ted by a 34Mbit server network twelfth frames. which is itself supported by a 140Mbit server network. Starting As illustrated in Figure 21.5, each superframe consists of 12 from the second order, the equipment no longer contains an analog repeating frames. One frame corresponds to 125 microseconds; one to digital converter and only deals with digital frames. The equip- superframe is thus 1.5 milliseconds in duration. Each frame contains ment essentially consists of multiplexers, combining into new one synchronisation bit to allow the receiving equipment to decode, frames the frames of four systems of the preceding order, called demultiplex, and allocate the incoming bits to the appropriate chan- tributaries. nels. During the construction of an nth order multiplexer starting from From the above follows that each superframe contains a 12-bit frames with order n-1, we are confronted with the problem of word, composed of individual bits coming from each of the 12 anisochronism of the tributaries. In effect, the frames to be grouped frames. The framing bits are called BFf, and the signalling bits BFs. come from different equipments which are geographically distinct BFfs are the odd numbered framing bits, whilst BFss are the even 64kbit/s 2Mbit/s 8Mbit/s 34Mbit/s 140Mbit/s 565Mbit/s Figure 21.4 Hierarchy of digital systems The T carrier framing and coding formats 21/7 Superframe Frame 5 Frame 6 Frame with signalling Framing Channel 24 Channel 1 Channel 2 5.2 u sec 5.85 usec 5.2 usec Information bits Information bits Information bits Framing bit Signalling bit Signalling bit Figure 21.5 D4 framing patterns numbered bits. The 12-bit word is used for synchronisation and for octet of bits. The sixth bit is also referred to as the A bit, while the identifying frames number 6 and 12, which contain channel signall- twelfth bit is also called the B bit. These combinations of bits allow ing bits. Each frame thus contains a BFf or BFs framing/signalling the end-user station equipment to carry out its signalling protocol, bit on 