563489578934

Sec. 8–2 Telephone Systems 575
Subscriber
lines
Remote
terminal
T1 or fiberoptic
lines
Subscriber
lines
Central
office
T1 or fiberoptic
lines
Remote
terminal
Subscriber
lines
T1 or fiberoptic
lines
Remote
terminal
Subscriber
lines
Figure 8–3
Telephone system with remote terminals.
on the receive line (shown at the bottom right of Fig. 8–4b) is balanced out and does not
appear on the transmit line (upper right). Consequently, self-oscillation (ringing feedback)
is prevented, even though there may be some coupling of the amplified transmitted signal to
the receive line at the distant end of the four-wire line or coupling from the transmit line to
the receive line along the four-wire path. As shown in Fig. 8–4a, the transmit VF signal is
converted to a DS-0 PCM signal, which is time-division-multiplexed with the PCM signals
from other subscribers attached to the RT. The TDM signal is sent over a DS-1 trunk to the
CO. Similarly, the received DS-1 signal from the CO is demultiplexed and decoded to
obtain the received VF audio for the subscriber. In a popular RT system first manufactured
by AT&T called SLC-96, 96 VF subscriber lines are digitized and multiplexed onto four T1
lines, and one additional T1 line is on standby in case one of the other T1 lines fails
[Chapman, 1984]. From Chapter 3, it is recalled that a T1 line requires two twisted pairs
(one for the transmit data and one for the receive data) and that each T1 line (1.544 Mbs)
carries the equivalent of 24 VF signals.
Let us now compare the twisted-pair requirements for systems with and without RTs.
If no RT is used, 96 pairs will be required to the CO for 96 subscribers, but if a SLC-96 RT
is used, only 10 pairs (5 T1 lines) are needed to the CO. This provides a pair savings (also
called pair gain) of 9.6 to 1. Furthermore, the RT may be located at any distance from the CO
(there is no 1300-Ω limit) because the pairs are used for DS-1 signaling with repeaters
spaced about every mile. Of course, fiber-optic lines can also be used to connect the RT to
the CO. For example, if two 560-Mbs fiber-optic lines are used (one for data sent in each
direction), this has a capacity of 8064 VF channels for DS-5 signaling. (See Table 3–8 and