Page 313 - Satellite Communications, Fourth Edition

Digital Signals 293

Figure 10.7 Bell T1 PCM format.

for a given channel must be separated in time by no more than 125 s.
As can be seen from Fig. 10.7a, this is also the frame period, and there-
fore, the bit rate for the T1 system is

193
R 1.544 Mb/s (10.7)
b
125 10 6
Signaling information is also carried as part of the digital stream.
Signaling refers to such data as number dialed, busy signals, and billing
information. Signaling can take place at a lower bit rate, and in the T1
system, the eighth bit for every channel, in every sixth frame, is replaced
by a signaling bit. This is illustrated in Fig. 10.7b. The time separation
between signaling bits is 6 125 s 750 s, and the signaling bit rate
is therefore 1/(750 s) 1.333 kb/s.

10.5 Bandwidth Requirements
In a satellite transmission system, the baseband signal is modulated
onto a carrier for transmission. Filtering of the signals takes place at a
number of stages. The baseband signal itself is band-limited by filter-
ing to prevent the generation of excessive sidebands in the modulation
process. The modulated signal undergoes bandpass filtering (BPF) as
part of the amplification process in the transmitter.