GB2439128A

GB2439128A - Providing communication in a network even if hidden nodes arise

Info

Publication number: GB2439128A
Application number: GB0611685A
Authority: GB
Inventors: Peter Strong
Original assignee: Siconnect Ltd
Current assignee: Siconnect Ltd
Priority date: 2006-06-13
Filing date: 2006-06-13
Publication date: 2007-12-19
Also published as: GB0611685D0; WO2007144600A1

Abstract

Communications apparatus is provided, for use in a communications network in which information is transmitted in frame form. The frame contains a payload portion and a signalling portion, the signalling portion comprising a first field for containing heard indications that a payload has been received by an apparatus in the network and a second field for containing acknowledgement indications that a payload has been received by the intended recipient apparatus in the network. The apparatus is operable to detect a heard indication indicating that a payload has been received by another communications apparatus and, if detection of said heard indication is not followed by detection of an acknowledgement indication, the apparatus is operable to remain synchronised on the basis of said heard indication. The invitation is applicable to the problem of hidden nodes.

Description

Powerline Communications Apparatus The present application relates to a

method and apparatus for transmitting data in a data network. It is particularly, but not exclusively, concerned with data communication in a data network comprising a plurality of stations interconnected by bus lines. The invention has particular application wherein the bus lines are implemented by means of electrical power lines or coaxial cable.

In communications technology, not only in powerline communications but also in some wireless applications, hidden nodes are a recognised problem.

Hidden nodes arise when a network is established, wherein a node in the network is not visible' to one or more other nodes in the network. A node is described as visible to another node when its communications activities are directly detectable and discernible by that node.

Figure 1 illustrates such a situation, in the context of a physical arrangement of powerline communications stations in a communications system. Five nodes 10 are illustrated, together with available communications paths 12 therebetween. It is notable that node 5 is only visible to node 4.

In this example, the above visibility situation arises because the physical relationship between stations 20 establishing the nodes is as illustrated in figure 2. In this figure, stations 20 are illustrated, with distinguishing numbers 1 to 5 corresponding to the nodes illustrated in figure 1. The physical connection between the stations is established by powerline communication, on a ring main 22. In this example, whereas stations 1 to 4 are positioned in close proximity, station 5 is rather farther away; its closest station is station 4. Due to the physical distance, the path length to other stations exceeds the practical limit for the implementation and so station 5 is hidden with respect to stations 1 to 3.

It will be recognised that hidden nodes may arise for other reasons than physical spacing between stations. For instance, attenuation may arise due to the use of junction boxes or other equipment, or other path effects such as due to network topology, reflection, multipath fading or noise can give rise to deterioration in signal path quality to the extent that communication between two nodes is rendered effectively impossible.

Although it may be possible to solve this problem by increasing signal to noise ratios in the network, there will be limitations on the ability for this approach to solve the problem, and so there is inevitably still the prospect of hidden nodes arising, and it is desirable to provide a means of communication in a network capable of enabling communication to take place.

A first aspect of the invention provides communications apparatus for use in a communications network in which information is transmitted in frame form, the frame containing a payload portion and a signalling portion, the signalling portion comprising a first field for containing heard indications that a payload has been received by an apparatus in the network and a second field for containing acknowledgement indications that a payload has been received by the intended recipient apparatus in the network, the apparatus being operable to detect a heard indication indicating that a payload has been received by another communications apparatus and, if detection of said heard indication is not preceded by detection of said payload, the apparatus being operable to resynchronise on the basis of said heard indication.

A second aspect of the invention provides a method of signalling in a communications network, the method comprising defining a frame containing a payload portion and a signalling portion, the signalling portion comprising a first field for containing heard indications that a payload has been received by an apparatus in the network and a second field for containing acknowledgement indications that a payload has been received by the intended recipient apparatus in the network, detecting a heard indication indicating that a payload has been received by another communications apparatus and, if detection of said heard indication is not preceded by detection of said payload itself, resynchronising on the basis of said heard indication.

A specific embodiment of the invention will now be described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates an example communications system in node diagram form; Figure 2 illustrates the physical arrangement of stations in the communications system illustrated in figure 1; Figure 3 illustrates a communications frame for use in the communications system illustrated in figures 1 and 2; Figure 4 illustrates a signalling portion of the communications frame illustrated in figure 3; and Figure 5 illustrates a process carried out at a plurality of stations in the system, in accordance with a specific embodiment of the invention.

Figures 1 and 2 have been described above in relation to the prior art example. For consistency, the invention will now be described by way of specific embodiment in relation to the arrangement illustrated in figures 1 and 2.

Each node 10 comprises a communications station capable of transmitting and receiving information in accordance with a frame based communications protocol. Figure 3 illustrates a typical example of a message frame structure, for use in conjunction with the specific embodiment.

The message frame consists of four sub-frames, named Fl, F2, F3 and F4. Fl and F2 are collectively the start of the frame, used for establishing access to the physical medium (the interconnections) and signalling the nature of information to follow. F3 is provided to contain the payload to be sent in the frame, and F4 is an end of frame, containing portions for signalling acknowledgements or transmission failures or busy indications, and for synchronising the next Message Frame.

F4 is illustrated in further detail in figure 4. F4 comprises a HEARD indication field, an ACK indication field, a SYNCH indication field and a BUSY indication field, each providing an opportunity for a station to send an identifiable signal indicating: HEARD: Sent by a station that has received an F3 frame with a valid CRC; but the frame was not addressed to this station.; ACK: Sent by a station that has successfully received an F3 frame portion addressed to it, and that has a valid CRC; BUSY: Sent by a station hat has received an F2 frame addressed to it containing a valid CRC but not able to process the frame due to resource issues.

SYNCH: Sent by a payload transmitting station to indicate release of the medium and commencement of the next frame.

The use of these bits in resolving problems associated with hidden station 5 will now be described with reference to figure 5 which illustrates processes carried out in parallel by stations of the network.

The example commences in the situation whereby station 1 intends to send a message to station 2. As can be seen in figure 1, station 5 is hidden from both of these stations and so could, if this situation is not addressed, attempt to gain control of the medium leading to collision.

In step S 1-2, the process commences at the point at which station I has won control of the medium, such as by means of non destructive bitwise arbitration, and sends payload in F3. F3 also contains a CRC check field, for use by message recipients.

As indicated by figure 1, all messages sent by station 1 to station 2 will also be received by stations 3 and 4. The behaviour of stations 2 and 4 are described here; for reasons of clarity the behaviour of station 3 is omitted but it will be understood that this will substantially be the same as the behaviour of station 4.

Stations 2 and 4, as recipients of the message, will perform a CRC check on the F3 payload on receipt, in respective steps S2-2 and S4-2. If the CRC check is correct, then a HEARD bit is transmitted by the respective stations (S2-4, S4-4).

Station 2 will then transmit the ACK bit if the CRC check is correct (S2-6), as this station is the one for which the message was intended. This ensures that station 1 can distinguish between other stations that merely heard the message, but were not intended to receive it, and the intended recipient: if station 2 does not send the ACK message, then station 1 will repeat the transmission (S 1-4).

While this is occurring, station 5, which has no direct communications path with station 1, receives the HEARD indication sent by stations 2, 3 and 4. The HEARD bit comprises a correlation pattern which enables station 5 to detect it as such, and is thus able to identify it as a HEARD indication. On the other hand, the ACK bit sent by receiving station 2 is not visible to station 5, and so cannot be relied upon.

On receipt of the HEARD bit, station 5 resynchronises using the receipt of the HEARD bit as a timing mechanism (step S 5-2). Then, the station waits through the passage of the ACK and SYNC timeslots (step S5-4), to avoid collision with other stations, and this way maintains synchronisation with the other stations.

The SYNC bit is transmitted by the transmitting node (station 1), and again may not be received by the hidden node (station 5). However, by pre-synchronising to the HEARD indication, and adding a fixed time offset, station 5 will be synchronised with the other stations and will therefore not corrupt the arbitration process carried out in the next frame.

If the hidden station requires access to the medium, it must arbitrate with other stations with which it has no direct means of communication. This is achieved by network control (which is distributed -i;e. there is no central controller) being led by station 4, which is visible to all nodes in the network. Station 4 will receive arbitration requests from all stations active in the network and seeking access to the medium. In order to ensure that arbitration is correctly executed, station 4 will echo all dominant arbitration bits, in order to ensure that requests made by station 5 are not missed by other stations, and vice versa. This way, all stations can arbitrate, even though they are not all visible to each other.

It will be appreciated that this process of echoing could impart some delay to the transmission of arbitration bits around the network. However, in the context of the arbitration bit length, this is expected to be insignificant. Moreover, stations are not expected to synchronise from the arbitration bits, and so no loss of synchronisation should result over the term of one frame.

The process described above can be used to form, in a distributed fashion, a HEARD map which represents a topology of the network. For instance, the following information can be collected in the present example: Station 1 identifies that it is in direct communication with station 2, 3 and 4; Station 2 identifies that it is in direct communication with station 1, 3 and 4; Station 3 identifies that it is in direct communication with station 1, 2 and 4; Station 4 identifies that it is in direct communication with station 1, 2, 3 and 5; Station 5 identifies that it is in direct communication with station 4 but that it is a hidden node with regard to stations 1, 2 and 3.

From this, station 5 can send an update message to all other stations on the network, announcing itself as a hidden node and requesting that they update their HEARD maps.

In fact, only station 4 will receive this message, which imparts further information.

Station 4 will deduce that station 5 is hidden with respect to station 1. The message will be sent on to all stations that station 4 is in communication with, and station 4, which already has visibility with all other nodes, will be able to update stations 1, 2, and 3 with the relevant information.

While the present example involves a network in which at least one station can be identified which is visible to all other stations, this may not always be the case.

However, it will be appreciated that by means of updates to the HEARD Map held at the stations, a model of the network topography can be held at each station. In that way, each station can make decisions on message routing. Indeed, the HEARD map preferably includes link quality information, enabling the best path' between two mutually hidden nodes to be determined. The 1-IEARD map will enable a station to identify those nodes with which it has direct contact, and those which will require one hop, or more in larger networks. The use of the network can then be optimised.

While the foregoing provides description of the invention in terms of apparatus and processes for establishing communication, the skilled reader will understand that other aspects of the invention also exist. For instance, the invention could be implemented by means of computer executable instructions, operable to configure a suitable computer to carry out steps in accordance with any aspect of the invention. The computer executable instructions may be provided by means of a computer program product, such as embodied on a computer readable storage medium, or as embodied on a signal for receipt by a computer. The signal may be initiated by a download instruction generated by a browser on the computer, or by means of an automatic update facility at the computer concerned.

It will be appreciated that the above represents only one of many alternative configurations of the invention, and should not be treated as limiting the scope of protection sought for the present invention. The scope of protection should be read as being defined by the claims appended hereto, with reference to the foregoing

description and the accompanying drawings.

Claims

CLAIMS: 1. Communications apparatus for use in a communications network

in which information is transmitted in frame form, the frame containing a payload portion and a signalling portion, the signalling portion comprising a first field for containing heard indications that a payload has been received by an apparatus in the network and a second field for containing acknowledgement indications that a payload has been received by the intended recipient apparatus in the network, the apparatus being operable to detect a heard indication indicating that a payload has been received by another communications apparatus and, if detection of said heard indication is not followed by detection of said acknowledgement indication, the apparatus being operable to remain synchronised on the basis of said heard indication.

2. Apparatus in accordance with claim 1 wherein said heard indication is identifiable as such other than by means of its temporal position.

3. Apparatus in accordance with claim 2 wherein said heard indication comprises a correlation pattern and said apparatus comprises heard indication detection means for detecting said correlation pattern.

4. Apparatus in accordance with any preceding claim and comprising network map storage means for storing information defining a map of the network, and network map determining means for determining, on the basis of received messages, the topology of the network.

5. Apparatus in accordance with claim 4 wherein said network map determining means is operable to send an update message to another apparatus on the network when information stored in said network map storage means changes.

6. Apparatus in accordance with claim 4 or claim 5 wherein said network map storage means is operable to store information relating to measures of communication reliability between apparatus in said network.

7. Powerline communications apparatus in accordance with any preceding claim.

8. A communications network comprising a plurality of communications apparatus in accordance with any preceding claim.

9. A method of signalling in a communications network, the method comprising defining a frame containing a payload portion and a signalling portion, the signalling portion comprising a first field for containing heard indications that a payload has been received by an apparatus in the network and a second field for containing acknowledgement indications that a payload has been received by the intended recipient apparatus in the network, detecting a heard indication indicating that a payload has been received by another communications apparatus and, if detection of said heard indication is not followed by detection of an acknowledgement indication, remaining synchronised on the basis of said heard indication.

10. A method in accordance with claim 9 wherein said step of detecting a heard indication comprises detecting a predetermined correlation pattern in said heard indication.

11. A method in accordance with claim 9 or claim 10 and comprising storing information defining a map of the network, and determining, on the basis of received messages, the topology of the network.

12. A method in accordance with claim 11 wherein said determining step includes sending an update message to another apparatus on the network when stored network map information changes.

13. A method in accordance with claim 11 or claim 12 wherein said storing step includes storing information relating to measures of communications performance between apparatus in said network.

14. A computer program product comprising computer executable instructions operable to configure general purpose computer controlled communications apparatus to perform a method in accordance with any one of claims 9 to 13.