Modems

Although the term "modem" is almost universally understood in concept, the actual operation of the modem is usually not. Outside of knowing that a modem is a device that modulates and demodulates digital signals into analog sounds, understanding how a modem does what it does is essential. You must understand the modem process to fully use and optimize it.

Modem Technologies

All devices you connect a modem to are called DTE. DTE devices can communicate with each other directly, but typically employ DCE, such as a modem, to manage the connection.

The current standard used to make a DTE/DCE connection is EIA/TIA-232-C. EIA/TIA-232 replaced RS-232 as the connection standard. You will be glad to know that RS-232 was approved and that all our computers, modems, and other devices aren't obsolete.

All traffic through the Public Switched Telephone Network (PSTN) is digitally encoded through a process of PCM encoding.

The DTE and DCE devices control their communication through five main signals sent between devices. The signals are one of two types, hardware flow control, shown in Table 6.1, and modem control, detailed in Table 6.2.

Table 6.1. Hardware Flow Control Signals

Signal	Purpose
RTS	Request to Send. Generated by DTE, indicates that the DTE has buffers to receive data from the DCE.
CTS	Clear to Send. Generated by DCE, indicates that the DCE has buffers to receive data from the DTE.

Table 6.2. Modem Control Signals

Signal	Purpose
DTR	Data Terminal Ready. Generated by DTE, informs the DCE that it is ready to receive an incoming call.
CD	Carrier Detect. Generated by DCE, indicates that DCE-to-DCE communication has been established.
DSR	Data Set Ready. Generated by DCE, informs the DTE that it is ready for use.

Modem Standards

Modem connections are governed by several standards that were developed to provide faster communication and higher levels of data integrity. There are a number of standards for error detection/correction and compression. As important as it is to know the standards, it is equally important to know that they only define the mechanism by which a specific task is to be accomplished. Compression standards, for example, can only define how compression works and the maximum compression that can be realized. They do not guarantee that all data will be, or even can be, compressed.

Error detection/correction standards fall into two groups, Microcom Networking Protocol (MNP) and Consultative Committee for International Telegraph and Telephone (CCITT). The CCITT changed its name to ITU-T (International Telecommunication Union Telecommunication Standard-ization Sector) in March 1993. The MNP standards are MNP 2-4 (for use in the public domain) and MNP 10 (for use with cellular technologies). The CCITT standard is also sometimes referred to as Link Access Procedure for Modems (LAP-M).

The main compression standards in use today are MNP 5, which provides a 2:1 ratio, V.42bis (4:1), and V.44 (6:1). The general rule is that any compression mechanism can be paired with any error detection/correction mechanism.