GB2504338A

GB2504338A - Channel contention for a shared medium

Info

Publication number: GB2504338A
Application number: GB1213349.2A
Authority: GB
Inventors: Geoffrey James Akwei Baddoo
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2012-07-26
Filing date: 2012-07-26
Publication date: 2014-01-29
Anticipated expiration: 2032-07-26
Also published as: GB2504338B; GB201213349D0

Abstract

A method of contending for access, by a communication device, of a shared communication medium for transmission of data in a communications network, involves the device setting a contention metric value S1-4, for comparison with any incoming control messages also indicative of attempts to contend for the medium S1-6, S1-8. Contention metric values from incoming control messages are compared with the selected contention metric value and, on the basis of the comparison S1-8, a decision is taken as to continuing for contention for the medium or discontinuing S1-24. The device can be said to have gained access to the medium when it has completed a requisite period of listening to the medium S1-12, with no incoming message received indicative of a stronger claim to the medium S1-14. The incoming control messages are messages broadcast by other devices contending for the medium. The contention metric value is a number chosen from a range of possible values indicating a current priority associated with the device, this number may be randomly chosen by the device from the possible range of values.

Description

I

METHODS OF CHANNEL CONTENTION IN COMMUNICATIONS NETWORKS, AND

APPARATUS THEREOF

Field

The present invention relates generally to channel contention in communications networks.

Backeround In communications networks where users (or devices) utilise a shared medium for communication, a media access control (MAC) protocol is employed to resolve contention between users seeking to access the medium simultaneously.

Essentially, the MAC protocol makes it possible for several users to share a physical medium. The MAC protocol manages when and how users transmit on the shared medium. This is achieved by detecting or avoiding data packet collisions if a packet mode contention based channel access method is used, or reserving resources to establish a logical channel if a circuit switched based channel access method is used.

Examples of channel access methods include pure ALOHA, slotted ALOHA, reservation ALOHA, and various Carrier Sense Multiple Access (CSMA) schemes.

The reservation ALOHA (or R-ALOHA) is a channel access method that allows a transmission slot to be temporarily owned" by a user which has successfully used it.

As a general rule, idle slots are considered available to all users in the network that may then implicitly reserve (or utilise) a slot on a contention basis. However, all reservations made in a reservation ALOHA channel are made for capacity which is used as a separate dedicated data channel. Furthermore, R-ALOHA requires one of the users in the network to function as a central master in order to allocate transmission resources.

CSMA is a probabilistic MAC protocol in which a user verifies the absence of other traffic before transmitting on a shared transmission medium. CSMA with collision detection (CSMAJCD) and CSMA with collision avoidance (CSMA/CA) are two modifications of CSMA. CSMAICD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, while CSMA/CA improves the performance of CSMA by reducing the probability of collisions on the channel.

However, it is known that CSMA schemes are susceptible to noise or interference in the channel.

US6538985 describes a contention resolution media access control protocol for a Focal area network (LAN) using orthogonal frequency divisional multiplexing (OFDM). Each LAN node is assigned a LAN-unique node identifier that is the index of a fast-Fourier transform (FFT) frequency bin or other discrete tone in the free band. The node identifier is played in each contention cycle by each node that has data to transmit.

Each contending node determines nodes that are contending for transfer of data over the media channel based on the presence of node identifiers within distinct frequency bins. Contention for the media channel is resolved by each contending node being given access to the media channel in accordance wEth a preset order in which contending nodes will transmit.

It is noted that channel capacity can be increased by reducing the contention time for a channel. Thus, it is desirable to reduce the ratio between the contention time and the time in which the channel is dedicated to a user for data transmission.

Summary

En a first aspect of the invention, there is provided a method of contending for access, by a communication device, of a communication medium shared with one or more other communication devices in a communications network for transmission of data, the method comprising selecting a contention metric value from a set of predefined contention metric values, monitoring the communications medium for receipt of a control message on the communications medium, such a control message comprising a contention metric value, and on receipt of such a control message, comparing the contention metric value of the received control message with the selected contention metric value, determining on the basis of said comparing whether the device can continue to contend for access to the communications medium, if, after said monitoring, it is determined that the device can continue to contend for access, transmitting a control message onto said communication medium, wherein said control message comprises said selected contention metric value1 further monitoring said communication medium for the receipt of control messages on said medium and, on detection of a further control message comprising a respective further contention metric value, comparing said selected contention metric value with said further contention metric value to determine whether said communication device can access said shared communication medium for transmission of data in said data portion of said packet.

In another embodiment, the method may further comprise selecting said set of predefined contention metric values from a plurality of sets of predefined contention metric values.

Each of said plurality of sets of predefined contention metric values may be associated with a priority level for transmission of data in said shared communication medium.

The control message may further comprise a priority lever associated with said selected said set of predefined contention metric values.

The further contention metric value may be selected from a further set of predefined contention metric values.

Said at least one further control message may comprise a further priority level associated with said further set of predefined contention metric values.

The method may further comprise comparing said priority level and said further priority level to determine whether said communication device can access said shared communication medium for transmission of data.

The method may further comprise terminating contention of said shared communication medium if said priority level is lower than said further priority level.

Said further contention metric value may be selected from said set of predefined contention metric values.

En one embodiment of the invention, the method may further comprise transmitting said data and said control message in the respective portions of said packet on said shared communication medium if it is determined that said communication device can access said shared communication medium.

The contention metric value may be higher than said further contention value.

The method may further comprise modifying said contention metric value to a highest value in said set of predefined contention metric values, thereby providing an indication that said shared communication medium is occupied.

In a second aspect of the invention, there is provided A communication device operable to contend for access of a shared communication medium for transmission of data in a data portion of a packet in a communications network comprising a plurality of communication devices, the communication device comprising a storage means for storing at least one set of predefined contention metric values, a selector operable to select a contention metric value from said at least one set of predefined contention metric values: a receiver for monitoring the communication medium for the presence of a control message, such a control message comprising a contention metric value, a contention metric comparing unit operable to compare a contention metric value on a received control message with a contention metric value selected by said selecting means, a contention process manager operable to decide, on the basis of a comparison conducted by the contention metric comparing unit, whether to continue contending for access to the communications medium, and a transmitter for transmitting a control message in a control portion of said packet via said shared communication medium to at least another one of said plurality of communication devices in said communications network, wherein said control message comprises said selected contention metric value, wherein the receiver is operable to monitor the medium for a first listening period for the presence of a control message on the medium, the comparing unit is operable to compare a contention metric on any received control message with the selected contention metric value, and the contention process manager is operable to discontinue contention on the basis that the comparison is indicative that a received control message wins the contention, and wherein the contention process manager is operable to determine that the device has access to the medium on completion of contention without discontinuation.

In a third aspect of the invention, there is provided a communication network comprising a plurality of communication devices in accordance with the preceding aspect.

One embodiment provides a computer program product comprising computer executable instructions which, when executed by a computer, cause the computer to perform a method as set out above. The computer program product may be embodied in a carrier medium, which may be a storage medium or a signal medium. A storage medium may include optical storage means, or magnetic storage means, or electronic storage means.

The described embodiments can be incorporated into a specific hardware device, a general purpose device configure by suitable software, or a combination of both.

Aspects can be embodied in a software product, either as a complete software implementation, or as an add-on component for modification or enhancement of existing software (such as a plug in). Such a software product could be embodied in a carrier medium, such as a storage medium (e.g. an optical disk or a mass storage memory such as a FLASH memory) or a signal medium (such as a download).

Specific hardware devices suitable for the embodiment could include an application specific device such as an ASIC, an FPGA or a DSP, or other dedicated functional hardware means. The reader will understand that none of the foregoing discussion of embodiment in software or hardware limits future implementation of the invention on yet to be discovered or defined means of execution.

Description of the drawings

Embodiments wilt now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a communications network according to one embodiment of the invention; Figure 2 illustrates a wireless communication device incorporating embodiments of the invention; Figure 3 illustrates a time diagram showing wireless communications devices contending for a transmission channel in the communications network of Figure 1, according to an embodiment of the invention; Figure 4 iLlustrates a flow diagram of a channel contention method according to the embodiment of Figure 3; Figure 5 illustrates a time diagram showing wireless communications devices contending for a transmission channeL in the communications network of Figure 1, according to another embodiment of the invention; and Figure 6 ilLustrates a flow diagram of a channel contention method according to the embodiment of Figure 5.

Detailed Description

Specific embodiments will be described in further detail in the following paragraphs on the basis of the attached figures. It will be appreciated that this is by way of example only, and should not be view as presenting any limitation on the scope of protection.

Figure 1 shows a communications network 10 according to an embodiment of the invention. The communications network 10 includes a plurality of wireless communication devices 14, 16, 18. The wireless communication devices -14, 16, 18 share one or more communication channels for transmitting data to each other.

In one embodiment of the invention, each of the wireless communication devices 14, 16, 15 is operable to broadcast control data on one or more broadcast communication channels to other wireLess communication devices in the communications network 10.

In an alternative embodiment, the devices in the communications network 10 are operable to broadcast control data on one or more of the shared communication channels.

In this illustrated example, there are three wireless communication devices in the communications network 10, but it will be appreciated that practical implementations may include more (or fewer) communication devices depending on the application.

Furthermore, in the embodiments described herein, the methods are performed in a wireless communications network, such as a Wireless Local Area Network (WLAN).

However, it will be appreciated by the skiLled person that the methods described herein can also be applied in a wired network, such a Local Area Network (LAN).

The wireless communication device 100 illustrated in Figure 2 is generally capable of establishing a wireless communication channel with one or more other device for data transmission and, according to embodiments of the invention, to contend for a shared channel for data transmission. The reader will appreciate that the actual implementation of the wireless communication device is non-specific, in that it could be an access point station or a user terminal.

The device 100 comprises a processor 120 operable to execute machine code instructions stored in a working memory 124 and/or retrievable from a mass storage device 122. By means of a general purpose bus 130, user operable input devices 136 are capable of communication with the processor 120. The user operable input devices 136 comprise, in this example, a keyboard and a mouse though it will be appreciated that any other input devices could also or alternatively be provided, such as another type of pointing device, a writing tablet, speech recognition means, or any other means by which a user input action can be interpreted and converted into data signals.

Audio/video output hardware devices 138 are further connected to the general purpose bus 130, for the output of information to a user. Audio/video output hardware devices 138 can include a visual display unit, a speaker or any other device capable of presenting information to a user.

Communications hardware devices 132, connected to the general purpose bus 130, are connected to antennas 134. In the illustrated embodiment in Figure 2, the working memory 124 stores user applications 126 which, when executed by the processor 120, cause the establishment of a user interface to enable communication of data to and from a user. The applications in this embodiment establish general purpose or specific computer implemented utilities that might habitually be used by a user.

Communications facilities 128 in accordance with the embodiments are also stored in the working memory 124, for establishing a communications protocol to enable data generated in the execution of one of the applications 126 to be processed and then passed to the communications hardware devices 132 for transmission and communication with another communications device. It wII be understood that the software defining the applications 126 and the communications facilities 128 may be partly stored in the working memory 124 and the mass storage device 122, for convenience. A memory manager could optionally be provided to enable this to be managed effectively, to take account of the possible different speeds of access to data stored in the working memory 124 and the mass storage device 122.

On execution by the processor 120 of processor executable instructions corresponding with the communications facilities 128, the processor 120 is operable to establish communication with another device in accordance with a recognised communications protocol.

While the communications facilities 128 are illustrated as a distinct software element, the reader will appreciate that software can be introduced to a computer in a number of different ways. For instance, a computer program product, consisting of a storage medium could be introduced to a computer, so that stored instructions can then be transferred to the computer. Equally, a signal could be sent to the computer bearing such instructions. Furthermore, in introducing a computer program product, the reader will appreciate that a piece of software may be composed of a number of components, some of which may be new, and others of which may be assumed to be provided in the computer already. For instance, a computer might be reasonably assumed to be supplied with an operating system of known type, and a computer program may be developed on the basis of the presence of such an operating system. The interaction between the computer program developed in that way, and facilities of the operating system, would lead to the definition of a communications facilities element such as iLlustrated in Figure 2. Thus! any computer program product may be developed as a new, stand-alone product, or as a plug-in to existing products.

Figure 3 illustrates an exampEe of two wireless communication devices contending for a transmission channel in the communications network of Figure 1, according to an embodiment of the invention.

As illustrated in Figure 3, each of the wireless communication devices 14, 16 broadcasts a control message comprising a contention metric value 14a, 16a over a short burst of time. In this example, the contention metric value is seLected randomly, prior to transmission thereof, by each of the wireless communication devices 14, 16 from a range of predefined values (for example, 0 to 99). In the example, the wireless communication device 14 has selected a contention metric value of 15, and the wireless communication device 16 has selected a contention metric value of 78.

The contention metric value is broadcasted repeatedly over a predetermined time period 14b, 16b. The timing of broadcasts includes a random variable, as will be described in due course. During the predetermined time period, each of the wireless communication devices 14, 16 listens" for control messages that are broadcasted by other devices in the network.

If no control messages are detected, the wireless communication device may have the option to: 1. stop broadcasting the control message and begin data transmission, or 2. continue broadcasting the control message and listen for control messages broadcasted by other devices in the network.

If a wireless communication device detects a control message comprising a contention metric value, higher than its own contention metric value, the wireless communication device will cease its attempt to contend for the shared channel. For example, as shown in Figure 3, the wireless communication device 14 ceases to broadcast the control message 14a as soon as it detects a control message containing a higher contention metric value.

However, if the wireless communication device does not detect any contention metric value which is higher than its own contention metric value, the wireless communication device can commence data transmission over the shared channel. For example as shown in Figure 3, the wireless communication device 16 begins data transmission over the shared channel 16c at the end of the second predetermined time period 16b.

The wireless communication device 16 also changes its contention metric value to the highest value of 99 in the predefined range to indicate that the shared channel 16 has been occupied.

The channel contention process, performed by a wireless communication device, according to the method described above will now be described with reference to the flow chart of Figure 4.

Step S1-2: The process commences with an initialisation process which includes predefining a range of contention metric values to be used in the network. A suitable range of values might be integers in the range of 0 to 99.

Step 51-4: The wireless communications device, seeking to contend for control of the channel, selects a contention metric from the predefined range of values.

Step 51-6: The wireless communication device listens to the channel for a period, known as the wait period. The wait period is of a randomly chosen length, selected from within a predetermined range. The listening is conducted for the existence of any control messages on the channel.

Step S1-8: If a control message is received in the wait period, the metric borne in this control message is compared against the device's own selected contention metric. If the received metric is higher than the device's own metric, then the device ceases contending for the channel (Step 51-24). Otherwise, the device proceeds.

Step Si-lu: Following the wait period, in which the device does not receive a control message with a metric higher than its own metric, the wireless communication device broadcasts a control message containing the selected metric value to the channel.

Step 51-12: The wireless communication device sets a new wait period, randomly selected again, and, for the duration of that wait period, the device listens to the channel for any further control messages.

Step 61-14: Again, if a control message is received in the wait period, the metric borne in this control message is compared against the device's own selected contention metric. If the received metric is higher than the device's own metric, then the device ceases contending for the channel (Step 51-24). Otherwise, the device proceeds.

Step 51-16: The device checks, against a stored record, the number of broadcast control messages it has sent in the current contention session. It the number is less than a requisite limit (e.g. 3), then the process reverts to Step 61-10. for a further broadcast. Otherwise, the process proceeds.

Step S1-18: The device listens to the channel one more time, this time for a wait period which is predetermined and fixed.

Step 51-20: Again, on this final opportunity, if a control message is received in the wait period, the metric borne in this control message is compared against the devices own selected contention metric. If the received metric is higher than the device's own metric, then the device ceases contending for the channel (Step S1-24).

Otherwise (Step 51-20) the device can conclude that it has successfully contended for access to the channel, and can begin transmitting data.

En another embodiment of the invention, the contention metric to be used by a communication device is selected (Step S1-4) with regard to the importance of the data transmission. That is, a priority level is built into the contention metric. The range of possible values of the contention metric is subdivided into sub-ranges. A lower sub-range is allocated to relatively low-priority transmissions; conversely, a sub-range comprising higher values for the contention metric is allocated to relatively high-priority transmissions. Table 1 illustrates this for two priority levels (A and B) and consequently two sub-ranges of possible contention metric values:

Table 1

Label Priority level Contention metric range A Low 0to99 B High lOOto 199 Thus, according to this, when a device selects its contention metric, it encodes within the contention metric a priority level for the transmission for which access to the channel is being sought.

FigureS illustrates a scenario whereby each of the wireless communication devices 14, 16,. 18 seeks to make a transmission. The intended transmissions of devices 14 and 18 are designated as high priority, while that of device 16 is designated low priority.

Thus, the contention metrics of devices 14 and 18 are selected from the range 100 to 199, while that of device 16 is selected from the range 0 to 99. Each device first broadcasts a control message 14a1, 16a1,18a1 at roughly the same time, and so none of the devices, in their first listening phases, detects the control messages of the other devices. This is not problematic, as the method as described builds in at least two listening phases per contention session.

The control messages are broadcasted over a randomly selected time period 14b, 16b, 18b. During the time period, each of the wireless communication devices 14, 16, 18 listens" for control messages that might be broadcast by other devices in the network.

If no control messages are detected within the two listening periods designated in the process, the wireless communication device may conclude that it has access to the channel, and can then begin data transmission.

If the wireless communication device detects a control message containing a contention metric which is higher than its own contention metric, the wireless communication device will cease its attempt to contend for the shared channel. For example, as shown in Figure 5, the wireless communication device 16 ceases to broadcast the control message 1 6a as soon as it detects a control message with a higher contention metric.

In this example, when the wireless communication device 14 detects that its contention metric value is lower than the contention metric value of wireless communication device 18, it immediately ceases to contend for the shared channel. As shown in Figure 5, the wireless communication device 14 ceases to broadcast the control message 14a2 as soon as it detects a control message containing a higher contention metric value.

However, if the wireless communication device does not detect any contention metric value which is higher than its own contention metric value, the wireless communication device can commence data transmission over the shared channel. For example, as shown in Figure 5, the wireless communication device 18 begins data transmission over the shared channel 18c at the end of the third predetermined time period 18b.

The wireless communication device 18 also changes its contention metric value to the highest value of 199 to indicate that the shared channel 18c has been occupied.

A feature of this approach is that the contention metric is selected from a predetermined sub-range of the possible range of values of the metric. This means that a high priority transmission will always win access to the channel, over a low priority transmission.

Another embodiment provides for a further field to be created in the control message, alongside the contention metric. According to this embodiment, the control message will thus include, not only a contention metric, but also a label, such as label A' or B' as set out in tabLe 1, indicating the priority level of the intended transmission. In this embodiment, the priority level need not be encoded in the contention metric. That is, all devices may select a contention metric from the same full range of values. In contending for access to the channel, according to this embodiment, a device will thus go through two steps, in assessing if a received control message indicates that the device cannot continue contending for the channel.

In a first step, the received control message is assessed for the priority label borne therein, and compared with the priority level of the device's own intended transmission.

If the priority label of the received message is higher than that of the device's own intended transmission, then the device cannot continue to contend for the channel. If the device's own priority level is higher than the intended transmission, then the other device will drop out instead.

If the device's own priority level is the same as that of the received control message, then a second assessment is made, as to the contention metric contained in the respective control messages. As before, if the device's own contention metric is lower, the device will drop out.

Figure 6 shows this in illustrative form.

By brief explanation, the process of figure 6 is similar to that of figure 4, but with a double layer of checks on a received control message. To that end, table 2 sets out correspondence between steps of figure 6 with those previously described with respect to figure 4:

Table 2

Figure 6 Figure 4 S2-2 S1-2 S2-4 S1-4 52-6 S1-6 --_____________ S2-14 Si-la S2-16 S1-12 S2-24 S1-16 S2-26 S1-18 S2-34 31-22 S2-36 31-24 References to those steps En the description below should be referred back to the

earlier description.

Step S2-8: In place of step S1-8, a check is made as to whether a control message has been received in the listening period. If not, then the process continues with step S2-14. Otherwise, in step S2-10, the priority level of the received message is checked against the priority level set for the device's own intended transmission. If this is higher, then the device ceases its attempt to contend (step 82-36). If it is lower, then the process continues with step S2-14. If they are the same, then a further check is made in step 32-12 as to the respective contention metrics. If the contention metric of the received message is higher, then the device ceases its attempt to contend (step 32-36). If the contention metric of the received message is lower, then the process continues with step 32-14.

Similarly, step 31-14 is replaced with steps 52-18, 52-20 and 52-22. Likewise, step 31-20 is replaced with steps S2-28, 52-30 and 32-32.

These three groups of steps each ensure that, if an incoming control message indicates a higher priority transmission, the device drops out. If an incoming control message indicates a lower priority transmission, the device continues to contend. If the intended transmission indicated by the incoming control message is of the same priority as that of the device, then the respective contention metrics govern the question as to whether the device continues to contend.

While the foregoing specific description of an embodiment of the invention has been provided for the benefit of the skilled reader, it will be understood that it should not be read as mandating any restriction on the scope of the invention. The invention shouFd be considered as characterised by the claims appended hereto, as interpreted with reference to, but not bound by, the supporting description.

Claims

CLAIMS: 1. A method of contending for access, by a communication device, of a communication medium shared with one or more other communication devices in a communications network for transmission of data, the method comprising: selecting a contention metric value from a set of predefined contention metric values, monitoring the communications medium for receipt of a control message on the communications medium, such a control message comprising a contention metric value, and on receipt of such a control message, comparing the contention metric value of the received control message with the selected contention metric value, determining on the basis of said comparLng whether the device can continue to contend for access to the communications medium, if, after said monitoring, it is determined that the device can continue to contend for access, transmitting a control message onto said communication medium, wherein said control message comprises said selected contention metric value, further monitoring said communication medium for the receipt of control messages on said medium and, on detection of a further control message comprising a respective further contention metric value, comparing said selected contention metric value with said further contention metric value to determine whether said communication device can access said shared communication medium for transmission of data in said data portion of said packet.
2. A method according to claim 1, further comprising selecting said set of predefined contention metric values from a plurality of sets of predefined contention metric values.
3. A method according to claim 2, wherein each of said plurality of sets of predefined contention metric values is associated with a priority level for transmission of data in said shared communication medium.
4. A method according to claim 3, wherein said contro' message further comprises a priority level associated with said selected said set of predefined contention metric values.
5. A method according to any one of the preceding claims, wherein said further contention metric value is selected from a further set of predefined contention metric values.
6. A method according to claim 5, wherein said at least one further control message comprises a further priority level associated with said further set of predetined contention metric values.
7. A method according to claim 6, further comprising comparing said priority level and said further priority level to determine whether said communication device can access said shared communication medium for transmission of data.
8. A method according to claim 7, further comprising terminating contention of said shared communication medium if said priority level is lower than said further priority level.
9. A method according to any one of claims 1 to 4, wherein said further contention metric value is selected from said set of predefined contention metric values.
10. A method according to any one of the preceding claims, further comprising transmitting said data and said control message in the respective portions of said packet on said shared communication medium if it is determined that said communication device can access said shared communication medium.
11. A method according to claim 10, wherein said comparing is on the basis that a lower contention metric value is indicative of a decision to cease contention for access to the communications medium.
12. A method according to claim 10 or claim 11, further comprising modifying said contention metric value to a highest value in said set of predefined contention metric values, thereby providing an indication that said shared communication medium is occupied.
13. A computer program product comprising computer executabLe instructions to cause a computer to become configured to perform a method according to any one of the preceding claims.
14. A computer product according to cLaim 13 comprising a computer readable storage medium.
IS. A computer program product according to claim 13 comprising a computer receivable signal.
16. A communication device operable to contend for access of a shared communication medium for transmission of data in a data portion of a packet in a communications network comprising a plurality of communication devices, the communication device comprising: a storage means for storing at least one set of predefined contention metric values; a selector operable to select a contention metric value from said at least one set of predefined contention metric values; a receiver for monitoring the communication medium for the presence of a control message, such a control message comprising a contention metric value; a contention metric comparing unit operable to compare a contention metric vaLue on a received control message with a contention metric value selected by said selecting means; a contention process manager operable to decide, on the basis of a comparison conducted by the contention metric comparing unit, whether to continue contending for access to the communications medium; and a transmitter for transmitting a control message in a control portion of said packet via said shared communication medium to at least another one of said plurality of communication devices in said communications network, wherein said control message comprises said selected contention metric value; wherein the receiver is operable to monitor the medium for a first listening period for the presence of a control message on the medium, the comparing unit is operable to compare a contention metric on any received control message with the selected contention metric value, and the contention process manager is operable to discontinue contention on the basis that the comparison is indicative that a received control message wins the contention, and wherein the contention process manager is operable to determine that the device has access to the medium on completion of contention without discontinuation.
17. A communication device according to claim 16, wherein said selector is further operable to select said at least one set of predefined contention metric values from a plurality of sets of predefined contention metric values stored in said storage means.
18. A communication device according to claim 17, wherein each of said plurality of sets of predefined contention metric values is associated with a priority level for transmission of data in said shared communication medium.
19. A communication device according to claim 18, wherein said control message further comprises a priority level associated with said selected said set of predefined contention metric values.
20. A communication device according to any one of claims 16 to 19, wherein said further contention metric value is selected from a further set of predefined contention metric values stored in said storage means.
21. A communication device according to claim 20, wherein said at least one further control message comprises a further priority level associated with said further set of predefined contention metric values.
22. A communication device according to claim 21, wherein said selector is operable to compare said priority level and said further priority level to determine whether said communication device can access said shared communication medium for transmission of said data signal.
23. A communication device according to claim 22, wherein said signal processor is operable to terminate contention of said shared communication medium if said priority level is lower than said further priority level.
24. A communication device according to any one of claims 16 to 23, wherein said further contention metric value is selected from said set of predefined contention metric values.
25. A communication device according to any one of claims 16 to 24, wherein said transmitter is operable to transmit said data in said data portion of said packet on said shared communication medium if it is determined that said communication device can access said shared communication medium.
26. A communication device according to claim 25, wherein said contention metric value is higher than said further contention value.
27. A communication device according to claim 25 or claim 25, wherein said signal processor is further operable to modifying said contention metric value to a highest value in said set of predefined contention metric values, thereby providing an indication that said shared channel is occupied