GB2529673A

GB2529673A - Method and device for data communication in a network

Info

Publication number: GB2529673A
Application number: GB1415257.3A
Authority: GB
Inventors: Pascal Viger; Romain Guignard; Stã Phane Baron
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2014-08-28
Filing date: 2014-08-28
Publication date: 2016-03-02
Anticipated expiration: 2034-08-28
Also published as: GB201415257D0; GB2529673B

Abstract

A method of sending data over a communication network is disclosed. A communication device accesses the network using a contention-type access mechanism based on computation of back-off values 311 for a plurality of access categories serving traffic queues at different priorities. Access category back-off values are computed based on the priority of the associated traffic queue. A synchronized back-off value 500 is computed and used to contend access to the network for transmitting data of one of the traffic queues. At least one of access category back-off values is adjusted 707 according to the synchronized back-off value. The adjustment may make the synchronized back-off value coincide with the value of at least one access category back-off value, preferably the one with the lowest back-off value. Instead, it may make the synchronized back-off value lower than a minimum back-off value. The adjustment may apply an offset value to at least one access category back-off value, the offset value being preferably the difference between the value of the lowest back-off value and the current synchronized value. The synchronized back-off value may be computed using a deterministic method, which may comprise messaging and/or pseudo-random generation, in accordance with the values of other devices in the network.

Description

Intellectual Property Office Application No. GB1415257.3 RTTVI Date:22 December 20t4 The following terms are registered trade marks and should be read as such wherever they occur in this document:

IEEE

Intellectual Property Office is an operating name of the Patent Office www.ipo.govuk

METHOD AND DEVICE FOR DATA COMMUNICATION

IN A NETWORK

FIELD OF THE INVENTION

The present invention relates generally to communication networks and more specifically to methods and devices for data communication over a network, the network being accessible by communication devices using a contention type access mechanism.

BACKGROUND OF THE INVENTION

Many wireless local area networks (WLANs), such as wireless communication networks using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), are founded on the principle of collision avoidance. Such networks may also conform to a communication standard such as a communication protocol of 802.11 type e.g. Medium Access Control (MAC).

The IEEE 802.11 MAC standard defines the way WLAN5 must work at the physical and medium access control (MAC) level. Typically, the 802.11 MAC (Medium Access Control) implements the well-known Distributed Coordination Function (DCF) which relies on a contention-based mechanism based on the so-called Carrier Sense Multiple Access with Collision Avoidance" (CSMA/CA) technique.

The 802.11 medium access protocol standard is mainly directed to the management of communication nodes waiting for the medium to become idle so as to try to access the medium.

Figure 1 illustrates a communication system in which several communication nodes exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN).

Access to the shared radio medium to send data frames is based on the CSMA/CA technique, for sensing the carrier and avoiding collision by separating concurrent transmissions in space and time.

Carrier sensing in CSMA/CA is performed by both physical and virtual mechanisms. Virtual carrier sensing is achieved by transmitting control frames to reserve the medium prior to transmission of data frames.

Then a transmitting or source node first attempts through the physical mechanism, to sense a medium that has been idle for at least one DIFS (standing for DCF lnterFrame Spacing) time period, before transmitting data frames.

However, if it is sensed that the shared radio medium is busy during the DIFS period, the transmitting node continues to wait until the radio medium becomes idle. To do so, it starts a countdown back-off counter designed to expire after a number of timeslots, chosen randomly in the interval [0,CW], CW (integer) being referred to as the Contention Window. This back-off mechanism or procedure is the basis of the collision avoidance mechanism that defers the transmission time for a random interval, thus reducing the probability of collisions on the shared channel. After the back-off time period, the transmitting node may send data or control frames if the medium is idle.

One problem of wireless data communications is that it is not possible for the transmitting node to listen while sending, thus preventing the transmitting node from detecting data corruption due to channel fading or interference or collision phenomena. A transmitting node remains unaware of the corruption of the data frames sent and continues to transmit the frames unnecessarily, thus wasting access time.

The Collision Avoidance mechanism of CSMA/CA thus provides positive acknowledgement (ACK) of the sent data frames by the receiving node if the frames are received successfully, to notify the transmitting node that no corruption of the sent data frames occurred.

The ACK is transmitted at the end of reception of the data frame, immediately after a period of time called Short lnterFrame Space (SIFS).

If the transmitting node does not receive the ACK within a specified ACK timeout or detects the transmission of a different frame on the channel, it may infer data frame loss. In that case, it generally reschedules the frame transmission according to the above-mentioned back-off procedure. However, this can be seen as a bandwidth waste if only the ACK has been corrupted but the data frames were correctly received by the receiving node.

To improve the Collision Avoidance efficiency of CSMA/CA, a four-way handshaking mechanism is optionally implemented. One implementation is known as the RTS/CTS exchange, defined in the 802.11 standard.

The RTS/CTS exchange consists in exchanging control frames to reserve the radio medium prior to transmitting data frames during a transmission opportunity called TXOP in the 802.11 standard as described below, thus protecting data transmissions from any further collisions.

As explained before, a back-off procedure is implemented in the communication nodes.

Figure 2 illustrates the behaviour of three groups of nodes during a conventional communication over the 802.11 medium: transmitting or source node 20, receiving or addressee or destination node 21 and other nodes 22 not involved in the current communication.

Upon starting the back-off process 270 prior to transmitting data, a station e.g. transmifting node 20, initializes its back-off time counter to a random value as explained above. The back-off time counter is decremented once every time slot interval 260 for as long as the radio medium is sensed idle (countdown starts from TO, 23 as shown in the Figure).

The time unit in the 802.11 standard is the slot time called aSlotTime' parameter. This parameter is specified by the PHY (physical) layer (for example, aSlotTime is equal to 9ps for the 802.1 in standard). All dedicated space durations (e.g. back-off) are multiples of this time unit.

The back-off time counter is frozen' or suspended when a transmission is detected on the radio medium channel (countdown is stopped at Ti, 24 for other nodes 22 having their back-off time counter decremented).

The countdown of the back-off time counter is resumed or reactivated when the radio medium is sensed idle anew, after a DIES time period. This is the case for the other nodes at T2, 25 as soon as the transmission opportunity TXOP granted to transmitting node 20 ends and the DIFS period 28 elapses. DIFS 28 (DCF inter-frame space) thus defines the minimum waiting time for a transmitting node before trying to transmit some data. In practice, DIES = SIES + 2 * aSlotTime.

Wien the back-off time counter reaches zero (26) at Ti, the timer expires, the corresponding node 20 will request access onto the medium in order to be granted a TXOP, and the back-off time counter is reinitialized 29 using a new random back-off value.

In the example of the Eigure implementing the RTS/CTS scheme, at Ti, the transmitting node 20 that wants to transmit data frames 230 sends a special short frame or message acting as a medium access request to reserve the radio medium, instead of the data frames themselves, just after the channel has been sensed idle for a DIFS or after the back-off period as explained above.

The medium access request is known as a Request-To-Send (RTS) message or frame. The RTS frame generally includes the address of the receiving node ("destination 21") and the duration for which the radio medium is to be reserved for transmitting the control frames (RTS/CTS) and the data frames 230.

Upon receiving the RTS frame and if the radio medium is sensed as being idle, the receiving node 21 responds, after a SIFS time period 27 (for example, SIFS is equal to 16 ys for the 802.11 n standard), with a medium access response, known as a Clear-To-Send (CTS) frame. The CTS flame indicates the remaining time required for transmitting the data frames, computed from the time point at which the CTS frame starts to be sent.

The CTS frame is considered by the transmitting node 20 as an acknowledgment of its request to reserve the shared radio medium for a given time duration.

Thus, the transmitting node 20 expects to receive a CTS frame 220 from the destination node 21 before sending data 230 using unique and unicast (one source address and one addressee or destination address) frames.

The transmitting node 20 is thus allowed to send the data frames 230 upon correctly receiving the CTS frame 220 and after a new SIFS time period 27. In IEEE 802.11, the data are sent as MAC (Medium Access Control) Service Data Units (MSDUs) encapsulated into MAC Protocol Data Units (MPDUs).

An ACK frame 240 is sent by the receiving node 21 after having correctly received the data frames sent, after a new SIES time period 27.

If the transmitting node 20 does not receive the ACK 240 within a specified ACK Timeout (generally within the TXOP), or if it detects the transmission of a different frame on the radio medium, it reschedules the frame transmission according to the back-off procedure.

Since the RTS/CTS four-way handshaking mechanism 210/220 is optional in the 802.11 standard, it is possible for the transmitting node 20 to send data frames 230 immediately upon its back-off time counter reaching zero (i.e. at Ti).

The requested time duration for transmission defined in the RTS and CTS frames defines the length of the granted transmission opportunity TXOP, and can be read by any listening node ("other nodes 22" in Figure 2) in the radio network.

To do so, each node has in memory a data structure known as the network allocation vector or NAV to store the time duration for which it is known that the medium will be busy. Wien listening to a control frame (RTS 210 or CTS 220) not addressed to itself, a listening node 22 updates its NAys (NAV 255 associated with RTS and NAV 250 associated with CTS) with the requested transmission time duration specified in the control frame. The listening node 22 thus keeps in memory the time duration for which the radio medium will remain busy.

Access to the radio medium for the other nodes 22 is consequently deferred 30 by suspending 31 their associated timer and then by later resuming 32 the timer when the NAV has expired.

This prevents the listening nodes 22 from transmitting any data or control frames during that period.

It is possible that the destination node 21 does not receive the RTS frame 210 correctly due to a message/frame collision or to fading. Even if it does receive it, the destination node 21 may not always respond with a CTS 220 because, for example, its NAV is set (i.e. another node has already reserved the medium). In any case, the transmitting node 20 enters into a new back-off procedure.

The RTS/CTS four-way handshaking mechanism is very efficient in terms of system performance, in particular with regard to large frames since it reduces the length of the messages involved in the contention process.

In detail, assuming perfect channel sensing by each communication node, collision may only occur when two (or more) frames are transmitted within the same time slot after a DIFS 28 (DCF inter-frame space) or when their own back-off counter has reached zero nearly at the same time Ti. If both transmitting nodes use the RTS/CTS mechanism, this collision can only occur for the RTS frames. Fortunately, such collision is early detected by the transmitting nodes since it is quickly determined that no CTS response has been received. As described above, the original IEEE 802.11 MAC always sends an acknowledgement (ACK) frame 240 after each data frame 230 received.

However collisions limit the optimal functioning of the radio network. In particular, simultaneous transmit attempts from a number of wireless stations lead to collisions. The back-off procedure for the family of IEEE 802.11 standards was first introduced for the DCF mode as the basic solution for collision avoidance, and further employed by the IEEE 802.lle to solve the problem of internal collisions between enhanced distributed channel access functions (EDCAFs). In the emerging IEEE 802.iin/ac standards, the back-off procedure is still used as the fundamental approach for supporting distributed access among mobile stations. The EDCA extends the legacy DCF function to provide a service differentiation and prioritization mechanism. This mechanism is still based on CSMA/CA, but also introduces Arbitration IFS (AIFS) to offer several levels of priority. Thus, the EDCA mechanism is limited to a soft" QoS, i.e. traffic prioritization. Therefore, the DCF does not provide an effective QoS mechanism but rather a kind of "best effort" mechanism. The EDCA mechanism appears effective for traffic classes with different priorities. However, for nodes having several classes of traffic, several back-off mechanisms are handled in parallel thus leading to increased collision risks. This aspect will be further described with reference to figure 3.

A further deficiency of the random back-off procedure lies in that, the randomly chosen value of the back-off slot count may degenerate the utility of the medium and thus degrade the performance of carrier sense multiple access (CSMA) technique, especially in case a large number of competing stations are involved (saturated traffic scenarios). This problem may become particularly critical in combination with the EDCA problematic mentioned above.

Collisions may be reduced or eliminated with upcoming deterministic back-off methods for driving the wireless medium access. Deterministic values are selected for the back-off slot counts of stations. Duplication among distributed slot counts can therefore be avoided and each station can exclusively access the medium at a time without colliding with others. The stations implementing a deterministic back-off methods form a back-off-synchronized community.

Different examples of deterministic back-off channel access may be envisioned: round-robin scheme (US2O11/007656 THOMSON LICENSING), determining a slot count based on a number of competing stations, next back-off advertisement through messaging (US 2009/0141738 HITACHI), distributed method based on back-off calculation from a known seed (W02012160474 CANON), etc... An example of the latter will be explained in relation with Figure 4.

In general, the effective use of deterministic back-off may rely on the ability of each station to "hear" transmissions from the other stations. This may be necessary in order to synchronize the back-off countdown amongst the nodes.

The deterministic back-off methods rely on DCF function but none of them have yet considered applying traffic classification as the EDCA mechanism. Indeed, applying traffic classification to a deterministic back-off method will lead to complicated management procedures since each node will require a plurality of back-offs (4 in case of EDCA) to be handled and each node of the community will need to know all the back-off values of the other nodes (4 per node in case of EDCA). Therefore, a too large number of back-off values need to be managed. Furthermore, upon a medium access, no means exists to know which queue is used and thus determine which back-off has triggered the current medium access, except when using a non-standard method which would prevent to integrate standard node in the network.

It is an aim of the present invention to simply manage traffic classification per priority (as EDCA) inside a back-off-synchronized community while improving radio medium access.

The present invention has been devised to address at least one of the foregoing concerns as well as their combination.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a communication device for a communication network, comprising: -a controller for accessing the communication network using a contention type access mechanism based on computation of back-off values; -a plurality of buffers for serving data traffic queues at different priorities; the controller comprising a plurality of back-off engines, each associated with one traffic queue and configured to compute a back-off value in accordance with the priority of the associated traffic queue; the controller comprising a back-off management module configured to compute a synchronized back-off value to be used to contend access to the communication network for transmitting data of one of the traffic queues; and wherein the device further comprises means for adjusting at least one of the plurality of back-off engines according to the synchronized back-off value computed by the back-off management module.

These features allow a device to simply manage traffic classification per priority (as EDCA) inside a back-off synchronized community while improving radio medium access. The device is able to manage a randomized prioritization for local traffic. The implementation of these features within a standard environment is easy, since only parameters need to be adjusted.

According to an embodiment, the means for adjusting at least one of the plurality of back-off engines makes coincide the synchronized back-off value computed by the back-off management module with a back-off value of the at least one back-off engine.

This feature allows having at least one of the plurality of buffers serving data traffic queues upon expiration of the synchronized back-off. In addition, no modification is performed in the medium access mechanism as the standard

B

mechanism is used: one of the plurality of back-off engines performs the request of medium access. Therefore no change to a state machine is required.

According to another embodiment, the means for adjusting at least one of the plurality of back-off engines makes the synchronized back-off value computed by the back-off management module lower than the minimum back-off value of the back-off engines.

This feature allows having a medium access request only performed by the synchronized back-off mechanism, so that a community medium access is issued according to the community protocol. In addition, the plurality of back-off engines are transparently prevented from requesting a medium access by themselves and therefore no change to their state machine is required.

In addition, this feature allows requesting a medium access for the community even if the plurality of buffers are empty (no pending data in queues).

Following a further embodiment, the device comprises means for selecting the back-off engine having the lowest back-off value among the plurality of back-off engines and wherein the means for adjusting at least one of the plurality of back-off engines makes the computed synchronized back-off value coincide with or lower than the back-off value of the selected back-off engine.

This feature allow to keep the standard traffic classification per priority (as EDCA), by keeping the relative priorities among the plurality of buffers.

In still another embodiment, the means for adjusting at least one of the plurality of back-off engines comprises means for applying an offset value.

The offset value is advantageously computed from the difference between the lowest back-off value among the plurality of back-off engines and a current synchronized back-off value and (6.) the means for applying the offset value adjusts all the back-off engines with said offset value.

In a specific embodiment, the means for adjusting at least one of the plurality of back-off engines comprises means for overwriting a register containing the back-off value of the at least one back-off engine.

According to a particular embodiment, the back-off management module computes the synchronized back-off value using a deterministic method in accordance with computed synchronized back-off values of other devices of the communication network. The deterministic method may comprise messaging and/or pseudo random generation for computing the synchronized back-off values.

According to a second aspect of the invention, there is provided a method of sending data over a communication network by a communication device using a contention type access mechanism based on computation of back-off values and further using different access categories serving data traffic queues at different priorities, the method comprising: -computing access category back-off values in accordance with the priority of each associated traffic queue; -computing a synchronized back-off value; -adjusting at least one of the access category back-off values according to the computed synchronized back-off value, and -contending access to the communication network using said computed synchronized back-off value for sending data of the traffic queue corresponding to said at least one of the adjusted access category back-off value.

These features allow to simply manage traffic classification per priority (as EDCA) inside a back-off synchronized community while improving radio medium access. The method enables a device to manage a randomized prioritization for local traffic. The implementation of these features within a standard environment is made easy, since only parameters need to be adjusted and therefore no change to a state machine is required. Since no overhead is required, the present method allows coping with any number of traffic queues.

According to an embodiment, the step of adjusting consists in making coincide the computed synchronized back-off value with the at least one of the access category back-off values.

According to another embodiment, the step of adjusting consists in making the computed synchronized back-off value lower than a minimum back-off value of the back-off engines.

Following a further embodiment, the method comprises a step of selecting the traffic queue having the lowest back-off value among the traffic queues and wherein the step of adjusting consists in making the computed synchronized back-off value coincide with or lower than the access category back-off value of the selected traffic queue.

In still another embodiment, the adjusting step comprises applying an offset value to adjust the at least one of the access category back-off value.

The offset value is advantageously computed from the difference between the lowest access category back-off value among the traffic queues and a current synchronized back-off value and the adjusting step comprises applying the offset value to all the access category back-off of the traffic queues.

In a specific embodiment, the adjusting step comprises overwriting a register containing the at least one access category back-off value.

According to a particular embodiment, the synchronized back-off value is computed using a deterministic method in accordance with synchronized back-off values of other devices of the communication network. The deterministic method may comprise messaging and/or pseudo random generation for computing the synchronized back-off values.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which: -Figure 1 illustrates a typical wireless communication system in which embodiments of the invention may be implemented; -Figure 2 is a timeline schematically illustrating a conventional communication mechanism according to the IEEE 802.11 standard; -Figures 3a, 3b and 3c illustrate the IEEE 802.lle EDCA involving access categories; -Figure 4 illustrates a collaborative group applying a deterministic back-off scheme according to an aspect of the present invention; -Figure 5 illustrates an embodiment of the present invention in a block diagram form; -Figure 6 is a block diagram illustrating components of a communication device in which embodiments of the invention may be implemented; -Figure 7 illustrates function blocks of the same communication device; -Figure 8 is an algorithm illustrating an embodiment of the present invention; and -Figure 9 details the algorithm of figure 8 according to a specific embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention provides communication methods and devices for data communication over a communication network, the physical medium of which being shared between a plurality of communication nodes or devices. An exemplary communication network is an IEEE 802.lle wireless ad-hoc network (and upper versions). The invention applies to any wireless network where a source node 101-107 sends data of a data stream to a receiving node 101-107 using a deterministic back-off scheme and supporting QoS per priority. In the accompanying figures, the same elements are represented by the same reference numbers.

The behaviour of communication nodes during a conventional communication over an 802.11 medium has been recalled above with reference to Figures 1 and 2, including the RTS/CTS four-way handshaking mechanism 210/220.

Figures 3a, 3b and 3c illustrate the IEEE 802.lle EDCA involving access categories.

In the IEEE 802.1 le standard providing quality of service enhancements to make more efficient use of the wireless channel, an optional feature, called Block ACK (acknowledgment per block), allows two or more data frames 230 to be transmitted before a Block ACK frame is returned to acknowledge the receipt of the data frames.

The Block ACK increases communication efficiency since only one signalling ACK frame is needed to acknowledge a block of frames, while every ACK frame originally used has a significant overhead for radio synchronization.

Block ACK is initiated through a setup and negotiation process between the transmitting or source node and the receiving node.

Once the Block ACK has been negotiated and established, multiple data frames can be transmitted in a contention free burst (or during the TXOP), with SIFS separation between the successive data frames. The Block ACK frame returned to the source node includes an acknowledgment bitmap, each information item (e.g. a bit) of which acknowledging the receipt of one of the data frames.

Another aspect that the IEEE 802.lle standard has improved regards the quality of service (QoS), for example latency of data traffic. In the original standard, a communication node includes only one transmission queue/buffer. However, since a subsequent data frame cannot be transmitted until the transmission/retransmission of a preceding frame ends, the delay in transmitting/retransmitting the preceding frame prevents the communication from having QoS.

The IEEE 802.lle has overturned this deficiency in providing quality of service (QoS) enhancements to make more efficient use of the wireless medium.

This standard relies on a coordination function, called hybrid coordination function (HCF), which has two modes of operation: enhanced distributed channel access (EDCA) and HCF controlled channel access (HCCA).

EDCA enhances or extends functionality of the original access DCF method: EDCA has been designed for support of prioritized traffic similar to DiffServ (Differentiated Services), which is a protocol for specifying and controlling network traffic by class so that certain types of traffic get precedence.

EDCA is the dominant channel access mechanism in WLAN5 because it features a distributed and easily deployed mechanism.

The above deficiency of failing to have satisfactory QoS due to delay in frame retransmission can be solved with a plurality of transmission queues/buffers.

QoS support in EDCA is achieved with the introduction of four Access Categories (ACs), and thereby of four corresponding transmission queues or buffers (310).

Each AC has its own transmission queue/buffer to store corresponding data frames to be transmitted on the network. The data frames, namely the MSDUs, incoming from an upper layer of the protocol stack are mapped onto one of the four AC queues/buffers and thus input in the mapped AC buffer.

Each AC has also its own set of channel access parameters, and is associated with a priority value, thus defining traffic of higher or lower priority of MSDUs.

That means that each AC (and corresponding buffer) acts as an independent DCF contending entity including its respective back-off engine 311. In other words, the ACs within the same communication node compete one with each other to access the wireless medium and to obtain a transmission opportunity, using the contention mechanism as explained above with reference to Figure 2 for example.

Service differentiation between the ACs is achieved by setting different contention window parameters (CWmin, CWmax), arbitrary interframe spaces (AIFS), and transmission opportunity duration limits (TXOP_Limit).

With EDCA, high priority traffic has a higher chance of being sent than low priority traffic: a node with high priority traffic waits a little less (low CW) before it sends its packet, on average, than a node with low priority traffic.

The four AC buffers (310) are shown in Figure 3a.

Buffers AC3 and AC2 are usually reserved for real-time applications (e.g., voice or video transmission). They have, respectively, the highest priority and the last-but-one highest priority.

Buffers AC1 and ACO are reserved for best effort and background traffic.

They have, respectively, the last-but-one lowest priority and the lowest priority.

Each data unit, MSDU, arriving at the MAC layer from an upper layer (e.g. Link layer) with a priority is mapped into an AC according to mapping rules. Figure 3b shows an example of mapping between eight priorities of traffic class (User Priorities or UP, 0-7 according IEEE 802.ld) and the four ACs. The data frame is then stored in the buffer corresponding to the mapped AC.

When the back-off procedure of an AC ends, the MAC controller (reference 704 in Figure 7 below) of the transmitting node transmits a data frame from this AC to the physical layer for transmission onto the wireless communication network.

Since the ACs operate concurrently in accessing the wireless medium, it may happen that two ACs of the same communication node have their back-off ending simultaneously. In such a situation, a virtual collision handler of the MAC controller operates a selection of the AC having the highest priority between the conflicting ACs, and gives up transmission of data frames from the ACs having lower priorities.

Then, the virtual collision handler commands those ACs having lower priorities to start again a back-off operation using an increased CW value.

Figure 3c illustrates configurations of a header of a MAC data frame and a QoS control field (300) included in the header of the IEEE 802.11 e MAC frame.

As represented in the Figure, the QoS control field 300 is made of two bytes, including the following information items: Bits BO to B3 are used to store a traffic identifier (TID) which identifies a traffic stream. The traffic identifier takes the value of the transmission priority value (User Priority UP, value between 0 and 7 -see Figure 3b) corresponding to the data conveyed by the data frame or takes the value of a traffic stream identifier (TSID, value between 8 and 15) for other data streams.

Bits B5 and B6 define the ack policy subfield which specifies the acknowledgment policy associated with the data frame. This subfield is used to determine how the data frame has to be acknowledged by the receiving node. Usually, it may take three different values as follows: -value equal to "Normal ACK" in the case where the transmitting node or source node requires a conventional acknowledgment to be sent (by the receiving node) after a shod interframe space (SIFS) period following the transmission of the data frame; -value equal to "No ACK" in the case where the source node does not require acknowledgment. That means that the receiving node takes no action upon receipt of the data frame; and -value equal to "Block ACK" as defined above. The receiving node takes no action immediately upon receiving the data frame, except the action of recording the state of reception in its scoreboard context. With such a value, the source node is expected to send a Block ACK request (BAR) frame, to which the receiving node responds using the procedure described below.

The other bits B4 and B7-B15 are of less importance for embodiments of the present invention.

The present inventors have envisaged using a collaborative medium access scheme with deterministic back-off for several nodes at a time, those nodes pertaining to a group of peer nodes, also called collaborative nodes. It is envisaged that there will also exist nodes outside the collaborative group, and that communication between nodes inside and nodes outside the group may be required. Nodes outside the group may be referred to as legacy nodes. A legacy environment typically describes a situation where nodes are independent and do not interact or cooperate with each other, as opposed to the collaborative group of nodes. A peer node may request access to the shared 802.11 type medium according to the 802.11 legacy protocol, and upon grant of access, the node may communicate with one or more peer nodes according to a collaborative protocol during the reserved talk time. Thus, if the back-off count reaches zero for one peer node among the group, said node reserves medium access (through classical RTSICTS scheme) for the group and lets the group share this granted 802.11 timeslot.

Figure 4 illustrates a collaborative group back-off slot positioning.

While targeting an improved communication service on top of classical 802.11 type CSMA/CA, attention of the inventors has been directed on the grouping of a set of peer nodes among a 802.11 legacy environment to allow a collaborative medium access.

More precisely, medium access collaboration is performed by watching the random values' used to initialize the back-off time counters among the group, in order that no back-off time count duplication occurs this is the deterministic back-off way of functioning. Each node may generate a virtual local image of its peers back-off time counters values by computing the back-off time of each of its peers, or have access by other means to said back-off values. Each node manages its own back-off time. Thus, each node of the group can predict the back-off time values of its peers and would attempt to access the wireless medium in a distinct back-off slot, avoiding any access collision among the group of peer nodes.

Figure 4 illustrates this concept. The idea comprises spreading the allocation of back-off counts among the group of peer nodes to give a collaborative group back-off positioning. This is effected by positioning the back-off time expiration of each node of the collaborative group in a different slot number (as example, by considering an absolute scale of back-off slots, the current back-off count being number n' 400, the scheme would select n+3' 410 and n+10' 420 for two distinct peer nodes). The sharing of back-off slot positioning information may be performed either by messaging on the wireless channel, or in a preferred embodiment by pseudo-random generation of a next back-off slot computation according to the collaborative group back-off slot positioning scheme. The count of expired back-off slots (said going from n to n+1) is performed by wireless channel spying by each node of the collaborative group, whereby all collaborative nodes can see the medium activity (mainly idle and busy periods).

The sharing of back-off slot positioning information allows the collaborative nodes to synchronize their access to the medium and avoid collisions. However various concurrent communications according EDCA behavior, wherein each EDCA queue has its own back-off engine, may potentially introduce a loss of synchronization of the back-off counters which are setup among the group, leading to medium collision inside the group.

Figure 5 illustrates an embodiment of the present invention and shows the enhancement over figure 3a.

According to an embodiment of the invention there is provided a collaborative access method or protocol for accessing a communication medium used by a plurality of communication terminals, comprising a medium allocation scheme for several nodes of a collaborative group at a time: each node of the group has a deterministic back-off value for requesting a transmission onto the medium. A SYNC back-off management module 500 (embodied in bloc 706 in figure 7) implements the management of the deterministic back-off value.

According to an embodiment of the present invention, this module is provided with a feature consisting in adjusting, for example by updating, the value of each AC back-off engine 311 in order that at least one AC back-off engine 311 will elapse at the same time as the SYNC back-off.

As a result, when a node of the group is granted to request access onto the communication medium by the collaborative scheme because at least one of the node's AC back-off has elapsed with the SYNC back-off, the node's SYNC back-off initiates the sending of RTS frame 210.

As at least one of AC back-off value is the same value as the SYNC back-off value, a transmission of pending data in corresponding AC buffer can follow after a CTS frame response.

As a result, by managing AC back-off values in accordance with SYNC back-off, the local node will never issue a medium access request (RTS) outside the scope of SYNC back-off. Embodiments of the present invention provide thus a simple answer to the drawbacks identified in the prior-art.

One main advantage of embodiments of the present invention is to still be able to reuse the hardware/state-machine of standard back-off mechanism, in particular the basic mechanism that enables, when a back-off value reaches zero, a medium access to be requested. Adjusting the back-off values is implemented simply by overwriting AC registers.

In addition, in a particular embodiment, it is possible to avoid having an additional back-off register for the SYNC back-off. A software module 706 may advantageously use one of the AC back-offs to set directly the SYNC value without requiring a separate SYNC back-off register, avoiding so the need of a specific hardware. Thus this current AC back-off register will drive the medium access request along with the SYNC value managed by the collaborative community.

According to another embodiment of the present invention, the back-off management module 500 is provided with a feature consisting in adjusng, for example by updating, the value of each AC back-off engine 311 in order that none of the AC back-off engine 311 will elapse at the same time as the SYNC back-off. Typically, a minimum value 1 of at least one of the AC back-off engine 311 is reached when the SYNC back-off is down to value 0.

As a result, when a node of the group is granted to request access onto the communication medium by the collaborative scheme because the node's SYNC back-off has elapsed, the node's SYNC back-off initiates the sending of RTS frame 210: this medium access request is performed through the SYNC backoff mechanisms, and not through the legacy AC back-off mechanisms.

As at least one of AC back-off value has a minimum value (typically the value is SYNC back-off value + 1, as stated before), a transmission of pending data in such AC buffer can follow after a CIS frame response.

One main advantage of said another embodiment of the present invention is to avoid to reuse the hardware/state-machine of standard back-off mechanism of AC buffers, but in contrary to use the specific hardware/state-machine of SYNC back-off mechanism. In particular the SYNC back-off mechanism enables, when the SYNC back-off value reaches zero, a medium access to be requested according the protocols of the group. For example, the RTS/CTS handshake of the group may differ from the legacy one performed by AC buffers in that frames may use different parameters like MAC addresses selection.

Further advantages will become apparent in regards to the detailed

description of module 706 (Figures 8 and 9).

Figure 6 schematically illustrates a communication device 600 of the radio network 100, configured to implement at least one embodiment of the present invention. The communication device 600 may be a device such as a micro-computer, a workstation or a light portable device. The communication device 600 comprises a communication bus 613 to which there are preferably connected: -a central processing unit 611, such as a microprocessor, denoted CPU; -a read only memory 607, denoted ROM, for storing computer programs for implementing the invention; -a random access memory 612, denoted RAM, for storing the executable code of methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing methods according to embodiments of the invention; and -at least one communication interface 602 connected to the radio communication network 100 over which digital data packets or frames are transmitted, for example a wireless communication network according to the 802.lln protocol. The data frames and aggregated frames are written from a FIFO sending memory in RAM 612 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 612 under the control of a software application running in the CPU 611.

Optionally, the communication device 600 may also include the following components: -a data storage means 604 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the invention; -a disk drive 605 for a disk 606, the disk drive being adapted to read data from the disk 606 or to write data onto said disk; -a screen 609 for displaying decoded data and/or serving as a graphical interface with the user, by means of a keyboard 610 or any other pointing means.

The communication device 600 can be connected to various peripherals, such as for example a digital camera 608, each being connected to an input/output card (not shown) so as to supply data to the communication device 600.

The communication bus provides communication and interoperability between the various elements included in the communication device 600 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 600 directly or by means of another element of the communication device 600.

The disk 606 can be replaced by any information medium such as for example a compact disk (CD-ROM), rewritable or not, a ZIP disk, a USB key or a memory card and, in general terms, by an information storage means that can be read by a microcomputer or by a microprocessor, integrated or not into the device, possibly removable and adapted to store one or more programs whose execution enables a method according to the invention to be implemented.

The executable code may be stored either in read only memory 607, on the hard disk 604 or on a removable digital medium such as for example a disk 606 as described previously. According to a variant, the executable code of the programs can be received by means of the communication network 100, via the interface 602, in order to be stored in one of the storage means of the communication device 600, such as the hard disk 604, before being executed.

The central processing unit 611 is adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to the invention, which instructions are stored in one of the aforementioned storage means. On powering up, the program or programs that are stored in a non-volatile memory, for example on the hard disk 604 or in the read only memory 607, are transferred into the random access memory 612, which then contains the executable code of the program or programs, as well as registers for storing the variables and parameters necessary for implementing the invention.

In the embodiment illustrated in figure 6, the device is a programmable apparatus which uses software to implement the invention. However, alternatively, the present invention may be implemented in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC).

Figure 7 is a block diagram schematically illustrating the architecture of a communication device or node 600 adapted to carry out, at least partially, the invention. As illustrated, node 600 comprises a physical (PHY) layer block 703, a MAC layer block 702, and an application layer block 701.

The PHY layer block 703 (here a 802.11 standardized PHY layer) has the task of formatting and sending or receiving frames over the radio medium used 100, such as a medium access request of the RTS type to reserve a transmission slot, a medium access response of the CTS type to acknowledge reservation of a transmission slot, as well as of MAC data frames and aggregated frames to/from that radio medium.

The MAC layer block or controller 702 comprises a standard MAC 802.11 layer 704 and three additional blocks 705 to 707 for carrying out, at least partially, the invention. The MAC layer block 702 may be implemented in software, which software is loaded into RAM 612 and executed by CPU 611.

The RTS/CTS module 705 has the task of managing Request-To-Send (RTS) and Clear-To-Send (CTS) messages, according to a collaborative medium access scheme.

The SYNC back-off management module 706 implements the management of the back-off (called SYNC) related to the collaborative medium access scheme defining a deterministic back-off of the local node as well as of the collaborative nodes.

The back-off value of the local node used in a deterministic collision avoidance scheme is therefore different from the ones of the other nodes forming the collaborative group.

Different methods may be employed for defining the deterministic back-off channel access as using a round-robin scheme, determining a slot count based on a number of competing stations, advertising of next back-off through messaging or distributed method based on back-off calculation e.g. from a known seed.

The EDCA back-offs management module 707 mainly implements the algorithm according to an embodiment of the invention, related to updating the EDCA back-off values of 802.11 MAC layer entity 704, according to the value of SYNC back-off.

Finally, the application layer block 701 implements an application that generates and receives data packets, for example data packets of a video stream. The application layer block 701 represents all the stack layers above MAC layer according ISO standardization.

Figure 8 is an algorithm of contending for access to a wireless medium, in accordance with an embodiment of the present invention.

Steps 801, 803, 805, 806, 808 and 809-811 represents the classical EDCA work. Upon arrival of a higher layer frame to transmit (step 801), the transmitting station invokes a back-off procedure (step 803) using a back-off counter to count down a random number of back-off time slots selected between 0 and OW (initially set to CWmin). Each transmission frame from the upper layers bears a priority value (0-7), which is passed down to the MAC layer, and thus the corresponding AC transmission queue is used with its own set of medium access parameters (like one back-off counter among the 4 counters). The transmitting station decrements the back-off counter(s) in progress by one (step 806) as long as the channel is sensed to be idle (steps 805). If the transmitting station senses the channel to be busy at any time during the back-off procedure, the transmitting station suspends its current back-off procedure and freezes its back-off counter(s) until the channel is sensed to be idle for an AIFS interval again.

Then, if the channel is still idle, the transmitting station resumes decrementing its remaining back-off counter(s) (step 806).

Once a back-off counter reaches zero, the transmitting station initiates an RTS transmission and awaits a CTS transmission from the receiving station.

In case a virtual collision is detected among the ACs (step 809 positive) in the virtual collision handler, the higher priority AC is selected (step 810). A transmission request is then initiated (step 811).

According to an aspect of the present invention, new steps 800, 802, 804 and test 807 (performed by module 706) are inserted inside the classical EDCA mechanism. Optionally, a step 812 may also be performed.

At step 800 a setup of the collaborative community is executed. At a first initialization time, a "start" management frame may be exchanged on the wireless medium in order to synchronize the deterministic back-off algorithm at each collaborative node. This may consist in resetting a pseudo-random generator for computing a suite of back-off numbers used by the collaborative community. The effective computation of a SYNC back-off, according to a deterministic scheme, for the local node is then performed in step 802. The computation of SYNC back-off of step 802 may be performed either at an initialization of the community communication phase, or upon elapsing of the SYNC back-off which typically occurs when the local node has talked on the wireless medium at steps 811 or 812.

In order to drive the medium access of the local node according the SYNC back-off, a step 804 is executed for updating the EDCA AC back-off in accordance with the SYNC back-off. An embodiment consists in aligning the lowest value of AC back-off counters (that are active or in progress) to the value of the SYNC back-off value. This procedure will be further explained with reference to figure 9. Another embodiment consists in aligning the lowest value of AC back-off counters (that are active or in progress) to a value greater than the SYNC back-off value. Additionally. the other active AC back-off counters may be shifted according to the alignment. By this means, the relative position amongst the AC back-offs will be kept, so that the EDCA priority remains unchanged. Therefore the access to the network is fully controlled by the SYNC back-off value while the EDCA priority is kept.

Steps 805 and 806 perform as usual in EDCA, checking if the medium is busy (step 805), and in the negative, executing a countdown of all back-off counters, including SYNC back-off (step 806).

A test 807 is here introduced which determines, in case of a back-off procedure is currently in progress for the SYNC back-off, if the SYNC back-off has elapsed. The fact that a back-off procedure for a SYNC back-off is in progress is determined by the MAC module 706. The process of waiting consists in jumping to step 805 (807 negative) until the remaining back-off slots expire (807 positive).

If no back-off procedure for SYNC back-off is in progress, meaning that no collaborative scheme is active, then the classical EDCA process is performed by bypassing test 807. Thus the test 808 checking the end of the AC back-off will conduct to test 805 (808 negative, link not shown in the figure) until an expiry of one of the AC back-off is detected (808 positive).

Otherwise, if SYNC back-off has elapsed (807 positive), the test 808 is executed consisting in verifying if one AC back-off has also elapsed.

If at least one AC back-off expired, then the method proceeds to steps 809- 810-811 in order to send a request to talk on the wireless medium, typically by issuing a RTS frame 210. According to said another embodiment, these steps are also performed if the AC back-off counters (that are active or in progress) were updated according to a value greater than the SYNC back-off value, and as a result at least one AC back-off counter reaches this value.

If the method determines that no AC back-off slots has expired, then the module 706 conducts to step 812 to issue a RTS request on the wireless medium. This case occurs when the collaborative community, synchronized by a deterministic back-off scheme, is active and has allocated a SYNC back-off for the present node whereas no higher layer data traffic is pending in the AC FIFO. Thus, optionally, the step 812 handles this case by still allowing sending a RTS frame even if no data is locally waiting to be transmitted. This action may be advantageous for the collaborative community, as other peer nodes require hearing this current node talking at the correct back-off timeslot in order to maintain the deterministic back-off synchronization.

Figure 9 details the algorithm of figure 8 according to a specific embodiment of the present invention. The present algorithm details step 804 of figure 8 consisting in updating at least one AC back-off with the deterministic back-off.

Basically, a re-alignment of all AC back-offs is performed in step 804 either upon a SYNC back-off modification (in step 802) or upon an AC back-off modification (in step 803). The latter case occurs for example when new data enters in an empty AC FIFO at step 801.

The present algorithm specifies an embodiment, in which step 803 is followed by a step 804 wherein an alignment of the AC back-offs is computed from the difference of the lowest AC back-off and the current SYNC back-off value.

In step 901, the minimum value among AC back-off in progress is determined. The idea here is to align this minimum value with the current value of SYNC back-off.

If this is already the case, the test 902 ends the computation of step 804. If not (902 negative), one of two cases occurs, which is tested in step 903: -if the determined minimum value is lesser than SYNC value (903 positive), then the step 905 is performed; -if the determined minimum value is greater than SYNC value (903 negative), then the step 904 is performed.

The difference or offset between the determined minimum value and SYNC back-off value is computed as MINVAL -SYNC_BACK-OFF in 904, respectively SYNC_BACK-OFF MINVAL in 905. Then all AC back-off counters are decreased, respectively increased, by that difference (step 904 resp. step 905).

In both cases, the expiration of the AC back-off having the minimum value and the SYNC back-off will occur at the same back-off slot time.

Advantageously, in that way, the medium access controller is still allowed to initiate a RTS frame transmission when an AC back-off counter is zero thanks to very limited hardware modification. In practice the algorithm may advantageously be implemented solely by software. In this view, adjusting the back-off values may be implemented simply by overwriting registers. This allows to re-use hardware/state-machine of standard back-off mechanism, that mechanism enabling, when a back-off value reaches zero, a medium access to be requested.

The previously described algorithms are only example of embodiments.

Other ways not shown in the figure may be envisaged. As an example, the medium access controller may be configured to only initiate a RTS frame transmission when the SYNC back-off counter is zero. The countdown of AC back-off can thus continue in negative way or be frozen to zero, so that no more AC back-off can trigger a transmission. The update of AC back-offs is performed uniquely at each SYNC back-off re-computation. As a result, the virtual handler (steps 809 and 810) is modified in order to take into account all less-than-U counters or zero counters in a transmission granted when the SYNC back-off is at zero.

Claims

CLAI MS1. A communication device for a communication network, comprising: -a controller for accessing the communication network using a contention type access mechanism based on computation of back-off values; -a plurality of buffers for serving data traffic queues at different priorities; -the controller comprising a plurality of back-off engines, each associated with one traffic queue and configured to compute a back-off value in accordance with the priority of the associated traffic queue; -the controller further comprising a back-off management module configured to compute a synchronized back-off value to be used to contend access to the communication network for transmitting data of one of the traffic queues; and wherein the device further comprises means for adjusting at least one of the plurality of back-off engines according to the synchronized back-off value computed by the back-off management module.
2. The device of claim 1, wherein the means for adjusting at least one of the plurality of back-off engines makes coincide the synchronized back-off value computed by the back-off management module with a back-off value of the at least one back-off engine.
3. The device of claim 1, wherein the means for adjusting at least one of the plurality of back-off engines makes the synchronized back-off value computed by the back-off management module lower than a minimum back-off value of the back-off engines.
4. The device of claim 1, comprising means for selecting the back-off engine having the lowest back-off value among the plurality of back-off engines and wherein the means for adjusting at least one of the plurality of back-off engines makes the computed synchronized back-off value coincide with or lower than the back-off value of the selected back-off engine.
5. The device of claim 1, wherein the means for adjusting at least one of the plurality of back-off engines comprises means for applying an offset value.
6. The device of claim 5, wherein the offset value is computed from the difference between the lowest back-off value among the plurality of back-off engines and a current synchronized back-off value.
7. The device of claim 6, wherein the means for applying the offset value adjusts all the back-off engines with said offset value.
8. The device of claim 1, wherein the means for adjusting at least one of the plurality of back-off engines comprises means for overwriting a register containing the back-off value of the at least one back-off engine.
9. The device of claim 1, wherein the back-off management module computes the synchronized back-off value using a deterministic method in accordance with computed synchronized back-off values of other devices of the communication network.
10. The device of claim 9, wherein the deterministic method comprises messaging and/or pseudo random generation for computing the synchronized back-off values.
11. A method of sending data over a communication network by a communication device using a contention type access mechanism based on computation of back-off values and further using different access categories serving data traffic queues at different priorities, the method comprising: -computing access category back-off values in accordance with the priority of each associated traffic queue; -computing a synchronized back-off value; -adjusting at least one of the access category back-off values according to the computed synchronized back-off value, and -contending access to the communication network using said computed synchronized back-off value for sending data of the traffic queue corresponding to said at least one of the adjusted access category back-off value.
12. The method of claim 11, wherein the step of adjusting consists in making coincide the computed synchronized back-off value with the at least one of the access category back-off values.
13. The method of claim 11, wherein the step of adjusting consists in making the computed synchronized back-off value lower than a minimum back-off value of the back-off engines.
14. The method of claim 11, comprising a step of selecting the traffic queue having the lowest back-off value among the traffic queues and wherein the step of adjusting consists in making the computed synchronized back-off value coincide with or lower than the access category back-off value of the selected traffic queue.
15. The method of claim 11, wherein the adjusting step comprises applying an offset value to adjust the at least one of the access category back-off value.
16. The method of claim 15, wherein the offset value is computed from the difference between the lowest access category back-off value among the traffic queues and a current synchronized back-off value.
17. The method of claim 16, wherein the adjusting step comprises applying the offset value to all the access category back-off of the traffic queues.
18. The method of claim 11, wherein the adjusting step comprises overwriting a register containing the at least one access category back-off value.
19. The method of claim 11, wherein the synchronized back-off value is computed using a deterministic method in accordance with synchronized back-off values of other devices of the communication network.
20. The method of claim 19, wherein the deterministic method comprises messaging and/or pseudo random generation for computing the synchronized back-off values.
21. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a communication device of a communication network, causes the communication device to perform the methods of claims 11 to 20.
22. A method for data communication in a network substantially as herein described with reference to, and as shown in, Figures 8 and 9 of the accompanying drawings.
23. A communication device in an a network substantially as herein described with reference to, and as shown in, Figures 5, 6 or 7 of the accompanying drawings.