WO1993016539A1

WO1993016539A1 - Communication method and apparatus

Info

Publication number: WO1993016539A1
Application number: PCT/NZ1993/000004
Authority: WO
Inventors: Russell William Gibbard; John Stewart Cameron Yaldwyn; Roger Neil Ballantine; Robert William Platts; Alan Christopher Sutherlan
Original assignee: HER MAJESTY THE QUEEN IN RIGHT OF THE GOVERNMENT OF NEW ZEALAND, acting by and through THE CHIEF OF DEFENCE FORCE
Priority date: 1992-02-07
Filing date: 1993-02-05
Publication date: 1993-08-19
Also published as: AU3464693A

Abstract

The invention relates to a method of communicating between devices allowing dynamic reconfiguration as devices are added to or removed from the network. Each device maintains a routing list listing the preferred communication path to each device. Beacon messages are transmitted by each device at regular intervals to inform other devices of their presence on the network. New beacon messages indicate a new device has been connected and non-receipt of beacon messages indicates a device has been disconnected. Preferably the routing lists include a path length parameter and the path having the shortest path length is stored for each device. There is also disclosed a method of transmitting voice or digital data over a radio link.

Description

COMMUNICATION METHOD AND APPARATUS FIELD OF THE INVENTION

The present invention relates to a method and apparatus for communicating between a number of devices. More particularly, but not exclusively, the present invention relates to a method of and an apparatus for communicating between units of an artillery fire control system.

BACKGROUND TO THE INVENTION

To the present time, when communicating between a plurality of devices, it has been necessary to configure the system so that messages sent from one device to another pass through the appropriate nodes of a netw . When a new device is connected or an existing device is dii inected, it is necessary to reconfigure the devices of t„ system to enable communication between the devices of the newly confederated network. It would be desirable for the devices to assume a configuration having the shortest path length between devices.

GB 2170079 discloses a communications network in which computers communicate via local area networks (LANs) and bridging devices connected to the LANs which are interconnected by a satellite backbone. The bridging devices generate routing lists indicating which LAN each listed computer is connected to. The bridging devices however only learn of the LAN to which a computer is connected once data is relayed from that computer. The bridging devices are not configured at start-up to know the location of each device on the network. Moreover, only bridging devices have routing lists, thus all devices do not know what other devices are connected to the network at any time. Further, the routing lists do not include a path length parameter to enable bridging devices to store the shortest path between devices. Also, as routing lists are not communicated between devices, they can not be scrutinised for errors.

SUBSTITUTE SHEET EP 404337 discloses a mesh connected network of host computers linked through switches. The switches contain completely configured routing lists. This invention relates to a statically configured configured systems rather than to a method of establishing and maintaining routing lists in a dynamic system.

EP 365337 discloses a routing engine for routing communications between a number of LANs. The routing engine records which LAN each source computer is on as it monitors communications. If a destination address is one of the computers listed in the routing engine the data packet will be directed to the appropriate LAN, otherwise it will be sent to all LANs. This specification does not disclose a method for generating complete routing lists for all devices of a network at start-up. Nor does it disclose the use of a path length parameter in the routing list or a method of reconfiguring the system should a link fail.

To the present time it has also been difficult to provide both voice and data communication on a single radio communication channel unless the analogue voice transmission is digitised so that both the information data and voice data can be transmitted in the same form. Digitalization of voice communications requires analogue to digital and digital to analogue conversion, as well as requiring data transmission at a much higher rate. The rate required for this type of transmission may exceed that available on a standard radio link.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a method of communicating which allows self-configuration of new devices connected to a network and self healing where devices ar removed from the network or transferred to another part of the network, or to at least provide the public with a useful choice.

SUBSTITUTESHEET It is a further object of the present invention to provide a communicating apparatus and method of communicating which enables voice communications and data communications to be transmitted over a single radio link without requiring digitization of the voice communication, or to at least provide the public with a useful choice.

According to one aspect of the invention, there is provided a method of communicating between a plurality of devices, wherein each device maintains a routing list containing the devices to which it can communicate and the directly connected device to which communications should be directed to communicate with each device, said method comprising the steps of:

i/ a newly connected device at connection sending a beacon message containing its own callsign to other directly connected devices

ii/ each of the directly connected devices updating its routing list to include an entry for the newly connected device;

iii/ each of the directly connected devices transmitting a message to the newly connected device informing the newly connected device of its presence and, in the case of a bridging device, the other devices which can be accessed through it;

iv/ said newly connected device updating its routing list in response to the transmitted messages to include listings for each of the directly connected devices, and for every other device known to these directly connected devices; and

v/ the routing lists of the remaining devices being updated to include an entry for the newly connected device.

SUBSTITUTE SHEET There is also provided apparatus for performing the method of the invention.

According to a further aspect of the invention there is provided a method of voice/data communication between a plurality of devices over a communications link comprising the steps of:

i/ a device requesting voice communication sending a digital start message to every other device, either directly or through other devices, requesting that data transmission cease to allow voice communication; ii/ each device acknowledging receipt of the request for voice communication, either directly to the device requesting voice communication or through other devices; iii/ the device requesting voice communication transmitting voice communications; iv/ each device, apart from the device requesting voice communication, receiving the voice communication and monitoring the transmission for a message indicating the return of data communication; and v/ at the end of voice communication, the device requesting voice communication sending a digital stop message to every other device informing the device that digital communication may resume.

Further aspects of the invention, which should be considered in all its novel aspects, will become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example of a possible embodiment with reference to the accompanying drawings in which:

Figure 1: shows a block diagram of the smart modem.

SUBSTITUTE SHEET Figure 2: shows a possible configuration of devices. Figure 3: shows a further possible configuration.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to an artillery fire control embodiment. It is to be appreciated, however, that the present invention may find application beyond the artillery fire control system described.

The "smart modem" referred to in the specification is shown in Figure 1. Central processing unit 1 can store to and retrieve data from memory 2. The central processing unit may consist of Motorola 68302 microprocessor using proprietary extended CCITT X.25 protocol. Central processing unit 1 may communicate with other devices via twisted pair line modem 3, radio modem 4, serial port 5 or telephone network modem 6. Twisted pair line modem 3 may transmit data modulated on a 32 kilohertz carrier at 2 or 4 kbps. in this way voice transmissions can be made on the lower part of the spectrum without interfering with data transmission. The smart modem may be connected to other devices via twisted pair line modem 3 by inexpensive twisted pair lines.

Alternatively CPU 1 may communicate with other devices via a radio link. Radio modem 4 may be connected to an appropriate radio unit to enable communication to another device by radio link. Data may be transmitted using fast frequency shift keying at 150 or 1200 bps. To avoid the need to digitise voice communication and use high frequency radio equipment the smart modem may send data only when voice communications are not in progress.

Serial port 5 allows direct communication to other digital devices. Serial communication may be according to RS232, RS422 or Mil-188 standards.

SUBSTITUTE SHEET Finally, central processing unit 1 may communicate with a telephone exchange via telephone network modem 6. This may enable an artillery battery to communicate with a regimental computer or commander via a line or cellular telephone link.

Display 7 may display status information and programme options of the smart modem. When a smart modem is used as a gun display unit, display 7 may display firing instructions. When a smart modem is used as a command post display 7 may display acknowledgements from gun display units. Keypad 8 allows selection of configuration options.

Speaker 9 enables voice communications from other devices to be heard and microphone 10 allows voice communications to be transmitted from the smart modem to other devices. Switch 11 may be depressed to initiate voice communications and- released to terminate voice communications.

Referring now to Figure 2, a possible artillery fire control configuration is shown. Units COl and C02 are smart modems as shown in Figure 1 used as command posts. Devices G01 to G04 are smart modems used as gun display units. In this example, separate smart modems are used as radio link bridging devices, although all other devices have radio ports. This is for strategic reasons to move radio transmission away from the battery positions.

B01 and B02 are field computers. These will typically be portable laptop type computers. BOl may be the main command computer and B02 a back up computer. Computer BOl may be linked to a regimental command post via a telephone port in modem COl o by radio link, etc.

Units GOl to G04 are gun display units. These devices display the firing instructions supplied from command post COl in conjunction with computer calculations from computer BOl.

SUBSTITUTE SHEET Computer BOl may be supplied with information regarding the characteristics of each gun and the ammunition available for eac gun. Upon the computer being supplied with information as to th location of each gun and the position of a desired target the computer may calculate the trajectory, charge, bearing, fuse setting and elevation required for a particular gun and lot of ammunition. Firing parameters for a number of targets may be stored in the computer awaiting a command to proceed with a mission. As a firing mission is selected the firing data for each gur may be sent to command post COl. The firing data for each gu may be transmitted to each gun display unit via lines L01 and L02 and radio link R.

The firing data for each gun display unit is transmitted with th unique address (callsign) of the gun display unit. In this way only the firing data pertaining to that gun display unit is received and displayed by the gun display unit. Other units may however enter a "spy mode" in which they can request information directed to another gun display unit. The gun display unit doe this merely by sending a "spy" request to the unit COl, which responds with the data that it had previously sent to the other gun display unit. This may be useful when a gun display unit fails so that instructions may be relayed between adjacent guns.

Firing command signals are sent as part of the firing data to th gun display units. For example a command to fire "at my command may be transmitted to the gun display units and displayed on the displays of the gun display units. Before the command is acknowledged the word "ACK" flashes on the display. Gunners may acknowledge receipt of the command on keypads on the gun display unit with the acknowledgment being transmitted back to the command post COl. The word "ACK" then stops flashing. A statu table displayed on the display of command post COl may indicate the acknowledgment of commands. Fire commands may be sent by voice. These commands may also be acknowledged at the gun display units.

SUBSTITUTE SHEET In the configuration shown in Figure 2 computer BOl is linked to smart modem command post COl via a serial link SI. Command post smart modem COl, gun display units G01 and G02 and smart modem SOI are connected by a twisted pair line L01. Smart modem SOI communicates with smart modem S02 via a radio link R, each smart modem communicating via a suitable radio unit. Smart modem S02, gun display unit G03 and command post smart modem C02 are connected by twisted pair line L02. Auxiliary computer B02 is connected to command post smart modem C02 via a serial link. Serial links SI and S2 may be RS232, RS422 or Mil-188 interfaces.

The present invention enables each device of the network to know at any time which devices are connected to the network and the port to which a device should communicate to relay communications to a desired device. The invention enables the devices of a network to configure themselves as the network is set up or a device removed. The network is configured so that the optimum connection paths between devices are established. The invention enables "self healing" so that should a device fail an alternative route can be established to route messages, where available. The design of the present invention allows any form of configuration of devices, such as daisy chain, star or random as the network is self configuring. This allows great flexibility in operation.

Data may be transmitted over twisted pair lines L01 and L02 modulated on a 32 kHz carrier at 2 or 4 kbps. Voice transmissions may be sent simultaneously on the lower frequency part of the spectrum.

So that standard radio units may be used, and digitization of voice communication is not required, transmission over radio links is either data or voice communication, but not both simultaneously.

SUBSTITUTESHEET Each device has a unique identification code (callsign) . This may be set manually or may be attributed to the device as it connects to the network. An enhanced X.25-based protocol is used in which a data frame includes fields for the sender's identification code and the identification code of the destination device. This enables a frame of data to be transmitted to one of several receivers.

An important feature of the invention is that each communicating device COl, G01, G02, SOI, S02, G03 and C02 stores a routing list. This routing list includes a listing for each device connected to the network (including devices such as computers BOl and B02) . Against each bridging device the port through which the bridging device communicates is recorded. This may be via a serial port as between BOl and COl; a line port as between COl and SOI; a radio port as between SOI and S02, or a telephone port. The routing list also records whether the connection between the device and another device is a direct link (as between COl and SOI) or whether an intermediate bridging device is required (as between COl and S02, where SOI is the bridging device) . A number is also recorded against each device in the routing list indicating the path length to that device. Each line or serial link carries a path length value of 2 whereas a radio link carries the value of 3. The radio link carries a value of 3 so that faster and more reliable links such as a line or serial link will be preferred if that option is available. It will be appreciated that in more complex configurations there may be multiple paths between one device and another device. In these situations the shortest path is selected.

A sample routing list for each device for the configuration shown in Figure 2 is given in Table 1.

SUBSTITUTE SHEET TABLE 1

COl GOl GQ2 SOI

BOl S dir 2 BOl COl 4 BOl COl 4 BOl L COl 4

COl - COl dir 2 COl dir 2 COl L dir 2

GOl L dir 2 GOl - GOl dir 2 GOl L dir 2

G02 L dir 2 G02 dir 2 G02 - G02 L dir 2

501 L dir 2 SOI dir 2 SOI dir 2 SOI -

502 L SOI 5 S02 SOI 5 S02 SOI 5 S02 R dir 3 G03 L SOI 7 G03 SOI 7 G03 SOI 7 G03 R S02 5 C02 L SOI 7 C02 SOI 7 C02 SOI 7 C02 R S02 5 B02 L SOI 9 B02 SOI 9 B02 SOI 9 B02 R S02 7

CQ2 GQ3 SQ2

BOl L S02 9 BOl S02 9 BOl R SOI 7

COl L S02 7 COl S02 7 COl R SOI 5

GOl L S02 7 GOl S02 7 GOl R SOI 5

G02 L S02 7 G02 S02 7 G02 R SOI 5

501 L S02 5 SOI S02 5 SOI R dir 3

502 L dir 2 S02 dir 2 S02 -

G03 L dir 2 G03 - G03 L dir 2

C02 - C02 dir 2 C02 L dir 2

B02 S dir 2 B02 C02 4 B02 L C02 4

At the top left is the routing list for device COl. The left hand column lists the devices which COl knows are on the network. The next column indicates whether the device is connected via COl's serial, line or radio link. The third column indicates whether the connection to the device is direct or through a bridging device. If the connection is through a bridging device then the directly connected bridging device to which communications should be sent is listed. The fourth column gives a value representing the path length.

SUBSTITUTE SHEET The first line indicates that device COl can communicate with device BOl via COl's serial port, the link is direct and the path length is 2. The link to auxiliary computer B02 is via the line port of device COl and the device to which a message to B02 should be sent in the first instance is smart modem SOI. The path length is 9 representing two line links (2 each), one serial link (2) and one radio link (3) . The other devices have corresponding routing lists.

The path length column is used to determine which route a signal should be sent by, if there is more than one path. A high value (ie. maximum value) in the path length field indicates a device can no longer be accessed by that route.

Various forms of message may be used to enable the network configuration to be maintained. A "beacon" message may be transmitted by a device to all directly connected devices. A beacon message is not addressed to any specific device, and so requires no specific acknowledgment according to the X.25 protocol.

A beacon message contains the sender's callsign, but the destination field is null, indicating that all devices can receive the message. The signal thus informs directly connected devices of the senders existence on the network.

Beacon messages may be transmitted at a fast rate when the devic is initially configured to ensure that all devices are alerted o its presence and a stable configuration can be rapidly achieved. Once a stable configuration has been reached beacons can be sent out at longer intervals so that other devices in the network are continually updated that the transmitting device remains on the network.

The data section of a beacon message contains the callsigns of the directly connected devices known to the transmitting device. Thus, if a device receiving the transmission finds itself absent from this list, then it will be apparent that the routing list o the transmitting device requires updating.

SUBSTITUTE SHEET An identification message may also be used. This has a similar format to a routing list update message, except that it is used to identify the sender. It is addressed to a particular device and so must be acknowledged. As beacon messages are not acknowledged they must be transmitted at regular intervals. An identification message is sent to a new device to inform it of the presence of the sender on the network. It will not be repeated if it is acknowledged.

An identify yourself message may be used to determine whether a device is still on the network. A short message may be transmitted to a device and, if not acknowledged, the sender can assume the device is no longer connected. This will result in a high value being assigned to the path length for that device. In the current implementation, a routing list update message (which must be acknowledged) is used for this purpose.

A routing list update message may be sent from a bridging device to a directly connected device to notify it of the devices that are connected directly or indirectly to other ports of the bridging device. This indicates to the device which other devices can be accessed by that bridging device. This message is sent from a directly connected bridging device to a new device in response to a beacon from the new device. This enables a newly connected device to create an up-to-date routing list.

When a routing list update message is sent a routing list acknowledgment message is returned to the transmitting device to confirm that a receiver has the most up to date version of the routing list.

When a new device is added to the network all routing lists must be updated to include an entry for the new device. Upon connection a newly connected device sends out a beacon message (with a null receiver call sign) at a fast rate (e.g. one message every 5 seconds) . All devices directly connected to the newly

SUBSTITUTE SHEET connected device receive the beacon messages and update their routing lists to include the newly connected device, if it is not already in their routing list. Bridging devices then send routing list update messages back to the newly connected device to let it know of their connection and the devices accessible through them. Other devices send identification messages to inform the newly connected device of their direct connection.

The identification message or routing list update message is repeated until acknowledgement is received. If no acknowledgement is received this indicates there is a fault on the line and the device is marked as "not connected" in the routing list (i.e: a high value in the path length).

The newly connected device forms an initial routing list from these communications consisting of the information provided by directly connected devices.

Upon receiving a routing list update message from a bridging device the newly connected device will be informed of all devices that can be accessed through that bridge. When the locally connected non-bridging devices send identification messages the newly connected device will know that these devices are directly connected to it, rather than through a bridging device. The routing list set up by the newly connected device contains the callsigns of all locally connected devices as well as those accessible via bridging devices.

Subsequent beacon messages sent by the newly connected device will include the callsigns of all devices that sent routing list update or identification messages to it. Any device that receives such a beacon will not respond with a routing list update (or identification) if it finds its own callsign included in the beacon.

Beacons continue to be sent at a fast rate (every 5 seconds) until no further identification or routing list update responses are received to a predetermined number of consecutive beacons.

SUBSTITUTE SHEET Beacon messages are then transmitted at a slower rate (for example every 25 seconds).

When bridging devices update their routing list they pass on the updates to devices connected to other ports in the form of routing list update messages. The routing list update message consists of the callsigns of devices connected to the bridging device and their updated path lengths. Bridging devices further downstream likewise transmit updated routing lists to devices now connected further downstream. Other devices merely update their own routing lists. In this way the update is propagated throughout the network.

When a device is disconnected the routing lists of all other devices must be updated to show that the device is no longer in the network. A device will assume that a directly connected device has been removed when a beacon message has not been received for a predetermined period. This period may be one minute where a slow beacon rate of one beacon per 25 seconds is used. Alternatively, a device will assume that a directly connected device has been removed if no response is received to a message sent to it, even after retrying. In either case, the bridging device will transmit an "identify yourself" message to the device. If no acknowledgment is received after a specified number of retries, the device will be flagged as disconnected in the routing list. This disconnection is reflected by a high value in the path length field.

Upon disconnection of a device, any previously connected bridging device updates its routing list and transmits this updated routing list to all devices connected to its other ports.

Directly connected bridging devices will likewise update their routing lists (by giving a high path length value to the device) and pass the updated routing list on to all directly connected devices downstream.

SUBSTITUTESHEET Where more than path exists between two devices (ie: a loop exists) a method is required to select the optimum path.

One option is for each device to store in its routing list all possible paths between it and a given device. The path having the shortest path length parameter will be utilised. Should one path fail then the path having the next lowest path length parameter is used. This method would however lengthen the routing list and thus the time required to configure the system.

According to a preferred embodiment only the path having the shortest path length will be entered in the routing list. Whenever a routing list update message is received by a device it compares the path length of each device in the message with the path length it has stored for that device in its routing list (taking into account the path length between communicating devices). If it has an entry for a device having an alternate path of shorter path length it sends a routing list update message back to the sender including the shorter path for that device. It should be noted that the path must not only be shorter but alternate (ie: not via the bridging device it is communicating with) . The sender updates its routing list, replacing the old entry for the shorter new entry, and sends a routing list update message including the new entry to other directly connected devices.

If a device fails the bridging devices on either side of the failed device will propagate routing list update messages back indicating that the devices on either side of the failed device have high path lengths. When a bridging device having an alternative path with a lower path length receives this message it sends back a routing list update message indicating the alternate path. This routing list update message propagates back to the device just prior to the failed device until all devices have updated routing lists including the alternate path.

If the device is restored the routing lists of converted devices will be updated to restore the entry to the new shorter path.

SUBSTITUTE SHEET The communications taking place upon the connection of a new device will now be described with reference to the connection of a gun display unit G04 to the network shown in Figure 2. Upon connection to the network G04 sends out a beacon message with an empty list of known call signs. This is received by the directly connected devices C02, G03 and S02.

When S02 receives G04's beacon message it adds the entry (G04 L dir 2) to its routing list. A routing list update message is transmitted from smart modem S02 to smart modem SOI to update SOl's routing list. This routing list update message consists of a list of the devices which can now be accessed through smart modem S02 along with the path length to each device (i.e.: B02:4, C02:2, G03:2, G04:2) . Smart modem S02 also transmits a routing list update message to G04 consisting of a list of devices which can be accessed downstream of S02 along with the- path length to each device (i.e.: B01:7, C01:5, G01:5, G02:5,. S01:3) .

Upon receipt of the routing list update message from S02, G04 forms the following entries in its routing list: (S02 L dir 2), (BOl L S02 9), (COl L S02 7), (GOl L S02 7), (G02 L S02 7), (SOI L S02 5). G04 then sends a routing list update acknowledgement message to S02.

G03 upon receiving G04's beacon adds the entry (G04 L dir 2) to its routing list. G03 then sends an identification message to G04. G04 receives the identification message from G03 and adds the entry (G03 L dir 2) to its routing list. It then sends a routing list update acknowledgement message to G03.

Upon receiving the beacon from G04, C02 adds the entry (G04 L dir 2) to its routing list. It then sends a routing list update message to G04 of the form (B02:2). Upon receiving the routing list update message from C02 G04 adds the entries (C02 L dir 2) and (B02 L C02 4) to its routing list. G04 then sends a routing list update acknowledgement message to C02.

SUBSTITUTE SHEET The directly connected devices now all have updated routing lists including G04. However, the other routing lists in the system now need to be updated. As mentioned before, S02 sends a routing list update message to SOI of the form (B02:4, C02:2, G03:2, G04:2). SOI adds the entry (G04 R S02 5) to its routing list. Only this entry is added because the rest are already present. SOI then sends a routing list update message to COl, GOl and G02 of the form (B02:7, C02:5, G03:5, G04:5, S02:3).

COl receives the routing list update message from SOI and adds the entry (G04 L SOI 7) as the other devices are already present. GOl likewise receives the routing list update message and adds the entry (G04 L SOI 7). G02 likewise receives the routing list update message and adds the entry (G04 L SOI 7). As a result of these communications the routing lists for the devices are as shown in Table 2.

TABLE 2

COl GOl G02 SOI

BOl S dir 2 BOl COl 4 BOl COl 4 BOl L COl 4

COl - COl dir 2 COl dir 2 COl L dir 2

GOl L dir 2 GOl - GOl dir 2 GOl L dir 2

G02 L dir 2 G02 dir 2 G02 - G02 L dir 2

501 L dir 2 SOI dir 2 SOI dir 2 SOI -

502 L SOI 5 S02 SOI 5 S02 SOI 5 S02 R dir 3 G03 L SOI 7 G03 SOI 7 G03 SOI 7 G03 R S02 5 C02 L SOI 7 C02 SOI 7 C02 SOI 7 C02 R S02 5 B02 L SOI 9 B02 SOI 9 B02 SOI 9 B02 R S02 7 G04 L SOI 7 G04 SOI 7 G04 SOI 7 G04 R S02 5

C02 GO3 GO4 S02

BOl L S02 9 BOl S02 9 BOl S02 9 BOl R SOI 7 COl L S02 7 COl S02 7 COl S02 7 COl R SOI 5 GOl L S02 7 GOl S02 7 GOl S02 7 GOl R SOI 5

SUBSTITUTESHEET G02 L S02 7 G02 S02 7 G02 S02 7 G02 R SOI 5

501 L S02 5 SOI S02 5 SOI S02 5 SOI R dir 3

502 L dir 2 S02 dir 2 S02 dir 2 S02 -

G03 L dir 2 G03 - G03 dir 2 G03 L dir 2

C02 - C02 dir 2 C02 dir 2 C02 L dir 2

B02 S dir 2 B02 C02 4 B02 C02 4 B02 L C02 4

G04 L dir 2 G04 dir 2 G04 - G04 L dir 2

The routing list tells the device if it is directly connected to another device or not. If it is directly connected the device may simply directly communicate with another device. Where however a device is connected to another device through a bridging device, the routing list tells the device to which bridging device the message should be sent in the first instance. Where more than one possible route is available the device will select the bridging device having the shortest path length to the selected device. Accordingly, if GOl wished to communicate with G03, it would refer to its routing list to determine where the message should be sent in the first instance. The routing list for GOl indicates that communications to G03 should be sent through SOI. Accordingly, the message would be addressed to SOI in the first instance. Upon receiving the message from GOl, SOI would look to its routing list, which indicates that transmissions to G03 should be sent via S02. Accordingly SOI would transmit the message to S02. S02 would then look to its routing list to find that it was directly connected to G03. The message would thus be transferred to G03.

If for example command post smart modems COl and C02 were linked by a line then the path between GOl and G03 would become direct. Accordingly, messages would be transferred via the line (a path length of 2). However, if the link between COl and C02 became faulty the path length value for the direct link between COl and C02 would become high.

CO2 would update its routing list to include a high path length value for COl and send a routing list update message with a high path length for COl to C02 (ie: C01:H) . S02 would update its

SUBSTITUTESHEET routing list to enter a high path length value for device COl and send a routing list update message having a high path length value for COl to SOI (ie: C01:H). SOI would check its path length for device COl and find that its length in its routing list is 2. SOI would send a routing list update message to S02 including the entry (C01:2). S02 would update its entry for COl to (COl R SOI 5) and send a routing list update message to C02 of the form (C01:5). C02 would update its routing list to include an,entry (COl L S02 7). The above is a simplified description excluding the updating of non-bridging devices and acknowledgment signals but illustrates how entries may be replaced as past length values change.

Accordingly, communications would be rerouted to the radio link between SOI and S02. Thus the network can rapidly reconfigure itself when changes occur to the network to provide the optimum communication path.

It will be apparent that if more than one device tries to transmit at the same time collisions may occur resulting in garbled communications. As a first precaution devices may monitor the communication channels and only transmit when no other device is transmitting on the communication channel. Collisions could however occur if two devices began transmitting simultaneously at the end of a burst of communication. According to the invention, each device wishing to transmit must wait a different length of time before transmitting. Accordingly, each device must have a different delay to ensure that no two devices commence transmission simultaneously.

As can be seen i Tables 1 and 2, the position of a device's own call sign ip, its own routing list is different for each device. Accordingly, the position of a device in its own routing list may be use to assign a delay to each device.

A problem with this method is that the call sign having the lowest value will always have the highest access priority. To overcome this problem a changing value may be added to shift

SUBSTITUTE SHEET access priorities. This value may be the position of the call sign of the last device to transmit on the network.

One further problem faced is the acknowledgement of messages passing through bridging devices. Where a number of bridging devices are used the bridging device will acknowledge receipt of the message from the previous device and then try to on-transmit the message.

However, the original transmitting device needs to know whether the final device to which the message was destined actually received the message. According to the invention, a higher level acknowledge message may be used. This higher level acknowledgement is different from a normal acknowledgement and indicates that a message has reached the device it was destined for. This message may be relayed between bridging devices in the normal way.

Referring now to Figure 3, a possible operational field configuration is shown. For convenience, this network employs two frequencies, one for local gun communications, and another for battlefield communications. Considering the battlefield network, a regimental command post computer 20 is connected to an artillery battery computer 21 via smart modems 22 and 23. Smart modem 19 supplies meteorological information to smart modems' 22 and 23 via a radio link. Smart modem 24 provides data from a mortar locating radar via a line connection. A forward observer 28 is connected to smart modem 23 by a radio link. Forward observers can send voice communications or data providing information as to the accuracy of fire.

A second battery computer 33 is connected on the same battlefield network via a radio link between smart modems 23 and 34.

The local gun network for the first battery consists of gun display units 25 connected through a radio link to smart modem 18, and gun display units 26, 27, 29, 30 and 31 connected via

SUBSTITUTE SHEET twisted pair lines to smart modem 18. For the second battery, gun units 34, 35 and 36 are connected by line to SM 37.

It will be appreciated that a number of artillery batteries may be connected via radio links between smart modems, lines or telephone connections. All batteries can likewise be connected to regimental command. In this way field activity can be directed from regimental command centre 20. Furthermore, should any battery computer fail, calculations can be performed by the computers of other batteries connected on the network. Furthermore each battery can be supplied with meteorological and mortar locating radar information.

The present invention allows for flexible interconnection of devices without requiring configuration by field staff. The network also determines the best route for messages to be transmitted and is self healing in case a link in the network becomes inoperative (provided an alternative link is available) . The invention also allows for the transmission of voice or data communications over all links of the network without requiring Digitalization of the voice communication or sophisticated radio equipment.

Where in the foregoing description reference has been made to integers or components having known equivalents then such integers are herein incorporated as if individually set forth.

Although this invention has been described by way of example and with reference to possible embodiments thereof it is to be appreciated that improvements and/or modifications may be made without departing from the scope of the invention as defined in the appended claims.

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Claims

CLAIMS :

1. A method of communicating between a plurality of devices, wherein each device maintains a routing list containing the devices to which it can communicate and the directly connected device to which communications should be directed to communicate with each device, said method comprising the steps of:

i/ a newly connected device at connection sending a beacon message containing its own callsign to other directly connected devices;

2. A method as claimed in claim 1 wherein each device in each device's routing list is assigned a path length parameter indicating the path length between devices.

3. A method as claimed in claim 2 wherein bridging devices transmit routing list update messages to directly connected

SUBSTITUTESHEET devices containing their current routing list whenever their routing lists are changed.

4. A method as claimed in claim 2 wherein each device records in its routing list the path to each device having the shortest path length parameter.

5. A method as claimed in claim 4 wherein each line link corresponds to a path length of 2 and each radio link corresponds to a path length of 3.

6. A method as claimed in claim 3 wherein bridging devices receiving routing list update messages check the path length for each device against the path length in their routing lists to see whether their routing list includes an alternate path of shorter length, and if so, transmit a routing list update message to the device which has just sent a routing list update message, which device substitutes the routing list entry having a shorter patn length.

7. A method as claimed in claim 1 wherein beacon messages are initially sent out at a fast rate, and subsequently at a slower rate.

8. A method as claimed in claim 7 wherein, if a beacon message is not received from a device for a predetermined period, a directly connected device assigns that device a high path length parameter and propagates that value to directly connected devices by routing list update messages.

9. A method as claimed in claim 8 wherein, if a beacon message is not received from a device for a predetermined period, a directly connected device firstly sends a message requesting tha the device identifies itself and if no response is received, the device is then assigned a high path length.

10. A method as claimed in claim 1 wherein the routing lists

SUBSTITUTE SHEET include a field for each device indicating the port through which communications should be directed to communicate with that device.

11. A method as claimed in claim 1 wherein each device waits a delay period after the termination of a previous communication proportional to its position in its own routing list before sending a communication to another device.

12. A method as claimed in claim 11 wherein the delay period is adjusted by a delay period proportional to the position in each devices routing list of the last device to transmit.

13. Apparatus operating in accordance with the method of any of one of the preceding claims.

14. A method of^"voice/data communication between a plurality of devices over a communication link comprising the steps of:

i/ a device requesting voice communication sending a digital start message to every other device, either directly or through other devices, requesting that data transmission cease to allow voice communication; ii/ each device acknowledging receipt of the request for voice communication, either directly to the device requesting voice communication or through other devices; iii/ the device requesting voice communication transmitting voice communications; iv/ each device, apart from the device requesting voice communication, receiving the voice communication and monitoring the transmission for a message indicating the return of data communications; and v/ at the end of voice communication, the device requesting voice communication sending a digital stop message to each other device informing the devices that digital communication may resume.

SUBSTITUTE SHEET

15. A method as claim in claim 14 wherein a digital start message is transmitted when a talk button has been pressed on the device requesting voice communication and a digital stop message is sent when the talk button is released.

16. A method as claimed in claim 14 wherein the communication link is a radio link.

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