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ISDN PRI vs T1 Lines for PBX Telephone
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 Services?
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 location.

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 Services
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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