0195136047.pdf

15.3 DIGITAL COMMUNICATION SYSTEMS 721

One frame, 193 bits, 125 μs

2 3 4 5 24

One framing bit

8 bits per message word

MSB LSB

1

182 3 4 5 6 7

....... 23
Time

Message time slots 22

Figure 15.3.10Frame structure for a D3 channel bank.

D3 channel

bank

64 kbit/s

each

Digital

subscriber

signals

1

2

..

.

24

..

.

7

Second

level

multiplexer

DS-2

First

level

multiplexer

DS-1

DS-1 lines

1.544 Mbit/s

each

Signals from

other DS-1

units

1

2

3

4

Third

level

multiplexer

DS-3

DS-2 lines

6.312 Mbit/s

each

Signals from

other DS-2

units

1

2

..

.

6

Fourth

level

multiplexer

DS-4

DS-3 lines

44.736 Mbit/s

each

Signals from

other DS-3

units

1

2

2

Fifth

level

multiplexer

DS-5

DS-4 lines

274.176 Mbit/s

each

DS-5 line

560.16 Mbit/s

output

Signals from

other DS-4units

1

Figure 15.3.11Digital TDM hierarchy for North American telephone communication system.

In digital speech transmission over telephone lines via PCM, a standard TDM hierarchy has been

established for accommodating multiple subscribers. Figure 15.3.11 illustrates the TDM hierarchy

for the North American telephone system. The output from the channel bank is a digital signal

(DS) on a line said to carry level 1 multiplexing. In the first level of the TDM hierarchy, 24

digital subscriber signals are time-division multiplexed into a single high-speed data stream of

1.544 Mbit/s (nominal bit rate). The resulting combined signal is usually called a DS-1 channel.

In the second level of TDM, four DS-1 channels are multiplexed into a DS-2 channel, having the

nominal bit rate of 6.312 Mbit/s. In a third level of hierarchy, seven DS-2 channels are combined

via TDM to produce a DS-3 channel, which has a nominal bit rate of 44.736 Mbit/s. Beyond

DS-3, there are two more levels, as shown in Figure 15.3.11. All multiplexers except the channel

bank are asynchronous.

In a mobile cellular radio system (see Section 15.2) for the transmission of speech signals,

since the available channel bandwidth per user is small and cannot support the high bit rates

needed by waveform encoding methods such as PCM, the analysis–synthesis method based on

linear predictive coding(LPC) is used to estimate the set of model parameters from short segments

of the speech signal. The speech-model parameters are then transmitted over the channel. With

LPC, a bit rate of 4800–9600 bit/s is achieved.

In mobile cellular communication systems, LPC speech compression is only needed for

the radio transmission between the mobile transcriber and the base station in any cell. At the

base station interface, the LPC-encoded speech is converted to analog form and resampled and

digitized (by using PCM) for transmission over the terrestrial telephone system. Thus, a speech