Improving Performance of MAC Layer by Using ... - CiteSeerX

Improving Performance of MAC Layer by Using Congestion Control/Avoidance Methods in Wireless Network* Song Ci Hamid Sharif Universityof Nebraska-Lincoln Universityof Nebraska-Lincoln EE Deptartment CEEN Department Lincoln, NE68588 Omaha, NE68182 [email protected] hsharif@ unomaha.edu

Keywords

its wired counterpart. In addition, wireless channel is errorprone, bursty channel, thus this makes the design of W L A N M A C layer protocol more difficult. Moreover, hidden station problem increases the design complexity [2]. Similarly, in connectionless wired packet network, the end user can not detect whether there is congestion happened inside the network due to finitebuffers and unpredictable load. Therefore, the design of both categories algorithms has to face the problem which is how to allocate the network resource dynamically.

Wireless LAN, QoS, MAC, IEEE802.11, Adaptive algorithms

ABSTRACT In this paper, adaptive fragment algorithms for IEEE 802.11 wireless LAN are proposed a n d studied. This work is inspired by studying end-to-end congestion control/avoidance m e t h o d s used in t r a n s p o r t layer. The throughput performance of proposed algorithms for IEEE 802.11 wireless LAN is simulated under different channel quality scenarios. According to the simulation results, the adaptive algorithms designed by using the m e t h o d s behind end-to-end congestion control/avoidance algorithms can improve the channel t h r o u g h p u t a n d reduce the end-to-end delay, even the channel is very noisy.

1.

Guevara Noubir CSEM Network Department Neuchatel, Switzerland [email protected]

The M A C layer protocol in W L A N will try to supply a collision flee access method for multiple users and it also has to use some means to improve link reliability to avoid retransmissions happen because re-transmissions will be very expensive in W L A N . In order to satisfy these new requirements for M A C layer protocol in W L A N , I E E E 802 project proposes the I E E E 802.11 W L A N standard. In this standard, Distributed Coordination Function (DCF) and Point Coordination Function (PCF) are specified to supply a collision free multiple access environment. A n d besides, M A C layer A C K (Acknowledgement) and N A V (Network Allocation Vector), new back-off method, R T S (Request to Send), C T S (Clear to Send), M A C layer fragmentation and defragmentation are specified to increase throughput and reliability of communications. The goal of adopting M A C layer fragmentation and de-fragmentation is to avoid retransmissions from higher layers. For example, due to delay and window size, the cost of T C P layer re-transmission will be much higher than that of M A C layer re-transmission. Similarly, the cost of congestion in connectionless packet network is also very high, thus, in transport layer, endto-end congestion control algorithms have to be adopted. There are some other congestion control/avoidence means like fair queuing and hop-by-hop flow control, but in this paper, only end-to-end congestion control/avoidence algorithms adopted by transport layer will be discussed.

INTRODUCTION

In recent years, there is an increasing need for more wireless d a t a services, such as voice, email, video and audio, accessed by laptop, paimtop a n d so on. As one of "last-hop" solutions, wireless LAN ( W L A N ) is adopted to connect mobile devices to high speed wired networks. In studying the issues faced by MAC algorithms, in some senses, they are very similar to issues faced by end-to-end congestion control algorithms in connectionless wired packet networks. Before this is explained further, it is necessary to compare and cont r a s t problems faced by these two categories of algorithms. First, the goal of these two categories of algorithms is to improve network throughput. In W L A N , the carrier sensing m e t h o d is much different from the way used in wired shared-media network like Ethernet. This is because collision is detected right away in Ethernet, but this is impossible in W L A N even though it is also a shared-media network like

*This paper is partially sponsored by M o C o R e P r o Project, University of Nebraska-Lincoln. Permission to make digitalor hard copies of all or part of thiswork for personal or classroom use is granted without fee provided that copies are not made or distributed for profitor commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise,to republish, to post on sm-versor to redistribute to lists, requires prior specific permission and/or a fee. SAC 2001, LainVegas, NV © 2001 ACM 1-58113-287-5/01/02...$5.00

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The rest of this paper is organized as follows. In the next sections, we discuss the possible application of T C P congestion control/avoidance algorithms in IEEE 802.11 W L A N . In section 3, four adaptive fragmentation algorithms for I E E E 802.11 W L A N are proposed in heuristics of studying T C P congestion control/avoidance algorithms. In section 4, the simulation model is described. The simulation results and analysis are presented in section 5. Finally, we will conclude with a summary.

2. PROBLEM DESCRIPTION In IEEE 802.!1 WLAN, there is a new enhancement for QoS provision, i.e., MAC layer fragmentation and de-fragmentation. The advantage of using fragmentation and de-fragmentation is that when end users send long data packets, the frame error rate is very large because of the characteristics of wireless channel. Thus, this causes a large number of re-transmission and then generate much overhead. According to [6, 7], the mean attempts of transmission(?,) is 1 ~= (1-py)

(1)

and the network throughput(C) is 1

¢= r

x

k

xps

(2)

Where, r : the coding rate such as 1, ~, ~ 5 etc. k : is equal to log 2 M, and M means M-array modulation scheme. p / : error probability of frame.

TCP layer, packet loss is thought as the result of congestion, similarly, packet loss is thought as bad packet received at receiver or packet destroyed by hidden stations in WLAN. In TCP end-to-end congestion control/avoidance algorithms, window size is used as a unit of controlling, i.e., through controlling the window size in sender or receiver or both, congestion will be controlled or avoided. In WLAN, packet size or fragment size or both are controlled in a similar ways to reduce the frame error rate. Here, four adaptive fragmentation algorithms are proposed in heuristic of studying congestion control/avoidance algorithms and their performance will be compared with the fragment adaptive algorithm using fragment size back-off [4]. The algorithm 1 described below is designed by using the method behind slow-start congestion control algorithm widely adopted in transport layer [9].

3.1

If(ACK lost or ACK time out) Ok+l = Ok + 2; if (0k+l < e)

In this work, we choose no coding and BPSK modulation case, i.e., r is set to 1 and k is set to 1. As P f = 1 - (1 --pb) N , Pb is bit error probability and N is the length of frame. Therefore,

Ok+l = e;

Else Ok+~ = Ok X2;

1 7 -- (1 - - p b ) N

Adaptive Fragmentation Algorithm 1

if (0k+l > 6) Ok+l = 6;

(3)

In this work, another type of throughput equation is used for simplification purpose, ¢ = V

In the above algorithm, the e is the optimal fixed fragment size in corresponding to different channel quality. The O is the adaptive fragmentation threshold used by sender.e is the total size of the long frame sent using fragmentation algorithms. If ACK is lost or time out, the adaptive fragmentation threshold will be back-off by half, i.e., in time k + 1, the threshold will be half of the threshold in time k. On the other hand, if ACK is received, the fragmentation threshold will be doubled in time k + 1. The fragraentation threshold will not be increased more than the frame size and it will also not be decreased less than the optimal fragmentation size.

(4)

T

Here, G is the total number of bytes received successfully and r is total simulation time. Therefore, using fragmentation will lower the frame error rate and then increase the network throughput. The other advantage of using MAC layer fragmentation is to reduce the total time of transmitting a longer packet. In this case, the sender is just waiting for a Short InterFrame Space (SIFS) after receiving an ACKnowledgement (ACK) of fragment from receiver rather than a Distributed Interframe Space (DIFS)(according to the specification of IEEE 802.11 W L A N standard, DIFS is always larger than SIFS). As wireless link is error prone and time-varying, this fragment size should not be a constant value like in wired network. We believe the adaptive algorithm enhances the throughput and reduces the end-to-end delay.

3.2

Adaptive Fragmentation Algorithm 2 If(ACK lost or ACK time out) generate n E [1, v]with uniform dist.; Ok+l = Ok + n;

if (0k+l < e)

Even though the IEEE 802.11 specifies the fragmentation approach, no adaptive fragmentation algorithm is given and there is no research work in this topic so far. As it is analyzed in Section 1, we suggest that the design methods behind T C P end-to-end congestion control/avoidance algorithms could help to improve the performance the MAC layer protocol of WLAN.

Ok+l = e;

Else generate m 6 [1, w] with uniform dist. Ok+l = O k x m;

if (Ok+l > e) 0k+l = 6;

3. ADAPTIVE ALGORITHMS SUMMARY

In the above algorithm, fragment size is increased or decreased exponentially in a random way. This algorithm is inspired by the random back-off algorithm employed by some MAC layer protocols. The discussion and comparison of

The similarity between issues faced by T C P and issues faced by WLAN is that the end user sends a packet into network without reservation and then wait an event happens. In

421

aOl / uOl

these two algorithms appear in [4]. v and to axe the maximum back-off window size and n and m axe the back-off step used in time k + 1.

all

3.3 Adaptive Fragmentation Algorithm 3 al0/ul0

If(ACK lost or ACK time out) generate n E [1, v]with uniform dist.;

Figure

Ok+l = Ok + n;

1: C h a n n e l

state

diagram for Gilbert-Elliot

Bursty Channel Model

if ( 0 k + 1 < e) O k + l = e;

Else Ok+~ = Ok + 6;

4.

if (0k+l > ~)

In this paper, the wireless channel model is characterized by Gilbert-Elliot Model [10]. This model uses a two-state ergodic Markov chain whose steady states here are AWGN channel with different N~--~. We choose 30dB difference between two steady states o~ Gilert-Elliot burst channel model, which are 10 -5 in good state and 10 -2 in bad state. In Figure.I, p01 is the arrival rate from good state to bad state, which obeys the Poisson distribution. Similarly, /~10 is the arrival rate from b a d state to good state, am is the trans i t i o n probability from good state to bad state and a m is the transition probability from bad state to good state, a00 is the transition probability from good state to good state and a l l is the transition probability of from bad state to bad state. P9 is probability of steady-state in good state, Pb is probability of steady-state in bad state and Perror is long-term (average) probability of errors. And they follow under equations [10]:

In above, fragment size is decreased in a random exponential way but increased in a additive way. Here 6 is the fragment increase step.

3A

Adaptive Fragmentation Algorithm 4 If(ACK lost or ACK time out) generate n E [1, v]with uniform dist.; 0k+~ = 0k + n;

if (0~+~ < ~)

Ok+l = e; Else if(0k < ~) 0k+l = 0 k x2; else Ok+~ = Ok + 6; if (0k+l > *)

SIMULATION MODELING

p s ( k + 1) = ps(k) *

aoo

q-pb(k) * al0

pb(k + 1) = pb(k) * alo + p b ( k ) * a l l

Ok+l = ~;

perro, = pg * 10 -5 + Pb * 10 -2

(5) a o l --{-aoo = 1

The algorithm 4 is designed by using the m e t h o d behind an improved version of slow-start algorithm [9]. There is a fragment size threshold in order to slow down the increase of fragment size after this threshold is reached. ~ is the threshold used to avoid the fragmentation size increased to too large and t o o fast.

3.5

al0 + all

Pg + Pb = 1

In this simulation, O P N E T modeler is chosen [8]. In [1], issues of simulation in IEEE 802.11 W L A N using O P N E T are discussed. But in their model, there are no noisy channel and fragmentation and de-fragmentation parts of I E E E 802.11 protocol. In the simulation for this work, the timevarying channel and fragmentation/de-fragmentation are implemented in order to a d a p t our requirements. The one-hop network topology is adopted in this simulation in order to verify the link performance. In this simulation, the delayinsensitive d a t a service traffic is used as source. Two different channel quality scenarios are used in order to check the throughput performance of the proposed adaptive algorithms.

Adaptive Fragmentation Algorithm 5 If(ACK lost or A C K 0k+l = e; Else

---- 1

time out)

if(0k < ~) 0k+l = 0 k X 2 ; else Ok+l = Ok + 6;

if (0k+l > ~) Ok.+l = g;

5.

RESULTS AND ANALYSIS

The parameters chosen for simulations are as following, because the d a t a frame size of IEEE 802.11 is specified between 0 and 2312 bytes, in the simulation of this work, is set to 2000 bytes; the values of v and to used in algorithms described before are set to 4 because of considera-

This algorithm is more conservative in decreasing fragment size and use the same way as algorithm 4 in increasing fragment size.

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0.5. From this figure, it is observed that under given channel quality, using fixed fragment size, 50 bytes can give the optimal throughput.

tions about computation complexity and real-time requirement, although other number could get the similar results; is set to optimal fragmentation size. Because the main concern here is to exam the adaptive fragmentation algorithms in heuristic of studying congestion control algorithms implemented in TCP, the method of how to get optimal frame size adaptively is out of the reach of this paper. So far, there are some research working on how to get the optimal frame size or fragmentation sizes [3, 5, 6]. Here, the optimal fragmentation size for each channel quality scenario is derived by using the method described in [5]. ~ is set to half of frame size, i.e., 1000 bytes.

Figure. 3 is derived from channel quality scenario I. Under this channel quality scenario, algorithm 1 can achieve the best throughput performance, which is better than the throughput achieved by using fixed optimal fragment size (50 bytes), and all proposed algorithms using the method behind end-to-end congestion control/avoidance algorithms can give better throughput performance than that achieved by using fixed fragment size. This is shown that when the channel quality is getting worse, using proposed adaptive fragmentation algorithms can get more gain in terms of throughput performance. There is more 15% increase in good throughput than the optimal throughput achieved by using using fixed fragmentation size in this channel quality.

Comparison ot'11~roughput Pmton'nance wilh Different Fb~ed Fmgmerd Size(Scenario I) 2o0o

16oo t ~ i i ........... i i i l• , 1~

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i .......... ,i .......... i, ..........

,

......... ] .......... i .......... ; .......... i .......... ; .......... { .......... ~.......... i ..........

CompQde, on of'll~roughput Perforrmmce with Oifle~nt Rxed Fragment S / z ~ (Scenario II) 700O ?

,~

i .......... : ! ~ . ~ , ~ . o ~ . . . . . . .

....... !

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. . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . •

...................

, .................

Figure 2: C o m p a r i s o n o f T h r o u g h p u t P e r f o r m a n c e w i t h Different Fixed F r a g m e n t Sizes (Scenario I) i ,~eula~on "time (mn.)

tsoo

Comparison of "n.lmughput Pedonmince of Adaptive Aloodthrns (Scenario I) :

F i g u r e 4: C o m p a r i s o n o f T h r o u g h p u t P e r f o r m a n c e with Different F i x e d F r a g m e n t Sizes (Scenario II)

of Adaptive X l g o ~ , ~ g ' i . f l o

Clx~pattson of ThtougtN~d P e f f o ~

II)

1 tloo

to

15

20

25

30

35

40

~

50

SlmulaUon 11me (rrdn.)

Figure 3: C o m p a r i s o n of T h r o u g h p u t P e r f o r m a n c e of Different A d a p t i v e A l g o r i t h m s (Scenario I)

:

15

!

20

25

~mul~on

Figure.2 shows the comparison of throughput performance using different fixed fragment sizes in channel quality scenario I, i.e., pg (probability of steady-state in good state) is 0.5 and Pb (probability of steady-state in bad state) is

30

35

40

Time (INn.)

F i g u r e 5: C o m p a r i s o n o f T h r o u g h p u t P e r f o r m a n c e o f Different A d a p t i v e A l g o r i t h m s (Scenario II)

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[5] S. Ci, H. Sharif, and G. Noubir. Frame Size Analysis for IEEE802.11 and Its Affect on Throughput. Proc. of Sixth International PDPTA Conference (PDPTA '~000), pages 2931-2937, Jun., 2000.

Figure.4 shows the comparison of throughput performance u.sing different fixed fragment sizes in channel quality scenario II, i.e.,pa is 0.7 and Pb is 0.3. From the figure, it is observed that under given channel quality, using fixed fragment size, 500 bytes can give the optimal throughput.

[6] E. Modiano. An Adaptive Algorithm for Optimizing the Packet Size Used in Wireless ARQ Protocols. Wireless Networks, 5:279--286, 1999.

Figure. 5 is derived from channel quality scenario II. Under this channel quality scenario, from above figures, algorithm 1 can achieve the best throughput performance, which is better than that the throughput achieved by using fixed optimal fragment size (500 bytes).

6.

[7] G. Noubir. Multimedia Access and Distribution. European Project Internal Note, 1999. [8] OPNET Inc. OPNET Modeler User Manual. 1997. [9] L. Perterson and B. Davie. Computer Network: A Syster~s Approach. Morgan Kanfmann Pulishers Inc., 1996.

SUMMARY

In this paper, several adaptive fragmentation algorithms inspired by studying TCP end-to-end congestion control/avoidance [10] S. Wilson. Digital Modulation and Coding. algorithms used in transport layer are proposed and studied. Prentice-Hall Inc., 1996. From the above results, we observe that the proposed adaptive fragmentation algorithms can improve the throughput performance of MAC layer protocol of WLAN under fading wireless channel, especially in the scenario that channel quality is getting worse. In general, in fading channel, the proposed adaptive fragmentation algorithm in heuristic of studying slow-start congestion control algorithm will offer better performance in both channel quality scenarios since the channel quality can be reflected more accurately in this algorithm. The proposed adaptive fragmentation algorithm proposed here is simple and can be easily implemented. Note that although the result throughput by adopting these algorithms are better than that of an optimal fixed size, in practice it is no optimal fixed size because we cannot have a single static two state model, so adaptation is very important. In future work, more efficient optimal fragmentation size estimation methods and more complex channel models should be considered.

7.

ACKNOWLEDGEMENTS

The authors of this paper would like to thank two anonymous reviewers for their valuable comments to help us to carefully make the presentation.

8.

REFERENCES

[1] R. Baldwin. IEEE80~.11 Wireless LAN Model Documentatioa OPNET modeler, 1998. [2] H. Chhaya and S. Gupta. Performance Modeling of Asynchoronous Data Transfer Methods of IEEE 802.11 MAC Protocol. Wireless Networks, 3:217-234, 1997.

[3] C. Chien, M. Srivastava, R. Jain, P. Lettieri, V. Aggarwal, and R. Sternowski. Adaptive Radio for Multimedia Wireless Links. I E E E Journal on Selected Areas in Communications, 17(5):793-813, 1999. [4] S. Ci and H. Sharif. Adaptive Approaches to Enhance Throughput of IEEE 802.11 Wireless LAN with Bursty Channel. Proc. of The ~5th Annual IEEE Conference on Local Computer Networks (LCN ~000), Nov., 2000.

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