GB2198313A

GB2198313A - Telecommunications switching system

Info

Publication number: GB2198313A
Application number: GB8627458A
Authority: GB
Inventors: Peter Howard Lisle; Keith Alan Oakley; John Hayden Mickle
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1986-11-17
Filing date: 1986-11-17
Publication date: 1988-06-08

Abstract

There is disclosed a telecommunications switching system for routing signals received from a plurality of incoming highways to a plurality of outgoing highways, in which system, switching elements are mounted on slide in units which in turn mounted in a shelf having a backplane communications bus, and wherein there are provided for the highways respective line termination units 31 mounted on slide in units in the or a shelf of the switching system 20. This avoids the disadvantage of the conventional arrangement where incoming highways are first terminated in equipment containing line termination units, this equipment being connected to switching equipment by means of large quantities of expensive couplings and cables. <IMAGE>

Description

TELECOMMUNICATIONS SWITCHING SYSTEM This invention relates to a telecomnunications switching system, and particularly, but not exclusively; to a system for switching between optical highways.

It is standard practice to terminate a transmission highway in a line termination unit (LTU). In the case of an optical fibre highway; the LTU comprises an optical receiver (or transmitter) which converts the pulses of electromagnetic radiation (laser light) to an electrical signal in digital form (or vice versa), and performs a protocol check to ensure that there are no errors in the received signal (or that the signal to be transmitted conforms to the protocol adopted for the transmission system).

In order to feed the electrical signal to an LTU for transmission, the LTU is provided with a conventional high quality coaxial cable connector, eg BNC or screw connector, and a similar connector is provided for outputting received signals. A group of LTUs would be coupled to the following equipment (e.g. aligner or time-switch of a switching system) by individual coaxial cables, and it is not unknown for all LTUs to be installed in the basement of an exchange building, and large bundles of coaxial cable routed to a plurality of switching systems in the building. It can thus be seen that there is great expense in such an arrangement, particularly where one or more switching systems in the building are a long way from the basement, perhaps many floors away.

It is an object of the present invention to provide a system which avoids such expensive cabling.

According to the present invention there is provided a telecommunications switching system for routing signals received from a plurality of incoming highways to a plurality of outgoing highways, in which system,switching elements are mounted on slide in units which in turn are mounted in a shelf having a backplane communications bus, and wherein there are provided for the highways respective line termination units mounted on slide in units in the or a shelf of the switching system.

The line termination units may interface directly with the communications bus.

Preferably at least one board has a plurality of line termination units mounted on it.

A telecommunications switching system in accordance with the present invention will now be described by way of example with reference to the drawings in which:- Figure 1 shows part of a system constructed with proprietary Line Termination Units; and Figure 2 shows the same part employing the present invention.

In Figure 1 a plurality of optical fibre transmission lines 10-1 to 10-N are directly terminated in a correspondin3 plurality of Line Termination Units (LTUs) 12-1 to 12-N, each of which has an electrical signal output terminal 14-1 to 14-N which is connected by means of a plug and socket coupling 16-1 to 16-N to a respective coaxial cable 18-1 to 18-N.

A large exchange 20, constituting a switching apparatus of the present invention, has a plurality of ports 22-1 to 22-N, each of which is connected to its respective coaxial cable 18 by means of a similar plug and socket coupling 24-1 to 24-N.

As is well known in the art, the lines 10 may carry transmissions at 2.048 Mbits/second, i.e. 32 channel PCM or 2M highway; in which case there is a one-to-one correspondence of line 10 to cable 18. However; it is known to use multiplexers (not shown) to multiplex x 2M highways onto a single line 10 (where x can be four, sixteen or sixty-four). In this case fewer lines 10 are used but there will always be a respective output terminal 14 for each 2M highway.

It is normal practice for optical fibre lines to an exchange building to install all LTUs in the basement.

Even if the proprietary LTUs are disposed close to the exchange there will be a large number of couplings; and a large amount of coaxial cable between the LTUs and the exchange, and both such items are not cheap. Furthermore, in some tall buildings another exchange may be installed in an upper floor, and then each cable 18 has to run from the basement to that upper floor with consequent extra expense of the coaxial cable.

It will be understood that a single 2M highway comprises a transmission path in each direction, and that the exchange 20 will have an equal number of output ports and input ports. Thus half of the LTUs will comprise a laser (not shown) for launching the optical signal into the fibre, and the other half will comprise an optical receiver (not shown). Conveniently a transmitter LTU may be associated with a receiver LTU in a composite construction corresponding to the two fibres of a 2M highway.

In Figure 2 the exchange 20 includes LTUs 30; similar to LTUs 12; mounted on printed circuit boards of slide in units 31 of the shelf system (not shown) in which the exchange 20 has been constructed. The slide in units 31 comprise conventional edge connectors 32 with which they connect to the shelf system backplane 34 and gain direct access to the communication bus of the exchange 20.

Not only does this arrangement omit the couplings 16 and 24 and the coaxial cable 18; but also the receiver circuitry of the LTU 30 is of simpler design than that of LTU 12 because it is interfacing directly with the communication bus in the backplane 34 instead of being required to drive a 75 ohms or 50 ohms coaxial cable; and the exchange 20 does not need to terminate such cables to match correctly their characteristic impedance and avoid reflections.

For economy of construction several LTUs are disposed on one board.

In the case mentioned above of the exchange on an upper floor of a tall building, the respective optical fibre lines 10 will come into the building in the usual manner and will be run to that upper floor, e.g. via the service ducts and be terminated by LTUs 30 in that exchange.

Whereas the above description relates to optical fibre transmission lines 10, it will be appreciated that the same problem exists with terminating e.g. coaxial cable transmission lines (highways) and the invention is equally applicable to such lines, the LTU 30 being designed to terminate such a transmission line.

As mentioned above, where several highways are multipexed then multiplexers and demultiplexers will be employed, and the present invention further envisages that such circuitry can be incorporated in the LTU function.

In other words, the LTU 30 will comprise the appropriate number of exchange-side inputs and outputs depending upon the level of multiplexing.

The invention has been described above in relation to an exchange utilizing switching of electrical signals. It will be appreciated that the invention is equally applicable to an exchange which utilizes switching of optical signals; and that communications between the slide in units by means of the backplane bus can be electrical or optical but the invention is not limited to any particular method of backplane communication.

Claims

1. A telecommunications switching system for routing signals received from a plurality of incoming highways to a plurality of outgoing highways, in which system, switching elements are mounted on slide in units which in turn are mounted in a shelf having a backplane communications bus, and wherein there are provided for the highways respective line termination units mounted on slide in Limits in the or a shelf of the switching system.

2. A system as claimed in claim 1 wherein each line termination unit interfaces directly with the communications bus.

3. A system as claimed in claim 1 or claim 2 wherein each line termination unit is mounted on a respective slide in unit.

4. A system as claimed in claim 1 or claim 2 wherein at least one slide in unit has a plurality of line termination units mounted thereon.

5. A system as claimed in any one of claims 1 to 4 for use with optical highways, wherein the line termination units comprise optical receivers anc transmitters.

6. A system as claimed in any one of the preceding claims wherein at least one pair of highways comprise a plurality of lower order highways and the respective line termination units comprise a respective multiplexer and demultiplexer.

7. A telecommunications switching system substantially as described herein with reference to the figure 2 of the drawing.