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DIGITAL CARRIER SYSTEMS AND NETWORKS 305
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T1 or E-1
Figure 7.3 Digital carrier as a replacement for multiple dedicated, leased - line networks.
Figure 7.3 , T - carrier can obviate the need for multiple voice, facsimile, data, video,
and image networks [6, 9] .
The signifi cance of T - carrier extends well beyond its practical advantages. Specifi
cally, and as the fi rst digital carrier system, it set the standards for digital transmission
and switching, including the use of Pulse Code Modulation (PCM) for
digitizing analog voice signals. T - carrier not only set the basis for the North American
digital hierarchy but also led to the development of similar standards, such as
E - carrier in Europe and J - carrier in Japan. Ultimately, the CCITT (now ITU - T)
developed international standards recommendations to ensure interconnectivity of
national networks. Although T - carrier, E - carrier, and J - carrier are very different in
terms of certain specifi cs of the protocols employed (e.g., transmission rates, encoding
techniques, and signaling and control methods), their basic characteristics are
much the same.
7.4.2 Channelized T1
The fundamental building block of T - carrier is a 64 - kbps channel, referred to as
DS - 0 ( Digital Signal level Zero ). Digital carrier is a channelized service, at least in
a standard voice implementation. In other words, a single high - capacity digital
circuit supports multiple logical channels, with each channel supporting a separate
conversation. A T1 circuit, for example, operates at 1.544 Mbps, supporting the
standard 24 time division multiplexed information - bearing channels, each with a bit
rate of 64 kbps (Figure 7.4 ). E - 1 supports 30 TDM channels of 64 kbps plus 2 separate
signaling and control channels; J - 1 supports 24 channels, as does T1.
The American National Standards Institute (ANSI) set the T - carrier hierarchy
standards (see Table 7.1 ) in its T1.107 specifi cations. Beginning at the T1 level, the
hierarchy progresses up to T4, which provides bandwidth of approximately 274 Mbps
in support of 4032 channels. Most end users subscribe to T1 services, because one
or more T1s generally satisfy their bandwidth requirements.
The process of transmitting data (voice, data, video, or image) in designated and
consistently repeated channels, or time slots , is known as byte interleaving . For
example, each voice conversation to be transmitted is accepted by the multiplexer,
assuming capacity is available, and is assigned a time slot. The eight - bit bytes associated
with that voice conversation are sent in the designated time slots 8000 times
per second, for a total of 64 kbps, which is a voice - grade channel. Those time slots
are reserved for that conversation, with the multiplexer providing the transmitting
device with regular and repeated access to them for the duration of the communication.
Time slots are reserved in both directions, because both real - time voice and
T - carrier are FDX in nature.
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