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Question: How fast is a residential phone line?

Question: How fast is a residential phone line? Bill Williams <[email protected]> sent us the following entertaining query: "Here’s a group of questions which have come up several times, drawing varying answers. Are there definitive answers? A) What is the standard for data transmission capability on a normal voice telephone line? B) What agency (federal or local) enforces this? C) How can a consumer find out if his or her residential phone line is up to the standard?"

Answer: First off, a quick bit of terminology. We generally talk about modem speeds, but that’s deceptive. Speed is a measure of distance divided by time – kilometers per hour, for instance. However, distance has nothing to do with modems. The correct term is throughput, since the question is how many bits a modem can transmit in a period of time, generally measured in kilobits per second or Kbps. In future issues of NetBITS, we’ll continue to cover issues surrounding the modems and other devices that so many of us rely on for our Internet connections.

Residential phone lines are specified to have a nominal throughput of 64 Kbps. "Nominal" in this case means that your phone, called Plain Old Telephone Service or POTS, is allotted 64 Kbps in the phone company’s internal switching network; it doesn’t mean that you can use all that bandwidth. (It’s also true that copper wire can carry much more data than that, but we’ll address that subject in future issues.)

There are a variety of standards – some enforced by federal powers, others accepted by industry organizations, others peculiar to individual pieces of phone equipment – that allow calls to travel seamlessly from one part of the world to another through many different switches, fiber optic lines, wire, and other systems. The details vary from country to country as to what a dial tone sounds like, along with some of the issues of voltage, rings, and so forth. But underlying principles allow voice calls to work to and from anywhere.

There’s no guaranteed way to test the throughput of your telephone line, although most connection programs (and a few modem displays) report the "speed" at which a connection has been made. Our late colleague Cary Lu was working on a book called The Race for Bandwidth for Microsoft Press at the time of his passing, and he had provided some insight into this issue. Only a small percentage – probably less than 10 percent – of all phone lines in the country are capable of going beyond 28.8 Kbps. In other words, that 33.6 Kbps or 56 Kbps modem you bought probably won’t increase your effective throughput much, if at all. In addition, some 56 Kbps modems are limited both by federal regulations – the Federal Communications Commission has set the maximum throughput at 53 Kbps for now – and by practical limitations in the quality of copper, switching stations, and differences in systems. (See Jeff Carlson’s article in NetBITS-008 and his followup in this issue for more on this subject.)

If you believe your phone line is delivering less bandwidth than it ought to be, most phone companies will come out and test the quality of your line (possibly for a fee). However, even if they find problems, improving the bandwidth capabilities of a phone line can be an expensive and frustrating process. It’s worth asking the phone company to check into the problems, especially if there’s no fee, but going beyond that may not be worth the expense.

Adam and Tonya Engst live in a relatively remote area and have run into these problems. Using 28.8 modems, they see throughputs of 26.4 Kbps at best, and usually less. The problems in their case are related to old, possibly damaged copper wires to their house, plus the effect of a pair gain device, which enables the telephone company to squeeze more calls onto a smaller number of wires. It’s an efficient use of resources for the telephone company, and the reduction in quality generally isn’t a problem for voice calls. However, data calls are more problematic, particularly as you try to increase throughput with the newer modems. [GF]


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