section. Up to several years ago, repeater sections were specified with a BER of 1
Today a BER of 1
is prevalent in the North American network.
It is interesting to note that PCM provides intelligible voice performance for an error
rate as low as 1 in 100 (1
). However, the bottom threshold (worst tolerable) BER is
one error in one thousand (1
) at system end-points. This value is required to ensure
correct operation of supervisory signaling. The reader should appreciate that such degraded
BER values are completely unsuitable for data transmission over the digital network.
Crosstalk is a major impairment in PCM wire-pair system, particularly when "go" and
"return" channels are carried in the same cable sheath. The major offender of single-
cable operation is near-end crosstalk (NEXT). When the two directions of transmission
are carried in separate cables or use shielded pairs in a common cable, far-end crosstalk
(FEXT) becomes dominant.
One characteristic has been found to be a major contributor to poor crosstalk coupling
loss. This is the capacitance imbalance between wire pairs. Stringent quality control during
cable manufacture is one measure to ensure that minimum balance values are met.
DIGITAL LOOP CARRIER
Digital subscriber loop carrier is a method of extending the metallic subscriber plant
by using one or more DS1 configurations. As an example, the SLC-96 uses four DS1
configurations to derive an equivalent of 96 voice channels.
The digital transmission facility used by a DLC system may be repeatered wire-pair
cable, optical fibers, either or both combined with digital multiplexers, or other appropriate
media. In Bellcore (now Telcordia) terminology, the central office termination (COT) is
the digital terminal colocated with the local serving switch. The RT is the remote terminal.
The RT must provide all of the features to a subscriber loop that the local serving switch
normally does, such as supervision, ringing, address signaling, both dial pulse and touch
tone, and so on (Ref. 7).
New Versions of DSL
ADSL, or asymmetric digital subscriber line, as described by Bellcore (Telcordia), pro-
vides 1.544 Mbps service "downstream," meaning from the local serving switch to the
subscriber, out to 18,000 ft (5500 m). In the upstream direction 16-kbps service is fur-
nished. Such service has taken on new life in providing a higher bit rate for Inter-
There is an ANSI version of ADSL that can provide 6-Mbps downstream service using
a complex digital waveform and devices called automatic equalizers to improve bandwidth
characteristics, particularly amplitude and phase distortion. The upstream bit rate can be
as high as 640 kbps. Some manufacturers purport to be able to extend this service out to
12,000 kft (3700 m).
Advantages and Issues of Digital Switching
There are both economic and technical advantages to digital switching; in this context
we refer to PCM switching. The economic advantages of time-division PCM switching
include the following: