UNIT-4 LECTURE-25Time-Division Multiplexing (1/2)A time-division multiplex (TDM) system enables the joint utilization of a common communication channel by a plurality of independent message sources without mutual interference among them

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Time-Division Multiplexing (2/2)A commutator is usually implemented using electronic switching circuitryThe purpose of a pulse modulator is to transform the multiplexed signal into a form suitable for transmission over the common channelAccurate equalization of both magnitude and phase responses of the channel is necessary to ensure a satisfactory operation of the systemTDM is immune to cross-talk

LECTURE-26Synchronization (1/1)The receiver requires a local clock to keep the same time as a distant standard clock at the transmitterOne possible procedure to synchronize the transmitter and receiver clocks is to set aside a code element or pulse at the end of a frameAn example receiver includes a circuit that would search for the pattern of 1s and 0s alternating at half the frame rate, and thereby establish synchronization between the transmitter and receiver

The T1 System (1/3)The T1 system carries 24 voice channels over separate pairs of wires with regenerative repeaters spaced at approximately 2-km intervalsThe T1 carrier system is basic to the North American Digital Switching HierarchyA voice signal is essentially limited to a band from 300 to 3100 HzIt is customary to pass the voice signal through a low-pass filter with a cutoff frequency of about 3.1 kHz prior to sampling

The T1 System (2/3)The filtered voice signal is usually sampled at 8 kHz, which is the standard sampling rate in telephone systems

The T1 System (3/3)Each T1 frame consists of a total of 193 bits. The duration of each bit equals 0.647 s, and the resulting transmission rate is 1.544 Mbps

LECTURE-27Digital Multiplexers (1/7)A digital multiplexer may enable us to combine several digital signals, such as computer outputs, digitized voice signals, digitized facsimile, and television signals, into a single data stream (at a considerably higher bit rate than any of the inputs)

Digital Multiplexers (2/7)An incoming bit stream at 64 kbps, irrespective of its origin, is called a digital signal zero (DS0)In the United States, Canada, and Japan the hierarchy follows the North American digital TDM hierarchyThe first-level hierarchy combines twenty-four DS0 bit streams to obtain a digital signal one (DS1) at 1.544 Mbps. Notice 24*64 kbps=1.536 Mbps. This bit streams are called the primary rate in the digital hierarchy

Digital Multiplexers (3/7)The second-level multiplexer combines four DS1 bit streams to obtain a digital signal two (DS2) at 6.312 Mbps. Notice 4*1.544 Mbps=6.176 Mbps.The third-level multiplexer combines seven DS2 bit streams to obtain a digital signal three (DS3) at 44.736 Mbps. Notice 7*6.312 Mbps=44.184 Mbps.The fourth-level multiplexer combines six DS3 bit streams to obtain a digital signal four (DS4) at 274.176 Mbps. Notice 6*44.736 Mbps=268.416 Mbps.

Digital Multiplexers (4/7)The fifth-level multiplexer combines two DS4 bit streams to obtain digital signal five (DS5) at 560.160 Mbps. Notice 2*274.176 Mbps=548.352 Mbps.The bit rate of a digital signal produced by any one these multiplexers is slightly higher than the prescribed multiple of the incoming bit rates because of bit stuffing built into the design of each multiplexer

Digital Multiplexers (5/7)Problems involved in the design of a digital multiplexerBit rate of each signal is not locked to a common clockThe multiplexed signal must include some form of framing so that its individual components can be identified at the receiverThe multiplexer has to handle small variations in the bit rates of the incoming digital signals

Digital Multiplexers (6/7)Example 3.3 AT&T M12 MultiplexerWhich is designed to combine four DS1 bit streams into one DS2 bit streamFigure 3.21Signal format of AT&T M12 multiplexer.

Digital Multiplexers (7/7)Each frame is subdivided into four subframes. The first subframe (first line in Fig. 3.21) is transmitted, then the second, the third, and the fourth, in that order

LECTURE-28 PAM/TDM SYSTEM

It is the combination of the concept of pulse amplitude modulation and the time division multiplexing.

Block Diagram of PAM/TDM

In this PAM experiment several message have been sampled, and their samplesinterlaced to form a composite, or time division multiplexed (TDM), signal(PAM/TDM). You will extract the samples belonging to individual channels, andthen reconstruct their messages

Signaling Rate in PAM/TDMSignaling rate of a TDM system is defined as the no. of pulses transmitted per second . it is represented by( r ) .r =Nfs (pulses/second).

BAND WIDTH OF PAM/TAM

B.W.=1/2(signaling rate)

Min. Transmission B. W.= NFs

AdvantageIt require some LPF so designing is simple & less distortion. Disadvantages

It requires perfect synchronization in terms of phase & frequency. Extra Pluses is also transmitted after each frame.

CROSS TORQUE Tg= .55/B.W.

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