The conventional TDM can waste the bandwidth of the communications line because the time slots in the frames are often unused. Vacant slots occur when an idle terminal has nothing to transmit in its slot. Statistical multiplexers dynamically allocate the timeslots (i.e. the bandwidth) among active terminals and dedicated time slots are not provided for each port.

A statistical multiplexer such as the Eloquence Esprit, dynamically allocates the bandwidth among the active equipment, varying the length of the frames in accordance with the input data, so that idle equipment does not waste the lines capacity. Data from each channel is buffered and organised into a frame before being sent. The frames also have headers, sequence numbers, and error-checking fields for the purposes of identification and control. Frames from the active channels are interleaved onto a single high-speed communications line. Flow control is used to prevent transmitting devices from sending data too fast into the multiplexer's buffers.

Statistical multiplexers will not be beneficial for applications with nonbursty, continuous traffic, since the statistical multiplexer cannot interleave all the data if the line is continuously busy. Notwithstanding, statistical multiplexors should be given serious consideration by organisations using long-haul leased lines..

Many communication devices need some way of regulating the movement of data between them. For example, a printer can generally receive data much faster than it can print, it must therefore have a handshake mechanism to stop a connected computer from sending more data than it can handle. This mechanism is called flow control and there are two methods, hardware flow control and software flow control.

Hardware (out-of-band) Flow Control
Devices implementing hardware flow control require two dedicated signals: an input to the device to control the flow of data from the device, and an output from the device to control the flow of data into the device. The device resumes its transmission if the control input is asserted, and pauses transmission if the control input is disasserted. Similarly the device can control the flow of data into the device by asserting/disasserting the control output.

Software (in-band) Flow Control
Software drivers in the device use special characters in the data stream to control the flow of data. ASCII codes DC1 and DC3 are often used, but they are usually called XON and XOFF. On a standard keyboard, these characters correspond to CTRL-Q (resume) and CTRL-S (pause). The device resumes its transmission if it receives XON and pauses transmission if it receives XOFF. Similarly the device can transmit these characters to control the flow of data into the device.

It is normal on a communication line for bursts of noise to cause occasional data errors - as in a telephone conversation when pops and clicks can sometimes be heard.

Data is transmitted by a statistical multiplexor in small packets. Each packet is checked for integrity by the receiving statistical multiplexor and if an error is detected the statistical multiplexor requests that the entire packet is re-transmitted. The error LED on the statistical multiplexor is used to indicate that a re-transmission is being performed. An occasional flash of the error LED is therefore normal and does not indicate a fault with the equipment. No data errors will occur since damaged packets are re-sent.

Excessive or constant flashing of the error LED may indicate an equipment fault or a communications line fault. In this case throughput may be poor since the statistical multiplexor is having to re-transmit large amounts of data.

Kilostream is a 64Kbps service, hence the Esprit and Express statistical multiplexers are suitable for connection since these both support a composite data rate of 64Kbps or more.

Data is transmitted by a statistical multiplexor in small packets. Each packet is checked for integrity by the receiving statistical multiplexor and if an error is detected the statistical multiplexor requests that the entire packet is re-transmitted. No data errors will occur since damaged packets are re-sent. This method however relies on the fact that the statistical multiplexor can control the flow of data from connected peripherals using flow control. If it can't, the RAM inside the statistical multiplexor used to buffer the data may overflow and data will be lost, causing the data errors.

If you are experiencing data errors, check that flow control is set-up correctly for both the statistical multiplexor and the connected peripherals. To ensure error-free operation you must choose either hardware and software flow control. Setting no flow control may appear to work, but error-free operation cannot be guaranteed since the statistical multiplexor cannot control the connected peripheral.

A typical cable to connect a channel port on the Escort2/Express/Esprit multiplexer to a terminal or printer is as follows:-