563489578934

Sec. 8–3 Digital Subscriber Lines (DSL) 577
Two-wire circuit
(transmit and receive)
Two-wire transmit
Z 1 Z 3
Four-wire circuit
Z 2 Z 4
Two-wire receive
(b) Balanced Wheatstone Bridge Model for a Hybrid Circuit
Figure 8–4(b) Remote terminal (RT).
Fig. 3–40.) Thus, an RT with a 560-Mbs fiber-optic link to the CO could serve 8064 subscribers.
Furthermore, in Sec. 8–7, we see that no optical repeaters will be required if the RT
is located within 35 miles (typically) of the CO.
Telephone companies have replaced their analog switches at the CO with digital
switches. Historically, at the CO an analog circuit switch was used to connect the wire pairs of
the calling and the called party (as shown in Fig. 8–2). These switches were controlled by hardwired
relay logic. Now, telephone offices use electronic switching systems (ESS). With ESS, a
digital computer controls the switching operation as directed by software called stored
program control. Moreover, modern ESS switches use digital switching instead of analog
switching. In a digitally switched CO, the customer’s VF signal is converted to PCM and is
time-division multiplexed with other PCM signals onto a high-speed digital line. (When RTs
are used, the conversion of the VF signal to PCM is carried out at the RT.) The digital CO
switches a call by placing the PCM data of the calling party into the TDM time slot that has
been assigned for the destination (called) party. This is called time-slot interchange (TSI). The
digital switch is less expensive on a per-customer basis than the analog switch (for a large number
of customers), and it allows switching of data and digitized video as well as PCM audio.
For toll calls, the local CO uses trunk lines that are connected to a distant CO. Multiple
calls are accommodated using either TDM or packets. A local digital CO acts much like a
RT attached to a distant CO. Interestingly, long-distance trunks account for less than 5% of
the total cost of the telephone network; most of the cost comes from the switching equipment.
8–3 DIGITAL SUBSCRIBER LINES (DSL)
How can high-speed data and video service be economically provided via wire technology?
High-speed data are easily transported from the CO to the RT via fiber-optic cables. The problem
is transporting the data to the customer over the “last mile.” It is usually not economical
to install a dedicated fiber or coaxial line from the RT directly to each customer. However, in