 carry a poll count and report polling exceptions. of accessing computers, the statistical TDM was developed. It was The standard HDLC format has no inherent mechanism for allow- first formulated in the early 1970s and in particular on the Arpanet ing these facilities as it was designed purely to control the link level network in the USA from which the X.25 packet switch standard functions. It is up to the designer to implement this in the informa- evolved (Davis, 1973). tion (data) fields in order to make the product as flexible as possible. As stated above, the main problems with using TDM techniques Figure 40.6 shows a basic HDLC Information frame with the on asynchronous data were lack of error detection and correction, unique flag identifier 7E (hexadecimal), the address field and the and poor efficiency on the composite line, especially since the control field, which in this case is indicating that the frame contains average asynchronous device is only active for 5 to 10% of the time. data. The solution for asynchronous multiplexers was to use a formal Figure 40.7 shows a modified HDLC information frame format, protocol between the multiplexer nodes which could concentrate used by a proprietary STDM manufactured by Dowty Case, other the data and provide the means of detecting and recovering from manufacturers using different encoding techniques. Here the infor- errors on the line. The protocol invariably chosen was based on a mation frame does not require an address field. It does, however, High Level Data Link Control (HDLC) which in turn had been require a mapping field for each block of 4 bytes for a given channel, derived from the IBM's SDLC protocol. This protocol had several which indicates whether the following bytes are data or control and design advantages over previous synchronous protocols such as timing codes e.g. DTR dropped, set channel loopbacks, set SPACE IBM's 3270, as follows: condition for 100mS. Given the mechanisms of a link protocol it was a small step to 1. It was solely concerned wit