Types of Modems

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.

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.