Modems, Modem Speeds

An RJ-11 Connection on a Modem. A modem must be able to both receive and transmit data. Communication in which there can be transmission in only one direction at a time is called half-duplex;

an example of this type of communication is a CB radio. A modem that can only communicate in one direction at a time is called a half-duplex modem. Communication that allows transmission in both directions at the same time is called full-duplex; regular voice phone conversations are an example of full-duplex communication. If a modem can communicate in both directions at the same time, it is called a full-duplex modem.

Modem Speeds

The speed at which a modem passes data over phone lines is partly determined by the transmission standard the modem is using. Modem speed is measured either in baud rate (named after the inventor, J. M. E. Baudot), which is the number of times a signal changes in one second, or in bits per second (bps); bps is the more common unit of measurement. For slower baud rates, one signal represents one bit (in which case baud rate is equal to bps rate); slower modems are often measured in baud rates. For faster baud rates, one signal can represent more than one bit, so faster modems are measured in bps (and baud rates may differ from bps rates). When measuring modem speed using baud rate, the number of bps will always be equal to or a multiple of the baud rate. The most commonly rated speeds of modems in use today are 14.4 Kbps, 28.8 Kbps, 33.6 Kbps, and 56.6 Kbps. The maximum speed of a modem is often written into the manufacturer’s name for the modem. To see what your modem rating is when using Windows 9x, click Start, point to Settings, click Control Panel, and double-click Modems. The installed modem is labeled as a 3334 33.6 FDSP Fax Modem. The 33.6 portion of the name indicates the maximum speed of the modem: 33.6 Kbps. Many factors contribute to limitations on modem speeds. One of these limitations is caused not by the limits of modem technology, but rather by the nature of regular phone lines and what they are designed to do. Analog phone lines were designed to provide only sufficient audio signal quality to support the transmission of the human voice. This limited quality of audio signals affects our ability to attain high transmission speeds for data. In addition to the limitations on analog lines, newer digital phone lines have limitations as well. Older phone lines (before 1940) were analog from beginning to end, with no digital components.

This is not true today; regular (analog) telephone lines are always analog as they leave a house or office building. However, the analog signals are almost always converted to a digital signal at some point in the transmission. These digital signals are then transmitted, using sophisticated computing equipment and methods, and converted back to analog signals at some point before traveling the last step between a local central telephone company office and the phone of the person receiving the call.

One limitation on modem speeds using regular phone lines results from the method used when an analog signal is converted to a digital signal somewhere along the phone line route. This is how it’s done: the phone company takes a sampling of the analog signal (8,000 samples every second) and converts each sample to an 8-bit byte. So, modem transmission is limited to this 8,000 bytes/second (64,000 bits/second) transmission of the digital signal.

Other factors further limit modem speed. For a modem to achieve this high a transmission rate, the conversion by the phone company from analog to digital can only take place once during the transmission. Also, if the lines in your neighborhood are multiplexed (several logical phone lines sharing a common physical line), the 56K speed cannot be attained. The line also often has some disturbance, called noise, which can be caused by lines bumping against one another in the wind, fluorescent lighting, nearby radios or TVs, bad wiring, lightning, and so forth. This reduction in line quality is called a dirty or noisy line. A line that consistently produces high-quality results is called a clean line.