with way D-Type. the Transmit and Receive circuits to indicate the state of the DTE or DCE respectively. No characters are transmitted or received on these circuits; they are simply turned ON or OFF to indicate their 40.7.2 current state. The Byte Timing circuit provides information pertaining to the The V.35 interface provides for speeds up to 48000bits/s at a grouping of bits into characters. When the byte timing circuit is not distance of up to 1 kilometre incorporating a partially balanced available a synchronising character is inserted before each string of interface to give a higher noise immunity. The V.35 connector is characters. Byte timing and SYNC characters are only required significantly different from the D type V.28 connector. It has a when exchanging physical layer control characters. When the physi- square profile with a staggered pin layout to prevent it being cal layer is being used to transmit link layer information on the coupled incorrectly and is known as the MRAC connector. V.35 transmit and receive circuits byte timing and synchronising charac- uses a balanced interface for TxD, RxD and the clocks but not the ters are not used. control signals. V.35 is however no longer recommended by CCITT who suggest using V.36 in its place for all new developments. 40.7.5 Physical interface 40.7.3 The X.21 physical interface is specified by V.11 and is basically a cut-down version of the V.36 interface. There are only two control The V.36 (similar to RS-449) interface provides for speeds up to signals (one in each direction) other than that the interface is the 72kbit/s at a distance of 1 kilometre. All control, data and clock same. The physical interface for X.21 is the 15 pin Dtype connector. 40/14 Multiplexers Multiplexers in communications tronic mail servers, local computers, printers, etc. The wide area networks network provides connectivity and interoperability between the LANs and to centralised computing resources i.e. a centralised stock distribution system or an aircraft booking system. This