe .2## ) falls 1dB below the maximum SNRout (i.e. 1dB below the value 2## ). ì By using the above definition it can be shown (...for you ...) that the threshold point occurs when
pe = "'."### where # is the number of bits per level.
? 10 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
SNRout (dB) γ=8
1dB
γ=7 γ=6 γ=5 SNRin,threshold for γ=8
6dB 6dB 6dB
SNRin (dB)
3.2. COMMENTS on THRESHOLD EFFECTS ì The onset of threshold in PCM will result in a sudden Å in the output noise power. ì P = Å Ê SNR38 = Å Ê SNRout reaches 6# dB and becomes independent of P ... above threshold: increasing signal power Ê no further improvement in SNRout ì The limiting value of SNRout depends only on the number of bits # per quantization levels
=318+6
signal
PSTN
? 11 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
4. INTRODUCTION to TELEPHONE NETWORK subscriber-A: 1784-382384
subscriber-B:20759 46266
Junction box (network Termination)
Junction box (network Termination)
PSTN
Twisted coper pair
Twisted coper pair
Note that, as calls are routed through the PSTN, they will be routed (multiplexed) through a hierarchy of switching centers ? 12 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Regional Center - Class 1 … … Sectional Center - Class 2 …
…
Primary Center - Class 3 …
… Toll Center - Class 4
… Local Loop
End Office - Class 5 …
PSTN
…
PSTN ? 13 ?
Local Loop
… Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì 1960 British Post Office (BPO) (currently BT) had established a 24-ch PCM system with objective the system to be available in 1968. Some of this work become the basis to the formation of a number of CCITT recommendations. ì In Europe, the original 24-ch PCM systems, which were designed mainly for up to 32Km transmission routes, have been replaced by 30-ch PCM systems.
PSTN
? 14 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì There are two different CCITT recommendations for PCM. The main differences between these two recommendations are shown in the following table: PCM CCITT RECOMMENDATIONS 1st Recommentation 2nd Recommentation Package Size 24-channels 30-channels Encoding Law .-law E-law .=255 (but they use .=100), E=)(Þ' bits bits bits # =7 56 samples ; # =8 samples # =8 samples FrameFA-signal is distributed FA-word is placed Alignment amongst several frames into a separate slot (TS0) Signalling information is Signalling information Signalling conveyed within each for all 30-channels Strategies speech-time-slot encoded and conveyed in a separate 8-bit TS (TS-16) PSTN
? 15 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
That is, 1st CCITT rec. (24-channels PCM) Ts=1/Fs=125µsec TS1 TS2
1
2
TS3 TS4
3
TS24
4
24
8 bits 8 bits 8 bits 8 bits
8 bits
1 193 bits bit = X=
Frame Alignment
1/6 bits Signaling Information 2nd CCITT rec. (30-channels PCM) Ts=1/Fs=125µsec TS0 TS1 8 bits
1
TS1 TS3
2
3
8 bits 8 bits 8 bits
TS15 TS16 TS17
15
8 bits
16
TS31
30 = 256 bits X=
8 bits
Signaling Information
Frame Alignment
[4bits kth user + 4bits (k+15)th user] 1 Ÿ k Ÿ 15 ? 16 ?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì Note: ˆ A-law=better than .-law (cheaper to produce and easy equipment maintenance, smaller quantization error in particular within the most significant part of the dynamic range). ˆ in 24-ch PCM the signalling information is conveyed within each speech time-slot (technique known as bit stealing). Result: a slight reduction in speech-coding performance.
PSTN
? 17 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Single-Channel Path of 2nd CCITT rec. (30-channels PCM)
Message signal bandwidth Fg=4kHz
3rd user (say)
HDB3 Line Codee
Uniform quantizer Q=28
A-law A=87.6
PCM HIGHWAY bits
γ=8 level Gray Code
8bits
Sampling Frequency Fs=8kHz
PAM HIGHWAY
PSTN
Ts=1/Fs=125µsec TS0 TS1
Bit rate=γ.Fs i.e. rb=64kbits/s
? 18 ?
Principles of Communication Theory & Systems
8 bits
1
TS1 TS3
2
3
8 bits 8 bits 8 bits
Frame Alignment
TS15 TS16 TS17
15
8 bits
TS31
16
30 8 bits
Signaling Information
Dr A. Manikas
Compact Lecture Notes
Implementation of 2nd PCM CCITT RECOMM. (First Level Mltplx )
PSTN
? 19 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
FDM Hierarchy CCITT recommendations
PSTN
? 20 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì Based on the 24-channels amd 30-channels PCM CCITT recommendations (primary multiplex groups) the core telephone network evolved from using Frequency Division Multiplex (FDM) technology to digital transmission and switching ì These two PCM CCITT recommendations have led to two PDH (Plesiochronous digital hierarchies) CCITT reccommendations for assembling the TDM telephony data streams from different calls. ì Plesiochronous means: "almost synchronous because bits are stuffed into the frames as padding and the calls location varies slightly - jitters - from frame to frame"
PSTN
? 21 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
PDH Hierarchy American DS-B
European CEPT-B
0
DS-0
64 kbits/s
CEPT-0
64 kbits/s
1
DS-1
1,544 kbits/s
CEPT-1
2,048 kbits/s
2
DS-2
6,312 kbits/s
CEPT-2
8,448 kbits/s
3
DS-3
44,736 kbits/s
CEPT-3 34,368 kbits/s
4
DS-4 274,176 kbits/s
CEPT-4 139,264 kbits/s
Hierarchical Level
5
PSTN
CEPT-5 565,148 kbits/s
? 22 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì The 24-channel PDH TDM CCITT recommendation (DS-x)
ì The 30-channel PDH TDM CCITT recommendations (CEPT-x)
PSTN
? 23 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Main disadvantage of PDH Networks ì PDH multiplexing was designed for point-to-point communications and channels cannot be added to, or extracted from, a higher multiplexing level demultiplexing down and then multiplexing up again, throught the entire PDH ì For instance, to isolate a particular call from DS4, say, it must be demultiplexed to DS1. ì i.e. this is a very complex procedure and needs very expensive equipment at every exchange to demultiplex and multiplex high speed lines ì American & European Telephone Systems are incompatible (therefore very expensive equipment required to translate one format to the other for transatlantic traffic ) ì Solution: SONET/SDH Signal Hierarchy PSTN
? 24 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
SDH (Synchronous Digital Hierarchy) ì The tranditional PDH standards are based on the DS (USA) and CEPT (Europe) PCM systems (24-channels and 30-channels PCM CCITT recommendation) ì PDH hierarchy is almost synchronous (extra bits are inserted into the digital signal stream to bring them to a common rate. ì In 1988 SDH (Synchronous Digital Hierarchy) was adopted by ITU and ETSI (European Telecommunications Standards Instritute) based on SONET (synchronous optical Networks) ì SDH signals have a common external timing i.e. SDH is synchronous
PSTN
? 25 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì The SDH standards used in Europe are STM-1 which provides 155 Mbits/sec STM-2 which provides 310 Mbits/sec STM-3 which provides 465 Mbits/sec STM-4 which provides 620 Mbits/sec etc (increments of 155 Mbits/sec ) ì The most important main standards are STM-1, STM-4 and STM-16. These are commercially available
? 26 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
SONET/SDH Hierarchy Hierarchical
Level
American
SONET STS-B
European
SDH STM-B
0
STS-3 œ 3 ‚ DS-3
STM-1 œ " ‚ CEPT-%
1
STS-12 œ 12 ‚ DS-3
STM-4 œ % ‚ CEPT-4
2
STS-48 œ 48 ‚ DS-3
STM-16 œ "' ‚ CEPT-%
Key Advantages ì it is simple to add and drop channels to meet customer requirements ì more bandwidth is available for network management ì equipment is smaller and cheaper ì network flexibility ì integrate and manage various types of traffic on a single fiber. PSTN
? 27 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
CORE Networks
Access Networks
PDH Nets SDH Nets Mobile Nets ATM Nets IP Nets Inteligent Networks etc. Network Gateways
POTS xDSL 2G 3G B-ISDN bluetooth ethernet GUI etc.
CORE Network No.2
Access Network No.1
Access Networks POTS xDSL 2G 3G B-ISDN bluetooth ethernet GUI etc.
Gateway Interface
CORE Network No.1
CORE Network No.3
Access Network No.3
? 28 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
5. xDSL-type Transmission Systems INTERNET
SERVICE PROVIDER
MODEM MODEM
To a POTS line Card LOCAL EXCHANGE (or a street-cabinet)
POTS Network (Narrowband Network)
LOCAL EXCHANGE (or a street-cabinet)
LOCAL EXCHANGE (or a street-cabinet) To a POTS line Card PSTN
? 29 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
INTERNET xDSL Spliter filter
g. E.
SERVICE PROVIDER
/S H SD ET N O
Broadband Network Spliter filter
xDSL
POTS Network (Narrowband Network)
LOCAL EXCHANGE (or a street-cabinet)
POTS Line-card
LOCAL EXCHANGE (or a street-cabinet) To a POTS line Card ? 30 ?
PSTN
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Frequency spectrum Upstream Downstream
Upstream
ADSL Spectrum
Downstream 640kb/s 8Mb/s In practice
POTS POTS
250kb/s
-1.1M
-138k
Upstream
-25k
-4k
In practice
2Mb/s
f (Hz)
4k
25k
138k
VDSL Spectrum
1.1M
Upstream
Downstream
Downstream
14 Mb/s
-10M,-30M PSTN
-3.94M
-3.56M
-138k
POTS
POTS
3 Mb/s
f 4k
-4k ? 31 ?
138k
3.56M 3.94M
10M-30M Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
Examples of Bit-rate Requirements for Various Applications ì In the following table the first number (bit-rate) indicates the minimum satisfactory rate for some RESIDENTIAL users, and the second number indicates the rate that will satisfy nearly all RESIDENTIAL users. Application
Downstream Bit Rate (bit/s)
Upstream Bit (bits/s)
Voice telephony Internet, online service access email High definition TV Broadcast video Movies on demand Music on demand Video phone Distance learning Shop Home Video Games
16k - 64k 14k - 3M 9k - 128k 12M - 24M 1.5M - 6M 1.5M - 6M 384k - 1.5M 128k - 1.5M 384k - 3M 128k - 1.5M 64k - 1.5M
16k - 64k 14k - 384k 9k - 64k 9k 9k 128k - 1.5M 128k - 3M 9k - 64k 64k - 1.5M
PSTN
? 32 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì It is clear from the above that for offering a near broadcast quality video service, telephony and fast Internet, the network transmission should support at least 5 Mbit/s. From POTS to xDSL ì The POTs coper network (designed and built to support telephony service) provides an extensive infrastructure of twisted pairs which connects approximately 30 million residential and business UK customers . ì Originally, customers wishing to transmit data were restricted to using modems which operated in the 4 kHz voice-band. The first voice-band modems were introduced in the 1950s and were capable of operating at 300 bit/s, but these rapidly developed to 28.8 (and more) kbit/s.
PSTN
? 33 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì As the core network evolved from using Frequency Division Multiplex (FDM) technology to digital transmission and switching, the rollout of the 64 kbit/s narrowband digital PSTN network meant that copper transmission in the access network was no longer restricted to 4 kHz bandwidth. Transmission equipment can now exploit the far greater bandwidth capabilities of the network infrastructure between the exchange and the customers' premises. ì In 1986 Basic Rate ISDN (ISDN2) was introduced. ISDN2 operates over a single copper pair and modulates data using one four level pulse to represent two binary bits (2B1Q). Data is sent simultaneously in both directions using echo cancelled hybrid transmission, and adaptive equalisation is used to automatically compensate for attenuation across the transmission band. ISDN2 uses 80 kHz bandwidth to deliver 160 kbit/s symmetric data over access lines up to approximately 5.5 km long (or up to 42 dB insertion loss at 100 kHz).
PSTN
? 34 ?
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì The evolution of digital transmission systems continued with the deployment 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 and MegaStreamTM over access lines up to 3.7 km in length (or up to 26 dB insertion loss at 100 kHz). To achieve this the system uses frequencies up to 290 kHz, and employs two or three copper pairs each operating at between 784 kbit/s and 1 Mbit/s. ì The most recent DSL system which is currently available from BT is ADSL. Unlike ISDN or HDSL, ADSL employs asymmetric data transmission. and is capable of delivering up to 8 Mbit/s downstream to the customer, and 640 kbit/s upstream to the exchange over a single copper pair. ADSL uses frequencies up to 1.1 MHz, but does not use the 4 kHz voice-band. Customers subscribing to ADSL derived data services can therefore continue to use basic telephony. Although ADSL is technically capable of delivering 8 Mbit/s downstream, the highest bit rate offered by BT's commercial ADSL data products (DataStream and IPStream) is 2 Mbit/s downstream and 250 kbit/s upstream. PSTN
? 35 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì VDSL is the next generation of digital copper transmission system, further increasing the capacity of a metallic twisted pair and using bandwidths of more 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 traffic to the customer is much higher than in the upstream direction from the customer. ì For example, the downstream traffic may carry a video channel or high speed Internet files, while the upstream traffic carries only a narrowband data channel for controlling video, or other control signals and low speed data. ? 36 ?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì ADSL operates over the local access twisted pair network between the local exchange 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/s ˆ Upstream (towards the exchange) bit rates up to 640 kbit/s ˆ Voiceband telephony service ì The operating frequency range of ADSL is limited to between 25 kHz and 1.1 MHz. The upstream channel is placed at the lower end of the available frequency band where it suffers less attenuation and crosstalk noise and is therefore easier to receive. The downstream channel bandwidth is much greater thus enabling the system to achieve higher transmission capacity for conveying services to the customer.
PSTN
? 37 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
MODULATOR: Discrete Multi-Tone modulation (DMT) ì The ANSI ADSL standard (T1.413) specifies DMT as the modulation technique. ì DMT is a multi-carrier modulation technique which employs a variant of Orthogonal Frequency Domain Multiplex modulation. The transmitter and receiver structures are shown below. Transmit Data
Band Pass Filter
Parallel to serial converter
Inverse discrete Fourier transform
Encoder and buffer
D/A To line
DMT transmitter
Band Pass Filter A/D From line
Serial to parallel converter
Discrete Fourier transform
Decoder and buffer
Receive Data
DMT receiver ? 38 ?
PSTN
Principles of Communication Theory & Systems
Dr A. Manikas
Compact Lecture Notes
ì The data stream to be transmitted is buffered into blocks of bits. ì The overall channel spectrum is divided into 256 independent sub-channels and the blocks of bits are assigned to each frequency sub-channel. ì The data is encoded by modulating each sub-carrier so that the amplitude and phase forms a QAM constellation for that sub-channel. ì Each sub-carrier can encode a variable but pre-determined number of data bits, thus forming different constellation sizes (see figure). QAM sub-channels
Signal Power
Variable constellation sizes per sub-channel
Frequency
PSTN
? 39 ?
Dr A. Manikas
Principles of Communication Theory & Systems
Compact Lecture Notes
ì The overall data capacity varies with frequency, and those sub-channels with higher capacity are assigned more bits. The number of bits allocated to each sub-channel is determined using an algorithm based on each subchannel's signal to noise ratio. All of these are performed by the first block of the DMT transmitter diagram. Twisted Copper Pair Line Attenuation
Bits per Channel
DMT Data Transmission
Frequency
Frequency
ì In the second block, the collection of N QAM symbols is then modulated by passing through the inverse discrete Fourier transform (IDFT) process. Real signals are then taken from the IDFT output, parallel-to-serial converted, digital-to-analogue converted, and finally band pass filtered before transmission. ì The demodulation process at the receiver is simply the reverse process. PSTN
? 40 ?
Dr A. Manikas
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