Data Communications Over Standard Telephone Lines

Networks (especially the Internet and large WANs, with nodes spread over a large geographic area) commonly transmit data across telephone lines. Although telephone companies are offering more digital lines (which are better suited to data transmission), most homes and businesses are still served by analog telephone lines. To transfer digital data over analog telephone lines, computers must use modems. When a computer sends data, its modem translates digital data into analog signals for transmission over standard telephone lines. At the receiving end, the computer's modem converts the analog signals back into digital data. The most important factors to consider when choosing a modem are internal versus external, transmission speed, data compression, and error correction. Modem transmission speeds are measured in bits per second (bps). Currently, the preferred standards for modems are 56.6 Kbps and higher.

Using digital connections, business networks and homes can transmit data many times faster than is possible over standard telephone lines. In areas where digital connection is not possible, homes and businesses are connected with standard analog lines, but high-speed digital lines are run between the telephone company's switching stations. The most popular digital telephone services are integrated services digital network (ISDN), T1, T3, and DSL. They offer faster data transfer rates and higher bandwidths than standard telephone lines.

Networks in the Home

New technologies enable homeowners to set up home networks to connect multiple computers. Home networks typically operate on existing media such as the home's telephone lines or cable wiring.

ISDN, T1, and T3

Many different kinds of digital services are offered by the phone companies. Some of the best known are called ISDN, T1, and T3. Of these, ISDN (integrated-service digital network) received the most attention in the past few years because it was the most affordable and the one most likely to make its way into homes and small businesses. ISDN, which stands for integrated services digital network, is a system that replaces all analog services with digital services.

When most people talk about ISDN, they are referring to a particular level of service called BRI (basic rate ISDN). BRI provides three communication channels on one line – two 64 or 56 Kbps data channels and one 19 Kbps channel that is used to set up and control calls. The two data channels can carry voice or data and they can be used simultaneously, so you can transmit data and carry on a conversation at the same time on the same line. Also, the channels can be combined so that BRI service can be used to transmit data at rates as high as 128 Kbps without compression.

Some telephone companies now offer BRI service in some locations – especially in large metropolitan areas. Installation can be expensive, but the cost of service is slowly coming down to compete with the basic rates offered for analog lines.

A higher level of service for ISDN is called primary rate ISDN, or PRI. In the United States, PRI provides twenty-four channels at 64 Kbps each, a total bandwidth of 1.544 Mbps. This level of bandwidth is also known as T1 service. In Europe, PRI service provides thirty-one data channels.

Although it is not specified by the ISDN standard, it is also possible to purchase lines from telephone companies that offer even more bandwidth. For example, a T3 line offers 672 channels of 64 Kbps each (plus control lines) for a total of 44.736 Mbps. Many telephone companies also offer services between the levels of BRI and PRI. Different businesses have all kinds of different needs for bandwidth, so telephone companies try to be as flexible as possible in their offerings.

DSL Technologies

One of the latest developments in connectivity is digital subscriber line (DSL). DSL is rapidly outpacing ISDN in areas where DSL is available because it is typically less expensive in terms of hardware, setup, and monthly costs. In fact, many local telephone companies are opting to develop DSL in their markets and are foregoing ISDN altogether.

Two key points that make DSL so attractive are its speed and its medium. DSL can achieve theoretical speeds up to 52 Mbps, a huge speed advantage over the fastest dial-up modems or even some digital connections. The second advantage is that DSL can use POTS lines, the standard copper wire used for telephone communications in most homes and businesses today. The typical home computer user can connect to the Internet or a private network through DSL at high transmission speeds, often for a cost that is competitive with standard dial-up connection. There are several types of DSL available in different markets, each offering different capabilities and rates:

Asymmetrical DSL (ADSL) uses discrete multitone (DMT) or carrierless amplitude phase (CAP) modulation.

Rate adaptive DSL (RADSL) adjusts the speed based on signal quality.

High-bit-rate DSL (HDSL) allows the telephone company to provide T1 speeds at a lower cost than T1 but requires two wire pairs.

ISDN DSL (IDSL) uses existing ISDN facilities.

Symmetric DSL (SDSL), a version of HDSL, uses a single pair of wires and provides slower transfer rates than HDSL.

Very-high-bit-rate DSL (VDSL) provides a high bandwidth with a commensurate cost and is geared primarily toward LAN and WAN connectivity.

The actual performance you can achieve with DSL depends on the type of DSL and the distance between the DSL modem and the telephone company's switch.