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Digital Multiplexers Digital signals are the sequences of binary 1 and 0 symbols. Digital multiplexing technique simultaneously transmits the symbols from many channels by interleaving, them.This is very much similar to time division multiplexing. In digital multiplexing there are no constraints like periodic sampling and waveform preservation. The digital multiplexing uses a binary multiplexers and their hierarchies. A binary multiplexer merges input bit from different sources into one signal for transmission via a digital communication system. The multiplexing of various digital signals can be bit by bit or by words or by characters.Additional pulses are inserted in the multiplexed data stream to identify the different channels or frames. These are called control bits. The multiplexer performs following operations. i. Establish the frame as the smallest time interval containing at least one bit from every input. Assign to each input a number of unique bit slots within the frame. Insert control bits for frame identification and synchronization. Make allowance for any variations of the input bit rates. Types of Digital Multiplexers There are following types of digital multiplexers Synchronous multiplexers : When single master clock governs all sources synchronous multiplexers are used. Since a single master clock is used there are no bit rate variations. Synchronous multiplexing has highest throughput efficiency. Synchronous multiplexer have the increased complexity because of master clock signal. Asynchronous multiplexers : Asynchronous multiplexers are used for digital data sources that operate in a start/stop mode producing bursts of characters with variable spacing between the bursts. Buffering and character interleaving makes it possible to merge these sources into a synchronous multiplexed bit stream. Quasi synchronous multiplexers : Quasi synchronous multiplexers are used when input bit rates have the same nominal value but vary within specified bounds. These multiplexers arranged in a hierarchy of increasing bit rates, constitute the building blocks of interconnected digital communication system. Multiplexing Hierarchies Fig. 1 shows the multiplexing hierarchy for digital communication. Chai AU , nel Yoice —7* K 1.5 Mb/s PCM __-_ pl 8 Second 64 kb/s level —t-4 6 Mb/s — | MUX ‘a | Third First level level B; Lm] 44 Mb/s —>+| 8. | Mux Voice i Fourth PCM | B, level bakb/s | Aa Cj 274 Mb/s c 7 MUX --—> & Fig. 1. Multiplexing hierarchy for digital communication In this hierarchy the third level is used, for multiplexing purposes and other three levels are designed for point to point transmission and multiplexing. The bit rate at the next level is more than the sum of all the channels multiplexed at the input of that level. Table 1. shows inputs and rates for a typical digital multiplexer. Analog input \ Coder PCM M Multiplexed ‘data " i PCM data 2 t — Codec i Line. p waveform LPF 1 generator e x 8 T N Codec N Channel ‘ Analog i PCM, output f data 1 D Codec . t e j m i u | t Codec |__.? H Pulse . 2 — LPF regenerator > I e x e r Codec N N Fig. 2 TOM/PCM system Multiple Channel Frame Alignment For TDM / PCM (I, System) The multiple channel! alignment is very important in TDM/PCM system. Fig. shows the TDM frame format of most widely used T1 system. Mulirame i i 1oms Frame sync bit Channel | Creninel 3 Frame Ieee BE aCe RDOEDBE i PEER OTOH T212Sps Fig, 193 digits 4 3 Multiple channel! frame alignment In T1 systen? As shown in the Fig. 3, this system contains a multiframe of 12 frames. The duration of the multiframe is 1.5 msec. Each frame consists of samples from 24 channels. Thus the samples of 24 channels are Time Division Multiplexed. Each channel sample is encoded into 8 bits. Thus the total bits of 24 channels will be 24 x 8=192 bits. This indicate the start of the next frame, the frame sync bit or 'S' bit is transmitted at the begining of each frame. Thus the total bits in one frame are (24 x 8) +1=193 bits. Calculation of bit rate : Each channel is normally sampled at § kHz rate. Thus the time between any two 1 . 000 He =125 microseconds. In the TDM system, the samples from each channel is transmitted in each successive frame. Hence the duration of the frame will also be 125 microseconds. This is shown in Fig. ~~ .3. Bits per frame = 24 channels/frame x 8 bits/channel + 1 frame sync bit successive samples of single channel will be = 193 bits . _ Number of bits per frame _ 193 bits Bit rate Ry = Time of one frame ~ 125x 10- seconds 1.544*105 bits/sec The signaling information is transmitted by replacing the 8" bit (i.e. LSB) in each channel by signaling bit in every sixth frame. Thus, Signaling period = Period of the signaling bit = 125x10 x6 = 750x10- sec —__1___ signaling period —!_ 750% 10°6 - = 1.3333 kbps. Signaling rate a > Example 1: The T, carrier system used in digital telephony multiplexes 24 voice channels based on 8 bit PCM. Each voice signal is usually put through a lowpass filter with cutoff frequency 3.4 kHz. The filtered signal is sampled at 8 kHz. In addition a single bit is added at the end of frame for the purpose of synchronization. ime =Example 2: Twenty four voice channels of 4 kHz bandwidth each sampled at Nyquist rate and encoded into 8 bit PCM are time division multiplexed with 1 bit/frame as synchronization bit. What is bit rate at the output of multiplexers? Solution : Given data is N = 24 channels Bandwidth W = 4 kHz v = 8 bits Nyquist rate = 2W =2x«4kHz =8kHz Twenty four voice channels are transmitted in one frame.Each of the channel has 8 bits. One bit is added in every frame for synchronization. Hence, Bits in one frame = 24 x8+1( Synchronization bit ) = 193 bits / frame. Frames are transmitted at the nyquist rate. Hence bit rate at the output of multiplexer will be, Bit rate = number of bits/frame x number of frames/sec. 193 x 8000 1.544 x 10° bits/sec. Thus the bit rate is 1,544 mbps. 
