Tuesday, June 07, 2005

What's The Bandwidth of a T1 Line?

T1 lines are high speed digital telecommunications circuits. They are typically used to carry digitized phone calls, including the traditional TDM (time division multiplexing) format used by legacy PBX systems and telecom carriers as well as the newer VoIP broadband phone. Other applications of T1 lines are dedicated broadband Internet service, ISP backbones to the Internet, audio and video transport, and point to point private lines between business locations. These applications all require what is called broadband or high bandwidth digital transmission speeds.

The basic bandwidth of a T1 line is 1.544 Mbps in both directions. It is a bidirectional or symmetrical service. It is also full duplex, allowing transmissions to take place simultaneously on the upstream and downstream paths without interference. This is especially important when T1 lines are used to carry telephone calls. Both parties need to be able to both talk and listen at the same time. Otherwise, phones would work more like walkie-talkies or CB radios where only one party can talk at a time.

The usable bandwidth of a T1 line is a little bit less than the line speed of 1.544 Mbps. That's because 8 Kbps of that speed is used for framing, which is a technique for keeping the line synchronized by constantly transmitting a unique framing pattern using a special framing bit. The equipment at the receiving end of each path in the T1 circuit looks for the framing pattern and uses it to know exactly where each frame starts. Deduct that 8 Kbps for framing and you have 1.536 Mbps of usable bandwidth, also called the payload. The payload is the bits that you get to put on the line.

The synchronous nature of a T1 line means that you can divide it into 24 unique channels. Each frame will have 1 framing bit and 24 channels of 8 payload bits each. Why this division? It's the original specification for digitized phone calls. Each channel carries one call of 8 bits sampled 8,000 times per second. That's 64,000 bits per second or 64 Kbps per channel. Take 24 channels at 64 Kbps per channel and you have 1,536 Kbps or 1.536 Mbps as the payload capacity of your T1 line.

Of course, you don't need to break the T1 line into channels if you don't need that feature to support multiple telephone lines for a PBX system or a bank of dial-up modems. You can have the T1 line set up as unchannelized, which gives you frames of 192 bits each. You get the same 8,000 frames per second. The 192 payload bits per frame times 8,000 frames per second is also 1.536 Mbps.

Need more bandwidth than this? Try bonding 2 T1 lines together for double the payload or 3.072 Mbps. You can continue to bond T1 lines up to 6 lines, which gives you 9.216 Mbps. Beyond that, you can move up to T3 or DS3 service which runs at 44.736 Mbps and gives you the equivalent of 28 T1 lines or 672 voice channels of 64 Kbps each. That's a usable bandwidth of 43.008 Mbps.

If your business has need for high bandwidth digital service, let T1 Rex help you find the lowest T1 line prices and T3 line prices with service level agreements.

Click to check pricing and features or get support from a Telarus product specialist.

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