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Computer Science
Computer Catlog
Comm Network - TOC

Physical Layer Standards


    The most commonly-used physical layer standards are those published by ISO, CCITT, IEEE, and EIA, many of which are inter-related. A large number of the existing standards deal with transmission over telephone lines. The CCITT V series of standards fall into this category and are by far the most-widely adopted. Below we will look at two very popular standards for connecting DTEs and DCEs: the analog standard RS-232 and the digital standard X.21.


RS-232


    RS-232 has dominated the computer industry as the most-widely used standard for physically connecting devices. It is an analog standard, defining the physical layer interface between a DTE and a DCE, and can support simplex, half-duplex, and fullduplex connections in synchronous as well as asynchronous mode. It originated in the late 1950s, and has been revised a number of times over the years. The latest revision, EIA-232-D, is based on CCITT's V.24 and V.28 standards and ISO's 2110 standard.

    RS-232 has two important limitations which reduce its usefulness: it is not suitable for distances of more than about 50 meters, and it has a maximum bandwidth of 20 kbps. Other similar standards have been devised to overcome these limitations. For example, RS-449 and EIA-530 can both support data rates of up to 2 mbps over longer distances.


CCITT X.21


    X.21 is a widely-accepted standard for interfacing a DTE to a DCE of a digital network. It can be used for connections of up to 1 km in length and data rates of up to 10 mbps (for distances less than 10 m). X.21 uses a connector based on the ISO 4903 standard.The connector provides 15 pins for connecting the circuits derived from the X.24 standard, as summarized in Figure 2.26. Unlike RS-232, the same transmit and receive circuits (T and R) are used for the exchange of control as well as data signals.

    The electrical characteristics of X.21 are defined by V.10/X.26 or V11/X.27. V.10 uses 4 to 6 Volts to represent binary value 0, and -4 to -6 Volts to represent binary value 1. It allows for connection distances of up to 1 km. X.21 bis is a variation of the X.21 standard with similarities to RS-232: it uses the V.24 circuits and is usually used with the 25-pin connector of ISO 2110.


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