The digital capacity of a plain old telephone service (POTS) line is 64 Kbs. There are 8000 timing bits added, so the actual throughput is 56 Kbps. This is enough to carry a single voice call or to connect a 56 Kbs modem. The POTS line is an analog line; it transmits and receives analog or sound-based signals.

A T1 line is a pure digital line. Conversations are converted (at a PBX) from analog (sound) to digital before being transmitted on the T1 line. If your office has a T1 line, it means that the phone company has brought a fiber optic line into your office (a T1 line might also come in on copper). A T1 line can carry 24 digitized voice channels, can carry data at a rate of 1.544 megabits per second, or as an integrated service it can handle a combination of fewer voice calls with some bandwidth given to Internet connections. If the T1 line is being used for telephone conversations, it plugs into the office's phone system. If it is carrying data it plugs into the network's router.

Sometimes one of the 24 lines is dedicated to caller ID; this line is usually connected to a computer that looks up customer records that are routed to the computer of the person taking the call.

A large company needs something more than a T1 line. A T3 line is also a pure digital line. It is the equivalent of 28 T1 lines or 672 POTS lines. T3 lines can handle 672 simultaneous voice calls or provide a high speed Internet connection of 44.736 Mbps. As an integrated service, it can handle a combination of fewer voice calls and provide high speed Internet connections. The T3 is typically used by high end data and voice customers.

Monthly costs for T1 and T3 lines vary, primarily depending on the distance from your local phone company's Customer Office. As a very rough comparison we can say a T1 might cost between $1,000 and $1,500 per month depending on who provides it and where it goes. The other end of the T1 line needs to be connected to an ISP, and the total cost is a combination of the fee the phone company charges and the fee the ISP charges. A T3 might cost $4,000 per month. Again, these cost figures are for example purposes only.
 
 

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