h the maintenance of the data link design a multiplexer that only transmitted data when there was data and did not have to get involved with device control. to sent. By scanning the input channels for activity and placing any 2. It had one common frame format for data, control and supervi- data present into a common buffer pool, a frame could be assembled sion with a unique flag indentifier for synchronisation. with data from different channels, plus any control information, and 3. It was bit oriented and therefore inherently transparent to data transmitted to the multiplexer at the other end of the link for structures. However, it used byte aligned boundaries to permit demultiplexing. byte interleaving, if required. Since HDLC uses a go-back-N frame sequencing count, any 4. It had a Frame Check Sequence (FCS), in the form of a Cyclic errors on the line due to a bad CRC or out of sequence frame would Redundancy Check sum (CRC), that allowed error checking of be detected and a request for the frame to be retransmitted issued the complete contents of all frames and thereby providing a from the remote end (also known as Automatic Repeat Request or mechanism for error recovery. ARQ). At that time, no standards existed for describing how the Address Control 128 bytes of user data Figure 40.6 Basic HDLC information frame Statistical time division multiplexing 40/7 Up to 128 bytes Control Start Control channel Channel X Channel y Channel Z, etc Figure 40.7 Proprietary HDLC information frame format (Dowty Case Communications Ltd.) frames should be constructed for multiple users and interface con- 40.3.2 Features trol information, SO every manufacturer developed their own pro- prietary standard, compared to the present development of LAPD By definition, statistical multiplexers have to concentrate many which overcomes these restrictions. The outcome of this was to see input channels onto a composite link, where the aggregate input a generation of proprietary statistical multiplexers before the estab- speeds may temporaril