What is a T1?

If you have ever searched for Business VoIP or have been serious about using any business telecom service then you will have come across the term T1. Over the course of the next few weeks we’ll endeavour to explain what a T1 is and delve down into the details of it for you techies out there.

So what is a T1? Well a T1 really defines 4 things.

  1. The physical medium used.
  2. The bandwidth or clock rate.
  3. The type of signaling and encoding used.
  4. The format or frame the data is transferred in.

With a T1 there are two physical mediums. One is 100 Ohm shielded twisted pair and the other is 75 Ohm Coaxial cable. Typically inside patch panels the 100 Ohm twisted pair is used as it is cheap and flexible but for transmission to the outside world a balun is used to match to 75 Ohm Coaxial cable which is typically used to connect to the ISP or PBX.

The bandwidth of a T1 is 1.544Mbits/sec that’s for one direction, one of the advantages of a T1 is that you get this dedicated bandwidth in both the uplink and downlink directions.

T1 uses a signal encoding called B8ZS which stands for Binary 8-zero Substitution – quite a mouthful. Without going into too much detail just now, this type of encoding allows data and a clock to be sent across the same cable – vital for synchronous data transfer.

Finally the format – Each T1 is split into 24 channels referred to DS0’s. Each channel is 8 bits wide and the frame is sent on a 8KHz boundary. The data capacity is therefore 24x8x8000 = 1.536Mbits/sec. Didn’t I say earlier that a T1 was 1.544Mbits/sec? Well there is something called a framing bit, this is 1 bit every frame that a receiver on the other end can lock onto to ensure that whatever equipment is there can extract the correct DS0’s in the correct order. This framing bit makes for an extra 8000 bits/sec giving a grand total of 1.544Mbits/sec.

T1’s are essential a for any business that employs more than about ten people at the one site. They are very reliable and give you a guaranteed bandwidth.

If you are starting a business or you already own a business and are planning on expanding, fill out this form for a quote on how much a T1 is – you maybe pleasantly surprised.

Next week or when we have time we’ll explain how the 8KHz frame rate came about and how we get 8 bits in a DS0.

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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