GB2224902A - Optical communication system - Google Patents

Abstract

In an optical fibre LAN each node is coupled to a central star-coupler (SC) via optical fibres (F1, F2) so that the system is a star network. However, each node in use has its receiver tuned to light of the wavelength sent out by the preceding node. Hence the system works like a ring. To minimize risk due to failure of the star-coupler assembly, two such assemblies each separately coupled to the nodes can be used, (Fig 4 not shown). Each star-coupler assembly is built up from 2x2 couplers. <IMAGE>

Description

OPTICAL COMMUNICATION SYSTEM The present invention relates to optical transmission systems of the Local area network (LAN) type.

According to one aspect of the present invention there is provided an optical local area network transmission system having a star topology and adapted to behave either as a ring or a bus in dependence on the employed protocol.

According to another aspect of the present invention there is provided an optical transmission system, which includes as a central element a star-coupler assembly and a number of nodes each coupled to the star-coupler assembly by one or more optical fibres, wherein the central element is a passive star-coupler assembly, wherein each said node has its receiver tuned to the wavelength of light transmitted from a different one of the nodes, wherein when a said node receives a message it checks to see if the message is for that node, wherein if the message is for that node the node accepts the message, and wherein if the message is not for that node the node's transmitter sends the message out modulated on to light at that node's transmitting wavelength, whereby the system, although of star topography, behaves in a manner analagous to a ring-type system.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of an optical multichannel network to which the invention is applicable; Fig. 2 is a schematic representation of an embodiment of the invention usable as the passive fibre network shown in Fig. 1; Fig. 3 is a schematic representation of a 4x4 optical star coupler usable in the system of Fig. 1; and Fig. 4 is a further embodiment of the invention, also usable as the passive fibre network of Fig. 1.

The present invention is basically concerned with producing a system such as a LAN in which a network with a star topology can be made to behave as if it is a ring or a bus by using a protocol suitable for a ring or a bus, respectively, as will be apparent from the following.

Such a system may be important in situations where a network is becoming fully loaded and has very little spare capacity so that a higher capacity network is being considered. In order that the already installed equipment is not to become obsolete, the new system must be able to carry more traffic and, at the same time, support the old protocols and equipment. An example of a standard which is under discussion for optical fibre LAN's is the FDDI (Fibre Distributed Data Interface) standard. FDDI was a token-passing protocol on a ring of point-to-point links, and runs at a high bit rate, such as 100 Mb/sec. There is one token circulating and any station can capture the token, which effectively gives it permission to use the ring for transmission. When the token is seized by a station, the station passes it on with a message attached to the front of it.

The message travels to the next station which checks to see if the message is for that station. If it is for that station it reads it and passes it on, while if the message is not for that station it is passed on to the next station. This continues until the message and token reach the originating station where the message is removed from the ring. The originating station then returns the token to the ring so that it is free for use by another.

As a background to the present invention, we now consider the optical multichannel network shown schematically in Fig. 1. This has a passive optical fibre network at the "centre" of the network, to which are connected a telecommunications exchange and a number of subscriber stations. The drawing includes a representation of the channel plan used in the network; it includes wavelength-division multiplex (WDM) channels, a signalling/reference channel and coherent optical frequency division multiplex (OFDM) channels.

In such a network, the optical fibre network which interconnects the various nodes is likely to be star-based.

In the drawing, some of the data sources and "sinks" are identified as WDM, in which case the wavelengths used are indicated by 1' 2' etc, while others are identified as OFDM, indicated by the frequencies f7, f8, etc used.

Thus in the network of Fig. 1 each node is allocated a fixed optical frequency determined by its laser transmitter. The receiver includes either an optical demultiplexer or a filter to give access to a single fixed channel, to tune to any one channel, or to access a number of channels simultaneously. Each node transmits on to the network, which distributes each channel to all of the nodes, whereat the receivers each select the channel or channels required.

Use of a totally passive interconnect based on single mode fibre and couplers minimises the maintenance requirements and allows the network to be used flexibly, for example as indicated.

We now turn to Fig. 2, which shows schematically one embodiment of the invention which can be used on the passive fibre network of Fig. 1. This includes eight nodes, designated node 1 to node 8, each connected by two optical fibres such as F1 and F2 for node 1 to a central passive star coupler SC. Of each such pair, one is used to convey light from star-coupler to node,and the other to convey light from node to star-coupler. These fibre links use single-mode fibre, and the star-coupler is assembled from a number of simple 2x2 couplers interconnected in the manner shown in Fg 3.

The network shown in Fig. 2 is set up to behave like a ring by tuning the receiver of each node to the transmitter of a different node in the array, which latter node thus behaves as if it was the previous node in a ring. In Fig. 2 it will be seen that nodes 4, 6 and 8 are not currently in use, the 'quasi ring network' actually including nodes 1, 2, 3, 5 and 7. Thus node 1 is set to receive wavelength > 7 (from node 7) and 7 transmit on wavelength > 1 node 2 receives on A (from node 1) and emits light on wavelength ) 2 and so on round the system.

Thus it will be seen that the arrangement behaves as if it were a ring, and it does not have to include all of the nodes. It can be dynamically reconfigured when necessary. The rest of the network, i.e. nodes 4, 6 and 8 in this case, can be carrying other traffic at the same time as the "quasi-ring" formed by nodes 1, 2, -3, 5 and 7.

If one of the nodes in the network fails, this is detected by the next node, which retunes its receiver to the wavelength of the node before the one which has failed. There is no limit to the number of simultaneous failures in a network such as shown in Fg 2, which is not the case with a dual ring system which relies on loop-back to minimise the impact of failure.

Fig. 3, which is believed to be self-explanatory, shows four 2x2 star-couplers SC1, SC2, SC3, SC4, interconnected so as to form a 4x4 coupler.

Similar methods of interconnections of individual star-couplers can be used to produce larger arrays, such as an 8x8 coupler for use in the system of Fig. 2.

In a system such as that of Fig. 2, there is a small risk of catastrophic failure caused by a fault in the central star-coupler. This is minimised by the use of a so-called distributed star-coupler such as that shown in Fig. 3, since the failure of one of the 2x2 couplers, or a fault in one of the fibres (cables) which interconnect those couplers, would not put the whole system out of use. An alternative way to deal with this problem is to use the system shown in Fig. 4, where we see two stars each with its own central coupler. Thus the two central couplers CC1, CC2 are each connected by its own pairs of fibres to the nodes.

In arrangements such as described above, all the nodes can be transmitting at once, so there is no need to allocate specific time slots to each node. The only control needed is a method of signalling to a node (or nodes) when another node wishes to "talk" to it (or them). This can be done by having a WDM channel received by all nodes at all times, which is used for signalling.

The standard protocol referred to above, and known as FDDI is usable is many different applications.

Thus an interconnection medium conforming to most of the FDDI protocol would be useful.

It confers 100 mBit/sec capacity which may be shared among the network users. Optical multichannel techniques give the operator further capacity and flexibility, and a single mode star network provides future proofing by allowing the use of many independent simultaneous 100 Mbit/sec (or more) channels without needing further cable installations. Further, with the forms of star-configured single-mode network described above the operator can use as much, or as little, of the network as is desired. Thus we show a way in which such a network can be used with a standard protocol economically and yet provide higher capacity services as and when required.

The FDDI standard referred to above relates to just one possibility in terms of protocol and speed and the invention is not to be considered as restricted thereto. As indicated above in one example there is shown the making a ring of point to point links using wavelength division multiplexing on a star topology network, and there is shown how station (node) or fibre (cable) failure are detected and how the resilience of the network to such failures is maintained.

Claims (8)

1. An optical local area network transmission system having a star topology and adapted to behave either as a ring or a bus in dependence on the employed protocol.

2. An optical transmission system, which includes as a central element a star-coupler assembly, and a number of nodes each coupled to the star-coupler assembly by one or more optical fibres, wherein the central element is a passive star-coupler assembly, wherein each said node has its receiver tuned to the wavelength of light transmitted from a different one of the nodes, wherein when a said node receives a message it checks to see if the message is for that node, wherein if the message is for that node the node accepts the message, and wherein if the message is not for that node the node's transmitter sends the message out modulated on to light at that node's transmitting wavelength, whereby the system, although a star topography, behaves in a manner analagous to a ring-type system.

3. A system as claimed in claim 2, wherein the optical fibres are of the single-mode type, there being separate fibres for the transmission of light from node to coupler and from coupler to node.

4. A system as claimed in claim 2 or 3, and wherein two central star-coupler assemblies are provided, each separately coupled to the nodes.

5. A system as claimed in claim 2, 3 or 4, wherein to set the system up in respect of the wavelengths used signalling messages are sent over the fibres to the nodes to be used.

6. A system as claimed in claim 2, 3, 4 or 5, wherein the star-coupler assembly consists of a number of 2x2 couplers interconnected in the manner indicated in Fig. 4.

7. A system as claimed in any one of claims 2 to 6 wherein if failure of a node or optical fibre cable section occurs, the node following the failed node or section returnes to the transmitted wavelength of the node before the failure.

8. An optical transmission system substantially as herein discribed with reference to the accompanying drawings.