GB2115649A - Improvements in or relating to a telecommunications network - Google Patents

Abstract

A network for carrying digital traffic includes a network switch (8, 18, 20) capable of switching all the traffic on an incoming line to a selected outgoing line in order to define a network configuration according to predetermined criteria which may include the evaluation of monitored network parameters, e.g. transmission quality, demand on specific routes, and available exchange capacity. <IMAGE>

Description

SPECIFICATION Improvements in or relating to a telecommunications network The present invention relates to a telecommunications network for carrying digital traffic comprising a plurality of lines or other signal routes which combine to interconnect a plurality of exchanges and switching centres.

A network of this type is suitable as the trunk network of a telephone system. The network may carry both intrinsically digital signals veg data or analogue signals which have been converted into digital form. In order to provide an acceptable level of service it will be necessary to provide, with a digital network, as has been the case with existing analogue networks, a degree of network planning to ensure that there are sufficient routes between exchanges and switching centres in order to cope with the demand. Hitherto this planning has always taken the form of long term planning with new exchanges, switching centres and lines being installed at intervals as demand increases. With such an arrangement it is probable that there will either be under or over capacity and only in a small number of situations will the most efficient use be being made of the available equipment.

In some cases manual patch panels have been provided to allow for rearrangement of certain circuits in a network, but such a panel cannot cope with a large number of circuit rearrangements and of necessity requires some interruption of traffic when its state is changed.

The network of the present invention is characterised in that it includes at least one automatically operable network switch which defines a junction of at least three routes and whereat some or all of the traffic on an incoming route may be switched to a selected out-going route, the network further comprising means for monitoring at least one netwcrk parameter and controlling the setting of the network switch(as) in order to define a network configuration according to predetermined criteria.

A network of the abovedefined type has the versatility to respond rapidly to varying traffic demands, to make the most efficient use of exchange equipment, to provide wideband circuits on demand and for short durations, to provide rapid restoration of service when a fault occurs, and to allow rearrangement of circuits to accommodate planned engineering work. In cases where the signal routes are provided by satellites it is particularly important to make the most efficient use of the available circuits as these are expensive to provide and are accordingly in short supply.

As the transmission rate of digital information along the lines is usually high, for example, 2, 8, 34 or 140 Mbits/s, these network switches are referred to as wideband switches.

Preferably the network parameter monitored is selected from the following group: the amount of traffic between given pairs of exchanges or switching centres, the quality of transmission on a given route, the capacity available at a given exchange or switching centre, the transmission rate on a route and the charge rate applicable to a given route.

One or all of these parameters of the network is or are evaluated by a control centre which is provided with information on the settings of the network switches as well as data on the routes and exchanges and switching centres. It is desirable to have routes separate from the network itself along which the monitored information is returned to the control centre in order to avoid a loss of effective control if a part of the network fails. The control centre which may include a microprocessor can have the criteria by which it evaluates the monitored information and other data modified from time to time in accordance with the objects of the network.

Accordingly the system may be made extremely versatile and adaptable to changing situations both of the technical state of the network and the commercial objects of the network provider.

In order that the invention may be well understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation showing the use of network switches at various levels of a network; Figure 2 is a diagram showing the basic structure of a network switch in sftu; Figure 3 shows more detail of the security structure for the switch of Figure 2; and Figure 4 shows a use of a network switch for switching onto protection circuits.

A network of routes capable of carrying digital traffic is provided between selected exchanges and switching centres. The network size may range from a trunk network system for carrying voice traffic converted to digital format over a large area to a single set of routes between at least two switching centres. An example of a network comprising relatively few routes where the application of a network switch gives significant improvements in service is a set of international routes provided by satellite or undersea cable circuits or both.

In the portion of a network shown in Figure 1 an exchange 1 has a plurality of outgoing lines 2, 3, 4, 5, 6, 7 along which digital traffic is routed at a transmission rate of 2 Mbit/s. Each line may either be carrying a number of voice conversations coded into digital form or it may be wholly or partially used for providing a data link eg a video signal may occupy the whole 2 Mbitis bandwidth.

The lines 5, 6 and 7 are input lines to a network switch 8 which has three outgoing lines 9, 10, 11.

In this example the network switch is being used to switch traffic into selected transmission rate circuits. However it is possible for the network switch to be used to select which of a number of possible geographical routes the traffic takes between the desired exchanges. Moreover the switch may fulfil a combination of these two forms of selection.

At the network switch 8, some or all of the traffic on any incoming line may be entirely switched onto a selected one of the outgoing lines 9, 10, 11. For example if the line 7 from the exchange is carrying a video signal that it is required to transmit without multiplexing with other traffic this can be switched to outgoing line 9. Whilst in many cases the switch 8 will be used to transfer the whole of the traffic on an incoming route onto the selected outgoing route, where the incoming traffic consists of a number of separate signals multiplexed together it is possible to arrange for some of the traffic to be diverted to different outgoing routes. For example where time division multiplexing is used the signals occupying specified time slots may take one route with the remainder being diverted to another route.The vacant time slots on the outgoing route may have signals diverted into them from other incoming routes. Similarly in cases where frequency division multiplexing is used the outgoing route may vary according to the carrier frequency of the signal concerned. The setting of the network switch 8 is controlled by a control centre which in this case is located at the exchange 1. A dedicated system 1 2 of routes carries control signals from the control centre to the individual switches in the network.

This system of routes 12 may also return information to the control centre from monitoring means (not shown) for indicating various network parameters as discussed later.

In Figure 1 the lines 10 and 11 from switch 8 together with 5 exchange lines 3 and 4, which are all carrying traffic at 2 Mbit/s, provide the inputs to a multiplexer which transmits combined traffic at a rate of 8 Mbit/s along output lines 14, 15, 16, 1 7. Another network switch 1 8 controlled from the exchange 1 via a control system route 1 2 is located in the 8 Mbit/s level of the network. The input and output routes from switch 1 8 all carry traffic at 8 Mbit/s. A further multiplexer 19 to merge traffic into a 34 Mbit/s stream is also provided and a network switch 20 is located at this level.

The control system of routes 12 conveys not only the signals to govern the settings of the switches 8, 18, 20 but also returns information to the control centre on the state of the switches and on at least one network parameter. The type of network parameters which may be monitored includes the demand for routes between given exchanges or switching centres, the quality of transmission on the various routes, the switching capacity at individual switching centres or exchanges, the transmission rate on a route, and the charge rate applicable to a given route, The particular network parameters which are monitored and the criteria by means of which they are evaluated at the control centre may vary according to the aims and objects of the particular network in which the switch is located.For example a network switch in a satellite network may be provided solely to put traffic onto those routes which are providing the best transmission quality at a given time. On the other hand network switches in a countrywide trunk network may be operated to give priority at certain times to the provision of wideband circuits to users or possibly to reroute traffic to take advantage of spare switching capacity elsewhere on the network to relieve pressure on exchanges and switching centres on a primary route between two locations on which there is heavy traffic. The provision of a separate route system 12 for control and monitoring purposes avoids a complete control failure if some of the network routes go out of service and moreover keeps the control centre informed of such failures.

The particular type of switch used may vary according to the nature of the routes which are to be switched, ie whether they are carried by coaxial cable, optical fibre or setellite, the number of routes connected to the network switch, and the type of digital coding and multiplexing being used on the traffic.

For optical fibre routes the network switch may be of the types described in our GB Patent Application No. 8123098, or GB Patent Application No. 2043293A or in an article entitled "Nonblocking 8 x 8 optical matrix switch for fibreoptic communication" published in Electronics Letters 22 May 1980 Vol 16 No. 11 Pages 422/3.

For coaxial cable or satellite routes crossbar switches or semiconductor switching devices may be used.

The control centre may include a computer or microprocessor which is provided with an operator terminal capable of displaying information on the state of the network and preferably of collating statistics and other data on the network parameters being monitored, and changes in the switch states. The operator terminal will also provide means whereby the criteria for evaluating the monitored network parameters may be changed. The computer is preferably provided with a printer for providing permanent records.

The following features may be provided in the control centre and its operator terminal: a. Secure password access.

b. Route set up and release.

c. Pre-programmed switch change.

d. Trace connections given the input or output system.

e. Display switch setting map.

f. Real time clock.

g. Display of alarm conditions.

h. Printer to provide permanent records.

The network switch 22 and associated control centre 23 illustrated in Figure 2 is a 256 x 256, 2 Mbit/s wideband switch for use in a satellite earth station to replace a manual patch panel. The switch may be used to provide inland re-routing capability and to rapidiy follow channelling changes within the TDMA satellite. Although this switch architecture is described with particular reference to satellite use it has many other applications in inland and other international networks. The switch of Figure 2 is adapted to interface with a High Density Bipolar digital signal.

This High Density Bipolar coding allows no more than three consecutive zeroes and is referred to as HDB-3.

Digital crosspoint devices can be used for a network switch in the following ways:- a. Convert the HDB-3 input signal to its equivalent binary form, switch in binary form and convert to HDB-3 at the output. A clock recovery circuit is required at the input interface for the code conversion. The recovered clock can then be transmitted on a parallel switched path for use at the output interface, thereby doubling the number of crosspoints. It is not possible to use a clock recovery circuit at the output interface because the binary signal does not contain a minimum defined clock content. It would be possible to incorporate a local synchronization utility in the wideband switch to provide triplicated waveforms to the input and output interfaces.This would however require an aligner to be incorporated into each input interface thereby incurring extra expense and the possibility of slip. Although slip may not be a problem for satellite systems which will be synchronized to + 1 part in 10", much higher slip rates could be experienced by users who chose not to synchronize to the network.

b. It is possible to convert the HDB-3 signal into two binary streams representing the positive and negative parts of the waveform. These two signals can then be routed in parallel through the switch. Preliminary cost studies have indicated that this technique will be more expensive than using unbalanced analogue switching, but less expensive than balanced analogue switching. This technique may be implemented by the use of an 8 x 8 digital space switch array implemented in an uncommitted logic array.

This latter approach is very attractive because the same functional design can be used at all levels in the transmission hierarchy even though different technologies would be required. For example, ISO-CMOS would be suitable for 2 Mbit/s and possibly 8 Mbit/s, low power Schottky for 34 Mbit/s and an ECL array for 140 Mbit/s. A common design would only require one functional validation and testing programme. Further savings arise from the fact that identical switch architectures and control structures can be used which will iower the development costs, reduce capital costs because of the larger numbers of common design and reduce operational costs by reducing the diversity of training and maintenance procedures.

Referring again to Figure 2 the control centre 23 provides a man-machine interface and a means of updating the switch via secured data links 24. A microprocessor 25 provides the local switch control and interfaces to the data links 24.

The local switch control will contain its own maintenance and diagnostic facilities to ensure correct switch operation and to locate faults. It will also be responsible for scanning the 2 Mbit/s inputs and performing routine checks on the alarm information in timeslot 0. Error rate monitoring (by detecting illegal HDB-3 symbol sequences) can also be performed in this way.

Figure 3 shows more detail of the switch security structure and its control. An input transformer 30 has two secondary windings which drive the duplicated switch planes (equivalent components in the two switch planes are distinguished by primes on components of one of the planes). Buffer amplifiers 34, 34' convert the HDB-3 signals into a pair of binary signals for transmission through the digital switch 22. A three stage, 31,32, 33; 31 32', 33' space switch 22 will be used, with standard IC's being used initially as crosspoint devices. Buffer amplifiers 35, 35' are used at the output of the switch to combine the two binary signals into a single HDB-3 waveform. The outputs from the buffers in each security plane are then combined in the transformer 36.Under normal operation both security planes are driving the output transformer with identical signals. Thus the loss of one signal will not cause a significant change in the line output signal. Misoperation of the switch can result in different waveforms appearing at the output of the two security planes. This is odtected by the scanning devices which allow any input to be compared with any output in each security plane. It also provides a means of interrogating the waveforms at each of the three switch stages so that a fault can be isolated down to a single card in most cases, or to one of two cards in the remaining cases.

The local switch control 25 is not duplicated, but is arranged such that failures do not affect the paths that have already been set up through the switch 22. To achieve this, each switch security plane has its own latches and a memory of its connections. This arrangement also allows a replacement card to be updated from its equivalent card in the other security plane.

The switch structure shown in Figure 4 is basically similar to that of Figures 2 and 3 but this switch 40 is intended to be used in a network for the protection of hypergroups. The switch may for this purpose be located in an unmanned coastal repeater station and permits the hypergroup traffic to be remotely re-routed onto protection circuits 42. The basic control structure for this switch is the same as for the switch of Figures 2 and 3. The switch 40 consists of a matrix of coaxial reed relays in order to provide the isolation and crosstalk performance required. Latching is preferably provided so that local switch control failures will not affect the switch matrix 40. In the example shown it is possible for any one of the seven working hypergroups 44 to be patched through onto any one of five protection circuits 42. Additionally it is possible to patch the protection circuits 42, which are not all routed to the same location, through as a tandem connection. The switch 40 is a pair of switches, one for the transmit and one for the receive direction.

For both the applications illustrated in Figures 2 to 4 the control centre 23 consists of a Monarch PABX (CDSS1) Operator's console modified to give four lines of display on a VDU 50 and to be capable of driving two modems (not shown) for the data links 24. The key functions of a keyboard 52 at the console 23 may be reassigned to suit the particular application of the network switch or switches it has to control. The console 23 also includes a microprocessor 54 and is provided with an interface for a printer 56.

Claims (5)

1. A telecommunications network for carrying digital traffic comprising a plurality of lines or other signal routes which combine to interconnect a plurality of exchanges and switching centres (1), characterised in that the network also includes at least one automatically operable network switch (8, 18, 20, 22, 40) which defines a junction of at least three routes and whereat some or all of the traffic on an incoming route may be switched to a selected outgoing route, the network further comprising means (12,23,24,25) for monitoring at least one network parameter and controlling the setting of the network switch(es) in order to define a network configuration according to predetermined criteria.

2. A network as claimed in Claim 1, characterised in that the or a network parameter monitored is selected from the following group: the amount of traffic between given pairs of exchanges or switching centres, the quality of transmission on a given route, the capacity available at a given exchange or switching centre (1), the transmission rate on a route and the charge rate applicable to a given route.

3. A network as claimed in Claim 1 or 2, characterised in that the controlling means comprises at least one control centre (23) the network including a system of routes (12, 24) independent of the network traffic routes for returning information from the monitoring means to the or an associated control centre and for controlling the switch settings.

4. A network as claimed in Claim 1,2 or 3, wherein the network further comprises multiplexers (13, 19) and lines adapted for transmission at various bit rates, all the lines at a network switch are adapted for transmission at the same rate.

5. A network as claimed in Claim 3, characterised in that network switches are located in at least two transmission rate levels of the network.