o alternatives were: the overheads taken into account, that results in a composite that Total = Mbit/s Total = Nbit/s Figure 40.5 Model of a multiplexer with overheads considered 40/6 Multiplexers must be faster than the input aggregate. The value of M is given by 5. It was being considered as an international standard. Equation 40.5, therefore M > N. There were still drawbacks however, in that early versions of M =A+B+C+C HDLC only allowed one virtual circuit (or user) per frame, and synchronisation header + control information (40.1) therefore modifications were made by many manufacturers to allow several users to share the same frame. In addition the multiplexers In summary it can be stated that time division multiplexing has needed to pass not just data but also various other types of informa- established itself as the most efficient technique for handling syn- tion such as: chronous protocols and data at different speeds, even though it suffered from the fact that bandwidth is always wasted. It is not 1. The status conditions of the V.24 (RS232) interface controls. however very effective at handing asynchronous data due to the 2. Test information, such as setting a specific remote channel into inflexibility and lack of error detection and correction. It is from this loopback for testing purposes. major disadvantage that the use of statistical multiplexers evolved. 3. Information for validation of a channel connection, to check that it is working end to end. 4. A SPACE condition for SO many milliseconds. 40.3 Statistical time division multiplexing 5. Information to set remote buffers into an XOFF state SO that no more data should be sent until further notification. 40.3.1 Principle of Operation It also had to allow the input channels to locally emulate some asynchronous polled protocols e.g. HP3000, so that polling could be Caught between the exponential growth of computing power and locally acknowledged rather than end-to-end. The link need only the use of asynchronous terminals as a cheap and convenient way