Comparative Study of QoS Scheduling Algorithms in Wireless Networks

International journal of Computer Science & Network Solutions

http://www.ijcsns.com

December.2013-Volume 1.No4 ISSN 2345-3397

Comparative Study of QoS Scheduling Algorithms in Wireless Networks A. J.Lakshmi, B. Dr.R.Latha Research Scholar, Department of computer Science & Applications, Bharathiar University, Coimbatore, India [email protected] Professor & Head, Department of Computer Applications, St.Peter ’s University, Avadi, Chennai -600 054, India

Abstract Wireless communications helped to simplify networking by enabling multiple users to simultaneously share resources without any wiring. Wireless networks are generally unpredictable compared to wired networks. Packet scheduling can guarantee quality of service and improve transmission rate in wireless networks. Wireless packet networks rely on a range of mechanisms to provide for Quality of Service (QoS), the core of which would be scheduling algorithms. This paper compares the performance of various wireless scheduling algorithms in wireless networks. The wireless scheduling algorithms are theoretically analyzed and comparison is done between each of them in specific areas, which are felt to be important and pertaining to QoS provisioning. Each wireless network has a different packet scheduling strategy and each has their own advantage and disadvantage. The desired qualities of QoS wireless scheduling algorithms are selected for this study. The concepts of IWFQ, CIF-Q and WPS algorithms are taken for this study. We compare their properties through theoretical analysis conclude that CIF-Q achieve all the properties of wireless service including short-term and long-term fairness, short-term and long-term throughput bounds, and tight delay bounds for channel access. (Karygiannis et al,2002)

Keywords: QoS, Packet Scheduling, Wireless Networks, IWPQ, CIF, WPS

I. INTRODUCTION:

With the development of wireless technologies, wireless networks are at the epicenter of this trend. At its broadest, a wireless network refers to any network not connected by cables, which is what enables the desired convenience and mobility for the user. This technique that helps industrialists and telecommunications networks to save the cost of cables for networking in specific premises in their installations. Administration of transmission station is done via radio waves. With the convergence of real time voice, video and data in multimedia applications, it is essential to provide QoS to the users in wireless networks.

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Figure1. Network Sample II. PACKET SCHEDULING:

The process of assigning user’s packets to appropriate shared resource to achieve some performance guarantee is called packet scheduling. Packet scheduling refers to the decision process used to select which packets should be serviced or dropped. Dropping of packets will be based on network characteristics like bandwidth, packet arrival rate, deadline of packet and packet size. Scheduling will be done in scheduler. A scheduler will find it difficult to handle all the packets coming in if the packet rate is high, if the bandwidth is too low or the packet size is large. So the scheduler will select certain packets based on various algorithms. Some of these algorithms have been selected for this survey. It’s not possible for every packet to reach at destination some may be dropped along the way due to the effect of network characteristics. It is an efficient method to enhance the system performance. Packet scheduling is mainly applied to guarantee quality of service and improved transmission rate in wireless networks. The main goal of packet scheduling algorithm is to maximize the system capacity while satisfying the QoS of users and achieving certain level of fairness. (Xiong et al ,2003)

Figure.2. Packet Scheduling

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III. GOALS OF PACKET SCHEDULING:

• • •

• •

• •

Provide per flow/aggregate QoS guarantees in terms of delay and bandwidth. Aggregate protection Flow/Aggregate identified by a subset of following fields in the packet header. o Source/destination IP address(32 bits) o Source/destination port number(16 bits) o Protocol type(8 bits) o Type of service(8 bits) Efficiency: The basic function of packet scheduling algorithms is scheduling the transmission order of packets queued in the system based on the available shared resource. Protection: Besides the guarantees of QoS, another desired property of a packet scheduling algorithm to treat the flows like providing individual virtual channels, such that the traffic characteristic of one flow will have as small effect to the service quality of other flows as possible. This property is sometimes referred as flow isolation. Flexibility: A packet scheduling algorithm shall be able to support users with widely different QoS requirements. Low complexity: A packet scheduling algorithm should have reasonable computational complexity to be implemented. (Zhu et al, 2012)

IV. QUALITY OF SERVICE (QoS) PARAMETERS

QoS provisioning encompasses providing Quality of Service to the end user in terms of several generic parameters, namely: A. Throughput or bandwidth Throughput is a measure of the date rate (bits per second) generated by the application. Bandwidth is the network resource that is allocated to traffic class depending on its requirements. B. Delay or latency Delay or latency would be time taken by the packets to transverse from the source to the destination. The main sources of delay can be further categorized into: source-processing delay, propagation delay, network delay and destination processing delay. C. Delay variation (delay jitter) Delay variation is the variation in the delay introduced by the components along the communication path.

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D. Packet loss or corruption rate Packet loss affects the perceived quality of the application. Several causes of packet loss or corruption would be bit errors in an erroneous wireless network, or insufficient buffers due to network congestion when the channel becomes overloaded due to out-of-profile or best effort traffic. V. WIRELESS SCHEDULING ALGORITHMS: Wireless scheduling algorithms, which schedule packets with reference to an error free system, are known as ideal reference system algorithms. A flow is said to be leading, lagging or in sync at any time instant by comparing its queue size with that of the one in an error-free system. The scheduler will thus perform scheduling and maintain fairness according to this knowledge. A. Idealized Wireless Fair Queuing (IWFQ) Idealized Wireless Fair Queuing (IWFQ) is an adoption of Weighted Fair Queuing (WFQ), which approximates Generalized Processor Sharing (GPS) on a packet level. An IWFQ model is defined with reference to an error-free WFQ service system. This scheme is idealized due to three factors: 1) Packets are never dropped or lost, 2) Channel state is always known, 3) Presence of a perfect MAC protocol. When no link suffers from bursty errors, IWFQ performs exactly like its wireline WFQ counterpart. When a packet n of flow i arrives at the scheduler, it is tagged with a service start time Si, n and a service finish time f i, n. S i, n = max {v (A(t)), f i, n - 1} f i, n = Si, n +L i, n / r i

Here, Li,n is the packet length, v(A(t)) is the system virtual time as defined in WFQ and ri is the service rate allocated to flow i. (Gupta et al,2000) Packets are stored in a non-decreasing order of finish times in each queue, and the scheduler would serve the packet with the smallest finish time.

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However, when there is an occurrence of a bad link in a wireless scenario, and the packet to be served cannot be served, the next packet with the smallest finish time would be served instead. This would perpetuate until the scheduler find a packet with a good link state. The essential difference is that in a wireless environment, there is implicit wireless compensation, whereby lagging packets may be discarded if the flow lags by too much without losing precedence in channel access, and users which experience error bursts will get precedence upon having good transmission conditions. To compensate for the short error-bursts, IWFQ performs local adjustments in the channel allocation by the use of tags. A flow, which is initially denied service due to burst errors in the packets, will eventually receive service, as its service tag does not change. IWFQ was the first algorithm to propose a structured adaptation of fair queuing to the wireless domain. And hence backlogged lagging flows will receive precedence over leading flows upon attainment of a good channel. Advantages: The advantages of IWFQ lie in the fact that it provides fairness in the system and QoS guarantees. Disadvantages: The disadvantage of IWFQ is its high implementation complexity due to the need to simulate an error free system and keep track of both services to the error-free and real system. And as IWFQ requires packet arrival times to compute virtual start times, the base station requires information of uplink packet arrival times, which is unknown. Furthermore, as absolute priority is given to packets with the smallest finish times, when a flow is compensated for its previous lagged service all other error-free flows would not be served at all, the delay and throughput guarantees are also closely coupled, which may lead to one affecting the other.

B. Channel-Condition independent Packet Fair Queuing (CIF-Q) Channel-Condition independent Packet Fair Queuing (CIF-Q) utilizes an error-free fair queuing reference system as it attempts to approximate the real service to the ideal error free system. With the location dependent errors in mind, CIF-Q tries to use the ideal system as the reference in order to perform wireless scheduling. Due to the discrepancies between users in a wireless networks, CIF-Q aims to provide delay, bandwidth and short term fairness guarantees for error free sessions and long term fairness guarantees among error sessions. In addition to that, sessions which have received additional services are permitted a graceful degradation in QoS. (Ford et al, 1962) 40




Upon arrival, each packet is assigned a start and finish tag based on the arrival time of the packet and the finish tag of the previous packet, and the packets are scheduled in increasing order of their start tags. In CIF-Q, packets are scheduled in accordance to their start tags. If in any situation whereby there are location-based errors, service is transferred to another flow, and the virtual time of that flow in the reference system is updated. Thus the virtual time of the flow is updated to record the normalized work such that: vi = vi +li / r i Whereby li k is the length of the kth packet of session i and ri is the rate of session i. As and when the selected flow is unable to be transmitted, the bandwidth is distributed among the other flows proportional to their flow rates. Furthermore, the CIF-Q is delay bounded as when a lagging session wants to leave, the lag variable is increased proportional to their rate, and a leading session is not allowed to leave until it has given up its lead. CIF-Q can provide short-term and long-term fairness, and bounded delay channel access. Service degradation for leading flows is linear. In general case, CIF-Q achieves the properties of wireless service, though in a pathological case, a lagging flow may capture the channel as in IWFQ. Advantages: The advantages of CIF-Q would be its fairness guarantees both short term and long term. Additionally, CIF-Q advantageous over IWFQ as it improves scheduling fairness by associating compensation rate and penalty rate with a flow’s allocated service rate and guaranteeing flows with error-free links with a minimal rate. In error-free links, CIF-Q also guarantees packet delay bounds as well flow throughput. Disadvantages: The disadvantages of CIF-Q are similar to that of IWFQ. First and foremost, CIF-Q requires packet arrival times to compute virtual start times; the base station requires information of uplink packet arrival times, which is unknown. The need to simulate a reference system also increases the complexity. C . Wireless Packet Scheduling (WPS) Wireless Packet Scheduling (WPS) is the approximation of IWFQ, which uses Weighted Round Robin (WRR) as its error-free reference algorithm. WRR is used in the implementation 41




due to its simplicity in implementation and its O (1) time to process a packet in wireless networks. Moreover, in comparison with other fair queuing algorithms, the performance is very similar. In WPS, to account for the bursting of wireless channels, WRR is modified with two different types of wireless compensation, 1) Intra-frame swapping, 2) Credits/debits. For intra-frame swapping, the algorithm will attempt to swap the frame of a flow with various error bursts with one, which is experiencing a good channel. For credits/debits, for a backlogged flow in a queue that is unable to intra-frame swap and unable to transmit, it is assigned a credit if there are one or other flows, which are able to use the slot. In turn, their credit is decremented. WPS generates a frame of slot allocation from the WRR spreading algorithm. In each slot of the frame, if the flow that is allocated the slot is backlogged but perceives a channel error, then WPS tries to swap the slot with a future slot allocation within the same frame. Advantages: The advantages of WPS are that it provides short term and long-term fairness guarantees as well as QoS guarantees. Moreover, as it uses WRR as its referencing system, the algorithm is simpler to implement. Disadvantages: The disadvantages of WPQ is similar to IWFQ and CIF-Q, as WFQ requires packet arrival times to compute virtual start times; the base station requires information of uplink packet arrival times, which is unknown. VI. COMPARITIVE STUDY OF WIRELESS SCHEDULING ALGORITHMS: This paper does the comparison of three scheduling algorithms IWFQ, CIF-Q and WPS. The Specific aspects of the wireless scheduling algorithms are analyzed and compared with each other. Comparison results agree with expected results based on theoretical analysis.

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TABLE I PERFORMANCE Aspects Analyzed

IWFQ

CIF-Q

WPS

EF Model

WFQ

STFQ

WRR-S

RiC(t2-t1)ri∑i€F Lp- Lp

Wi/∑j€FwjC(t2 -t1)

Throughpu RiC(t2-t1)-Lp t Wi(t1,t2) Delay dik

Lp/riC+Lp/C

Lp/riC+ ∑j€FLp/C

Fairness Index fr(t1,t2)

Lp/ri+ Lp/rj

Lp/ri+ Lp/rj

∑j€Fwj/wi Lp/C+ Lp/C (∑k€Fwk) ( Lp/wi+ Lp/wj)

The table I summarizes the performance of the error-free service model for the various algorithms. The average throughput of the algorithms are shown in the chart below by giving sample flow values, when the per-flow maximum sustained rate increases up to 300 kbps. No packet loss occurs. ( Gambiroza et al,2004) Cumulative Average throughput

300 T h r o u g h p u t

250 200 150 100 50 0 50 100 150 200 250 300 Rate Requested Per Flow (kbps)

Figure.3. Average throughput of the algorithms 43




Triangle represents CIF-Q, Big Square represents WPS and Small square represents IWFQ. As expected, Channel-Condition Independent Fair Queuing (CIF-Q) performance is the best considering its ability to perform compensation of lagging flows by preferentially swapping slots with leading flows, thus ensuring that in-sync flows are not significantly affected. VII. CONCLUSION: In this article we presents the comparative study and performance analysis of three wireless scheduling algorithms (IWFQ, CIF-Q and WPS) based on various aspects like delay bound, bandwidth guarantee, wireless link variability and short term fairness. The compensation model in IWFQ is implicit. If a flow perceives channel error, it retains its tag (and hence, precedence for transmission when it becomes error free). A leading flow may be starved out of channel access for long periods of time. Thus the compensation model in IWFQ does not support graceful degradation of service. Additionally, in-sync flows may be affected during the compensation that is granted to lagging flows. The study of these algorithms shows that CIF-Q keeps on improving lagging flows. As a consequence of its compensation model, CIF-Q provides a graceful linear degradation in service for leading flows. Additionally, it performs compensation of lagging flows by preferentially swapping slots with leading flows, thus ensuring that in-sync flows are not significantly affected. CIF-Q thus overcomes two of the main drawbacks of IWFQ and WPS. References i. ii.

L.R. Ford and D.R. Fulkerson, Flows in Networks, Princeton University Press, 1962. P. Gupta, P. R. Kumar, The capacity of wireless networks, IEEE Transactions on Information Theory, Vol. 46(2), 2000, pp. 388–404.

iii.

Xiong, Qing, Weijia Jia, and Chanle Wu. "Packet scheduling using bidirectional concurrent transmission in WiMax mesh networks." Wireless Communications, Networking and Mobile Computing, 2007. WiCom 2007. International Conference on. IEEE, 2007.

iv.

T. Karygiannis, L. Owens, Wireless Network Security 802.11, Bluetooth and Handheld Devices, NIST Special Publication 800-48, Nov. 2002.

v.

Zhu, Xiaomin, et al. "An improved security-aware packet scheduling algorithm in real-time wireless networks." Information Processing Letters 112.7 (2012): 282-288.

vi.

V. Gambiroza, B. Sadeghi, E. W. Knightly, End-to-end performance and fairness in multihop wireless backhaul networks, Proceedings of ACM MobiCom’2004, pp. 289–301.

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