This section discusses modems that users use to receive and send their data and information through communications channels. All communications must flow through some types of channels. Most of the computer communications are transmitted via ordinary telephone lines. Therefore, many require devices to convert the digital signals (represented by the presence or absence of an electronic pulse) into analog signals. This converter is called a modem.

The modem is an acronym for "MOdulator- DEModulator." A modem converts the digital signals into continuous analog signals (Modulation), and converts from analog to digital (Demodulation). The modem enables digital microcomputers to communicate across analog telephone lines. Both voice communications and data communications can be carried over the same telephone line.

Microcomputers require modems to send and receive messages over telephone lines. However, not all computer communications should have converters such as modems. Computer systems connected by coaxial or fiber- optic cables can send and receive digital signals directly through these circuits. Most modems today are both data and fax modems in that they can transfer computer data and also acting as fax machines.

Communications Speeds

The modem handles the dialing and answering of the call and controls transmission speed. Communications speeds are expressed in bits per second (bps). A baud is commonly and erroneously used to specify bits per second for modem speed. This is not always true. Modems are classified by the speed at which they operate. The modems used with microcomputers usually operate at 2400, 9600, 14400, and 28800 bits per second.

The higher the speed, the faster users can transmit a document and therefore the cheaper your line costs. For example, transmitting a 100-page single-spaced report takes 25 minutes at 2400 bps. It takes 6 1/4 minutes at 9600 bps, about 4 1/6 minutes at 14400 bps, and 2 1/12 minutes at 28800 bps.

External vs. Internal Modems

• External Modem: This is a modem separated from the system unit in the computer case. It is connected to the serial port of the computer by means of a cable. It is connected to the telephone wall jack by another cable.

• Internal Modem: An internal modem is a circuit board (a modem card) that can be added to the system unit of the computer. It takes one of the expansion slots.

Standard vs. Intelligent Modems

• Standard Modems: Most modems used today are called standard modems. These modems are usually operated by commands entered from a microcomputer keyboard. Users control the functions (dialing, etc.) of a modem through the keyboard. Modems may use different command languages to control their functions, but most modems (called "Hayes compatible") use a popular language developed by a company "Hayes Microcomputer Products, Inc." The command ATDSX4, for example, indicates that pay attention (AT), dial stored telephone number (DS), and then display the progress of the call (X4).

• Intelligent Modems: Intelligent modems are also called advanced modems. These modems can accept new instructions and then respond to the commands while transmitting data and information. These can be done by microprocessor chips and internal read only memory (ROM) contained in the modem. Of course, these modems are more expensive.

Short-Haul and Wireless Modems

• Short-Haul Modems: Short- haul modems are devices that transmit signals down the cable through any COM1 port. They sometimes are called modem eliminators, because they do not require an external power source. They are also called line drivers that can send data for a distance of more than one mile. This type of modem can be used within or across several buildings in a company or a university campus.

• Wireless Modems: Wireless modems transmit the data signals through the air instead of by using a cable. They sometimes are called a radiofrequency modem. This type of modem is designed to work with cellular technology, and wireless local area networks. Wireless modems are not yet perfected, but the technology is rapidly improving.

Many different types of modems exist in market today. There are several standards for modems, and modems that conform to a standard can communicate with other modems having the same standard. Many new modems currently developed support several standards.

V.22/V.22bis Modems

V.22 is the Consultative Committee on International Telegraph and Telephone (CCITT) standard for 600 bps or 1,200 bps full-duplex modems for many years, but this standard is becoming obsolete. The modem uses frequency modulation (FSK). V.22bis is the CCITT standard for 2400 bps full-duplex modems. It uses QAM modulation.

V.32/V.32bis Modems (High Speed)

V.32 is the CCITT standard for 4,800 bps and 9,600 bps full-duplex modems over dial-up telephone circuits. It uses QAM modulation. V32bis is the CCITT standard that extends V.32 to data rate of 14,400 bits per second.

V.34/V.34bis Modems (Error Checking)

V.34 is the CCITT standard for the digital (not analog) transmission telephone network beyond a local loop. It supports the data rates up to 28,800 bps. V.34bis is an extension of the V.34 standard. This modem provides a better error checking and voice transmissions. It supports the higher data transmission rates of 33,600 bps.

V.42/V.42bis Modems (Data Compression)

V.42 is the CCITT standard for error correction that uses two protocols. The main protocol is LAPM and secondary protocol is MNP Classes 2 through 4. V.42bis is the CCITT standard for error correction and data compression. It provides a compression technique to increase transmission speed around 4 times the bps rating by using a dictionary of 4- byte character combinations.

Baud

This is a switching speed or signaling rate of a line. It refers to the number of transitions (frequency changes) made per second. Baud equals to bits per second at only low speeds. (e.g., 300 baud is the same as 300 bps. However, the V.22 modem can generate 1,200 bps at 600 baud.)

COM1 Port

Logical name assigned to serial port #1 in DOS and OS/2. COM ports are usually connected to a modem or mouse. DOS version 3.3 or higher supports up to COM4, and OS/2 supports eight COM ports. Also, called RS232 port. See LPT1 port.

LPT1 Port

Logical name assigned to parallel port #1 in DOS and OS/2. This port is usually connected to a printer. A second parallel device is assigned LPT2.

RS232

A 25-wire electrical interface between a computer and a peripheral device, such as a modem, a mouse, etc. It is an EIA (Electronic Industries Association) standard for serial transmission that uses a 25-pin DB-25 or a 9-pin DB-9 connector.

Frequency Modulation

A communications transmission technique that modulates a data signal into a fixed carrier frequency by modifying the carrier frequency.

FSK (Frequency Shift Keying)

A simple communications modulation technique that merges binary data into a carrier frequency. It usually creates only two changes in the frequency, one for the 0 bit and another for the 1 bit.

QAM

?Refers to Quadrature Amplitude Modulation. A modulation technique that generates 4 bits per baud.

LAPM

Refers Link Access Procedure Modem. The protocol defined in CCITT V.42. It uses LAPD methods (LAP-D channel) used for the data channel of an ISDN transmission.

MNP

Refers to Microcom Networking Protocol. This is a data compression protocol that has been a standard in the United States. It was developed from microcom, Inc.