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The function of modems, how they work and what the term ‘modem’ actually stands for

A modem, simply put, MOdulates and DEModulates your signal between your computer over the telephone system and to your ISP, hence the term MODEM. Most know a modem is used to communicate between computers, but a modem does more than that. The telephone system can only send analog signals back and forth, but Internet activity is sent using digital pulses.

A digital signal is binary, which means the signal can carry a value of only 1 or 0, where as analog signals can carry any range of values. Your computer, when accessing the Internet, shoots digital pulses at the modem that will, in turn, translate those digital pulses into analog signals to travel across your telephone line outside your home. Then, once the analog signal reaches your ISP’s modem, it is converted back into digital to be sent out across major, high speed backbones to fulfill your request. The translation between analog and digital is known as modulization and demodulization.

Modems communicate with an RS-232, or serial, interface with an RJ-11 telephone cord jack (usually two) and come in both internal and external varieties. To make communication a little easier, a set of standards was set forth to make sure a modem from one manufacturer can ‘speak’ with a modem from another manufacturer.

In the early 80’s, Hayes Microcomputers developed a standard by which all other modems were judged. In fact, your company was not ‘cool’ unless it conformed to the Hayes standard. Hayes-compatible modems first sent and received data a the blazing speed of 300 bits per second, considerably slower than any modem available on the market today. As times progressed, so did the standards, beginning with the International Telecommunications Union, or ITU.

In the late 80’s the ITU developed a series of specifications for modems known as the V series. Beginning from the V.22 standard of 2400 bits per second, to V.32 (9600 bits per second), to V.32bis (14,400 bits per second), to V.32terbo (19,200 bits per second), to V.FastClass (28,800 bits per second), to V.34 (28,800 bits per second), to V.42 (57,600 bits per second) and to the currently standardized modem bit rate of 56,600 per second, V.90.

Engineers soon learned that compression could greatly increase the speed at which data could be transferred by allowing sound to carry more than one single bit of data at a time. So, although a modem might advertise 56K, with correct compression (and a good quality telephone line), sound waves can transfer at upwards of 115,200 bits per second.

Asynchronous modems

There are different types of modems for use in different types of networks. The most widespread type of modem is an asynchronous modem, also known as “async”. With asynchronous modems, data is transferred one bit at a time, due to its serial communication interface. Each and every character, number, symbol, whatever is transferred into a string of 8 bits (a byte) and is separated by a start-of-character bit and stop bit, adding to the size of the data. The receiving computer can use the start/stop bit as a timing function so it can be ready to receive the next bit.

The actual rate at which an asynchronous modem can send is 28,800, but with current compression standards, data can be transferred at upwards of 115,200 bits per second. Asynchronous modems, in an effort to provide error checking, use a parity bit. With parity checking, the amount of bits send MUST match exactly the amount of bits received. Otherwise, the receiving computer knows the data has been corrupted (or not all received) and will attempt to have the sending computer resend.

Synchronous modems

To separate groups of bits, synchronous relies in a timing scheme while special characters are used to signal the beginning of the synchronization. As you can see, because of the timing scheme to signal each bit, no start/stop bit is needed, increasing performance of the transmission. Synchronous modems group bits together into a block, also known as a frame for easy transmission. If an error is detected, a retransmission is requested, similar to asynchronous. Digital networks often use synchronous modems, and due to increased cost, usually are not found in common homes. If you do not know specifically what type of modem you have, you most likely have an asynchronous modem.


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