ISDN PRI vs T1 Lines for PBX
What is ISDN? How does ISDN
PRI differ from T1?
By: John Shepler
PBX phone systems and other telecommunications
equipment often supports both PRI or Primary Rate Interface and
T1 lines. What are differences between these dedicated line services
and how do you choose the right one for your application?
Dedicated Line Varieties
It's easy for one hardware interface to support T1, Channelized
T1 and ISDN PRI because they are all variations on the same basic
circuit. The differences are in how that circuit is configured
for particular applications.
What's Common in These Dedicated Line
The circuit that is provided at the demarcation point is based
on the electrical specifications for the T1 trunk line originally
developed for the Bell System. It runs at 1.544 Mbps with separate
pairs of copper wire for transmit and receive. That's 4 wires
total. T1 trunks may be carried on phone wires, coaxial cable,
microwave or dropped off by a SONET fiber optic ring.
PRI - Primary Rate Interface
PRI is also known as ISDN PRI or sometime T1 PRI. ISDN stands
for Integrated Services Digital Network. There are two speeds
of service offered: BRI or Basic Rate Interface and PRI or Primary
Rate Interface. BRI is a low capacity service intended for residential
and small business applications. PRI is the high capacity service
carried on T1 trunk lines between telco central offices and your
PRI divides a T1 digital signal into 24
channels of 64 Kbps capacity per channel. 23 of these channels
can be assigned as one telephone call each, the equivalent of
having 23 separate telephone lines. The 24th channel is used
for signaling information and special features such as caller
ID and hotel / motel information services. It is a popular service
for call centers which need the ANI automatic number identification
or caller ID data.
T1 service can be provided as channelized or unchannelized. In
the channelized T1 version, there are 24 channels. Each channel
can be a telephone call. T1 gives you 24 phone lines in place
of the 23 that can be accommodated with PRI. However, since there
is no separate signaling channel, the signaling information that
tells when a phone is on hook or off hook is carried within each
channel by using or "robbing" the least significant
bit. Unfortunately, channelized T1 doesn't provide any capability
for ANI or caller ID data. You need PRI service for that.
Unchannelized T1 treats all 24 T1 line
channels as one big combined channel for carrying Internet service,
point to point data download or VoIP broadband phone. This service
is intended to be handled by digital routers, not PBX telephone
systems. There are no dedicated phone channels or signaling assigned
for switched telephone service.
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