GB2224901A

GB2224901A - Optical fibre networks

Info

Publication number: GB2224901A
Application number: GB8826484A
Authority: GB
Inventors: Nigel Baker; John Stuart Fraser Lee
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-11-11
Filing date: 1988-11-11
Publication date: 1990-05-16
Also published as: GB8826484D0

Abstract

A passive optical fibre network is divided into regions (three regions as shown), respective star coupler means (3:7, 8) being associated with each region. Full interconnection cabling (fibre) is only required between the regions. The star coupler means for each region can comprise a separate coupler (7) for the transmitter nodes (T) and a separate coupler (8) for the receiver nodes (R), in which case the optical losses are nominally equal for each possible transmitter to receiver path and wavelength division multiplexing can be employed. Upon failure of one coupler or one inter-coupler fibre the remaining regions can still communicate. <IMAGE>

Description

OPTICAL FIBRE NETWORKS This invention relates to optical fibre networks and in particular to passive optical fibre networks.

Within a passive optical fibre network there are no electrical components. Electrical components may, however, be coupled to transmitter/receiver nodes of such a network. The physical interconnection topology required for a plurality of transmitter/receiver nodes such that, for example, any transmitter may communicate with any receiver can vary in dependence on the application. For applications involving wavelength division multiplexing, a topology that has as little variation in path loss between any individual optical transmitter and receiver as possible is necessary. This requirement is fulfilled by use of a single star coupler to which each transmitter and receiver are directly coupled.A signal transmitted by any one transmitter to the star coupler will be supplied to each receiver coupled to the star coupler and since only one coupler is involved the optical losses are nominally equal for each possible transmitter to receiver path and there is sufficient similarity between received signal levels for wavelength division multiplexing. The use of single star coupler topology has two basic disadvantages. Firstly, as the number of communicating nodes increases, the cabling becomes increasingly unyieldy and difficult to implement, and secondly failure of the coupler means failure of the whole system and no communication between any of the transmitters and receivers.

According to the present invention there is provided an optical fibre network whereby any one of a plurality of transmitter nodes can be interconnected to any one of a plurality of receiver nodes, the network comprising n network regions (n > 2) each including respective numbers of said pluralities of transmitter and receiver nodes, and comprising respective star coupler means associated with each region and optical fibre means interconnecting each star coupler means to the other star coupler means.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a known interconnection topology of a plurality of transmitters and receivers within a designated area in a passive optical fibre network using a single star coupler; Fig. 2 illustrates a passive optical fibre network for coupling transmitters and receivers within the designated area, the area being divided into three regions with a star coupler associated with each region, and Fig. 3 illustrates a passive optical fibre network for coupling transmitters and receivers disposed in three regions of a designated area, two star couplers being associated with each region.

Fig. 1 shows the conventional single star coupler approach. Each transmitter node T and receiver node R within an area A is coupled to a single multiport transmissive star coupler 1 via a respective length of optical fibre 2 which may be single or multimode. The coupler may be either single or multimode.

To overcome the problem of total failure of the system resulting from failure of such a single star coupler, the area A is subdivided into three, as illustrated, regions B. Associated with each region B is a respective multiport transmissive star coupler 3.

The nodes disposed within any particular region B are coupled to the respective star coupler 3. The regions corresponding to node geographical concentration. Such an arrangement facilitates cabling and in the event of failure of the coupler 3 means that the nodes connected to the remaining couplers can still communicate.

However, since the losses involved in coupling any one transmitter node to any receiver node are not the same, there is not enough similarity in received signal levels for wavelength division multiplexing. A signal transmitted by transmitter node 4 is subject to loss in one coupler 3 if receiver node 5 is involved, but is subject to loss in two couplers 3 if receiver node 6 is involved.

The network arrangement illustrated in Fig. 3 also involves three regions but the losses involved are nominally equal for each possible transmitter node to receiver node path. This is achieved by having two transmissive star couplers 7 and 8 at each region B.

Each of the communicating nodes is connected directly to a coupler. The transmitting nodes T are connected to a coupler 7 and the receiving nodes are connected to a coupler 8. Within each region B the couplers 7 and 8 are connected via an optical fibre 9, between regions B each transmitting node coupler 7 is connected to each other receiver node coupler 8 via respective optical fibres 10. The pairs of couplers 7, 8 serving each region B are connected to the pairs of couplers in each other region via respective pairs of optical fibres 10 as illustrated although if required more than two such optical fibres 10 may be employed. The couplers 7 and 8 can be conventional multiport star couplers.

As a result of using two transmissive star couplers at each region the optical losses are nominally equal for each possible transmitter to receiver path since two couplers are involved in each interconnection path within a region or between separate regions. In comparison with the arrangement of Fig. 1 using a single central star coupler, in Figs. 2 and 3 full interconnection cabling is only required between each of the regions rather than from each node to a central point.

With the arrangement of Fig. 3 failure of one coupler of a pair means that the other coupler can still transmit to or receive from.the other regions so that communication with the region including the failed coupler is not totally lost. In the event of failure at either coupler or one intercoupler fibre 10 the remaining regions can still communicate.

The arrangement of Fig. 3 is a fibre optic interconnection topology which fulfills the requirement for low path loss variation while allowing the nodes to be interconnected by means of only a few fibres, the interconnected regions corresponding to the node geographical concentration for the particular application. Minimum cabling is required between the regions. The arrangements of both Figs. 2 and 3 may be considered to comprise three-region distributed central star topology but only Fig. 3 involves nominally equal optical losses for each possible transmitter to receiver path.

The arrangement of Fig. 3 involves three regions although a two region arrangement can be envisaged as can arrangements with more than three regions. With four or more regions, however, the interconnections become increasingly complex. A three region arrangement is considered to be the optimum.

Claims

CLAIMS:

1. An optical fibre network whereby any one of a plurality of transmitter nodes can be interconnected to any one of a plurality of receiver nodes1 the network comprising n network regions (n ss 2) each including respective numbers of said pluralities of transmitter and receiver nodes, and comprising respective star coupler means associated with each region and optical fibre means interconnecting each star coupler means to the other star coupler means.

2. An optical fibre network as claimed in claim 1, wherein the star coupler means comprise first and second star couplers for each region, the respective transmitter nodes being connected to the first star coupler and the respective receiver nodes being connected to the second star coupler, and comprising respective optical fibre means interconnecting the first and second star couplers in each region, each first coupler being connected to the second couplers associated with the other regions by said optical fibre means.

3. An optical fibre network as claimed in claim 2 and comprising three network regions

4. An optical fibre network as claimed in claim 1 and wherein in each transmission path between any one said transmitter node and any one said receiver node there are two star couplers whereby for all possible transmission paths the optical losses are nominally equal and wavelength division multiplexing can be employed.

5. An optical fibre network as claimed in claim 1 wherein each star coupler means comprises a respective single star coupler to which said respective numbers of said pluralities of transmitter and receiver nodes for each region are connected.

6. An optical fibre network substantially as herein described with respect to Fig. 2 or Fig. 3 of the accompanying drawings.