ve any data to send. However, time slots in the form of band- chronous traffic and the only rules for configuration would involve width still have to be allocated since the TDM cannot recognise the working out the total aggregate input and formulating the best absence of real data; this is the trade-off against data transparency 'scanning' method to cater for all the different speeds, SO that they and speed. could mapped on to the available composite bandwidth. According To summarise, the technique of sending asynchronous data by to the design of the multiplexer a small percentage of the bandwidth TDM is to use a frame synchronised, byte interleaved method with had to be allocated for the synchronisation pattern, or sequence, SO facilities for control information using a redundant bits. the general design rule was to limit the overhead to around 1% or 2 The advantage is transparency to data format; any format of bits in a 200 bit frame. In addition, if the status of the V.24 (RS232) asynchronous data can be sent, at any speed. control lines such as Data Terminal Ready (circuit 108/1 or 2) or The disadvantages are: Request to Send (circuit 105), then this had to be allocated band- width as if it were data. As an example, if a half duplex circuit is 1. Fixed speeds; it was usually difficult to change any speeds being employed, which means that the data and control signals have without completely re-programming the multiplexers. to be kept contiguous, then the control information could require as Poor efficiency; input channels always have a fixed allocation much bandwidth as the data. The result is that the bandwidth of composite bandwidth, whether they have data to send or not. actually available for data could be less than the theoretical maxi- As asynchronous data is by definition 'bursty' interactive type mum by up to 50%. The technique developed to overcome this traffic, there will almost certainly be long periods of inactivity problem is to use multi-frames with two types of controls, high and which still has