Data T1 Lines

First let me state there are no physical differences between a T1 for Voice and a T1 for data. They use the same physical medium, they have the same bandwidth, the same signal coding and the same frame format. The only difference is that it is now data instead of voice. What is the data rate of a t1 line you may ask – it is 1.544Mbits/s regardless of the traffic that is sent as that is the actual clock rate.

We’ll confine our discussion to data sent via the internet.

First lets recap on the T1 frame. One frame every 8KHz, 24 DSO’s in a frame and 8 bits in a DS0. This can be seen in the diagram below. If we are talking about a T1 to a PBX then each DS0 carries one voice call, with that call occupying the same DS0 every frame until the call is terminated.

T1 frame structure






With data transfers the DS0’s in a frame can be grouped together. If for example we grouped 6 DS0’s together then we would have a bandwidth of 384Kbits per second. This kind of generates it’s own frame inside the T1 frame T1 subframe







Now we have created a second sub frame how do we know when one of these sub frames starts or stops? This is where we need to discuss IP Packets.

Next time round we’ll look at these IP packets and how they are formatted.

WhichVoIP Team


About the author  ⁄ Calum

Calum has been creating articles and blogs for WhichVoIP for many years. He has vast knowledge of the telecommunicators sector.

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