GB2199209A

GB2199209A - Optical communication systems

Info

Publication number: GB2199209A
Application number: GB8630281A
Authority: GB
Inventors: David Philip Martin Chown; Isobel Jennifer Goddard
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-12-18
Filing date: 1986-12-18
Publication date: 1988-06-29
Anticipated expiration: 2006-12-18
Also published as: GB8630281D0; GB2199209B

Abstract

An optical local area network consists of a star-type arrangement of optical couplers starting from a single central coupler. Each of the couplers is a passive device, with all switching, control and amplification at the nodes. The terminals served are connected to the nodes, each of which may serve more than one terminal. Couplers, other than the central one may handle only incoming links or only outgoing links (Fig 2 not shown). <IMAGE>

Description

OPTICAL COMMUNICATION SYSTEMS The present invention relates to optical communication systems of the local area network type.

One of the simplest forms of communications network is of the star-type, and in fact a typical telephone exchange is such a network. However, when one attempts to extend this principle to an optical system with a single star coupler at the centre, difficulties arise because the arrangement is complex to connect to its users. Further it usually uses more transmission medium than does a simple ring arrangement. Hence it has been proposed that a hierarchical array of star couplers could be used.

As an example of a known optical fibre local area network using star couplers, we refer to European Patent Application No. 0179550A2 (TRW Inc.) which describes such networks. One of these networks is a hierarchical array with a pair of central star couplers which interconnects a number of other star couplers each of which serves a plurality of system nodes. Each of the couplers in this system is a controlled star coupler which includes both the star coupler itself and a control box therefor. Hence control of the network is distributed among the star couplers.

Such an arrangement requires the star coupler units each to be complex and expensive, and it is an object of the present invention to provide a system of this general type in which the disadvantages of complexity and expense are at least minimised.

According to the present invention there is provided an optical communication system, which includes a hierarchical array of interconnected optical couplers the innermost layer of which is a single optical coupler and the outermost layer of which includes a number of optical couplers each of which gives access to a plurality of nodes of the- system, wherein said optical couplers are also passive couplers and all switching, control, and amplification functions are performed at the system nodes.

The feature which distinguishes this arrangement from the prior art referred to above is that the star couplers used are passive, i.e. relatively simple and inexpensive, whereas the switching, control; and amplification functions are concentrated at the nodes.

Some of these functions must in any case be provided at the nodes, so that the extra functions called for by the use of our arrangement are relatively simple. Further, with our arrangement it is possible for the system to be initially installed with less than the maximum number of nodes, in which case expansion to the maximum number of nodes is easy, and does not disturb existing connections.

It will be appreciated that the limitation on our arrangement is fixed by the levels of attenuation introduced by the couplers and the interconnecting transmission media. With the present couplers and transmission media, three layers of couplers can be comfortably provided, and in the future a larger number of such layers should be possible.

Two embodiments of the invention will now be described with reference to the accompanying highly schematic Figs. 1 and 2.

Both of the arrangements to be described herein exploit hierarchical toplogies, but for convenience we refer to the arrangement of Fig. 1 as a hierarchical star arrangement and that of Fig. 2 as a distributed star arrangement.

The hierarchical star arrangement, Fig. 1, has a single central star coupler 1 with each of its four ports made up of an input optical link and an output optical link. Each such port gives access to a further coupler such as 2, and those further couplers are, like the coupler 1, qf the so-called 4 x 4 type.

Each of the couplers such as 2 has its three ports not connected to the central coupler 1 connected to yet a further similar coupler such as 3. Each of the couplers such as 3 has its other three ports connected to a node N.

Thus we have a total of seventeen optical couplers via which thirty-six system nodes are interconnected. Such a topology is relatively simple to install since the central coupler only has to have eight fibres connected to it, and yet it serves the relatively large number of thirty-six nodes. This is also convenient as it suits building and site layouts better than does a single star.

The couplers used in the system of Fig. 1 are simple couplers with no switching, control and amplification functions; i.e. they are purely passive.

All the switching, control and amplification functions are concentrated at the system nodes. Since these have in any case to be "intelligent", to use contemporary terminology, it is relatively cheap for them to execute the various functions referred to above. These functions will, of course, operate in accordcance with the system protocol adopted.

It should be noted that each of the nodes of the system might well serve two or more system terminals.

In installing the system it must be borne in mind that a lower path loss exists between nodes on th same coupler then between nodes on different couplers.

Thus a large dynamic range is required of the node receivers, and any cross-talk due to a close node on a connection from a distant node will be more of a nuisance crosstalk due to a distant node on a channel from a close node. Interference may in some cases occur due to the fact that light may pass between two nodes over two or more different routes. Careful design of the receiver should, however, enable the effects of such interference to be overcome.

We now consider the so-called distributed star topology, Fig. 2, which is also of a hierarchical type.

This requires a much reduced dynamic range for its receivers compared with the arrangement of Fig. 1 since it behaves like a single (larger) central coupler in certain ways. As compared with the arrangement of Fig.

1, each of the 4 x 4 star couplers except the central one is replaced by two (N-l) x 1 couplers, one for traffic collection and one for traffic distribution. Each of these couplers has its (N-l) links connected to the next layer out and its 1 link connected to the adjacent inner layer coupler.

With this arrangement light travelling between any pair of nodes is routed via the central coupler, so that light travelling between any two nodes is subjected to the same, relatively high, path loss. For the examples shown, the hierarchical star topology has 5dB more path loss than the distributed star for the same number of nodes served.

In the arrangements described, N=4, but other sizes of couplers can be used. Further, couplers in which N is relatively large can be assembled in known manner from a number of smaller couplers.

Claims

1. An optical communication -system, which includes a hierarchical array of interconnected optical couplers the innermost layer of which is a single optical coupler and the outermost layer of which includes a number of optical couplers each of which gives access to a plurality of nodes of the system, wherein said optical couplers are also passive couplers and all switching, control, and amplification functions are performed at the system nodes.

2. A system as claimed in claim 1, in which each said optical coupler has a number N of ports each formed by an input link and an output link.

3. A system as claimed in claim 1, in which the single central optical coupler has N ports each formed by an input link and an output link, and in which each other coupler has N single link ports so that separate incoming and outgoing links and couplers are used for each connection to be set up.

4. A system as claimed in claim 1, 2 or 3, and in which N=4.

5. An optical communication system, substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawing.

Amendments to the claims have been filed as follows

6. An optical communication system, which includes a hierarchical array of interconnected optical couplers the innermost layer of which is a single optical coupler and the outermost layer of which includes a number of optical couplers each of which can give access to a plurality of nodes of the system, wherein the optical couplers are interconnected by optical fIbres without optical repeaters, wherein the system includes at least three layers including said innermost layer, and wherein said optical couplers are passive couplers and all switching, control, and awpliflcation functions are performed at the system nodes.

7. An optical communication system, which includes a hierarchical array of interconnected optical couplers the innermost layer of which is a single optical coupler ano the outermost layer of which includes s number of optical couplers each of which can give access to a plurality of nodes of the system, wherein the optical couplers are interconnected by optical fires without optical repeaters, wherein said single optical coupler forrIng sai innermost layer has a number of ports each formed by an input link and an output link which ports each extend to an optical coupler of the next layer of the hierarchy, wherein within each said optical coupler the ports thereof are interconnected in such 2 way that an optical signal inputted to an':: one of that coupler's input ports is outputted from any one of that couplers output ports wherein the optical coupler other than the single coupler which forms said innermost layer each has M single-link ports with half of those couplers having 1 input link and (M-1) output links an the other half having (M-1) input links and one output link, such that all connections are set up via all layers of the hierarchy including the innermost layer, such that every nooe in the system has access to all said nodes including itself, and wherein al@ of the optical couplers are passive couplers and all switching, control, n and amclication functions are performed at te system notes.