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Chapter 8

Chapter 8. Digital Transmission Systems Part 1. 1 Basics of PCM. About 50% or less of the traffic carried on the PSTN is voice traffic which is initially analog . These analog signals must be converted to a digital format compatible with the digital network .

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Chapter 8

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  1. Chapter 8 Digital Transmission Systems Part 1 Bahman R. Alyaei

  2. 1 Basics of PCM • About 50% or less of the traffic carried on the PSTN is voice traffic which is initially analog. • These analog signals must be converted to a digital format compatible with the digital network. • The remaining of the PSTNtraffic is digital data, a great portion of which is Internet-related that is already digital. Bahman R. Alyaei

  3. Continue… • In Digital Networks, the transmission facilities and the switches are digital. • The digital waveform on the PSTN is based on Pulse Code Modulation (PCM). • In the design of PCM systemsfor PSTN, the error rate is maintained at BER = 1×10−3. Bahman R. Alyaei

  4. Continue… • In PCM system, the incoming band limited voice signal (W = 4kHz) is • Sampled according to the Nyquist Ratefs = 2W = 8000 samples/sec, • Each sample is then quantized through Non-uniform Quantizer, • Then each quantized sample is encoded by a k =8-bit ADC. • The bit rateRb=kfs = 64 kbps. Bahman R. Alyaei

  5. Continue… • Then, the transmission bandwidth for a single voice channel is approximately BT =64 kHz. The process of digitization Bahman R. Alyaei

  6. Logarithmic law “Compression” Linear Quantizer “Mid-rise” or “Mid-tread” Input signal Input signal 2 Quantization • A Non-linear quantizer consists of a logarithmic law circuit called compressor followed by a linear quantizer. • On the receiver side, the inverse of the logarithmic law circuit is used, that is called expander. Non-linear quantizer Bahman R. Alyaei

  7. 2.1 Companding • Companding stands for two words • Compression • Expansion. • Compression: takes place on the transmit side of the Transceiver Circuit, it reduces the dynamic range with little loss of fidelity • Expansion: takes place on the receive side of the Transceiver Circuit, and it returns the signal to its normal condition. Bahman R. Alyaei

  8. Continue… • This is done by favoring low-level speech over higher-level speech. • Hence, more code segments are assigned to speech bursts at low level than at the higher levels, progressively more as level goes down. Bahman R. Alyaei

  9. Continue… A simple graphic representation of compression. Six-bit coding, eight six bit sequences per segment. Bahman R. Alyaei

  10. Continue… • The figure in the previous slide explains the non-linear quantization process, where each sample is encoded with k = 6-bits. • The total number of levels L = 2k = 256. • Note that eight coded sequences are assigned to each level grouping. • The smallest range rises only 0.0666 V from the origin (0 V). • The largest range extends over 0.5 V, and it is assigned only eight coded sequences. Bahman R. Alyaei

  11. Continue… • There are two types of Non-linear Qunatizers: • A-law: European. where xis the signal input amplitude and A = 87. 6 for E1 system Bahman R. Alyaei

  12. A-law used for companding in Europe Bahman R. Alyaei

  13. The 13-segment approximation of the A -law curve used with E1 PCM equipment Bahman R. Alyaei

  14. Continue… • The figure in the previous slide shows the compandingcurve and resulting coding for the European E1 system. • Note that the curve consists of linear piecewise segments, seven above and seven below the origin. • The segment just above and the segment just below the origin each consists of two linear elements. Bahman R. Alyaei

  15. Continue… • Counting the collinear elements by the origin, there are 16 segments. • Each segment has 16, 8-bit PCM codewords assigned. • These are the codewords that identify the voltage level of a sample at some moment in time. • The first bit (MSB) tells the distant-end receiver if that sample is a positive or a negative voltage. Bahman R. Alyaei

  16. Continue… • Note that all the PCM words above the origin start with a binary 1, and those below the origin start with a binary 0. • The next three bits identify the segment. • There are 8segments above the origin and 8 below (23 = 8). • The last 4 bits, shown in the figure as XXXX, identify where in the segment that voltage line is located. Bahman R. Alyaei

  17. Continue… • Example: the code word 11010100 is +ve and located in segment 4. The European E1 system, coding of segment 4 (positive) Bahman R. Alyaei

  18. Continue… • µ-law: North American. • Where xis the signal input amplitude and µ = 255 for DS1 system (New). • µ = 100 for T1 system (Old). Bahman R. Alyaei

  19. μ-Law used for companding in the United Sates. Bahman R. Alyaei

  20. Piecewise linear approximation of the µ-law logarithmic curve Bahman R. Alyaei

  21. Continue… • The figure in the previous slide shows an equivalent logarithmic curve for the North American DS1 system. • It uses a 15-segment approximation of the logarithmic µ -law curve (µ = 255). • The segments cutting the origin are collinear and are counted as one. Bahman R. Alyaei

  22. Continue… • The first code element (MSB), indicates to the distant end whether the sample voltage is positive or negative. • The next three elements (bits) identify the segment. • The last four elements (bits) identify the actual quantum level inside the segment. Bahman R. Alyaei

  23. Eight-level coding of the North American DS1 PCM system. Note that there are actually only 255 quantizing steps because steps 0 and 1 use the same bit sequence, thus avoiding a code sequence with no transitions (i.e., all 0s) Bahman R. Alyaei

  24. 3 PCM Codec or TDM System • Codec is a contraction of the word group Coder–Decoder. • Codec: isa TDM transceiver. • Codec accepts multiple voice channels. • It digitizes (ADC) and multiplexes the information; and delivers a serial bit stream to the trunkline or link. Bahman R. Alyaei

  25. Continue… • It receives voice signals from the telephone sets in the local network through the subscriber loop. • It also accepts a serial bit stream from the link, demultiplexes the digital information, and performs Digital-to-Analog Conversion (DAC). Bahman R. Alyaei

  26. Simplified functional block diagram of a PCM codec or TDM system Bahman R. Alyaei

  27. Continue… • The voice channel to be transmitted is passed through a 3.4-kHz LPF. • The output of the LPF is fed to a S/H Circuit to generate PAM signal. • The PAM signal (PAM highway) fed to a Channel Gate Circuit. • The release of the PAM highway samples is under the control of the Channel Gate which is controlled by a pulse derived from the transmit clock. Bahman R. Alyaei

  28. Continue… • The input to the Coder is the PAM highway. • It accepts a sample of each (n) channel gate in sequence and then generates the appropriate 8-bit signal character corresponding to each sample, which is the basic PCM signal. Bahman R. Alyaei

  29. Continue… • The coder output (nPCM signals) is fed to the Digit Combiner where framing-alignmentsignals and necessary supervisory signaling digits corresponding to each channel are inserted in the appropriate time slots. • On the receive side, the Codec accepts the serial PCM bit stream through the Digit Separator. Bahman R. Alyaei

  30. Continue… • At the Digit Separator, the nsignals are regenerated and split, delivering the PCM signal to four locations to carry out the following processing functions: • Timing recovery, • Decoding, • Frame alignment, • Signaling (supervisory). Bahman R. Alyaei

  31. Continue… • Timing Recovery keeps the receive clock in synchronism with the far-end transmit clock. • The receive clock provides the necessary gating pulses for the receive side of the PCMCodec. • The Frame-Alignment Circuit senses the presence of the frame-alignment signal at the correct time interval, thus providing the receive terminal with frame alignment. Bahman R. Alyaei

  32. Continue… • The Decoder, under control of the receive clock, decodes the code character signals corresponding to each channel. • The output of the Decoder is the reconstituted pulses making up a PAM highway. • The Channel Gate accepts the PAM highway, gating the n-channelPAM highway in sequence under control of the receive clock. Bahman R. Alyaei

  33. Continue… • The output of the Channel Gate is fed in turn to each channel filter, thus enabling the reconstituted analog voice signal to reach the appropriate voice path. • Gating pulses extract signaling information in the signaling processor and apply this information to each of the reconstituted voice channels with the supervisory signaling interface as required by the analog telephone system in question. Bahman R. Alyaei

  34. Continue… • There are two varieties of PCM Codec: • The North American, called DS1 or T1. • The European, called E1. • Both DS1 and E1 are TDM systems. • The frame duration of E1 and DS1 is Tf = 1/fs = 1/8000 = 125 μS. Bahman R. Alyaei

  35. 4 E1 System Bahman R. Alyaei

  36. Multiframe for E1 carrier Bahman R. Alyaei

  37. Continue… • E1PCM system is a 32-channel system. • One frame = 32-channels. • Each channel consists of 8-bits. • We say that, each channel is allotted an 8-bit time slot (TS), TS0 through TS31. • Total No. of TSper frame= 32. • TS = Ts/Total no. of channels = 125μ/32 = 3.906 μS. Bahman R. Alyaei

  38. Continue… • The total no. of bits per E1 frame is Total no. of bits = 8×32 = 256 bits. • The E1bit rate to the line is Rb = 256×8000 = 2.048 Mbps. • Of the 32 channels (32 TS): • 30 channels transmit speech (or data) derived from incoming telephone trunks • The remaining 2 channels transmitsynchronization-alignment and signaling information. Bahman R. Alyaei

  39. Continue… • E1 can be adapted for CCS, providing 31 data channels and employing a single synchronization channel,  and  the signaling protocol being transmitted on a different physical channel. (Today) Bahman R. Alyaei

  40. Continue… • In TS0, a synchronizing code or word which is called Frame Alignment Word (FAW) is transmitted every second frame, occupying digits 2 through 8 as follows: *0011011. • This allows the receiver to lock onto the start of each frame and match up each channel in turn. Bahman R. Alyaei

  41. Continue… • For the alignment mechanism to be maintained, the FAW does not need to be transmitted in every frame, only even frames. • In those frames (odd frames) without the synchronizing word called Frame Service Word (FSW), the second bit of TS0 is frozen at a 1 so that in these frames the synchronizing word cannot be imitated (mimicked). Bahman R. Alyaei

  42. Continue… • It is therefore available for other functions, such as the transmission of the Alarms and Supervisory Signals. • An alarm must be sent to the transmitter when a device detects any of the following at the • Multiplexer • A power failure. • A failure of the codec. • Demultiplexer: • Loss of the signal. • Loss of frame alignment. • BER greater than 10−3. Bahman R. Alyaei

  43. Continue… • The remaining bits in positions 4 to 8 (Spare bits) of TS0 (odd frames) can be used in a number of ways, such as • Transmission of supervisory information signals between exchanges. • In specific point-to-point applications • To establish a data link based on messages for operations management • Maintenance or monitoring of the transmission quality, and so on. Bahman R. Alyaei

  44. Continue… • E1 allow for a full Cyclic Redundancy Check (CRC) to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits (information), but this is not always used. • TS16 is used to carry the call-control signaling between the exchanges at either end of the PCM route. Bahman R. Alyaei

  45. Continue… • Framing and basic timing should be distinguished. • Framing: ensures that the PCM receiver is aligned regarding the beginning and end of a bit sequence or frame. • Timing: refers to the synchronization of the receiver clock, specifically, that it is in step with its companion far-end transmit clock. Bahman R. Alyaei

  46. 4.1 Enhancements to E1 • In order to enhance signaling information (dial pulses) for all 30 channels to be transmitted, the concept of Multiframe is introduced. • The Multiframe consists of 16 frames numbered 0-15. • A Multiframe is divided into two parts: • Sub-multiframe I (SMF-I): frames 0-7. • Sub-multiframe II (SMF-II): frames 8-15. Bahman R. Alyaei

  47. Continue… • In Frame 0, TS16 contains the Multiframe Alignment Word (MFAW) and Multiframe Service Word (MFSW). • In Frames 1-15, TS16 contains signalling information for two channels. • The duration of each Multiframe is 2 mS (125 µS x 16). Bahman R. Alyaei

  48. 5 Digital Signal 1 (DS1) DS1 signal format Bahman R. Alyaei

  49. Continue… • DS1PCM system is a 24-channel system. • One frame = 24-channels. • Each channel consists of 8-bits. • We say that, each channel is allotted an 8-bit time slot (TS), TS1 through TS24. • Total No. of TSper frame= 24. • TS = (8)(Ts/Total no. of bits per frame) = (8)(125μ/193) = 5.181 μS. Bahman R. Alyaei

  50. Continue… • The DS1 signal format, has one bit added as a framing bit called an “S” bit for synchronization. • The total no. of bits per DS1 frame is Total no. of bits = 8×24 + 1 = 193 bits. • The DS1bit rate to the line is Rb = 193×8000 = 1.544 Mbps. • In DS1, all the 24 channels are used totransmit speech (or data) derived from incoming telephone trunks. Bahman R. Alyaei

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