CN1585404A - Distributive competition control of withdraw of radio local network - Google Patents
Distributive competition control of withdraw of radio local network Download PDFInfo
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Abstract
First the method checks channel, and then uses the checked stop numbers of retreat-register and the initialized retreat time slot number to get the initial estimate value. After the initial estimate value of capacity factor of time slot is made smoothing processing, the probability of outgoing data frame is obtained. Then the transmission action of current data frame of node is adjectively adjusted according to transmission probability. Finally the competing window after transmission is selected according to if it is transmitted or if it hits to data of other node.
Description
Technical field
The present invention relates to communication technical field, particularly a kind of WLAN (wireless local area network) distributed contention control back-off method.
Background technology
At present, because WLAN (wireless local area network) need not any fixing wire structures, under the underdeveloped state of public telecommunication network, WLAN (wireless local area network) has obtained using widely in the environment that is not suitable for connecting up as campus, hospital, airport, large supermarket etc.But along with the progression of wireless local network user increases, the load of network is more and more heavier, how to utilize limited bandwidth resources to improve the utilance of WLAN (wireless local area network), has become the technical problem that needs to be resolved hurrily that WLAN (wireless local area network) develops.
At present but the access protocol of employing wireless local area network (LAN) has: adhere to that based on p model dynamically adjusts the AOB of sending probability (Asymptotically Optimal Backoff) method, distributed wireless medium access control DFWMAC (Distributed Foundation Wireless Media Access Control) method, channel division reservation multiple access inserts SRMA (split-channel reservation multipleaccess), multiple access access protocol MACA (multiple access with collisionavoidance) is avoided in conflict, multiple access access protocol MACAW is avoided in improved conflict, the bottom obtains agreements such as multiple access access protocol FAMA (floor acquisition multiple access) and IEEE802.11, and the WLAN (wireless local area network) of indication of the present invention adopts the IEEE802.11 agreement competitive channel based on CSMA/CA.
In WLAN (wireless local area network),, cause the part waste of radio resources because all nodes adopt distributed coordination function DCF (Distributed Coordination Function) competition wireless channel based on the IEEE802.11 agreement.Waste of radio resources is mainly concentrated two parts: time slot waste that idle time slot caused in the backoff procedure before Frame sent and transmission back are because the time slot waste that Frame bumps and causes.Be head it off, in the WLAN (wireless local area network) access protocol, introduced back-off method.Existing back-off method mainly contains three kinds: the binary exponential backoff method that is adopted in a kind of IEEE802.11 of being standard agreement (Binary Exponential Backoff algorithm) is called for short BEB; Second kind of back-off method (Distributed Contention ControlMechanism) that is based on distributed contention control mechanism is called for short DCC; The third back-off method (SlowCW Decrease) that is based on " keeping out of the way slowly " mechanism is called for short SD_DCF.
Described binary exponential backoff method BEB is adopted by IEEE802.11 working group and is applied in the IEEE802.11 agreement of standard.As shown in figure 11.The core concept of binary exponential backoff method is that whole time shaft is divided into several slots, each node can only be competed access channel in the zero hour of time slot, after sending successfully each time, node all can be reduced to minimum competition window with the competition window of oneself, after bumping, again competition window is enlarged to the twice of current competition window, till maximum allows competition window.Here said competition window is that a time slot is selected the zone, each node is all chosen by even distribution in this zone and is kept out of the way timeslot number, and give and to be kept out of the way register assignment with the timeslot number of choosing of keeping out of the way, being used to write down needs the idle timeslot number waited for before sending node sends.The binary exponential backoff method is applicable to the number of users situation seldom that competition inserts in the network.But, because the universal and WLAN (wireless local area network) of WLAN (wireless local area network) is the cloth net mode flexibly, selected WLAN (wireless local area network) to increase by geometric progression as the number of users of access network in recent years, it is more and more busier that network becomes, and is operated in congested overload state under a lot of situations.After network entered congestion state, there was the defective of two aspects in the binary exponential backoff method: the one, and it is cost that the size of increase competition window will bump with the packet that node sends; The 2nd, after each node data bag sent successfully, application register did not write down the current Congestion Level SPCC of network, and then utilizes network congestion degree information that the competition window of this node reasonably is set.Therefore, when the network user increased severely, serious deterioration all took place in the throughput of binary exponential backoff method and delay performance, and the WLAN (wireless local area network) that is unsuitable for high load capacity is used.
Described back-off method DCC based on distributed contention control mechanism, propose by people such as professors LucianoBononi, its core concept is a mechanism of having introduced " virtual collision ", when sending Frame at every turn, utilize the backoff procedure before Frame in the high load capacity network sends to write down the current race condition of network, decide after keeping out of the way end, whether to send this Frame according to the degree of contention of network.Though this method has overcome original BEB method still sends Frame blindly when network is busy shortcoming, make network can not enter the state that continues deterioration.But the DCC method does not still solve the original drawback of BEB method, oneself competition window blindly is kept to minimum value after promptly at every turn sending successfully, and reasonably choosing competition window according to the current race condition of network.Therefore be operated in higher load condition following time when network, each Frame successfully sends all needs experience repeatedly to keep out of the way, and causes time delay increase and throughput performance decrease always.
Described back-off method SD_DCF based on " keeping out of the way slowly " mechanism, be propositions such as strong professor Buddhist nun the earliest by IEEE802.11 working group, this method is mainly used in the problem that solution is repeatedly collided when a plurality of competition nodes are competed single Traffic Channel under the high load capacity environment when network work, its core concept is after each Frame sends successfully, the δ that the value that will keep out of the way register is reduced to current competition window doubly, but the value that will not keep out of the way register is changed to minimum value.This method has overcome DCC method and standard agreement BEB method and has sent the shortcoming that the numerical value that all will keep out of the way register after the success is changed to minimum backoff window at every turn, can reduce effectively when in the high load capacity network, sending Frame and keep out of the way number of times and collision probability, improve utilization ratio of wireless resources.But, again because after sending successfully at every turn, only be to keep out of the way half that contention window is changed to competition window when successfully sending simply, and at each current race condition of estimation network in real time that sends before the Frame in the backoff procedure not, to plan the transmission action of own present frame.So when network was in higher load condition, this method can not guarantee the successful access of present frame, on the contrary, wait is blindly kept out of the way and is finished the back and be about to Frame and mail to the further deterioration that wireless channel only can cause the wireless communication environment.
Summary of the invention
The objective of the invention is to avoid the IEEE802.11 agreement of CSMA/CA based on the carrier sense conflict, overcome the deficiency of above-mentioned prior art, propose a kind of WLAN (wireless local area network) distributed contention control back-off method (Distributed Contention Control algorithm Based on Slow CWdecrease) and be called for short SD_DCC, to solve in the high load capacity network, after not realizing the real-time estimation of the preceding network race condition of data transmission simultaneously and send successfully, existing back-off method reasonably chooses the technical problem of competition window according to the current race condition of network.
Technical solution of the present invention is:
Based on the thought of keeping out of the way slowly, be by the current race condition of estimation network, the current data frame of adjusting node adaptively sends action, whether bump after whether sending or send according to this Frame and choose the competition window that sends posterior nodal point with the Frame of other node, the initial value of register is kept out of the way in setting, to determine that node sends the timeslot number of keeping out of the way of Frame once more, its step is as follows:
1. keep out of the way initialization of register
When node sends data, sending node is at first intercepted the time slot channel, if current time slots channel idle, then sending node directly sends data, if channel busy, then node one of picked at random in minimum competition window is kept out of the way timeslot number Init_Backoff, utilizes this to keep out of the way timeslot number Init_Backoff and carries out initialization to keeping out of the way register.
2. detection wireless channel
Whenever time slot of channel idle, sending node subtracts 1 with the numerical value of keeping out of the way register of oneself, and busy if channel becomes, sending node stops to keep out of the way reducing of register, keeps out of the way the counter that register stops times N um_Busy_Times and adds 1 being used to write down simultaneously
3. estimate the wireless network race condition
Utilize to detect resulting keep out of the way register stop number of times (Num_Busy_Times) and the initialized timeslot number (Init_Backoff) of keeping out of the way, draw the initial estimate (Slot_Utilization) of slot efficiency by following formula,
In the formula, k is the statistics number of slot efficiency;
4. the initial estimate to slot efficiency carries out smoothing processing
Utilize following exponentially weighted moving average (EWMA) formula, the estimated value of slot efficiency is carried out smoothing processing upgrade, promptly
Slot_Utilization(k)＝0.9×Slot_Utilization(k)+0.1×Slot_Utilization(k-1)???(2)
In the formula: k is the statistics number of time slot rate
Slot_Utilization (k) is the initial estimate of the k time slot efficiency
0.9 be the smoothing factor of exponential weighted moving average model(EWMA)
Slot_Utilization (k) is the k time slot efficiency through obtaining after the smoothing processing;
5. utilize result after the smoothing processing to obtain the sending probability of Frame to be sent
The numerical value Slot_Utilization (k) that obtains after the smoothing processing according to slot efficiency draws each Frame sending probability P_T by following formula
P_T(k)＝(1- Slot_Utilization(k)
Num_Att)????????????????(3)
In the formula: k is the statistics number of slot efficiency
Slot_Utilization (k) is the numerical value after the slot efficiency smoothing processing
P_T (k) is the sending probability of current data frame
Num_Att is the number of times that retransfers of Frame to be sent;
6. move according to the transmission of sending probability decision node current data frame
When each sending probability P_T calculate finish after, the counter that is used to write down slot efficiency statistics number k is added 1; Sending node produces between [0...1] and meets equally distributed random number, after this random number and sending probability P_T compared, select the transmission of current data frame is moved by sending node, and whether virtual collision or true collision have taken place, change the size of competition window according to this Frame;
7. determine to send once more the timeslot number of keeping out of the way of Frame according to competition window, repeat the 2-7 step.
The present invention has following advantage:
(1) technical scheme that combines with distributed access control thought of the thought that to keep out of the way slowly of the present invention owing to adopted, back-off method to existing IEEE802.11 agreement improves, therefore, can intactly combine with existing IEEE802.11 standard agreement, need not any extra hardware spending and frame structure and change;
(2) back-off method of the present invention is owing to adopt sending node real-time monitored network race condition before each the transmission, according to the real-time estimation of network race condition dynamically being adjusted the transmission action of sending node to current data frame, so can guarantee when the network bouncing busy hour, node can be adjusted the transmission action of oneself adaptively, and change the numerical value of this node competition window according to the transmission result of current data frame, make the IEEE802.11 agreement adopt throughput performance behind the back-off method of the present invention and throughput gain performance all be higher than far away existing IEEE802.11 agreement adopt other back-off methods the performance that can obtain, as accompanying drawing 3, accompanying drawing 4, accompanying drawing 7, shown in the accompanying drawing 8;
(3) back-off method of the present invention is owing to can more accurately write down the current race condition of network in real time, and after node successfully sends data, reasonably choose the competition window of this node, so the technical problem of competition window reasonably is set after having avoided original back-off method can not realize the network race condition estimation before data send simultaneously and send successfully at every turn, even can guarantee that node still can obtain than better delay performance of existing back-off method and time delay gain performance in the high load capacity network environment, shown in accompanying drawing 5, accompanying drawing 6, accompanying drawing 9, accompanying drawing 10.
Description of drawings
Fig. 1 is the combination principle figure of the present invention and IEEE802.11 standard agreement
Fig. 2 is realization flow figure of the present invention
Fig. 3 is that the node number is to different back-off method throughput Effect on Performance simulation curve figure
What Fig. 4 was the node number to different back-off method throughput gain performance influences simulation curve figure
What Fig. 5 was the node number to different back-off method delay performances influences simulation curve figure
What Fig. 6 was the node number to different back-off method time delay gain performance influences simulation curve figure
Fig. 7 is that access way is to different back-off method throughput Effect on Performance simulation curve figure
What Fig. 8 was an access way to different back-off method throughput gain performance influences simulation curve figure
What Fig. 9 was an access way to different back-off method delay performances influences simulation curve figure
What Figure 10 was an access way to different back-off method time delay gain performance influences simulation curve figure
Figure 11 is the combination principle figure of existing binary exponential backoff algorithm BEB and IEEE802.11 standard agreement
Embodiment
Below in conjunction with accompanying drawing method of the present invention and effect are described in further detail
With reference to Fig. 1, back-off method of the present invention is embedded in the IEEE802.11 standard agreement, finishes the control of node equity Frame access channel to be sent by this agreement.Back-off method SD_DCC of the present invention comprises access control, virtual collision reprocessing competition window based on virtual collision thought, send the successful back processing competition window after based on the processing competition window of keeping out of the way thought slowly, true collision.
When node has Frame etc. to be sent, at first carrying out Frame according to the access control flow process of IEEE802.11 agreement handles, then, before the Frame access channel, utilize back-off method SD_DCC of the present invention to send the change that front nodal point sends the control of action to Frame and sends the back competition window.The control that this Frame sends action is based on virtual collision thought, and when node had data etc. to be sent, sending node is the monitoring wireless channel at first; If channel idle can directly send data, if channel busy, then node one of picked at random in minimum competition window is kept out of the way timeslot number, utilizes this to keep out of the way timeslot number and carries out initialization to keeping out of the way register; Whenever time slot of channel idle, the numerical value of keeping out of the way register subtracts 1 automatically, and is busy if channel becomes, and keeps out of the way register and stop automatically, keeps out of the way the counter that register stops number of times and add 1 being used to write down simultaneously; When the numerical value of keeping out of the way register reduces to zero, sending node calculates slot efficiency and these two important parameters of sending probability P_T, and with probability P _ T this frame is sent to wireless channel respectively, think that with probability 1-P_T virtual collision has taken place present frame, sends to wireless channel and terminate in this time slot with present frame.If after with probability P _ T this frame being sent to wireless channel, this frame is access channel and be received node and receive successfully, then adopts based on keeping out of the way thought slowly competition window is set, and is about to half that competition window becomes current competition window; When true collision takes place the Frame that sends when frame to be sent and other node, then handle by the competition window after the true collision, promptly competition window increases and is twice, and till maximum contention window, and number of retransmissions is increased by 1; If think that with probability 1-P_T virtual collision has taken place present frame, and terminate in this time slot when present frame sent to wireless channel, then adopt the competition window behind the virtual collision to handle, promptly at first consider to increase competition window, when competition window reaches maximum, again number of retransmissions is increased by 1, otherwise number of retransmissions is constant, competition window is increased be twice.
As follows referring to Fig. 2 implementation procedure of the present invention:
Node is not when having Frame etc. to be sent, whether detect has Frame etc. to be sent always in oneself the frame data buffer district, and avoid the mode of CSMA/CA to monitor shared wireless channel by the method for detection signal energy based on the carrier sense conflict always, there is Frame etc. to be sent if node is found the frame data buffer district, then enters keeping out of the way the stage before sending.
When node sends data, sending node is at first intercepted wireless channel, if the current time slots wireless channel free time, then sending node directly sends data, if wireless channel is busy, then node one of picked at random in minimum competition window is kept out of the way timeslot number Init_Backoff, utilize this to keep out of the way timeslot number Init_Backoff and carry out initialization to keeping out of the way register, and whether detect current channel idle.
Be reduced to before 0 keeping out of the way register, whenever time slot of channel idle, sending node subtracts 1 with the numerical value of keeping out of the way register of oneself, if it is busy that channel becomes, sending node stops to keep out of the way reducing of register, keeps out of the way the counter that register stops times N um_Busy_Times and adds 1 being used to write down simultaneously.
When network is in height during busy state, wait for blindly and keep out of the way register to be reduced to the collision probability that will wait Frame to be sent to send on the wireless channel in 0 back very big at once, when taking place, collision can cause the waste of time slot.So, the present invention at first utilized before sending node sends to wireless channel with Frame and add up the race condition that the information that obtains is estimated wireless network in the backoff procedure before transmission, and according to estimating that the network race condition that obtains decides the action to current Frame to be sent, reduces the collision of Frame on wireless channel.The decision of this transmission Frame action is to finish by calculating slot efficiency and Frame sending probability.When keeping out of the way register and be kept to 0, sending node reads keeps out of the way total degree Num_Busy_Times (k) that register stops to reduce and the initialization value Init_Backoff (k) that keeps out of the way register in the whole backoff procedure, and utilizes following formula to obtain the initialization estimated value Slot_Utilization (k) of slot efficiency:
In the formula, k is the statistics number of slot efficiency
Excessive for fear of single estimated value and the actual busy extent deviation of network, adopt exponentially weighted moving average (EWMA) EWMA model to calculate the slot efficiency estimated value carry out smoothing processing again:
Slot_Utilization(k)＝0.9×Slot_Utilization(k)+0.1×Slot_Utilization(k-1)??(2)
In the formula: k is the statistics number of slot efficiency
Slot_Utilization (k) is the initial estimate of the k time slot efficiency
0.9 be the smoothing factor of exponential weighted moving average model(EWMA)
Slot_Utilization (k) is the k time slot efficiency through obtaining after the smoothing processing.
Employing 0.9 is according to the emulation under the high load capacity network environment as choosing of smoothing processing coefficient, and its protocol capabilities preferably determines.
The numerical value Slot_Utilization (k) that obtains after the smoothing processing according to slot efficiency draws each Frame sending probability P_T by following formula
P_T(k)＝(1- Slot_Utilization(k)
Num_Att)????????????(3)
In the formula: k is the statistics number of slot efficiency
Slot_Utilization (k) is the numerical value after the slot efficiency smoothing processing
P_T (k) is the sending probability of current data frame
Num_Att is the number of times that retransfers of Frame to be sent
See from the definition of sending probability, under identical slot efficiency condition, the number of retransmissions of Frame is big more, the sending probability of Frame is big more, has avoided adopting in standard IEEE 802.11 agreements of binary exponential backoff method BEB and has not considered the defective of number of retransmissions to the influence of node access channel priority.
After drawing sending probability P_T, sending node can be according to the transmission action of this sending probability decision current data frame.At this moment, sending node produces and meets equally distributed random number between [0...1] by calling the random number generation function of writing, this random number and sending probability P_T are compared, automatically choose three different Frames according to sending node as a result relatively and send action: (1) is if pseudo random number during greater than sending probability P_T, explanation is very little according to this Frame probability that success sends in this time slot that the network busy extent calculates, be that current network is very busy, therefore we stop truly to send to Frame on the channel, and think that virtual collision has taken place for it, to reduce the load of network.When Frame generation virtual collision, sending node judges at first whether competition window has reached maximum contention window, if competition window do not reach maximum contention window, sending node only increases competition window and is twice, and number of retransmissions is constant; Otherwise competition window is constant, and number of retransmissions adds 1; (2) if pseudo random number during less than sending probability P_T, illustrate that the probability of this Frame success transmission in this time slot that calculates according to the network busy extent is very big, then sending node sends to physical channel in current time slots with Frame; (3) if send failure, promptly sending node is not received the acknowledgement frame of destination node in the time of setting, and sending node increases competition window and is twice till maximum contention window, and the number of retransmissions of Frame is added 1; If send successfully, promptly send Frame and in the time of setting, receive sending the acknowledgement frame of Frame, this sending node is kept to competition window half of current competition window automatically.Because in the high load capacity network, node sends the success of data and does not represent weakening of network race condition, in fact after network enters congestion state, the network race condition changes very little in the transmission at interval of node two continuous frames, so sending node sends all can think that the race condition of network changes little and the numerical value of competition window is kept to half of current competition window after the success at every turn.
At last, according to the definite timeslot number of keeping out of the way that sends Frame once more of competition window, promptly picked at random is kept out of the way timeslot number in the competition window of having set, and utilizes the selected timeslot number of keeping out of the way to carry out initialization to keeping out of the way register; Whether whether sending node detects again has Frame wait for to retransmit or has new Frame etc. to be sent in data buffer area, turn back to send before the wireless channel detected state.
In order to check the IEEE802.11 standard agreement to use the performance of various back-off methods, SD_DCC method of the present invention is used in respectively with existing binary exponential backoff method BEB, pure slow back-off method SD_DCF, pure distributed competition control method DCC in the back-off method of IEEE802.11 standard agreement and carries out emulation, this emulation has mainly been studied under high load capacity busy network environment, by competition node number and two network parameters of different access waies, use after the SD_DCC method of the present invention influence to the IEEE802.11 protocol capabilities.
Simulation software is OPNET PL8.0, and emulation experiment is finished on the desktop computer that a dominant frequency is 2.4G, and concrete parameter is as shown in table 1:
Table 1: performance simulation parameter
| Parameter | Numerical value | Parameter | Numerical value |
| Data rate | ??11Mbps | Slot length | ????20us |
| Frame length | ??8184bit | ??SIFS | ????10us |
| The mac frame head | ??272bits | ??DIFS | ????50us |
| The physics frame head | ??128bits | Long frame number of retransmissions | ????4 |
| ??RTS | 160bits+ physics frame head | Short frame number of retransmissions | ????7 |
| ??CTS | 112bits+ physics frame head | Minimum competition window | ????15 |
| Propagation delay | ??1us | Maximum contention window | ????1023 |
Frame length is obeyed the negative exponent distribution in the table.In order to describe performance of the present invention quantitatively, adopt the end-to-end time delay of normalized throughput, throughput gain, Frame, four indexs commonly used of end-to-end time delay gain to assess its performance in the present invention.
As seen from Figure 3, existing binary exponential backoff method BEB, all descend along with increasing of node number based on the normalization throughput performance curve of pure slow back-off method SD_DCF and pure distributed competition access control method DCC and SD_DCC method of the present invention, wherein the BEB method descends the fastest, next is the DCC method, when SD_DCF is less than 65 nodes at number of network node, its performance is better than the DCC method, and after the node number surpasses 65 nodes, its performance is not as the DCC method, and the decline of the throughput performance curve of SD_DCC method of the present invention is the slowest.This simulation curve shows that throughput performance of the present invention all is better than other three kinds of back-off methods under the diverse network scale.
As seen from Figure 4, along with increasing of node number in the network, SD_DCC method advantage of the present invention is more and more obvious, when the node number is counted less than 65 nodes, the throughput gain performance of SD_DCF is higher than the DCC method always, though after the node number surpasses 65 nodes, the performance of DCC method slightly is better than the SD_DCF method, but no matter under any network size, the throughput gain performance of SD_DCC method of the present invention all is better than other 3 kinds of back-off methods.In the curve of Fig. 4, when the node number is respectively 50,60,100, the IEEE802.11 agreement throughput gain of using behind the SD_DCC of the present invention is respectively 28.569%, 28.791% and 29.593%, and its performance is far superior to use the performance of IEEE802.11 agreement after existing DCC method and the SD_DCF method.Though, at the node number is 80 o'clock, the IEEE802.11 agreement throughput gain of using the SD_DCC method is that the throughput gain of using the IEEE802.11 agreement of SD_DCC method at 60 o'clock is compared to some extent and descended with the node number, but its throughput is still much larger than the throughput of using agreement after DCC method and the SD_DCF method.
As can be seen from Figure 5, along with increasing of interstitial content in the network, the time delay of various back-off methods all increases to some extent, and wherein the growth of the time delay of BEB method is the fastest, and DCC and SD_DCF take second place, and SD_DCC method time delay of the present invention increases the slowest.This simulation curve shows that the present invention can effectively reduce the end-to-end time delay of Frame in the IEEE802.11 agreement.
As can be seen from Figure 6, be example so that 65 nodes to be arranged in the network, the present invention is used in employing under the unusual busy condition of network can obtain 19.749% time delay gain.
As can be seen from Figure 7, because the SD_DCC method of the present invention current congestion state of estimation network effectively, so the throughput performance of IEEE802.11 agreement behind application the present invention obviously is better than using the throughput performance of IEEE802.11 agreement after the BEB method.
Use the throughput yield value of SD_DCC method of the present invention in the IEEE802.11 agreement when as can be seen from Figure 8, the IEEE802.11 agreement throughput yield value after the application SD_DCC method of the present invention is obviously greater than employing RTS/CTS access way under basic access way bas mode.Be example there to be 65 nodes in the network equally, adopting SD_DCC method of the present invention can obtain maximum throughput gain under the basic access way bas of IEEE802.11 is 28.569%, both made under the access way of the short frame reservation of RTS/CTS channel, using SD_DCC method of the present invention compares with using the BEB method, the present invention still can obtain 23.08% throughput gain in the IEEE802.11 agreement, this explanation is in the busy network of high load capacity, the present invention can reduce the energy resource consumption of node effectively, improves the utilization ratio of wireless network bandwidth.
Fig. 9 shown in the IEEE802.11 agreement, adopts the short frame reservation of basic access way bas and RTS/CTS to insert two kinds of different access waies, uses after existing BEB method and the SD_DCC method of the present invention comparison to IEEE802.11 agreement delay performance.No matter can clearly be seen that from Fig. 9, be existing BEB method or SD_DCC method of the present invention, introduces after the short frame reservation of the RTS/CTS access way, because the minimizing of collision frequency, time delay all decreases.But after adopting SD_DCC method of the present invention, the IEEE802.11 agreement can more effectively avoid different nodes to send the collision of Frame.Therefore, both made under basic access way bas, adopt the delay performance of IEEE802.11 agreement after the SD_DCC method of the present invention, still the delay performance height that is obtained than IEEE802.11 agreement after using the BEB method under the short frame reservation of the RTS/CTS channel access way condition.
Figure 10 shown under short frame appointment mode of RTS/CTS and basic access way bas condition, uses SD_DCC method of the present invention after IEEE802.11 time delay that agreement obtains gain.As can be seen from Figure 10, no matter be under the basic bas of the access mode or under the short frame appointment mode of RTS/CTS, the time delay of SD_DCC method of the present invention is all less than the time delay of binary exponential backoff BEB method under two kinds of access waies, and after the node number is greater than 20, along with increasing of interstitial content in the network, the time delay gain that the present invention obtained is increasing; Simultaneously we from two time delay gain curves of Figure 10 as can be seen, because the RTS/CTS access way can reduce collision effectively, so under basic access way, use the time delay gain of IEEE802.11 agreement after the SD_DCC method of the present invention, greater than under the RTS/CTS access way, use the time delay gain that the IEEE802.11 agreement is obtained after the SD_DCC method of the present invention.
The technical term explanation that relates among the present invention:
1. node: node is meant the various portable terminals in the WLAN (wireless local area network) in the present invention, as mobile phone, laptop computer etc.;
2.RTS/CTS: be meant that sending node utilizes defined request transmit frame RTS and the access way that allows transmit frame CTS reservation wireless channel in the IEEE802.11 standard agreement before sending Frame.
3. normalized throughput (throughput): normalized throughput is meant the ratio of the mutual data bit number of whole network success in the unit interval with the wireless channel data rate in the emulation.
4. throughput gain (Gt): the throughput gain is meant the normalization throughput of SD_DCC method of the present invention and the normalization throughput difference of IEEE802.11 agreement BEB method in the emulation, and with the normalization throughput of the difference that calculates divided by IEEE802.11 agreement BEB method.
5. the end-to-end time delay of Frame (delay): the end-to-end time delay of Frame is meant that the buffer queue that enters sending node from first bit of Frame begins successfully to receive time interval of last bit of this packet to destination node in the emulation.
6. time delay gain (Gd) end to end: time delay gain end to end is meant the end-to-end time delay difference of the end-to-end time delay and the IEEE802.11 agreement BEB method Frame of SD_DCC method Frame of the present invention in the emulation, and the difference that calculates is divided by the end-to-end time delay of IEEE802.11 agreement BEB method Frame.
7. time slot: time slot is meant in the IEEE802.11 agreement among the present invention, in order to coordinate of the competition of a plurality of nodes to single physical channel, with length sometime whole time shaft is cut apart, the time span unit that is used for axle sliced time here promptly is called a time slot.
Claims (4)
1. WLAN (wireless local area network) distributed contention control back-off method, be by the current race condition of estimation network, the current data frame of adjusting node adaptively sends action, whether bump after whether sending or send according to this Frame and choose the competition window that sends posterior nodal point with the Frame of other node, the initial value of register is kept out of the way in setting, to determine that node sends the timeslot number of keeping out of the way of Frame once more, its step is as follows:
(1) keeps out of the way initialization of register
When node sends data, sending node is at first intercepted the time slot channel, if current time slots channel idle, then sending node directly sends data, if channel busy, then node one of picked at random in minimum competition window is kept out of the way timeslot number Init_Backoff, utilizes this to keep out of the way timeslot number Init_Backoff and carries out initialization to keeping out of the way register;
(2) detect wireless channel
Whenever time slot of channel idle, sending node subtracts 1 with the numerical value of keeping out of the way register of oneself, and busy if channel becomes, sending node stops to keep out of the way reducing of register, keeps out of the way the counter that register stops times N um_Busy_Times and adds 1 being used to write down simultaneously;
(3) estimate the wireless network race condition
Utilize to detect resulting keep out of the way register stop number of times (Num_Busy_Times) and the initialized timeslot number (Init_Backoff) of keeping out of the way, draw the initial estimate of slot efficiency by following formula,
In the formula, k is the statistics number of slot efficiency;
(4) initial estimate to slot efficiency carries out smoothing processing
Utilize following exponentially weighted moving average (EWMA) formula, the estimated value of slot efficiency is carried out smoothing processing upgrade, promptly
Slot_Utilization(k)＝0.9×Slot_Utilization(k)+0.1×Slot_Utilization(k-1)(2)
In the formula: k is the statistics number of slot efficiency
Slot_Utilization (k) is the initial estimate of the k time slot efficiency
0.9 be the smoothing factor of exponential weighted moving average model(EWMA)
Slot_Utilization (k) is the k time slot efficiency through obtaining after the smoothing processing;
(5) utilize result after the smoothing processing to obtain the sending probability of Frame to be sent
The numerical value Slot_Utilization (k) that obtains after the smoothing processing according to slot efficiency draws each Frame sending probability by following formula,
P_T(k)＝(1- Slot_Utilization(k)
Num_Att)??????(3)
In the formula: k is the statistics number of slot efficiency
Slot_Utilization (k) is the numerical value after the slot efficiency smoothing processing
P_T (k) is the sending probability of current data frame
Num_Att is the number of times that retransfers of Frame to be sent;
(6) move according to the transmission of sending probability decision node current data frame
When each sending probability P_T calculate finish after, the counter that is used to write down slot efficiency statistics number k is added 1; Sending node produces between [0...1] and meets equally distributed random number, after this random number and sending probability P_T compared, select the transmission of current data frame is moved by sending node, and whether virtual collision or true collision have taken place, change the size of competition window according to this Frame;
(7) keep out of the way timeslot number Init_Backoff according to what competition window determined to send once more Frame, repeat (2～7) step.
2. WLAN (wireless local area network) distributed contention control back-off method according to claim 1, wherein the generation of random number is by calling random number generation function Rnd () in the software code, produce [0...1] and meet equally distributed pseudo random number, () representative function symbol.
3. WLAN (wireless local area network) distributed contention control back-off method according to claim 1, wherein competition window is changed is whether successfully to send data by the written program code according to node to come will represent automatically the variable of competition window to carry out corresponding multiplication and division computing to realize to sending node.
4. according to claim 1 or 3 described WLAN (wireless local area network) distributed contention control back-off methods, wherein sending node carries out according to the following procedure to the change of competition window:
(1) if pseudo random number during greater than sending probability P_T, i.e. Frame virtual collision, Stop node sends at the Frame of this time slot, and at this moment, if competition window does not also reach maximum contention window, sending node can increase competition window and is twice; If competition window has reached maximum contention window, competition window is constant, and the Frame number of retransmissions adds 1;
(2) if pseudo random number during less than sending probability P_T, node sends Frame at this time slot, if send failure, be that Frame bumps, sending node is not received the acknowledgement frame of destination node in the time of setting, sending node increases competition window and is twice till maximum contention window, and the number of retransmissions of Frame is added 1; If send successfully, promptly Frame does not bump, and sends Frame and receives in the time of setting sending the acknowledgement frame of Frame, and this sending node is kept to competition window half of current competition window automatically.
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