Eloquence - Frequently Asked Questions

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General Frequently Asked Questions

What is a multiplexer ?

A multiplexer allows more than one device to share a communications line. This can result in substantial savings by decreasing the number of lines used. Multiplexers are normally installed in pairs at either end of the communications line. Data from many different devices including phones, faxes and computers can be combined into a single high-speed data stream and demultiplexed (separated and restored) at the other end.
There are two main types: Frequency Division Multiplexers (FDMs) and Time Division Multiplexers (TDMs). All Eloquence multiplexers are TDM type since they generally allow faster bit rates and potentially more channels than FDM at less cost.
A TDM uses time as a reference for multiplexing data. Data from each port is placed into "timeslots" in a frame. The frame also contains synchronisation data so that the receiving TDM can determine the position of the timeslots and hence the destination port for the data.

Is it easy to configure an Eloquence multiplexer ?

Yes ! All Eloquence multiplexers have a serial "supervisor port". This can be connected to most terminal types ( a VT100 for example). Even if you don't have a terminal, you can use the serial port on a PC and run a terminal emulation package (such as Windows Terminal).
Configuring the multiplexer is easily accomplished using menu driven selection screens. Most items allow you to scroll through all available options, making configuration simple. When you've finished, all settings will be stored in non-volatile memory and will always be restored automatically each time the multiplexer is powered up.

Which interface type should I choose ?

This depends on what type of communications line you want to connect to. Eloquence can supply three different types of interface:
X.21/V.11
A balanced interface using only one clock reference for both transmit and receive data. Can be used at high data rates (over 1Mbps) and long cable distances (over 100 metres). Typical use is the 64K Kilostream service provided by BT (British Telecom) in the UK.
V.35
A balanced interface for clock and data lines only, flag signals being unbalanced. It has separate clock reference lines for both transmit and receive data. Officially it can only be used at data rates up to 48Kbps, but some people do run them much faster. More popular in Europe (outside UK) and in the USA.
V.24
An unbalanced interface officially running at data rates up to 19.2 Kbps, though some people do run them at higher rates. Cable lengths should be restricted to 15 metres. Both transmit and receive clock references are available if required. Typically used on modems.

What's the difference between a statistical multiplexer and a TDM ?

A conventional TDM divides the bandwidth of the communications line into "timeslots". The TDM cyclically scans the input signals (incoming data) from connected equipment. Bits, bytes, or blocks of data are peeled off and interleaved together into frames on a single higher-speed communications line. Synchronisation data is also included in the frame so that the receiving TDM can ensure correct demultiplexing of the data.
In certain equipment (such as a terminal) it may be likely that there are periods of time when there is no data transmission activity. Because a TDM demultiplexes by the position of data in each frame, the absence of activity could result in the misinterpretation of data. To prevent this problem from happening, a TDM inserts null characters into each frame when there is no activity from an input channel. At the receiving TDM, the null characters maintain the positioning within the frame required for demultiplexing to occur correctly. They are "stripped", however, by the receiving multiplexer and are not output to devices attached to the TDM.
Although the use of null characters ensures that demultiplexing occurs correctly, it also indicates that the multiplexing process is not as efficient as it could be. This inefficiency resulted in the development of a different type of multiplexer for use when using equipment with periods of inactivity (e.g. asynchronous terminals). This type of multiplexer, referred to as a statistical multiplexer (statmux), provides a much higher level of line utilisation efficiency than TDMs in certain situations.
In comparison to TDMs, which use fixed frames with data positioned in each frame, statistical multiplexers use variable-length frames. 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.
Since a statisitical multiplexer takes advantage of idle times, they are seldom beneficial for applications with nonbursty, continuous traffic, since the multiplexer cannot interleave the traffic. They also introduce greater delay as the data is buffered and prepared for transmission.

What's the difference between AC15 and E&M signalling ?

Signalling is the command "language" of any network, whether it is analogue or digital. Even the switch operated by a telephone handset when it is 'on or off the hook' is a signalling device, indicating to the exchange whether the instrument is available for calls.
E&M Signalling
This is a simple form of signalling, where the signalling leads are separated from the speech path.
In the forward direction, the M lead (from the word 'Mouth') is connected to ground when a call is originated (e.g. telephone picked up, line seized). It may pulse between closed and open states when the telephone is dialling. When the remote site answers, the E lead (from the word 'Ear') gets looped to ground to indicate that the call has been answered.
In the backward direction, the E lead is connected to ground when the exchange wants to originate a call (e.g. RING the telephone). The called party (the 'phone) indicates that it has been answered by connecting the M lead to ground.
E&M can be implemented with two-wire or four-wire speech circuits. The E and M leads are not used for voice.
AC15 Signalling
AC15 is based on the 'tone on idle' principle. This means that while the circuit is idle (carrying no traffic), there is a permanent tone (2.28KHz) on the line. This tone is only removed when a 'seize' takes place.
A tone ON is exactly the same as an un-looped E or M lead. A tone OFF is exactly the same as a looped E or M lead.
Simplified interconnections (i.e. LESS wires) are required for AC15, however, it does require more complicated hardware.

What is Synchronous Tunnelling ?

In a typical network, routers carry routable protocols from one local area network (LAN) to another, either directly or over wide area links (WAN). In some situations there is a requirement to carry protocols that are non-routable and that are carried over synchronous lines. Synchronous tunnelling is a mechanism for carrying these protocols through a network of routers over another routable protocol. The word tunnelling refers to the fact that the traffic is passed from one point in the network to another, through a tunnel between two routers.
Synchronous tunnelling is normally a proprietary option in routers. No standards exist for tunnelling generalised synchronous data through a network. A synchronous protocol such as HDLC is normally supported, since this allows the router to packectise the synchronous data.
An example of a situation where synchronous tunnelling might be used is to connect two Esprit statistical multiplexers. Normally the Esprit is connected over a dedicated synchronous point-to-point link, but since the Esprit communicates using HDLC it is suitable for connection through routers that support synchronous tunnelling. HDLC frames from the Esprit are packetised by the router and sent through a TCP/IP network to a remote router that directs them to the other Esprit. In this way, customers that already have TCP/IP LANs or WANs can tunnel Esprit data over the existing network without need of a dedicated connection.