 pairs being used for transmission of the layer 1 signals to and from the network and 2 pairs for power LAYER 2 I.440/1 feeding which are optional. Power feeding is also possible over the transmit and receive pairs via the phantom, and may be used to provide power from the network to the terminal in order, for example, to maintain a basic telephone service in the event of local LAYER 3 I.450/1 power failure. However to save power fed from the local exchange, both in the local network and the NT1, an activation/deactivation procedure is defined. Deactivation is also provided to reduce elec- Figure 44.6 Relationship between CCITT Interface tromagnetic radiation. Recommendations A connect/disconnect indication is used by Layer 2 to determine if a terminal is plugged into the bus. This is necessary as each ment (TE) is connected at the end of up to about 1km of cable. In terminal is given a random, unique identity (its TEI value) by the the point to multipoint mode up to 8 terminals can be connected in network. If disconnected the terminal must forget its original ident- parallel anywhere along the bus, but the bus length is now limited ity to prevent duplication when reconnected. The connect/discon- to 200 metres. nect indication is achieved by monitoring the presence of DC power Over this bus passes the two B channels, which are transparent on the bus. 64kbit/s user channels, as well as the D channel, which is the The layer 1 structure for supporting these functions is shown in 16kbit/s signalling channel, and other bits used for miscellaneous Figure 44.7. The dots demarcate those parts of the frame that are purposes such as frame synchronisation. The B channels contain the independently d.c. balanced. The frame is 48 bits long and lasts 48 bits in 250 microseconds NT to TE DLFF. EDAFA N EDL.FL. TE to NT DL.FL. L.D . F. A L. L.DL. L.DL. L.DL.FL. F = Framing bit N = Bit set to a binary value N = = DC balancing bit B1 = Bit within B channel 1 channel bit B2 = Bit within B channel 2 = D echo - channel bit Bit used for