GB2173977A

GB2173977A - Local area network

Info

Publication number: GB2173977A
Application number: GB08510162A
Authority: GB
Inventors: Timothy Richard Allen; Stephen Mark Illman; Martin Sharpe
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-04-20
Filing date: 1985-04-20
Publication date: 1986-10-22
Also published as: GB2173977B

Abstract

In a local area network for use in high system integrity circumstances, e.g. for ship-board use, there is a number of nodes 1,2,...16 each serving a number of subscribers' lines, and each connected via respective links to a number, three in the present case, of other nodes. When the system is to be used, one node, e.g. the lowest- numbered node, is selected to be the master node. The system then configures itself under software control as a mesh in which every other node is connected to the master node, in each case using the minimum number of links. This system is a damage-tolerent, and any node can in fact function as a master node. If nodes or links fail the system reconfigures to take account of the failures, and in extreme cases of damage may reconfigure itself into two or more "sub-systems" each with its own master node. In the system shown there are two separate loops, nodes 1,6,4,8,2,15 and nodes,7,9,10,11,14,3,5,16,12, 13 with interloop links and a couple of bypass links on the larger loops. <IMAGE>

Description

SPECIFICATION Local area network The present invention relates to a telecommunication system of the local area network (LAN) type, and especially to such a system in which a high degree of security of communication between the various subscribers is needed.

One example of a circumstance in which a high degree of security is needed is the internal communication system of a ship, especially a warship. Here the system includes a number of nodes, each of which serves more than one subscriber, interconnected by links so that a LAN is set up. An object of the invention is to provide a system of the above type, in which a high degree of security is attained.

According to the invention, there is provided a telecommunication system of the local area network type, which includes system nodes each serving a plurality of subscribers' terminals and bidirectional links so interconnecting the nodes that each of said node is connected via respective ones of said links to at least three other nodes, wherein in any connection to be set up between two of said nodes, one of these nodes is the master node in respect of the connection between that pair of nodes.

According to the invention, there is also provided a telecommunication system of the local area network type, which includes system nodes each serving a plurality of subscribers' terminals and bidirectional links so interconnecting the nodes that each said node is connected via respective ones of said links to at least three other nodes, wherein the arrangement of the nodes and the links is such that the system includes two closed loops with some of the links each extending between a node of one of the loops and a node of the other of said loops, wherein there are one or more additional links each of which interconnects two non-adjacent nodes in the same one of said loops, such that each said node is connected via respective ones of said links to at least three other nodes, and wherein when a mesh of connections is to be set up between the nodes of the system the routes to be used for the connections between respective pairs of said nodes are so chosen as to minimise the number of links to be used for each connection between a pair of said nodes.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a schematic diagram representing a system embodying the invenion.

Figs. 2 to 6 are explanatory flow diagrams explanatory of various aspects of the operation of the system of Fig. 1.

Figs. 7 and 8 show possible conditions of the system of Fig. 1 when operating under damage conditions.

Glossary of Terms This glossary explains the somewhat specialised terms used in the forthcoming specific description.

NODE Intelligent unit capable of receiving, interpreting and transmitting signalling information to other nodes via two or more LINKS.

LINK A physical corinection between two NODES capable of carrying messages in either direction.

MESH A netword of NODES interconnected by LINKS.

HARDWARE NUMBER A unique arbitrary number assigned to each NODE in the MESH.

SOFTWARENUMBERA number held by each NODE comprising the NODE NUMBER and the LINK NUMBER for the node. - NODE NUMBER The HARDWARE NUMBER of the NODE considered to be the master.

LINK NUMBER The number of links between the NODE and the MASTER NODE.

PRIMARY LINK The LINK on each NODE which leads to the next level up in the NETWORK.

NETWORK The configured connection which links each NODE to the MASTER NODE by the most efficient method.

MNSA Master Node Set-up Attempt. A message containing the SOFTWARE NUMBER of a NODE.

MNSA is sent to an adjacent NODE in order to determine which is nearest to the MASTER NODE.

CHANGE The message used to trigger the generation of MNSA messages throughout the MESH.

MASTER NODE The NODE to which all others are connected. Potentially all NODES may become the Master node. Selection is based on a simle decision such as the NODE with the lowest HARDWARE NUMBER.

In Figs. 1,7 and 8, the rings with numbers inside them each represent one of the nodes in the network, the number being the node's hardware number Each double line between two nodes with an arrow-head appended represents a primary link of the network, while each single line between two nodes represents a link not functioning as a primary link. As will be seen, links of the network may or may not at any time be primary links.

In use of the system at any one time, the lowest numbered node is regarded as a master node; thus in Fig. 1, node 1 is the master node. The rectangles appended to the node rings each convey further information about that node in the condition of the network shown. In each casb the upper left portion of the rectangle contains the node number for the current master node, while the other number indicates a link number appropriate to that connection. Note that iri Fig. 7, the master node is node 2, while in Fig. 8 two connections are in existence, with nodes 1 and 3 the master nodes.

Note that in all of these cases the master node is the one with the lowest hardware number.

In the system described herein, see Fig. 1, there is a hierarchy of nodes in a mesh such that each node is connectable to the current master node via the smallest possible number of connecting links. This is achieved in the system of Fig. 1 by providing two loops of nodes, an outer loop consisting of nodes 5, 3,14,11,10, 9, 7,13,12 and 16, and an inner loop consisting of nodes 1,6,4,8,2 and 15. Each node is connected via respective links to three other nodes, usually to two nodes in its own loop and one in the other loop. Two exceptions to this exist in Fig. 1, due to the fact that the loops do not have the same number of nodes. These exceptions are indicated by the links between nodes 13 and 16, and between links 10 and 14. It will also be seen that these four nodes do not have direct connections to the inner loop.

Although we have described the invention in its application to a system using two loops, it is also applicable to systems using other configurations.

The nodes are intelligent devices, in that each of them includes its own data processing device, for instance a microprocessor. The algorithms used are such as to define a hierarchy of nodes in a mesh, such that each node is connected to a master node via the smallest number of interconnecting links.

Such an algorithm assumes that any one of the nodes can function as a master node, and selection is based on the lowest hardware number which is connected to the mesh. Note that this selection is arbitrary, and could be easily be based on the node with the highest hardware number. The fact that each node is usuable as a master node is important, especially for operations under damage conditions.

When the network has been established, each node has knowledge of the hardware number of the master node, the number of links between itself and the master node, and which links leaves the node towards the master node. Thus the links thus specified are the primary links. In the condition shown in Fig. 1 a mesh has been set up based on three links per node with an established network.

The controlling algorithm needs no knowledge of the physical topology of the mesh nor is it dependent on the number of links per node.

As will have been seen from the glossary given above, there are in general two types of message used in connection establishment. The first of these is a change message, which contains two pieces of information, a type number identifying the message as a change, and a number to uniquely identify the message. This latter number normally includes two parts, a hardware number of the originating node and a message sequence number generated by the originating node. The response of a node to a change message is set out in Fig. 2. From this flow diagram it will be seen that a change message floods through the mesh to all other nodes, in a manner akin to saturation routing. In response to a CHANGE all nodes assume they are master, however when the network mesh is established the master node will be the one with the lowest number irrespective of which node originated the CHANGE.

The other sort of message referred to above is a master node set-up message, or MNSA, which is emitted from a node when that node receives a change message. An MNSA message contains two pieces of information, one of which is a type number which identifies it as an MNSA and the other of which is the current software number of the transmitting node. Fig. 3 shows the response to a node when it receives an MNSA. In this diagram, the abbreviations RX is used for "received", or "receiving".

It will be noted that in the condition of Fig. 1, all nodes are connected to the master node, either directly or indirectly, and that every nodes has access to every other node via the links currently in use. With such a system, all connections are set up under the control of the master node, whichever node originates the connection.

The algorithm which controls the system, as indicated above, is used to establish a mesh involving all of the nodes. When such a mesh is set up it is hierarchial in nature in that the various nodes of the system are interconnected, see for instance Fig. 1, in a "tree-like" structure with the current master node at the apex of the tree.

In general the algorithm is triggered to operate when one of three conditions exist: (i) A node recognises that a link which was faulty has become operational.

(ii) A node detects that its primary link, i.e. the link which connects it to, or towards, the master link, has failed.

(iii) A new node has "powered-up" while it is connected to an existing mesh.

The flow diagram of Fig. 4 applies to the action of the new node joining the mesh. Note that here also, when the node detects a power-up condition in a line or the like connected to the port, the node assumes that it is the master node.

The action of a node when it detects a primary link failure is set out in Fig. 5.

Fig. 6 indicates in flow diagrams form what happens when a node detects that a previously faulty link is now serviceable, and is back in service.

In a network of the type described herein, especially where used for military purposes, links and possibly nodes will on occasions cease to be serviceable. When this happens, it is possible for the connections to be maintained. Thus for a series of faults which break the lower ring of nodes, the ring reconfigurates itself so that another node becomes the master node. In this case the new node is node 2. In reconfiguring to the condition shown in Fig. 7, depending upon timing, node 6 could have become connected to node 4 instead of to node 5.

Fig. 8 shows another example, in which the system has settled itself after fault detection, as two mesh's based respectively on nodes 1 and 3 as the mster nodes.

In a system such as described above, the node which becomes the master node may, if the nodes have to be synchronous, be used to provide a master clock. If the master node uses its local clock as the transmit clock on all of the its links, and all other nodes use the clock extracted from the receive leg of the primary link as their own transmit clock on all links, the clocks at all nodes become synchronous with the master node clock.

With each node supporting a number of subscribers, it is possible to provide a mesh-wide conference system using a hierarchy of mixers based on the network. Each such mixer is a specialised conference circuit located at one of the nodes. Thus, see Fig. 1, taking node 13 as an example, at that node, the inputs from all subscribers supported by node 13 are mixed using one of a number of known conferencing mixing techniques. The output of the conference mixer at node 13 is then routed via the primary link to the next node up the hierarchy, node 12.

At node 12, the input from the node 13 is combined with the input from node 16 and the local subscribers. The resulting output is routed via primary link for node 12 to node 1,which in this case is the master node. At the master node the inputs from all of the links are combined with the input from the local subscribers to provide a single output. This output is then routed back down all of the primary links to all other nodes in the mesh. At each node the combined output is fed to each subscriber as required.

In general, when a connection is to be set up between two nodes, one of these nodes is, in effect, a master node in respect of the connection between these nodes.

In the system described above, each node is directly connected to three other nodes by respective ones of the links. It will be appreciated that in a more complex network embodying the invention, some at least of the nodes could be connected to more than three other nodes.

Claims

1. A telecommunication system of the local area network type, which includes system nodes each serving a plurality of subscribers' terminals and bidirectional links so interconnecting the nodes that each said niode is connected via respective ones of said links to at least three other nodes, wherein in any connection to be set up between two of said nodes, one of these nodes is the master node in respect of the connection between that pair of nodes.

2. A system as claimed in claim 1, wherein when a mesh of connections is to be set up between the nodes of the system the routes to be used for the connections between respective pairs of the nodes involved in that connection are so chosen as to minimise the number of links to be used for each connection between a pair of said nodes.

3. A telecommunication system of the local area network type, which includes system nodes each serving a plurality of subscribers' terminals and bidirectional links so interconnecting the nodes that each of said node is connected via respective ones of said links to at least three other nodes, wherein the arrangement of the nodes and te links is such that the system includes two closed loops with some of the links each extending between a node of one of the loops and a node of the other of said loops, wherein there are one or more additional links each of which interconnects two non-adjacent nodes in the same one of said loops, such that each said node is connected via respective ones of said links to at least three other nodes, and wherein when a mesh of connections is to be set up between the nodes of the system the routes to be used for the connections between respective pairs of said nodes are so chosen as to minimise the number of links to be used for each connection between a pair of said nodes.

4. A system as claimed in claim 3, wherein when a said mesh of connections is to be set up the setting up commences from one of said nodes which acts as a master node in respect of that mesh, and wherein the routes to be set up via the minimum number of links are selected locally at the nodes.

5. A system as claimed in claim 4, wherein under fault conditions when one or more links or nodes is faulty one or more of said nodes can function as master nodes dependent on the position of the faulty node(s) or link(s), and wherein when two or more of said nodes act as master links, each acts as a master link with respect of a different set of said nodes.

6. A telecommunication system of the local area network type, which includes a number of system nodes each serving a plurality of subscribers' terminals, and bidirectional links interconnecting the nodes in such a way that each said node is connecte via respective ones of said links to at least three other nodes, wherein when a mesh connection involving the nodes is to be established this mesh is set up from one of the nodes which then function as a master node in respect of that mesh connection, wherein each connection between two of said nodes is set up via the smallest possible number of said links, wherein each said node includes test means for testing the condition of the links serving that node, wherein the selection of the routes to be used between nodes is effected, and wherein a said node which acts as a master node for the establishment of said mesh connection signals to the other nodes that a said mesh connection is to be set up, each node responds to information relating to the links associated therewith to select the routes via which the said nodes are to be interconnected, which selection is so effected as to minimise the number of links used in any one nodeto-node connection, and transmits set up information to those nodes.

7. A system as claimed in any one of the preceding claims, wherein when one or more links and/or nodes is faulty the system reconfigures so that as many as possible of the serviceable nodes are connected to the system, and wherein such reconfiguration may involve two or ore nodes functioning as master nodes.

8. A telecommunication system of the local area network type, substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed, and have the following effect:~ (b) New or textually amended claims have been filed as follows:- New claim 9 added 5th June 1986.

7. A system as claimed in any one of the preceding claims, wherein when one or more links and/or nodes is faulty the system reconfigures so that as many as possible of the serviceable nodes are connected to the system, and wherein such reconfiguration may involve two or more nodes functioning as master nodes.

8. A telecommunication system of the local area network type, substantially as decribed with reference to the accompanying drawings.

9. A telecommunication system of the local area network type, which includes a number of nodes each of which serves a plurality of system subscribers, each said node being connected via respective links to at least three other nodes, wherein the system includes a plurality of loops in which each said node is connected via respective links to the adjacent nodes in its own loop, wherein each said node is also connected via at least one other link either to a node in a different loop orto a non-adjacent node in its own loop, depending on whether the respective loops have the same or different number of nodes, wherein when the system is to be used one of the nodes is selected as the master node, whereafter the system configurates itself under software control or a mesh in which the nodes are connected to the master node, in each case using the minimum number of links, and wherein any one of said nodes can function as a master node so that under fault conditions the system can reconfigure itself to like according to the fault conditions.