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Channelized vs Unchannelized T1 Lines
How you format your T1 line depends on the needs of your voice or data application.

By: John Shepler

A T1 line is a T1 line, right? Well, yes and no. T1 line service can be formatted in different ways to suit its intended application. What you would set up to serve your analog PBX system is likely different that what you want for dedicated Internet service.

First of all, there are certain characteristics of T1 lines that you'll always find. They are defined by standards that were originally created by AT&T and modernized by ANSI (American National Standards Institute). International standards are provided by the ITU (International Telecommunications Union).

A T1 line is a synchronous digital transmission running at 1.544 Mbps with certain signal characteristics. It is divided into frames of 192 data bits plus 1 framing bit for a total of 193 bits. There are 8,000 of these frames per second. That gives you 1.536 Megabits of data per second.

Now's where the various flavors of T1 come into play. Many companies use that 1.536 Mbps of bandwidth for what's called dedicated Internet service. The line is dedicated to Internet service and is not shared with other customers. It often is shared within the company by plugging it into a router that has a T1 CSU (Channel Service Unit) and using the router output to feed the corporate network.

Using a T1 line as a data pipe is also called "Unchannelized T1" because the 192 bits or 24 bytes per frame are not divided up further by the T1 line equipment.

Unchannelized T1 is a fairly recent development. When T1 or Trunk Level 1 was being put into service in the 1950s, it was designed to carry telephone traffic. Each call is separate and distinct, so the T1 frame was divided into 24 channels of 8 bits each. Running at 8 Kbps, that gives each channel a bandwidth of 64 Kbps or just right for one toll quality telephone call. Using a T1 line in this fashion is called "Channelized T1."

Channelized T1 is still popular today to provide multiple telephone lines to a PBX system. It replaces up to 24 separate pairs of telephone wires. A single T1 line combining all those phone lines into a single 4 wire line is often considerably less expensive that running them all separately. A PBX system with a T1 interface card will assign the channels as needed to support up to 24 simultaneous telephone calls.

For a PBX system or Key Telephone System that expects separate phone lines, a device called a "Channel Bank" will do the T1 multiplexing and demultiplexing and conversion between analog and digital formats. A typical channel bank has one connector for the T1 line input and 24 pairs of standard telephone lines as the output. The point is that whatever you connect to those phone lines doesn't know whether they came from the phone company as separate wires or were carried on the T1 line.

An excellent reference manual on the intricacies of T1 is "T1, A Survival Guide" by Matthew S. Gast and published by O'Reilly.

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