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Primary Rate Interface (PRI) and T1 Feature
Description
PRI
PRI is a common configuration for a DS1 facility. A DS1 facility consists
of 24 channels, sometimes referred to as DS0 channels, each with a capacity
of 64 kbps. DS1 refers to the twenty-four 64-kbps channels, plus framing and
signaling bits, multiplexed together to form a 1.544-Mbps Digital Signal Level
1 signal. When used for PRI, a channel can be designated as either a B-channel
(bearer channel) or a D-channel (data or delta channel).
A B-channel is used to carry user information, such as the voice or data content
of a call, between the system and the far-end switch. Each B-channel provides
access to one or more network services. The system supports Call-by-Call Service
selection, which allows multiple network services over the same B-channels.
The D-channel conveys signaling required to set up, control, and clear calls
made over all of the B-channels.
The most common configuration of a DS1 facility for PRI consists of 23 B-channels
and 1 D-channel, although other combinations are possible. Each PRI must include
a D-channel, but may include fewer than 23 B-channels. The remaining channels
cannot be used for any other purpose.
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The MERLIN MAGIX Integrated System does not support multiple PRI facilities
sharing one D-channel (as allowed with Non-Facility Associated Signaling).
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Up to three DS1 carrier facilities (maximum of two in one carrier),
and therefore three PRIs, can be connected to the system through separate 100D,
100R INA, or 100DCD modules, each of which occupies a slot in the system carrier.
In terms of system capacity, each DS1 channel counts as a line/trunk, so the
maximum number of B-channels supported by the system is 69. Signaling for 69
B-channels is provided over three separate D-channels, using up 72 of the system's
80-line capacity.
PRI Lines and B-Channel Groups
PRI tandem trunks are available to network MERLIN MAGIX Integrated Systems
with one another or with MERLIN LEGEND (Release 6.0 or later), DEFINITY ECS,
DEFINITY BCS, or DEFINITY ProLogix Solutions systems, providing smoother operation,
additional features, and easier management than the networking available in
earlier releases. Tandem PRI trunks are private trunks that may use central
office switches in the PSTN for amplification over long distances but do not
use central office facilities for switching. For details, see Tandem
PRI Trunks.
PRI service offers the following benefits:
- Network Subscriber Service Options. The system supports network
services from AT&T, MCI, and the DMS-100 5ESS network of local exchange
carriers. These services are described in Type of Service. Tandem networking
is supported by using PRI, T1-emulated voice, and T1-emulated data trunks
as tandem trunks in private networks of systems.
- Speed. Data calls to outside destinations can be established on
the same B-channels used for voice calls if the service allows. Dedicated,
conditioned lines/trunks are not needed. By supporting high-speed digital
data transmission, PRI provides the capability for videoconferencing and Group
IV (G4) fax.
- Dynamic B-Channel Assignment. An individual B-channel can be removed
from service without blocking ISDN calls to or from any extension.
- Improved Toll Restriction. The ways that Toll Restriction can be
bypassed are limited on PRI lines/trunks. Specifically, three types of toll
abuse are eliminated with PRI 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 PRI 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 PRI 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 PRI 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.
The system's implementation of PRI provides the following features:
- Support for Caller Identification. The system supports AT&T's
Automatic Number Identification (ANI) service and similar services from MCI
and local exchange carriers. Customers who subscribe to one of these services
can identify the incoming caller on a PRI line/trunk by telephone number or
billing number. The ANI can be programmed as extension-based or facility-based.
Extension-based ANI results in a more PBX-like performance from the system.
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The availability of the caller identification information may be limited
by local-serving (caller's) jurisdiction, availability, or service provider.
Blocking Caller ID is possible system-wide by accessing the PRI Lines
menu. For information, see System Programming , Chapter 3, "Copy
Telephone Number to Send."
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- Routing by Dial Plan. Routing by dial plan supports call handling
similar to DID. For example, you can specify that calls received from a particular
area code should be routed to a specific person or group responsible for accounts
in the area.
Routing by dial plan 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.
- Call-by-Call Service Selection. This feature maximizes use of communications
lines, providing more services with fewer lines. Call-by-Call Service selection
provides more than one PRI service (such as VPN service and OUTWATS) for each
B-channel. Based on the number dialed and the bearer capability (voice, data,
or both), the system chooses which service is used. If a caller requests operator
service, the system bypasses Call-by-Call Service selection.
- Restriction Code Handling for FTS2000 Network. FTS2000 network users
can have restriction codes applied to their extensions. A person who attempts
to place a call that exceeds the set restriction level must first enter a
restriction code. If no code is entered, the FTS2000 network prompts the user
to enter the code from the telephone dialpad. The system allows a restriction
code to be entered with the Account Code Entry feature. This is especially
useful for data calls.
- Networked Tandem PRI Trunks (Hybrid/PBX mode only). Networked PRI
trunks allow cost-effective use of trunks by remote networked users, transparent
dialing of extensions connected to networked systems, shared automated attendant
systems under some conditions, and flexible display features for MLX display
telephones. PRI dial plan routing is not applied to calls received on tandem
PRI trunks. For additional information, see this section and Uniform
Dial Plan Features.
