antial when there are many drops per line. A special type of modem called an acoustic coupler acoustically connects the modem to the DDD network. The coupler employs FSK modulation and couples acoustically with a telephone handset. Acoustic couplers are useful for portable applications, and they are typically priced below conventional modems. However, they are lim- ited to transmission speeds of 1200 baud. Modems designed for private facilities are called limited distance or short-haul modems. Private facilities are typically those provided by the user for communication within a building or between build- 20 TELECOMMUNICATIONS ings in the same complex. These modems are less expensive than long- haul modems and eliminate the cost of lines provided by the tele- phone company. The transmission range is usually limited to 4 or 5 miles. 2.4 INDUSTRY STANDARDS Data communications standards have been established by several industry standards organizations. The Electronic Industries As- sociation (EIA) and the Consultative Committee for International Telegraph and Telephone (CCITT) have established standards for modem parameters and the electrical interface that provides a con- nection between data terminal equipment (DTE) and data circuit ter- minating equipment (DCE); such as a modem (see Fig. 2-3). The interface passes digital data and control signals between the devices, but can differ electrically depending on application. The EIA Standard RS-232C is the most common electrical inter- face standard used throughout the industry; however, some connec- tions support functions that differ among vendor products. The physical RS-232C interface is a 25-pin connector that supports trans- mission at data rates up to 20 k bps at distances to 50 feet between DTE and DCE. EIA RS-449 is an improved interface standard com- patible with RS-232C, but it accommodates extended data rates to 2 m bps at distances up to 200 feet between DTE and DCE. The added connections of the 37-pin interface extend control functions. The U.S. Governme