How Much Bandwidth In A T1 Line? Here's the story behind the numbers of how fast a T1 line runs and how of that is available for your use
By: John Shepler
When we talk about a T1 line, we’re talking about a dedicated bandwidth connection that runs at 1.5 Mbps in both directions, upload and download. But is it exactly 1.5 Mbps or something in that range?
Line Bit Rate vs Payload
The 1.5 Mbps designation is a shorthand. The T1 line actually runs at a bit rate of 1.544 Mbps. But you don’t get to use all of that. The bandwidth that is available for your use, called the payload, is 1.536 Mbps.
So, why the odd numbers and where does the rest of the bandwidth go? To understand that, we need to deconstruct the T1 line and see what’s going on in there. A little history will also provided some valuable insights.
The History of T1 Lines
T1 is one of several technical specifications for a family of services known at T-carrier. It’s an invention of Bell Labs that was originally meant for the phone companies. Back in the 1950’s all phone lines were either analog copper or analog signals riding on radio carriers in a process called frequency division multiplexing. It worked something like the AM radio band today. If each telephone call is like a radio station, then the radio band can hold lots of different stations assigned to different channels. You pick the one you want to listen by selecting the proper frequency or channel.
As you know, analog based frequency division multiplexing has its limitations. Stations interfere and there is often atmospheric noise. If you remember making long distance calls 40 or 50 years ago, you’ll also remember that there was a lot of hiss and you could sometimes hear other calls interfering through a process called cross-talk. Those effects went away when digital telephony was installed.
What Makes Up A T1 Line?
T1 lines use standard twisted pair copper wiring but the signals are digital, not analog. It starts with a stream of bits arranged as 24 channels of 8 bits each, being sampled at 8 KHz, for a total of 64 Kbps per channel. Multiply 24 channels by 64 Kbps and you get 1,536 Kbps or 1.536 Mbps. Aha! That’s the payload value of a T1 line.
What’s important about having so many 64 Kbps channels? It turns out that each 64 Kbps channel is exactly the right size to carry one telephone call. The channels are each designated DS0 and the collection of 24 is called a DS1.
That all makes sense, considering the telephone company heritage of the T1 line. But what causes the difference between a 1.544 Mbps line rate and a 1.536 Kbps payload?
Overhead Is The Difference
Subtract those two numbers and you get 8 Kbps. Those 8,000 bits per second are the overhead needed to run the line. T1 lines are precisely synchronized at both ends so the terminal equipment knows where the channels are in the bitstream. It’s actually one framing bit in a total frame of 193 bits being sampled 8,000 times per second. That one bit per frame keeps everything in lock-step and is also used to support error detection and notification.
So, when you hear that you’re getting 1.5 Mbps bandwidth on a T1 line you now know that your actual usable bandwidth is 1.536 Mbps. More importantly, it’s a full duplex service offering both upload and download bandwidth of 1.536 Mbps. That bandwidth is maintained by the line at all times for your exclusive use. Whatever you aren’t using at the moment idles while waiting for the next payload bits.
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