CN102256317B - Wireless channel access control method - Google Patents

Abstract

The invention discloses a wireless channel access control method, which comprises the following steps of: 1, distinguishing priorities of different access types in a network; 2, setting different packet retransmission probabilities aiming at different priorities, and dynamically resolving transmission probabilities of different priority nodes; and 3, competitively transmitting data in the same time slot according to the resolved transmission probabilities of different priority information. By the method, the transmission probabilities of different priority information can be dynamically acquired by adjusting the packet retransmission probabilities of the different priority nodes, so that the length of a time slot to be transmitted is dynamically adjusted through nodes with higher transmission probabilities, that important information has a higher priority is effectively guaranteed, the information can be transmitted faster, and the channel can be most effectively utilized on the basis of quality of service (QoS).

Description

The wireless channel access control method

Technical field

The invention belongs to wireless communication technology field, specifically, relate to a kind of channel access control method that is applicable to wireless network.

Background technology

New network technology such as wireless Ad Hoc (Ad hoc) net, wireless sensor network, WLAN (wireless local area network) and Wireless Mesh network are subjected to the extensive attention of academia and industrial quarters in recent years.These wireless networks adopt the shared channel of competition usually, can be used on places such as the burst scene of accident and meeting that people wish the information of sharing rapidly, office.

Procotol refers to have the level characteristic in order to carry out the exchanges data in the network and to set up rule or agreement.The MAC agreement is the important component part of wireless network protocol, is to be grouped in the major control person who sends and receive on the wireless channel.In wireless network, what the MAC agreement need be considered is how to take full advantage of channel, avoids conflict.Present widely used agreement mainly contains Carrier Sense Multiple Access/collision detection (CSMA/CA) agreement and time division multiple access (TDMA) mechanism.

Understand the present invention and background technology thereof for the ease of those of ordinary skill in the art, when background technology of the present invention is described, at first make as giving a definition:

(1) shared channel: the common channel that uses of a plurality of nodes.Channel Sharing can adopt modes such as frequency division multiplexing or time division multiplexing.

(2) service quality (QoS, Quality of Service): a kind of security mechanism of network, with a kind of technology that solves problems such as network delay and obstruction.Under normal circumstances, if network only is used for specific timeless application system, do not need QoS.But it is just very necessary for key application and multimedia application.When network over loading or when congested, QoS can guarantee that the important service amount is not postponed or abandons, and guarantees the efficient operation of network.

(3) distributed coordination function (DCF, Distributed Coordination Function): a kind of basic medium access protocal, allow to use CSMA/CA and a stochastic backoff process, realize that on the PHY of compatibility automatic medium is shared.The parameter that has some to select in the DCF agreement, the user can adjust to improve performance to it at different network condition.

(4) (CSMA/CA is avoided in Carrier Sense Multiple Access/conflict, Carrier Sense Multiple Access with Collision Avoidance): a kind of agreement that is operated in the MAC layer, in the shared channel of a plurality of user's shared resources of support, before sending data, carried out the mechanism of the availability detection of network earlier by the sender.

(5) priority: pending as a plurality of data etc. is to determine the parameter of the priority level of each data access channel.

(6) time slot (aSlotTime): the periodicity period of the unique identification of any energy and definition.

(7) AIFS: the time span of the access style of different priorities wait channel idle in startup or before waking backoff procedure up.

(8) DIFS:DCF frame period, the website of use DCF transmits to the back at channel idle DIFS and back off time.

(9) data collision/conflict: when certain receiving node B was receiving the grouping that sending node A sends, other node also sent grouping simultaneously in the network, thereby made Node B can't correctly receive the phenomenon of the grouping that sending node A sends.

(10) backoff procedure: in the CSMA/CD agreement, in case detect conflict, in order to reduce the probability of conflict again, need to wait for a random time, and then attempt transmission.The process of this wait random time is backoff procedure.

(11) backoff counter (BC, Backoff Counter): to a record of back off time, through after the initialization, whenever will subtract 1 through its value of channel idle time slot in the backoff procedure.The value of backoff counter directly affects the length of the time of delay of generation.When the value of backoff counter was big, the random back time value of generation was in general longer; When the value of backoff counter hour, the random time value of generation is generally shorter.The value of node backoff counter is more little, and the ability that it seizes channel is just more strong; Otherwise it is just more weak that it seizes the ability of channel.That is to say that the value of backoff counter has reflected the ability of node access channel.

(12) request sends (RTS, Request to Send) grouping and allows and sends (CTS, Clear to Send) grouping: be that the wireless MAC layer protocol disturbs the pair of control grouping of introducing for reducing concealed nodes.Before sending packet, sending node and receiving node are established the relation of reception that sends by this to the control grouping earlier.Receive the neighbor node of RTS grouping or CTS grouping, in RTS grouping or CTS grouping, do not allow to send packet in the official hour section, thereby avoided the interference to receiving node.

(13) contention window value (W): backoff counter is got an integer value randomly as its initial value on interval [0, W], wherein W is the current contention window value of DCF.

(14) frozen state: in the EDCA agreement, when node listens to channel when changing busy condition over to, backoff procedure will " be freezed ", namely stop backoff procedure, and backoff counter also stops to successively decrease.

(15) p adheres to: be suitable for the time slot channel, when certain station was ready for sending information, it is monitor channel at first, if idle, just transmitted information with Probability p, and postponed transmission with probability (1-p).If this station listens to channel for busy, just wait until that next time slot repeats said process again.

(16) throughput: sometime, between two nodes in network, offer the remaining bandwidth of application.Namely there be not under the situation of LOF the maximum rate that equipment can be accepted.

In existing wireless network, great majority have adopted IEEE 802.11 DCF agreements (to see document: IEEE 802.11.Wireless LAN medium access control (MAC) and physical layer (PHY) specification.IEEE Std.802.11,2007), but this agreement can't provide QoS service support for the transmission of multimedia real time business in the network, thereby IEEE 802.11 working groups have further released EDCA (Enhanced Distributed Channel Access) agreement on the basis of DCF agreement.Different with all kinds of professional fair competition shared channel resources among the DCF, the channel that EDCA can provide prioritization for dissimilar business inserts the transmission service, so that the real time business of high priority obtains to transmit than the MAC of low priority common service priority in the network.

For those of ordinary skill in the art can better understand the present invention, now in conjunction with Fig. 1 and Fig. 2 IEEE 802.11DCF of the prior art and EDCA agreement are simply introduced.

The EDCA agreement is supported to expand through QoS on DCF agreement basis, and its basic channel access way still keeps consistent with DCF.Adopt the network of DCF agreement, each node is with CSMA/CA mode access wireless channel competitively, and the carrier wave of channel detects and the transmission of data is base unit with time slot (aSlotTime) all.When node had data to send, node was at first intercepted channel.If channel continuous idle DIFS (the DCF inter-frame space) time, then node is initiated transmission at channel immediately, otherwise, will wait for after channel idle, starting backoff procedure.DCF has defined two kinds of data-transmission modes, is respectively fundamental mode and RTS/CTS pattern.Competition sends the probability that produces the conflict collision between the node in order to reduce, and in time adjusts the incentive degree of node channel competition, and DCF has adopted binary exponential backoff (Binary Exponential Backoff, BEB) algorithm.When node entered backoff procedure before sending, node started back off algorithm, and at this moment, (BackoffCounter BC) gets an integer value as its initial value to backoff counter randomly on interval [0, W], wherein W is the current contention window value of DCF.The size of W is dynamically to change the minimum contention window value W of its span in-system define _MinWith maximum contention window value W _MaxBetween.BC is through after the initialization, whenever through a channel idle time slot its value subtracted 1.When node listens to channel when changing busy condition over to, backoff procedure will " be freezed ", and BC stops to successively decrease, and wake backoff procedure after the time again up at the DIFS that channel recovers the space.When BC was decremented to 0, node was initiated transfer of data or the transmission of RTS frame at channel.If there is the BC of two or two above nodes to be decremented to 0 simultaneously, these nodes will be carried out transmission simultaneously, therefore produce collision at channel, and the data that node sends can't be able to correct reception.The value that the node of generation data collision will upgrade competition window is W _New=2 * (W _Old+ 1)-1, and reinitializes the value of BC, enter retransmission processes.If number of retransmissions surpasses the maximum retransmission L of system's regulation, the MAC layer will be abandoned the transmission of these data, enter the preparation that sends next data.Fig. 1 has provided the schematic diagram of 802.11DCF data transmission procedure.

In the DCF agreement, each node has only a MAC entity, and all dissimilar business datums all send by this protocol entity access channel.For the channel that dissimilar business datums is provided differentiation inserts the transmission service, the EDCA agreement expands to 4 with the MAC entity of each node, and the access style of 4 different priorities of formation (Access Category, AC).Different AC adopts different parameters that its channel of control is set and inserts transmission course, and these parameters comprise W _Min/ W _Max, AIFS and TXOPlimit.

AIFS is expanded by DIFS, and its size is determined by following formula:

AIFS[AC _i]=SIFS+AIFSN[AC _i] * aSlotTime formula (1)

Wherein, AIFS[AC _i] nonnegative integer relevant with concrete AC.Different AIFS puts and makes the AC of different priorities wait for that in startup or before waking backoff procedure up the time span of channel idle does not wait, and AIFS is more little, and its stand-by period is more short.Under the square one, less AIFS arranges and can improve backoff counter and be decremented to 0 speed, and therefore, the AC of high priority adopts less AIFS to arrange usually.Fig. 2 has provided the schematic diagram that different AIFS arrange to be influenced backoff procedure, has supposed among the figure that AIFSN is respectively two different priorities AC of 2 and 3.What pay particular attention to is, different with the DCF agreement, in the EDCA agreement, namely carry out BC after backoff procedure is waken up and subtract 1, and BC is decremented to after 0, node also needs one of back-off wait to keep out of the way time slot to initiate transmission again and (see document: Xiong LX again, Mao GQ, " Saturated throughput analysis of IEEE 802.11e EDCA ", ComputerNetwork, 3047-3068,2007).

Major defect based on the wireless channel access control method of EDCA agreement is: can not guarantee priority service QoS transmission.When a large amount of high priority message access channel, because the time slot of transmission is the value of fixing, therefore can not guarantee the transmission of high-priority service effectively.

Summary of the invention

The objective of the invention is to have proposed a kind of wireless channel access control method in order to solve the problems referred to above that existing wireless channel access control method exists.

To achieve these goals, technical solution of the present invention is: a kind of wireless channel access control method specifically may further comprise the steps:

Step 1: access styles different in the network is carried out prioritization;

Step 2: at different priority different packet retransmissions present probability is set, the sending probability of different priorities node is carried out dynamic solution;

Step 3: according to the sending probability of the different priorities node of trying to achieve, competition sends data in same time slot.

As a kind of preferable scheme, the concrete solution procedure of the described sending probability of step 2 is as follows:

$P=((N-k)q_a+{\mathrm{kq}}_r)e^{-((N-k)q_a+{\mathrm{kq}}_r)},$ Wherein, P is the sending probability of different priorities node, and N is the sum of node, and k is for waiting for the node number that retransmits, q _aBe the probability that has new grouping to arrive and transmit, q _rBe the packet retransmissions present probability.

The invention has the beneficial effects as follows: method of the present invention is by adjusting the packet retransmissions present probability of different priorities node, can dynamically obtain the sending probability of different priorities information, make the slot lengths to be sent such as the dynamic adjustment of node with higher sending probability, and then guaranteed that effectively important information has higher priority, can access quickly and deliver, can realize the most effective utilization of guaranteed qos basis upper signal channel.

Description of drawings

Fig. 1 is prior art 802.11DCF schematic diagram of mechanism.

Fig. 2 is prior art EDCA backoff procedure schematic diagram.

Fig. 3 is the schematic flow sheet of control method of the present invention.

Embodiment

The present invention is described further below in conjunction with accompanying drawing and specific embodiment:

The parameters of the described wireless channel access control method of present embodiment meets IEEE 802.11 standards.According to the IEEE802.11p standard, defined W _MinAnd W _MaxValue, the EDCA parameter of high priority message correspondence is: AIFSN=2, W _Min=7, W _Max=15.The EDCA parameter of low priority correspondence is AIFSN=3, W _Min=15, W _Max=31.The size of AIFS is determined by following formula among Fig. 2:

AIFS[AC _i]=SIFS+AIFSN[AC _i] * aSlotTime formula (2)

Different AIFS is arranged so that the AC of different priorities waits for that in startup or before waking backoff procedure up the time span of channel idle does not wait, and AIFS is more little, and its stand-by period is more short.Under the square one, less AIFS arranges and can improve backoff counter and be decremented to 0 speed, and therefore, the AC of high priority adopts less AIFS to arrange usually.

The schematic flow sheet of wireless channel access control method supposes to have in the network great deal of nodes need send information as shown in Figure 3, and the urgency level difference of information, may further comprise the steps:

Step 1: access styles different in the network is carried out prioritization;

Step 2: at different priority different packet retransmissions present probability is set, the sending probability of different priorities node is carried out dynamic solution;

Step 3: according to the sending probability of the different priorities node of trying to achieve, competition sends data in same time slot.

Said process is the wireless channel access control method, the essence of the method is: by adjusting the packet retransmissions present probability sending probability of different priorities information is carried out dynamic solution, make the slot lengths to be sent such as the dynamic adjustment of node with higher sending probability, emergence message with high priority is preferentially delivered, thereby when having improved channel utilization, supported the transmission of multipriority QoS of survice.

Wireless channel access control method in the above-mentioned steps 2, as shown in Figure 3, the concrete calculating formula process of its sending probability is as follows:

Total N node in the network makes k wait for the node number that retransmits the zero hour at each time slot, and each node is with fixing probability q _rRetransmission packet is N-k and the node number of new grouping arrival can be arranged till success in follow-up time slot, and each node has new grouping to arrive and the probability of transmission is q _a=1-e ^-λ/NUnder the condition of given k, make Q _r(i k) waits for i terminal being arranged at the probability of current time slots transmission, Q in the node that retransmits for k _a(i k) has i node at the probability of current time slots transmission, then for having in the N-k node:

$\left\{\begin{array}{c}{Q}_r(i,k)=\left(\begin{array}{c}k\\ i\end{array}\right){(1-q_r)}^{k-i}{q}_r^i\\ Q_a(i,k)=\left(\begin{array}{c}N-k\\ i\end{array}\right){(1-q_a)}^{N-k-i}{q}_a^i\end{array}\right.$ Formula (3)

Then the probability P that success sends in a time slot is:

$P=Q_a(1,k)Q_r(0,k)+Q_a(0,k)Q_r(1,k)$

$\≈((N-k)q_a+{\mathrm{kq}}_r)e^{-((N-k)q_a+{\mathrm{kq}}_r)}$ Formula (4)

By formula (4) as can be known, by adjusting packet retransmissions present probability q _rThe size of value can obtain the sending probability of different priorities node.

For the design of simplification system, make q=q _a=q _r, suppose that namely retransmission probability and new grouping arrive and the probability of transmission equates, then

$P=((N-k)q_a+{\mathrm{kq}}_r)e^{-((N-k)q_a+{\mathrm{kq}}_r)}={\mathrm{Nqe}}^{-\mathrm{Nq}}$ Formula (5)

By formula (5) as can be known, system can obtain maximum throughput q/e=0.368 when Nq=1, and namely when q=1/N, system obtains maximum throughput.

From the above, when q=1/N, system can obtain maximum throughput.To cause sending probability P sudden change in order preventing because node is counted the N sudden change, thereby to influence the stability of method, make the throughput of system can not be stabilized in the maximum edge.The system of setting up departments has N node to have data to transmit, and estimates that at m time slot the node number is M, and each node is with probability

Carry out packet retransmissions present, up to the transfer of data success.The present invention adopts a kind of exponentially weighted moving average (EWMA) algorithm (EWMA, Exponentially Weighted Moving Average) model that estimated value is carried out smoothing processing, and detailed process is as follows:

$p_{\mathrm{avg}}^m=\mathrm{\θ}\×p_{\mathrm{curr}}^m+(1-\mathrm{\θ})\×p_{\mathrm{avg}}^{m-1}$ Formula (6)

In the formula (6), θ is the smoothing processing coefficient of EWMA algorithm; M represents the sequence number of current time slots,

The packet retransmissions present probability of current time slots,

The packet retransmissions present probability of previous time slot,

Representation node is chosen in the moment probability that current time slots sends data,

Wherein, M is the estimated value that current time slots is sent the node number of data.

By the packet retransmissions present probability is carried out smoothing processing, and then can realize the smoothing processing to sending probability P, and then make method of the present invention have stronger stability.

Competition in the above-mentioned steps 3 sends in the data procedures specifically can adopt the rollback model, adopts a kind of simple state transition model here.Specific as follows: as when node has data to send, at first channel to be intercepted; If channel idle, then initiate transmission with probability P at channel, otherwise, starting backoff counter and enter and keeps out of the way state, is 0 up to the value of backoff counter, initiates transfer of data again, node for different priorities, the backoff counter of priority node is decremented to 0 earlier than the backoff counter of low priority node, and namely FEFO is kept out of the way state, transfers to the transmission state of carrying out.

Be specifically described with the example that is applied as of method of the present invention in wireless self-organization network (Wireless Ad-hoc Network) below.

In wireless self-organization network, increase along with active node quantity, there are up to a hundred nodes to need competitive channel, in order to guarantee that emergence message has better priority, it is priority 1 that emergence message is set, out of Memory is priority 2, and the information of different priorities is with different sending probabilities while competitive channels, and the computational methods of its probability such as formula (3) can get.The sending probability of supposing these two kinds of information is p _i, p _j(p _i＞p _j), simultaneously according to the generation frequency of the emergence message that produces in the unit interval and general information etc., adjust p dynamically _i, p _jValue.Guaranteeing to realize the maximum throughput of overall system performance optimum and system under the preferential prerequisite of transmitting of emergence message.

Those of ordinary skill in the art will appreciate that embodiment described here is in order to help reader understanding's principle of the present invention, should to be understood that protection scope of the present invention is not limited to such special statement and embodiment.Those of ordinary skill in the art can make various other various concrete distortion and combinations that do not break away from essence of the present invention according to these technology enlightenments disclosed by the invention, and these distortion and combination are still in protection scope of the present invention.

Claims (4)

1. wireless channel access control method specifically may further comprise the steps:

Step 1: access styles different in the network is carried out prioritization;

Step 2: at different priority different packet retransmissions present probability is set, the sending probability of different priorities node is carried out dynamic solution; The concrete solution procedure of described sending probability is as follows: $P={((N-k)q_a+{\mathrm{kq}}_r)e}^{-((N-k)q_a+{\mathrm{kq}}_r)},$ Wherein, P is the sending probability of different priorities node, and N is the sum of node, and k is for waiting for the node number that retransmits, q _aBe the probability that has new grouping to arrive and transmit, q _rBe the packet retransmissions present probability;

Step 3: according to the sending probability of the different priorities node of trying to achieve, competition sends data in same time slot.

2. wireless channel access control method according to claim 1 is characterized in that, described q _a=q _r=q.

3. wireless channel access control method according to claim 2 is characterized in that, and is described

4. according to the described arbitrary wireless channel access control method of claim 1 to 3, it is characterized in that the described packet retransmissions present probability of step 2 is made smoothing processing, and detailed process is as follows: $p_{\mathrm{avg}}^m=\mathrm{\θ}\×p_{\mathrm{curr}}^m+(1-\mathrm{\θ})\×p_{\mathrm{avg}}^{m-1},$ Wherein, θ is the smoothing processing coefficient, and m represents the sequence number of current time slots,

The packet retransmissions present probability of current time slots,

The packet retransmissions present probability of previous time slot,

Representation node is chosen in the moment probability that current time slots sends data,

Wherein, M is the estimated value that current time slots is sent the node number of data.

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