© N. Tadayon and S. Zokaei. 2010
Received: 30 September 2009
Accepted: 14 March 2010
Published: 26 April 2010
IEEE802.11 access protocol uses CSMA/CA in its Medium Access control layer as the main access function, which carries several deficiencies. In these networks, as the number of active stations increases, delay and throughput degrade severely. As far as throughput and service delay are vital elements in Quality of Service (QoS) determination, such degradation could lead to intolerable situations and reduce the efficiency of WLANs. Networks (WLANs). Studies proved this problem arises due to constant initial backoff windows size ( ), which is an important parameter in determination of network behavior. In this paper, we introduce a new method to tune this parameter adaptively according to changes in channel load. In this method, we do tune this parameter after every transmission using a feedback from transmission channel. Later it will be proven that applying this method in MAC layer enhances network stability; delay trend in all traffic classes exhibits a considerable reduction when compared with simple Enhanced Distributed Coordination Access (EDCA) scenarios. Also throughput exhibits a salient improvement in level. In other word, QoS improves. Especially, with the aid of this method, delay variations in all decrease considerably and more smoothen delay trends are achieved.
In recent years, desires to utilize Local Area Network (LAN) for communication increased dramatically. Undoubtedly, one of the most important classes of these access networks is IEEE 802.11 that was innovated in 1999 . IEEE 802.11 networks work based on a contention-based access mechanism namely Carrier Sense Multiple Access supported with collision avoidance capability (CSMA/CA). This was the subject of investigation for many researchers during these years [2–4]. As time went by and new, delay sensitive services emerged with real-time requirements, attentions were attracted toward applying diff-serve model on IEEE 802.11 Medium Access control (MAC) layer. Henceforth, many literatures focused on this subject [5–13].
Arrival of IEEE 802.11e standard into scene was a clear response to these efforts that have not been stopped yet. In this paper, we follow the idea behind [14–16] which is improving the 802.11 MAC performance using a channel adaptive backoff scheme. We investigate the merits and shortcomings of Distributed Coordination Function (DCF) and Enhanced Distributed Coordination Access (EDCA) and verify their dependency on network parameters. It would be claimed that using the legacy exponential backoff technique in DCF and EDCA leads to destructive dependence of network performance on initial backoff windows size, number of stations, and network load. This is apparently a drawback from network viewpoint. By applying this adaptive method, each station could periodically estimate network load (by continuously hearing to channel activities). That helps us directly in tuning of . Simulation results confirm the suitability of this method.
We apply our proposed method on EDCA of IEEE 802.11e to probe its effect on different traffic classes. As DCF is a specific case of EDCA, the totality of argument is reserved.
2. Legacy IEEE 802.11
In this section, we do summarize DCF performance and then limitations of this protocol on supporting Quality of service (QoS) will be discussed.
The IEEE 802.11 MAC layer protocol is a distributed coordination function and works based on carrier sense multiple access technique. In this technique, each station transmits its MAC service data units (MSDUs) after sensing the channel and ensuring that no transmission is in progress. In case two or more stations find channel idle and hence transmit simultaneously, the collision occurrence is inevitable. Therefore, IEEE 802.11 working group devised a mechanism, namely, collision avoidance, to reduce the collision probability. In this mechanism, stations start a backoff procedure before transmission; to that end, they should keep silent for a random amount of time immediately after channel remains idle for DCF Inter frame Space (DIFS). The DIFS value considered to be around 34 s in IEEE 802.11a standard. Upon the expiration of this random time, stations are allowed to transmit. The length of this random time should be multiple of slot length. In fact, each station carries a parameter, namely, contention window from which this random time is to be extracted.
Upon the correct reception of each data frame, recipient terminal transmits an acknowledgement packet back to sender confirming the correct reception of previous data frame. In case a collision occurs, the contention window size is multiplied by a persistent factor (PF) mentioned in standard. This mechanism is named exponential backoff. It would be titled truncated exponential backoff scheme in case there is an upper bound on contention windows size.
All frames that would not be acknowledged during a predefined amount of time (ACK-Timeout) should be scheduled for retransmission; but with a doubled contention window size. This procedure definitely lessens the collision probability when several stations are attempting to access the channel.
Stations that deferred their channel access due to medium busyness, do not initiate a new random backoff time; instead, they continue to count down their most recent frozen values as soon as channel remains idle for at least DIFS. Finally, after each successfully transmitted MSDU, a new random backoff procedure needs to be initiated regardless of the fact that whether the transmitter queue is empty or containing an MSDU (ready to send). This routine is entitled post backoff because of its initiation after each transmission not before that.
Essentially, there is one case in which no backoff procedure is required to be performed, that is, when, the last post backoff has already been finished while the queue is still empty. Thus, an arrived MSDU from higher layer would be immediately transmitted without need to perform a new backoff routine. All other MSDUs coming after last one should be transmitted after this backoff procedure.
In order to reduce the collision length in long frames, the standard suggested fragmentation scheme. In this scheme, long MSDUs should be fragmented to a series of smaller units to be transmitted sequentially one by one and acknowledged as well. The principal benefit of fragmentation is that, in case of collision occurrence, errors are identified in a swifter manner. Apparently, fragmentation's intrinsic drawback is the huge overhead it imposes on network.
In order to confront with hidden terminal problem, which is one of the most prevalent difficulties in CSMA/CA, Request to Send/Clear to Send (RTS/CTS) mechanism is devised. In this mechanism, sender station transmits a short control frame, namely, RTS prior to sending its data frame. Then, RTS recipient replies with another control frame, namely, CTS. Both RTS and CTS frames contain information about the length of following data frame. Following to reception of RTS by terminals in proximity of sender and reception of CTS by hidden terminals in proximity of receiver, all terminals should refrain from sending another frame in order to avoid collision occurrence. In fact, this mechanism helps in protecting system against sending long collided frames, especially in situations where hidden terminal problem is probable. Using Fragmentation, several smaller frames would be transmitted in series whereas using RTS/CTS method, a long frame would be transmitted but with less overhead and in a faster way.
3. Quality of Service Provisioning Mechanism in 802.11e Access Protocol
In order to support QoS in 802.11 wireless LANs, many proposals have been presented up to now and a lot more are under studying. Recently, IEEE 802.11 task group approved a new standard by adding few enhancements to MAC layer of IEEE 802.11. The result is a new enhanced distribution coordination function, namely, EDCA. IEEE 802.11e constituted from two distinct access phases, namely, contention period (CP) phase and contention free period (CFP) phase. These two phases alternate steadily in a superframe framework over time. Like DCF, EDCA is a contention-based protocol that is utilized in CP phase.
Now we commence with a definition of access category (AC). AC is the classification of different traffic classes in order to serve them with different requirements. To each AC in a station, a distinct EDCA is dedicated. They perform backoff procedure and act independently from each other. Here, backoff procedure starts immediately after channel stays idle for AIFS duration. Depending on physical characteristic of each AC, Arbitration Inter Frame Space (AIFS) might extend from DIFS, which is the bottom value, to larger amounts. Immediately after waiting for AIFS, each backoff entity sets its counter to an integer value extracted uniformly from interval where Contention Window (CW) in each AC varies from a minimum value, namely, , up to the bound .
A delicate difference between DCF and EDCA is regarding the exact time that frozen counters start to decrease. In DCF, decrement occurs at the edge of first slot coming immediately after DIFS idle time, whereas in EDCA the counter reduction is accomplished at the first edge of last AIFS idle slot. Although this little difference may not lead to a tangible influence on performance, it makes the analysis more convenient in EDCA case.
3.2. EDCA Performance Evaluation
In order to gain deeper understanding of our dynamic tuning scheme, we establish a set of simulations to study EDCA basic performance. The aim of this section is to prove that differentiation mechanism is only a tradeoff between different classes and cannot improve the overall level of QoS in network. In other word, using differentiation methods, when one of QoS metrics (Throughput, Delay, Utilization...) improves in some classes, we should certainly expect to see degradation in other classes (on the same metric).
We utilized predefined model of IEEE 802.11e existing in Opnet modeler.14 . In this set of simulations, we apply constant traffic load to a group of stations. Traffic is generated based on exponential interarrival rate and constant payload size, the condition that happens in many real situations. We set Mean interarrival time to 0.005 second and mean packet size to 1500 byte, what envisaged being a high load condition. In order to set up a stable working condition, an offset time should elapse after simulation beginning and before traffic generation in stations. To evaluate EDCA performance, three traffic categories have been defined; first, interactive multimedia class; second, interactive voice class; third, Best effort class.
Physical Layer Parameters.
Transmit Power (W)
Packet Reception Power Threshold (dBm)
RTS Threshold (byte)
Fragmentation Threshold (byte)
CTS to Self Option
Short Retry Limit
Long Retry Limit
Max Receiver Lifetime (s)
Buffer Size (bits)
Now we have enough tools in hand to follow our aim, which is our novel adaptive tuning method.
4. Adaptive Method of Tuning Initial Backoff Windows Size
4.1. Schematic Diagram
To accomplish our task, we have drawn a schematic cyclic flowchart. Let us proceed through its all stages to derive appropriate practical equations. Up to now, we have derived a suitable equation for third stage (14). Now, its first stage's turn!
4.1.1. Stage I
This is a simple, linear, and effective equation and permits each station to estimate the number of contending terminal every time required. In cases of small busyness probability, (light contention) is a suitable and tight approximation for ; thus, a linear equation with least difficulty in computations is obtained. As and are closely equal, we could apply them interchangeably.
4.1.2. Stage II
Clearly, and q are both constant values that are tunable depending on requirements. The larger results in more variable and instantaneous and hence an unreliable behavior with greater variations, but with swifter acting accomplished. Despite that, the smaller value leads to stable behavior with little variation and slow movements.
4.1.3. Stage III
At this stage we apply (15) to adaptively compute as a function of (which is an estimation of channel traffic). Finally, this initial window value will be exploited by stations for next transmissions.
4.1.4. Stage IV
As it is evident, our goal from exploiting several summations in this set of equations is to have a smoother as far as possible. The principal philosophy behind using the right hand side equation is that, upon an abrupt change in of previous stage, the increases automatically to provide more dependency on exactly former rather than farther ones (refer to equation ).
5. Simulation Results
In this section, we present simulation results of adaptive tuning method proposed in last section. Again, all results are achieved by applying network parameters mentioned in Tables 1 and 2. In addition, all traffic generation parameters are the same as what mentioned in Section 3.2 except interarrival time that is set to be 0.005 s in these set of simulations.
In this paper, we presented a novel adaptive tuning method. We proved that applying this method on MAC layer of 802.11 family networks improves network performance and stability in a sensible manner. Early in this paper, we have established a simple set of simulations on 802.11e networks to give readers a wider perspective of what followed latter. Then simulation results of our adaptive method are presented and compared with simple legacy EDCA results. The comparisons demonstrated that delay, throughput, retransmission attempt, and utilization were all positively impacted when applying it in backoff procedure of EDCA. These are all network quantities that could be considered. The differences between results were far enough to decisively vote to suitability of this method.
In the coming paper, we proposed a new QoS improving method based on prioritization. We showed that it is possible to combine -ATT and differentiation methods in order to achieve better QoS in 802.11e networks.
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