AWPP: A New Scheme for Wireless Access Control Proportional to Traffic Priority and Rate
© T. Lagkas and P. Chatzimisios. 2011
Received: 30 November 2010
Accepted: 20 February 2011
Published: 13 March 2011
Cutting-edge wireless networking approaches are required to efficiently differentiate traffic and handle it according to its special characteristics. The current Medium Access Control (MAC) scheme which is expected to be sufficiently supported by well-known networking vendors comes from the IEEE 802.11e workgroup. The standardized solution is the Hybrid Coordination Function (HCF), that includes the mandatory Enhanced Distributed Channel Access (EDCA) protocol and the optional Hybrid Control Channel Access (HCCA) protocol. These two protocols greatly differ in nature and they both have significant limitations. The objective of this work is the development of a high-performance MAC scheme for wireless networks, capable of providing predictable Quality of Service (QoS) via an efficient traffic differentiation algorithm in proportion to the traffic priority and generation rate. The proposed Adaptive Weighted and Prioritized Polling (AWPP) protocol is analyzed, and its superior deterministic operation is revealed.
There is no doubt that the current trend in the telecommunications market is the extensive adoption of wireless networking solutions. It is expected that in the following years all types of wireless networks will form a significant part of the overall networking infrastructure. In addition to this tendency, the nature of the network applications changes requiring considerably more resources. In particular, multimedia traffic load greatly increases; thus, efficiently serving multiple demanding streams becomes challenging. Furthermore, modern users expect to experience high quality communications independently of the flows' nature or the network type.
The effort to provide qualitative services for all kinds of traffic to wireless network users has lately created a large research area. The barriers we need to overcome are significant; the available bandwidth is limited due to the nature of the signal transmission and legal restrictions, the wireless links are not reliable with increased bit error rate, the communication range varies and affects the transmission rate and the link quality, and the user mobility raises major issues. A clear-cut solution at the physical layer would be the maximization of the bit rate in conjunction with the minimization of the transmission errors. There has been definitely great development towards this objective with the introduction of modern techniques and standards (e.g., the IEEE 802.11n standard  proposed for wireless local area networks and achievable data rate around 200 Mbps). However, the increasing requirements for total QoS support necessitate aggregate approaches. Specifically, the access control of the shared wireless medium plays a crucial role in the final quality of the provided services.
The most well-known present scheme which provides QoS supportive MAC for WLANs (Wireless Local Area Networks) is HCF . The latter comprises a distributed protocol known as EDCA and an optional resource reservation centralized protocol called HCCA. EDCA is capable of differentiating traffic; however, it suffers from low channel utilization which leads to limited performance. On the other hand, HCCA is able to guarantee QoS to constant bit rate traffic streams, but it demands predefined requests for resources while it considers no priorities.
Recently, intensive research work has been noticed in the field of optimizing QoS provision in wireless networks through medium access control. A significant number of proposals are oriented towards the improvement of existing well-known standards (like the IEEE 802.11e), trying to enhance the overall performance while retaining compatibility to a great degree [3–8]. On the other hand, some new schemes have been lately introduced, which attempt to maximize the network efficiency regarding QoS support [9–13]. A survey of MAC protocols for multimedia traffic in wireless networks that have put the basis for the modern schemes is presented in .
This paper presents a novel resource distribution mechanism for centralized wireless local area networks, that does not require predefined resource reservation and is capable of providing predictable QoS to traffic flows of different type. The proposed AWPP protocol employs the frame structure and the basic polling scheme that were introduced with the high-performance Priority Oriented Adaptive Polling (POAP) protocol . Moreover, AWPP introduces a deterministic traffic differentiation technique that operates in proportion to the buffered packets' priorities and the traffic generation rate. The main idea of the presented protocol is to efficiently share the scarce available bandwidth according to well-defined QoS principles. Specifically, the key objective is to assign transmission opportunities in absolute accordance to the weighted traffic priority and the packet arrival rate of each individual flow. By this manner, we succeed on effectively supporting multimedia streams, while being able to predict and configure resources allocation and network behavior based on the characteristics of the served traffic.
This paper is organized in six sections. In Section 2, the EDCA, HCCA, and POAP protocols are discussed, which are used as reference points in this work. Section 3 thoroughly presents the proposed AWPP protocol. In Section 4, an analytical approach on the AWPP operation is provided. The developed simulation scenario and the comparison results are presented and commented in Section 5. Finally, the conclusions can be found in Section 6.
2. Related Work
The presentation of the AWPP protocol adopts as reference points the well-known EDCA and HCCA protocols, which are the parts of the dominant IEEE 802.11e standard, as well as the very effective POAP protocol, which sets the basic structure for AWPP. These three protocols are briefly described in the current section.
2.1. The EDCA Protocol
The mandatory MAC protocol of the IEEE 802.11e standard is EDCA. It is actually a QoS supportive enhanced version of the legacy IEEE 802.11 MAC protocol, that is the Distributed Coordination Function (DCF). The operation of EDCA is based on the adoption of packet priorities according to the DiffServ model .
EDCA employs the CSMA/CA algorithm. Its operation bases on station contention for medium access using a backoff procedure. The latter involves waiting intervals of different length, called Arbitrary Distributed Interframe Spaces (AIFSs), and backoff intervals of different length, called Contention Windows (CWs), according to the priority of the corresponding packet buffer, called Access Category (AC). These different values of the intervals' length impose different access probabilities for the traffic packets based on their priorities. This way, traffic can be differentiated and QoS can be supported. Additionally, EDCA implements a collision avoidance technique using a two-way handshake, called RTS/CTS (Request To Send/Clear To Send). This technique handles to some degree the serious hidden station problem.
The operation of EDCA exhibits significant deficiencies regarding its QoS capabilities. To be more specific, the use of backoff intervals leads to waste of resources, while the hidden station problem, which is still present despite the adoption of the RTS/CTS mechanism, increases the collision rate, thus, decreasing the overall performance. Moreover, QoS support gets problematic due to the exponential backoff procedure. Specifically, it is inefficient to penalize the already delayed collided packets with even longer waiting times. Furthermore, EDCA is shown not to be able to share the available bandwidth fairly . The reasons for the lack of efficiency of EDCA are described in . As a conclusion, EDCA can certainly differentiate traffic and hence provide some QoS, but it reveals great performance limitations.
2.2. The HCCA Protocol
The optional part of the IEEE 802.11e HCF scheme is the HCCA protocol. This is a centralized protocol which uses the so-called Hybrid Coordinator (HC) to perform medium access control. The HC is considered by the standard to be collocated with the Access Point (AP).
The HCCA resource reservation mechanism defines that every Traffic Stream (TS) communicates its Traffic Specifications (TSPECs) to the AP. The TSPECs include the MAC Service Data Unit (MSDU) size and the maximum Required Service Interval (RSI). The standardized scheduler calculates first the minimum value of all the RSIs and then chooses the highest submultiple value of the beacon interval duration as the selected Service Interval (SI), which is less than the minimum of all the maximum RSIs.
A basic weakness of the HCCA protocol is related with its nature. HCCA is an optional part of HCF that can guarantee QoS via resource reservation to fixed traffic flows of known resource requirements. The IEEE 802.11e standard actually proposes HCCA for the exclusive handling of multimedia streams. Regarding the resource allocation algorithm, the constant TXOPs lead to limited support for Variable Bit Rate (VBR) traffic. Furthermore, HCCA considers no traffic priorities. It handles simply the QoS requests in time order and denies service to traffic flows that at that moment cannot be given the whole requested resources.
2.3. The POAP Protocol
Polling a Station That Has No Packets for Transmission (Figure 1(a)).
Polling a Station That Has Packets for Transmission (Figure 1(b)).
Polling Failure or Feedback Failure (Figure 1(c)).
If the polling fails, then the AP has to wait for the maximum polling cycle before polling again, because it must be sure that it will not collide with a possible ongoing transmission. When polling succeeds, but then the AP fails to receive any of the following packets, it has to wait for the maximum polling cycle before the new poll, similarly to the polling failure case.
where is the buffer index, is a preset weight, is the normalized buffer priority of buffer , is a preset weight, and is the normalized number of packets contained in buffer . The main idea is that both the buffer priority and the current buffer load affect the chance to transmit a packet from the specific buffer, but the contribution of each one of these two factors is controlled by different weights.
where is the normalized priority score of station , is a preset weight, and is the normalized time elapsed since the last poll of station . The factor is employed in order to ensure some fairness among the stations regarding medium access. The AP is further favored, because of its central role, by multiplying its nonnormalized polling probability with the weight .
POAP has been shown to achieve high performance, exhibiting great medium utilization and providing sufficient QoS support. However, the nature of its algorithmic operation makes it very hard to predict to what degree a traffic flow will be favored in comparison to another traffic flow or a station in comparison to another station. To be more specific, the decision-making mechanism in POAP mainly depends on a combination of the buffered packet priorities and the current buffered load. The fact that the buffer load is an alternating factor and the use of the mathematical operation of addition in (5) and (7) in order to combine the priority and load coefficients do not allow the estimation of the ratio of the bandwidth that a traffic flow will be provided with and do not finally ensure the proportional contribution of each coefficient. For example, if in a station a buffer is expected to carry the same load (which cannot be calculated in advance) with another buffer of a higher priority, then we cannot estimate based on (5) at what degree the second buffer will be favored in relation to the first one. Thus, it becomes challenging to set the weights to suitable values, which procedure was eventually carried out in a heuristic manner. At this point, it should be noticed that AWPP comes to provide weighted traffic differentiation proportional to traffic priority and rate allowing the analytical estimation of the network metrics and generally a more deterministic behavior.
3. The AWPP Protocol
3.1. The "Packet to Transmit'' Algorithm
3.2. The "Station to Poll'' Algorithm
The AP implements an algorithm responsible to decide each time which station to poll in a QoS provision basis, similarly to the "packet to transmit" algorithm. To be more specific, the objective here is to proportionally favor stations that have high-priority buffered traffic and exhibit high traffic rate, according to the same concept that was described in the previous subsection. Thus, the polling decision should mainly depend on the stations' BTI values. Furthermore, since the AP itself is considered to participate in the polling contention, it should be probably served with higher medium access chances, since it plays a central role in the network by connecting it externally. For this reason, the AP_ExtraPriority parameter (default value 1) is introduced. Specifically, when the AP calculates its buffers' BSW values, which then give the AP's BTI value, it adds the AP_ExtraPriority to each buffer's priority, which means that the exponent in (8) is considered to be equal to BP[i]+AP_ExtraPriority for the AP's packet buffers.
4. Analytical Approach on the AWPP Operation
Characteristics of the traffic flows.
Bit rate per flow (kbps)
Data packet total size (bits)
Notice that in reality the data packet size and the traffic bit rate need not to be fixed. However, in this study constant values are used for comparative reasons. The protocol is expected to operate according to the same principles when serving variable bit rate flows, too. In this scenario, there are three different bidirectional traffic flows between the AP and each wireless station. Someone could possibly assume that the LP flows correspond to web traffic, the MP flows correspond to video traffic, and the HP flows correspond to voice traffic. It should be mentioned that in order to retain traffic symmetry and produce more explanatory results, the AP flows are not favored in this scenario, that is AP_ExtraPriority and for AWPP and POAP are set to 0 and 1, respectively, Furthermore, the network bit rate was considered to be equal to 36 Mbps, which corresponds to the typical ERP-OFDM-16 QAM mode of the widely used IEEE 802.11g physical layer . The stations are placed at distances of 60 m of each other, leading to an estimated signal propagation delay of 0.2 μ s. Lastly, the network observation interval is set to 60 s.
It should be mentioned that the BAU value is in fact the upper limit of the respective throughput. Apparently, when BAU is higher than the required bandwidth, then the residual bandwidth becomes available to the lower priority traffic. At this point, the proportional distribution of resources also becomes clear. Specifically, (14) and (19) reveal that according to AWPP, the HP traffic deserves 4 times more bandwidth than the MP traffic, since the former's priority is higher by 2, the priority factor equals 2, and they exhibit the same rate, whereas the HP traffic deserves 32 times more bandwidth than the LP traffic, since the former's priority is higher by 6, the priority factor equals 2, and the latter exhibits 2 times higher rate.
Since POLL packet total size is equal to 272 bits, DATA packet total size is equal to 10192 bits, STATUS packet total size equal to 352 bits, and Total Bandwidth is equal to 36 Mbps, (16) results in UB equal to 33.732 Mbps. Finally, the traffic throughput is equal to the traffic load, when the traffic load is lower than the BAU value, while in case the traffic load is higher than BAU, then the traffic throughput equals BAU, as it is already explained.
5. Simulation Results
This section presents the simulation results regarding the performance of the AWPP protocol compared to POAP, EDCA, and HCCA. The simulated network scenario was described in the previous section. The four protocols were simulated on the same specialized developed in C++ event-based simulation framework, adapted to the operational characteristics of each one. The matching of the analytical and simulation results presented in the previous sections validates both the analytical model and the simulator as well. The condition of any wireless link was modeled using a finite-state machine with three states (good, bad, and hidden) based on the work of Zorzi et al. . Note that the relative performance of the four protocols is not affected by the channel status, because in good channel conditions the performance of all protocols improves, whereas in bad conditions all protocols perform worse. Hence, the comparative results are actually the same and conclusions can be drawn whatever the case. The default parameter values for the four protocols were used. The simulation results presented in this section are produced by a statistical analysis based on the "sequential simulation" method .
This work proposed the Adaptive Weighted and Prioritized Polling (AWPP) protocol capable of efficiently supporting total QoS in wireless networks. The presented analytical approach has proven that AWPP succeeds to provide deterministic traffic differentiation proportional to traffic priority and rate. The simulation results, which coincide with the analytical results, have shown that AWPP serves the different types of traffic more efficiently than the effective POAP protocol, the dominant EDCA protocol, and the specialized HCCA protocol. AWPP is also shown to achieve superior total network performance. As future work, we intend to study extended network scenarios that involve traffic flows characterized by limited duration and bursty nature. Moreover, the special features of the introduced scheme could be adapted into the medium access control mechanism of the emerging wireless broadband networks. Specifically, a possible integration of the AWPP resource managing engine into the respective module of the IEEE 802.16 wireless broadband network will be examined.
This work was partially supported by the State Scholarships Foundation of Greece.
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