DS-1 (T1)

DS-1 (T1) refers to a dedicated leased line made up of 24 DS-0 channels with a bandwidth of 1.544 Mbps. Although DS-1 and T1 are generally referred to interchangeably, DS-1 is actually referring to the specific formatting of the data and T1 is referring to the actual wires that carry this information. But how exactly do 24 DS-0 channels pipe into a DS-1?

First of all, each DS-0 is 64kbps in size. This number is derived when you multiply the size of an 8 bit PCM coded word by the 8 kHz (8000 times per second) sampling rate.

DS-0 = 64kbps = 8 bits x 8 kHz

If you were to multiply the 24 channels of a DS-1 by 64kbps (the size of each DS-0), you only come up with 1.536 Mbps total. This is something I noticed before, and was what prompted me to do further reading to understand why a DS-1 (T1) is said to have 1.544 Mbps and not 1.536 Mbps of total bandwidth. Interestingly, I found that 1.536 is the actual data transfer rate of a DS-1 on a T1, but it is not the total data rate because there is a framing bit added to each frame.

A frame, which is made up of the 24 DS-0 channels, has 192 bits. This number is derived when you multiply the 24 channels by the 8 bits contained in each.

DS-1 frame = 192 bits = 24 DS-0 channels x 8 bits each

In order to synchronize the data at the receiving end, they add a framing bit to each frame, and this brings the total to 193 bits per frame. Because the sampling rate is 8 kHz, when the 193 bits is multiplied by 8 kHz, you arrive at the total aggregate line rate of 1.544 Mbps. So, in other words, there are 8000 frames, each made up of 193 bits, which includes 24 8 bit channels and a framing bit, that make up the 1.544 million bits per second of a T1 line.

Total aggregate bandwidth = 1.544 Mbps = 193 bits per frame x 8 kHz

But what about Clear Channel Capacity? Basically, Clear Channel Capacity refers to the amount of a DS-1 frame that can actually hold data. Robbed bit signaling is used in order to signal whether a line is "on" or "off" hook. This uses the eighth bit of every sixth frame in order to signal this status, with a 1 representing on hook and a 0 representing off. So, when a DS-1 is used to carry data and not voice, this has an impact on the amount of data that can be held in a frame. Because each frame needs to contain the same amount of data, there are only 7 bits, and not 8, available in each PCM encoded word of a DS-0. So, although each DS-0 has a 64 kbps data rate (8 bits X 8 kHz), only 56 kbps is available for data (7 bits X 8 kHz). This means that the clear channel aggregate data rate is only 1.344 Mbps, which is derived when you multiply the 24 DS-0 channels by 56 kbps each.

So, clear channel capacity explains why digital services only offer data rates in multiples of 56 kbps, since the other 8 kbps is lost to signaling.

Clear Channel Capacity = 1.344 Mbps = 56kbps x 24 DS-0 channels

T1 circuits connect to T1 CSU or T1 DSU/CSU equipment. For more information on DSU/CSU equipment, visit Verilink's products page. ComTest Technologies can provide you with sales and support of WAN connectivity products from Verilink.


This page was written by Will Twiggs, an associate with ComTest Technologies, Inc. For more information or if you have questions about this material, you can contact the author at william@comtest.com

Data and Telecommunications Systems and Applications by Charles N. Thurwachter, Jr.

Telecommunications, 3rd Edition, by Warren Hioki

This page created by Will Twiggs
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Last updated on 12/18/2000