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Primary Rate Interface (PRI) and T1 Feature
Description
T1
A DS1 facility programmed as a T1 line/trunk uses 24 channels, sometimes referred
to as DS0 channels each with a capacity of 64 kbps. Signaling must be in-band
signaling, however, which limits the data rate for each channel to 56 kbps when
the channels are programmed for Switched 56.
T1 channels can be programmed either to emulate voice tie trunks or data tie
trunks. These trunks can be used as tandem trunks linking networked systems.
In addition, you can use drop-and-insert equipment to supply fractional T1 use.
See the Network Reference for more information.
T1 channels can be programmed to emulate the following types of connections:
- Loop-start
- Ground-start
- T1-emulated data (56-kbps data)
- Ear & Mouth (E&M) tie trunk
- Direct Inward Dial
T1 service provides the following benefits:
- Speed. Data calls to outside or network destinations can be made
by programming a channel for T1 Switched 56 Data. This service must be supported
on the far end. By allowing high-speed digital data transmission, T1 provides
the capability for videoconferencing and Group IV (G4) fax.
- Improved Toll Restriction. The ways in which Toll Restriction can
be bypassed are limited on T1 lines/trunks. Specifically, three types of toll
abuse are eliminated with T1 service:
- Because dialing is in the form of out-of-band messages that must be generated
by the system, a user cannot use a Touch-Tone generating device, such as a
pocket dialer, to send dialed digits directly through the system to the line/trunk.
- Without T1 service, Toll Restriction can be deceived by dialing digits
on a loop-start line before the far-end switch applies dial tone. These initial
digits may indicate a local call to the system's toll-restriction checking
while the subsequent digits, those actually recognized by the far-end switch,
may produce a toll call. This is not possible with T1 service because every
digit screened and passed on by the system's Toll Restriction is guaranteed
to be received by the far-end switch.
- A T1 line's far-end disconnect signal provides a reliable indication when
a call ends, and a new call cannot be initiated until the line has been released
from the prior call on both ends. This prevents a user on a loop-start line,
waiting off hook for the restoration of dial tone after a previous call, from
placing a second call before Toll Restriction is reapplied.
- Reliable Indication of Far-End Disconnect. This prevents an incoming
call from being blocked because a line is not released when a call is ended.
T1 supports routing by dial plan on Switched 56 data channels that are connected
to the public switched telephone network. Routing by dial plan supports call
handling similar to Direct Inward Dial (DID). It performs digit analysis on
incoming calls, matches to Called Party Numbers (CdPNs), and delivers the calls
to the destinations based on the respective Called Party Numbers. It also allows
multiple calls to the same directory number--that is, multiple concurrent incoming
calls with the same Called Party Number can be delivered to a destination simultaneously.
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When T1-emulated tie facilities are used as tandem tie trunks, digit
manipulation can be performed through UDP routing. See Uniform
Dial Plan Features for details.
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