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Computer Science
Computer Catlog
Comm Network Catlog

Network Components
Network Types
The OSI Model
Protocol Notations
Physical Layer
Transmission Media
Digitization and Synchronization
Physical Layer Standards
DataLink Layer
Error Checking
Retrans - Flow Control
Sliding Window Protocol
Data Link Layer Standards
Network Layer
Switching Methods
Congestion Control
Network Sub layers
Transport Layer
Transport Protocol
Transport Layer Standards
Session Layer
Session Layer Role
Session Protocol
Presentation Layer
Abstract Syntax Notation
Application Layer
Common Application
Specific Application
Message Handling
IEEE 802 Standards
ANSI FDDI Standard
Frame Relay
Broadband ISDN & ATM

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 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.


    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.