low of data) rather than by Signal Ground discarding packets. Individual packets on a virtual circuit are rarely autonomous, DTE Common Return generally they are only part of a message. Transmit Receive Physical layer standards Control Indication There are two standards applicable to the physical layer X.21 and Signal Element Timing X.21 bis. X.21 allows the user to access the network via a high speed serial interface. X.21 bis allows the user to access the network using Byte Timing a lower speed (<20000bit/s) serial interface. X.21bis is the most common method of connection to an X.25 network. The full X.21 protocol provides all the services of the first three layers: physical, data link, and network services. X.25 only uses the X.21 physical interface specifications. The circuits function is defined in CCITT Recommendations X.24 40.7.1 X.21 bis and their electrical characteristics are defined in CCITT Recom- mendations X.27 for DCE and X.26 for DTE. X.21 bis is based upon CCITT Recommendations V.24/V.28 where Signal Element Timing provides bit timing. Byte Timing is op- V.24 specifies the control signals such as Request to Send (RTS tional and therefore not always supplied. All timing is provided by circuit 104) and Clear to Send (CTS circuit 105). V.28 specifies the the DCE. electrical interface in terms of voltage levels, impedances and the The DTE uses the Transmit and Receive circuits to exchange data rise and fall times of the signals. It is suitable for speeds up to and control messages with the DCE. Unlike the V.24 and RS232- 19200bits/s. Higher speeds can be achieved using the V.35 and V.36 C/D standards X.21 specifies that control information be exchanged interfaces which were specifically designed for that purpose. The using characters transferred on the interchange circuits, rather than EIA RS232-C and D standards are almost identical to the V.24/V.28 using circuit states. standard but also include the connector specifications which is a 25 The Control and Indication circuits are used in conjunction 