Qos Support For Wireless Lans

University Of Jordan Faculty of King Abdullah II for Information technology Computer Science Department Computer Network

A New Hybrid Medium Access Mechanism with Integrated QoS Support for Wireless LANs

Group Names: Ayman Alfetyani 8070251 Mohammed Musaddaq 8081244 Mohammad Ghanem 8060467 Supervised By: Dr. Wesam Almobaideen First Semester, 2008-2009 Dec.2008

Abstract In nearly all networks we are using video, audio and different data formats, accordingly QoS is performed in wired networks. Therefore, we want to discuss 1

some techniques to support QoS in wireless networks which are focused in the IEEE 802.11e standard. We go through Hybrid Coordination Function (HCF) with its two techniques, the EDCA and HCCA. We will discuss then a new two methods which are Priority Oriented Adaptive Polling (POAP) and Priority Oriented Adaptive Control with QoS Guarantee (POAC-QG). Finally we have developed the POAC-QG to become more efficient.

1. Introduction The need for the quality of multimedia data in wireless environment emerges the need for good quality data transmission. At the beginning we start focusing on the first techniques that support QoS which is HCF (Hybrid Coordination Function) scheme defined by IEEE 802.11e. It consists of two medium access mechanisms: contentionbased (Enhanced Distributed Channel Access - EDCA), and contention-free protocol (Hybrid Control Channel Access - HCCA). Because EDCA and HCCA operations have some limitations regarding the provided QoS, we introduce the POHA (Priority Oriented Hybrid Access) which is a complete medium control protocol, it combines a polling based (POAP – Priority Oriented Adaptive Polling) and a TDMA access scheme (POAC-QG - Priority Oriented Adaptive Control with QoS Guarantee). This protocol provides QoS for all types of communications that optimizes channel utilization and supports multiple streams with best possible quality and dynamic resource assignment. After some modifications we reach to better performance for POAC-QG protocol.

2. Overview of HCF The 802.11 standard specifies a Medium Access Control (MAC) Layer, which provides a lot of functions that support the operation of 802.11 based WLANs. In general the MAC Layer manages and maintains communications between 802.11 stations by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium. The 802.11 MAC Layer uses an 802.11 Physical (PHY) Layer, such as 802.11b or 802.11a, to perform the tasks of carrier sensing, transmission, and receiving of 802.11 frames. The IEEE 802.11 standard specifies two medium access control (MAC) mechanisms: the mandatory distributed coordination (DCF) and the optional point coordination function (PCF).But is 802.11 enough for QoS? DCF can not provide QoS trivially and PCF is not enough because only one frame can be sent at each polling. Point Coordinator (PC) does not know the QoS requirement of traffic and can not guarantee the delay and jitter bound. To this appears IEEE 802.11e for improvement IEEE 802.11 providing QoS about to • Minimize Latency • Maximize throughput • Define traffic models for both Ad-hoc and Infrastructure. The 802.11e enhances the DCF and the PCF, through a new coordination function: the Hybrid Coordination Function (HCF). There are two methods of channel access, similar to those defined in 802.11 for HCF Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA). Both EDCA and HCCA define Traffic Classes (TC). For example, emails could be assigned to a low priority class, and Voice over Wireless LAN (VoWLAN) could be assigned to a high priority class.[1][2][3] [8] 2

2.1 Overview of EDCA EDCA (Enhanced Distributed Channel Access) enhanced (DCF) of 802.11e provides differentiated channel access to frames of different priorities, in IEEE 802.11e has provided differentiated channel access mechanism, when call demand rises for important festivals or large scale natural disasters such as earthquakes, the delay of voice will increase and the QoS of voice nodes will drop down rapidly. In order to guarantee the QoS of emergency voice calls in congested situation, a higher priority for these calls will be required. QoS can be interpreted as the ability of a network to provide some consistent services for multimedia delivery. As to the approaches to QoS provision for the emergency calls, a priority number control method is often used. In this method, users are classified into multiply priority classes and the network services higher priority traffic before lower priority traffic.[2]

Figure 1. Introduction to the difference of DCF vs. EDCA 2.1.1

EDCA : Frame priorities and ACs

In IEEE 802.1le EDCA provided differentiated (DCF) for frames with 8 different priorities (0 to 7) by enhancing the DCF and implement four access categories (ACs), Each frame with priority is mapped into an AC. [2] UP

AC

1 2 0 (default) 3 4 5 6 7

0 0

Designation (Informative) Background Background

1

Best Effect

1 2 2 3 3

Video Video Video Voice Voice

Low Priority

High

3

2.1.2

EDCA : Contention Parameters Priority & Four transmission queues

Table 1 Priorities & Access categories for EDCF The back-off counter is selected from [ 1, 1+CW[AC]], instead of [0, CW] as in the DCF. Figure 2 shows the timing diagram of the EDCF channel access. The values of AIFS[AC], CWmin[AC], and CWmax[AC], which are referred to as the EDCF parameters, are announced by the AP via beacon frames. The AP can adapt these parameters dynamically depending on network conditions. Basically, the smaller AIFS [AC] and CWmin[AC], the shorter the channel access delay for the corresponding priority, and hence the more capacity share for a given traffic condition. However, the probability of collisions increases when operating with smaller CWmin[AC]. These parameters can be used in order to differentiate the channel access among different priority traffic. [2] Figure 2 IEEE 802.11e EDCF channel access "In table 1 shows the IEEE 802.11e MAC with four transmission queues, where each queue behaves as a single enhanced DCF contending entity, i.e., an AC, where each queue has its own AIFS and maintains its own Back-off Counter BC. When there is more than one AC finishing the back-off at the same time, the collision is handled in a virtual manner. That is, the highest priority frame among the colliding frames is chosen and transmitted, and the others perform a back-off with increased CW values."[2]

2.1.3 EDCA: TXOP and CFB A TXOP is coming to improve the throughput performance. It is introduced in IEEE 802.11e for both EDCF and HCF. A TXOP is a time period for station to continue transmitting after end the time specific to transmissions onto the wireless medium, which is defined by a starting time and a maximum duration. This means when a station has gained an EDCF-TXOP, it is allowed to send more than one frame without contending for the medium again. Hence, the node can use the medium as long as the total transmission time does not exceed its TXOP Limit bound determined by the access point (AP). This multiple frame transmission is referred as contention free burst (CFB). Figure 3 shows the transmission of two QoS data frames during an EDCF TXOP, where the whole transmission time for two data and ACK frame is less than the EDCF TXOP limit announced by the AP. As multiple MSDU transmission honors the TXOP limit, the worst-case delay performance is not be affected by allowing the CFB. We show below that CFB increases the system throughput without degrading other system performance measures unacceptably as long as the EDCF TXOP limit value is properly determined [5].

Figure 3 CFB timing structure 4

2.2 Overview of HCCA HCCA is based on a Contention-Free Period (CFP) during which the access point uses polling for controlling the traffic in the WLAN, like PCF. The differences between HCCA and PCF are the following: • HCCA can poll stations also during the Contention Period (CP). • HCCA supports scheduling of packets based on the QoS requirements. • Stations can communicate their QoS requirements (data rate, delay, packet size…) to the access point. In HCCA, the Access Point (AP) polls the stations to grant them HCCA-TXOP (Transmission Opportunity: A time interval during which a station is allowed to transmit). The AP uses the traffic specifications (TSPECs), which describe the characteristics of the TSs (Traffic Streams), in order to schedule HCCA-TXOPs. More accurate, a TXOP should be long enough to transmit all packets generated during a SI (Service Interval: the time interval between any two successive TXOPs allocated to a station). By this way, HCCA can guarantee QoS in many cases. However, since its scheduler considers fixed TXOPs, it is unable to dynamically assign resources to traffic with changing demands, while this kind of traffic is generated by numerous multimedia network applications. Furthermore, using polling packets and acknowledgements are bandwidth costly. Finally, HCCA fails to efficiently differentiate the TSs, because it does not employ real-time traffic priorities. [1]

3. Overview of POHA Protocol "The introduced POHA scheme considers a super-frame separated into consecutive Polling-based Random Access (PRA) and Resource Reservation Access (RRA) periods. POAP is the protocol operating during the PRA periods. It is a new general purpose polling based access scheme able to serve different types of traffic by adopting packet priorities in order to provide QoS. During the RRA periods, the POACQG protocol takes place. This is a resource reservation access scheme that supports exclusively real-time fic of strict QoS demands. It is based on a new TDMA scheme, which is employed to reduce the bandwidth waste due to polls, keep the stations synchronized by dividing the RRA period into slots, and keep them informed of the time, source and destination of the coming transmissions."[1]

3.1 Overview of POAP 3.1.1 The polling scheme In this protocol[6] the AP polls the stations so that they are permitted to transmit. This sceme of polling eliminates the collisions and causes low overhead. This protocol uses POLL, NO_DATA, and STATUS control packets with transmission duration tPOLL, tNO_DATA, tSTATUS respectively to keep stations informed of the network status and minimize the idle intervals. The main purposes of the control packets are keeping the concerned stations informed of the network’s status and minimize the idle intervals. The AP needs to 5

monitor the transmissions so that as soon as the completion of a communication it can proceed to the next poll. Because of this reason, it has to get knowledge of the actual duration of the specific polling cycle. In order to obtain this knowledge, the AP must successfully detect the NO_DATA packet or the STATUS ACK packet, which contains the duration of the following data transmission, or the STATUS ACK-NACK packet from the destination station or the DATA packet from the polled station. In all the proceeding cases, the AP is aware of the polling cycle duration with getting the information from different sources and can proceed to the next poll without wasting any time. [6] In fact, whenever POLL is successfully received, then it is most likely that the AP will obtain the necessary feedback. Furthermore, our purpose at the end is to poll stations that are actually active in a way that QoS is provided in the network. For this reason, it is needed that the AP needs to be aware of the stations’ status so as to avoid wasting time polling inactive stations, and to favor the high priority traffic, and provide fairness between machines. A STATUS packet is marked as ACK or NACK according to the specific case. The transmission duration of a DATA packet is tDATA and the propagation delay is tPROP_DELAY. All possible polling events are depicted in Figure 4. The AP polls an inactive station (Figure 4a): The AP sends POLL to the mobile station at time t and waits for feedback. The station responds with a NO_DATA packet, which is Figure 4 The POAP station choice received by the AP at t+ tPOLL+ tNO_DATA mechanism +2tPROP_DELAY. Then, the latter will initiate a new polling procedure. The AP polls an active station (Figure 4b): In this situation the AP sends POLL to the station at time t and waits for feedback. The station replies with a STATUS packet marked as ACK, which carries the destination address plus the size of the following DATA packet. After that, the polled station starts transmitting the DATA packet directly to the destination. Now upon successful reception, the destination will broadcast a STATUS packet marked as ACK. In the other case, if the reception fails but the station has realized the STATUS packet sent from source and the DATA that followed which are destined to it, it responds with a STATUS packet marked as NACK. Sending a NACK is not wasted time, since either way the stations had to wait for a possible ACK. The AP proceed to a new poll at time t+ tPOLL+ tDATA + 2tSTATUS + 4tPROP_DELAY . Note that we consider variable DATA packet size thus, tDATA is not static. The communication fails (Figure 4c): In this case the station does not successfully receive the POLL packet, in other words, the polling fails. The AP must wait for the maximum polling cycle before proceeding to the next new poll, since it has to be certain that no collide will happen with possible on going transmission. When successfully receiving the POLL packet from the 6

polled station, but the AP fails to receive any feedback, it waits for the maximum polling cycle similarly to the previous case. The duration of the maximum polling cycle is tPOLL+ tMAX_DATA + 2tSTATUS + 4tPROP_DELAY , where tMAX_DATA is the duration of the largest allowed DATA packet. At the end of this cycle, it is certainly that the medium is idle in any event. When such a communication failure occurs, the AP lowers the probability of choosing this station in the new polling procedure by assuming a bad link between them. However, it is almost likely that the AP will receive some feedback either from the polled or the destination station. [6]

3.1.2. The priority model: POAP mainly adopts the packet priorities concept which is used by IEEE 802.11e to retain compatibility. However, it is differentiated in such a way that chooses a packet to send. Firstly, for each packet buffer, the priority (access category) has to be considered, so that high priority traffic is favored. Also, the number of packets in each buffer has to be examined before choosing a packet in order to provide low packet delays, low packet drops and fairness among the ACs. More specifically, heavy loaded buffers should have higher probabilities of transmitting. [6] Lastly, the earlier generated packets must be favored among others. The mechanism of the respective packet choice is depicted in Figure 5. We initially start by examining if there are any buffered packets, otherwise, the polled station will reply with a NO_DATA message. Then, each one of the four buffers must be examined in Figure5 The POAP packet choice mechanism order to calculate its normalized priority (PPR) and normalized number of buffered packets (PB). To be specific, assume that the priority of buffer i is equal to p[i] =i+ 1, so that it is not null for AC[0], then it holds: PPR [i]=P[i]/ P[k]. Also, if b[i] is the number of packets carried by buffer i, then it holds: PB [i]=b[i]/ b [k]. The buffer priority and the number of buffered packets must have different contribution to the final buffer choice probability (P). Therefore, we use the weights WPR (default 6) and WB (default 2) for PPR and PB, respectively. The non-normalized choice probability for buffer i is: P [i]= WPR × PPR [i] + WB × PB [i]. When WPR is high compared to WB, it is most probably that a high priority buffer will be chosen for transmission regardless of the buffer load (number of packets). On the other hand, the situation is altered if WB is increased. It is by choosing a packet from a highly loaded buffer. The default values favor packet priorities over buffer load by a ratio of 3. The normalized choice probability equals to: P[i]/ P[k]. The station chooses to send the earliest generated packet in it after the buffer selection. Before the AP can decide which station to poll, it has to be well informed of their buffers’ status. However, this feedback should not cause a great overhead to improve performance. As a result of that we exploit the ACK and NACK messages, which are already useful. More accurate, 7

STATUS packet provides acknowledging receptions, it also carries its source’s priority score, which is analyzed below. When a station broadcasts a STATUS packet, it includes its priority score in this packet, which is an indication of the status of its buffered traffic. The priority score depends on both the priority of each buffer and the number of packets it contains. For station i, the priority score is: PS [i]= P[k] × b[K]. So, any time a STATUS packet is broadcasted, the AP examines it in order to update the stored priority score of the transmitting station. By this way, the model provides efficient feedback for the AP with minimum network overhead. The gathered priority scores are then used by the AP to implement a high performance polling mechanism. [6]

3.2 Overview of POAC-QG The need that has led to the development of this protocol is the necessity for bandwidth saving, traffic differentiation, and strict QoS with efficient VBR traffic support. The superframe is separated into real-time traffic (RT) and background traffic (BT) periods. POAC-QG operates during the RT periods, which are contention free. A contention based access mechanism can be used during the BT periods. The 802.1le superframe is suitable for adapting POAC-QG into it. The CFPs and CAPs correspond to the RT periods, and the CPs during which EDCA takes place correspond to the BT periods. [7] POAC-QG is based on TDMA scheme but not polling. The concept is to reduce the bandwidth waste due to polls, keeping the stations synchronized by dividing the RT period into slots, and keeping them informed of the time, source and destination of the coming transmissions. Thus, we could employ a potential power saving model, since stations can stay in "sleep" mode during the RT period and "wake" only to exchange data. The AP uses the beacon signal to inform the stations of the assigned slots and the SI. Stations send their QoS requests for every TS during the BT periods or the last RT slots assigned to them. [7] Bandwidth is assigned to the new requesting TSs, but before that these must be sorted according to their priorities (highest priority first). The corresponding algorithm starts with the highest priority TS and checks if there is enough available bandwidth to serve the specific TS with maximum quality level. Otherwise, the QoS requirements of the lower quality level are checked. If neither the minimum quality level can be supported, then the TS is rejected and the next priority TS is examined. As soon as no bandwidth left to serve a TS with minimum quality, the quality levels of the previously examined higher priority TSs are lowered in order to save some bandwidth priority TSs. When the quality levels of the high priority TSs are lowered, then we check if it becomes possible to increase the quality of the low priority TSs. By this way, the best combination of supported quality levels is provided. [7]

POAC-QG modifications: This protocol gives the available bandwidth to the highest priority traffic streams. But whenever the available bandwidth is not enough the AP examines the high and low quality traffics in the highest priority traffic steam if there is not enough bandwidth for them it examines another low priority traffic stream if the bandwidth still not enough for it lowers the already higher priority processed traffic steams to serve the last new low priority traffic stream. In our work, when the AP examines the high priority traffic steam

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to give it bandwidth and the bandwidth is not enough it immediately lowers the already higher priority processed traffic steams to serve this new higher priority traffic stream.

Simulation results: By comparing POHA and HCF [1] via simulation it is clear that POHA is the best be seeing the following diagrams:

Figure6: Average packet delay versus file transfer (FT) throughput in POHA (POAP) and HCF (EDCA)

Figure7: Average packet delay versus video throughput in POHA (POAC-QG) and HCF (HCCA)

Figure8: Average packet delay versus remote database (RDB) throughput in POHA (POAP) and HCF (EDCA)

Figure9: Aggregate time the TSs were served versus number of offered TSs in POHA (POAC-QG) and HCF (HCCA)

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Figure10: Average packet delay versus voice throughput in POHA (POAC-QG) and HCF (HCCA)

Conclusion: POHA protocol is an efficient medium access mechanism which is able to provide integrated QoS for all types of traffic in wireless LANs. It is an optimized alternative of the IEEE 802.11e HCF scheme. It combines a polling-based and a resource reservation access method, which combination ensures extended QoS support by lowering delays, and having high throughput. And by the proposed modification to POAC-QG it will show efficient performance. At the end it is shown that POHA has better performance than HCF in all cases. [1]

References [1] T. D. Lagkas, G. I. Papadimitriou, Senior Member, IEEE, P. Nicopolitidis, and A. S. Pomportsis, A New Hybrid Medium Access Mechanism with Integrated QoS Support for Wireless LANs 2007. [2] CUI Lu-mlng, OKADA KBzunorl, KATO Toshihiko, CHEN Xlr&yl Performance evaluation on IEEE 802.1le considering emergency calls in congested situation , Volume 14, Supplement, October 2007. [3] Thomas D. Lagkas, Member, IEEE, Georgios I. Papadimitriou, Senior Member, IEEE, Petros Nicopolitidis, and Andreas S. Pomportsis A Novel Method of Serving Multimedia and Background Traffic inWireless LANs, 2008. [4]IEEE 802.1 le WG, IEEE Standard for Information Technology-Telecommunications and Information Exchange Between System-LAN/MAN Specific Requirement-Part 11 Wireless Medium Access Control and Physical Layer specifications, Amendment 8: Medium Access Control Quality of Service Enhancements,Nov.2005. 10

[5] Sunghyun C, Javier D P, Sai S N, et al. IEEE 802.e contention-based channel access (EDCF) performance evaluation. ICC’03, 2003,115 1-1 156 [6] T. D. Lagkas, G. I. Papadimitriou, P.Nicopolitidis, and A. S. Pomportsis, "Priority Oriented Adaptive Polling for wireless LANs," Proceedings of IEEE ISCC '06, Sardinia, Italy, June 2006, pp. 7 19-724. [7] T. D. Lagkas, G. I. Papadimitriou,P.Nicopolitidis, and A.S. Pomportsis,”Priority Oriented Adaptive Control with QoS Guarantee for wireless LANs,” IEEE Transactions on Vehicular Technology,vol.56,no.4,jul.2007,pp.1761-1772. [8] wikipedia

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