Study of the TCP Upstream/Downstream Unfairness Issue with Per-flow Queueing over Infrastructure-mode WLANs

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1 Study of the TCP Upstream/Downstream Unfairness Issue with Per-flow Queueing over Infrastructure-mode WLANs Yi Wu, Zhisheng Niu, and Junli Zheng State Key Lab on Microwave and Digital Communications Department of Electronic Engineering, Tsinghua University, Beijing, P.R. China Abstract - Fairness is an important issue in WLANs due to their shared media nature. Essentially, IEEE. MAC protocols have been designed to provide fair access for all the competing mobile hosts. However, the fairness at MAC layer can not be maintained at TCP layer in the presence of both mobile senders and receivers accessing to the wired networks. In this paper we investigate the TCP upstream/downstream unfairness issue over WLANs with per-flow queueing employed at the Access Point. The interactions between the IEEE. MAC protocol and TCP are evaluated through analysis and simulation. Based on the derived analytical model, an efficient solution is proposed to be implemented at the Access Point for fairness achievement. Index Terms: Wireless TCP, Fairness,. DCF, Cross-layer. I. INTRODUCTION Emerging as an attractive alternative, or complement, to wired LANs [], the IEEE. WLAN family has been gaining great popularity for data applications in public areas where people can obtain access to the Internet, e.g., campuses, offices, homes and hot spots such as airports, hotel halls, cafes, etc. Being free to access to an existing network with high data rate in some hot spots is perhaps one of the killer-applications of WLANs. To provide a more reliable and convenient network connection for mobile clients, the infrastructure mode of WLAN. is employed, in which all the mobile stations communicate with each other or communicate with a wired network by first going through an Access Point (AP) []. Compared with the alternative ad-hoc mode in which the mobile stations talk to each other directly, the infrastructure-mode WLANs offer the advantage of scalability, centralized security management and improved reach. As in the infrastructure-mode, the DCF of IEEE. protocol allows equal access to the media for all competitive mobile nodes. Therefore, fairness is ensured at the MAC level when communications are confined to the local WLAN or when all data flows passing through the AP are in the same direction. However, considering the case in which several downstream and upstream flows exist simultaneously, the data packets in the forward path of downstream flows and the ACK packets in the reverse path of upstream flows are mixed in the buffer of AP, competing equally with other mobile stations for access to the WLAN. This mixture of data flows and ACK flows will cause great asymmetric effect between the upload and download TCP connections. Thus the equal access nature of the IEEE. DCF protocol contrarily results in significant unfairness of the upper TCP layer. In a nutshell, the most relevant TCP unfairness is that TCP flows in downstream are penalized with respect to the TCP flows in upstream because of the greedy closed loop control nature of TCP. Therefore, the bandwidth allocation among data flows and ACK flows at AP needs to be carefully designed. Fairness issues in WLANs have been widely investigated, mostly concentrating on the particular QoS differentiation and bandwidth allocation at MAC layer []- []. However, it is insufficient to confine the fairness consideration to the MAC layer. As which has been pointed out by Pilosof [], in the presence of current IEEE.b WLAN with DCF access, fairness of MAC layer can not be maintained at TCP layer from the viewpoint of user applications. To the best of the authors knowledge, reference [] is the first paper to focus on TCP fairness in the infrastructure-mode IEEE. WLAN which contains both mobile senders and receivers. The work in [] is based on the assumption that all packets in the downlink direction (including TCP data segments and ACKs) are integrated in the same queue of AP without differentiation. It is worth noting that the TCP upstream/downstream unfairness problem in the case of per-flow queueing at AP has not been investigated by any previous work. This paper focuses on the TCP unfairness issue over infrastructure-mode WLAN with per-flow queueing. The interactions between TCP and IEEE. MAC protocol are investigated by analysis and simulation. With TCP behavior modeling and queueing analysis at the AP, we propose a bandwidth allocation strategy for the downlink buffer of AP to ensure the TCP fairness among all the upstream and downstream flows. The proposed strategy is derived from model analysis and the results have been

2 validated by simulations on ns-. The rest of this paper is organized as follows. In Section II, the related works are discussed. In Section III, the TCP unfairness problem over infrastructure-mode. WLAN with DCF is described. In Section IV, based on modeling of the interacting behaviors of TCP and IEEE. MAC protocols, the TCP unfairness problem is analyzed. In Section V, derived from the analytical model, an efficient bandwidth allocation strategy for the downlink buffer of AP is proposed to improve TCP fairness. Finally, conclusions are presented in Section VI. Server High Speed Wired LAN Mbps Mobile Station Access Point TCP data flows of Uplink ACK flows of Uplink WLAN.b Mbps ACK flows of Downlink TCP data flows of Downlink II. RELATED WORKS There have been several research papers addressing the fairness issue of IEEE. based WLANs. Tang and Gerla [] provide an insight into the interactions between TCP and MAC layer protocols and discuss the fair sharing problem of MAC under TCP in wireless ad hoc networks. T. Nandagopal [] proposes a general analytical framework for translating a given fairness requirement into a matching backoff scheme for contention resolution. Vaidya [] presents a fully distributed algorithm for fair scheduling in WLAN, where it emulates Self-clocked Fair Queueing in a distributed manner and chooses a backoff interval to achieve the weighted fairness among different flows. In [], Yang develops a scheduling scheme by which an extra adaptive delay is inserted before new packet transmission at MAC layer. By the modification of MAC protocol, the fairness among TCP flows is improved across wireless ad hoc and wired networks. In [], by adjusting the contention window, a distributed weighted fair queueing mechanism is proposed to allocate the link s bandwidth among different flows proportionally to their weighted fairness. However, few of the above works present the effect of available buffer at the AP and the unfairness problem between upstream and downstream flows. As the first paper to consider the unfairness between upstream and downstream TCP flows, Reference [] provides analytical models for the TCP behavior over WLANs in a particular scenario that a finite single queue is employed at the AP s downlink buffer. Also, the authors propose an advertised window manipulation scheme by modifying the receiver window field of the ACKs passing through the AP. In this way, the rates of upstream TCP flows are limited by a pre-decided value in order to achieve the fairness between upstream and downstream flows. The method of advertised window manipulation is conservative because it tries to achieve fairness by limit all TCP flows with the same upper bound which leads to the total throughput degradation. Furthermore, modifying the ACK content at an intermediate AP breaks the end-to-end TCP semantics and may cause more undesirable influence especially when passing through heterogenous networks. Mobile Station Mobile Station Mobile Station Fig.. Illustration of the network topology. Since ACK flows are quite different from TCP data flows in the respect of packet length and loss influence on the TCP source behavior, the technology of single queue operation for all different flows in the downlink direction is obviously inefficient for buffer utilization and bandwidth allocation at AP. Consequently, it is intuitive to introduce the technology of per-flow queueing operation [9] to the buffer management of AP. Therefore, a key problem arises: could fair queueing provide fairness in the bottleneck links? In this paper the unfairness issue between upstream and downstream TCP flows over WLAN with per-flow queueing at the AP s downlink buffer is investigated. III. TCP UNFAIRNESS OVER. In this section we illustrate the particular unfairness problem between upstream and downstream TCP flows over an infrastructure-mode WLAN. The particular hot spot scenario we consider is shown in Fig., where multiple TCP flows are assumed to span across a WLAN and a high-speed wired network connected through an AP. In such a scenario, the AP works as a gateway and provides access to the high-speed wired network for a set of mobile stations. For mobile stations in the WLAN, the downstream TCP flows may be some data applications such as file downloading or video streaming from a fixed server in the wired network. On the other hand, the upstream TCP flows may also exist for file uploading to a fixed server or for peer to peer data transmission. It is noted that such a hot spot scenario is very popular for WLAN s applications today. For example, in the stages of Olympic Games, people may need to exchange important information with a database in the wired network by accessing through a WLAN. In order to better understand the upstream/downstream unfairness problem and analyze the interaction between TCP and the IEEE. MAC protocols, we conduct a comprehensive simulation study using the ns- simulator

3 T C P T h r o u g h p u t ( K b p s ) 9 queue limit: D D D D D D D D D9 D U (a) downstream flows and upstream flows TC P Throughput (Kbps) queue limit: D D D D D D D D D9D U U U U U (b) downstream flows and upstream flows Fig.. TCP throughput for multiple flows with single queueing at AP. []. Without loss of generality, the multiple mobile stations are set separately as senders and receivers to communicate with the particular fixed server in the wired networks. As shown in Fig., all the TCP data packets of the downstream flows and the reverse ACKs of the upstream flows have to pass through the AP for access to the WLAN. At MAC layer, the DCF with the infrastructure mode of IEEE. is implemented. Since the wired network is assumed to be a Fast Ethernet with Mbps, the downlink access to the WLAN.b with Mbps becomes the bottleneck. Therefore, the queueing behavior of the AP downlink buffer appears to be the key factor of the TCP performance. It is noted that the unfairness problem occurs only when the propagation delay in the wired network is much smaller than the queueing delay and transmission delay in the WLAN. Otherwise, if the propagation delay of the wired network is large enough, the AP s downlink buffer would not become the bottleneck because then the TCP throughput is limited by the dominant round trip time. Consequently, there is no unfairness induced by the WLAN. In this paper we focus on the scenario where the propagation delay of the wired network is small (set to be ms in simulations) and this is a resonable assumption for the high-speed optical networks. In order to identify the relevant problem and to analyze the TCP unfairness issue, we focus on the up/down throughput ratio R which is defined as the ratio of upstream TCP throughput to downstream TCP throughput. Denote by R u the average upstream TCP throughput and by R d the average downstream TCP throughput, we have R = R u /R d. If the up/down throughput ratio R approaches to, a perfect upstream/downstream fairness is obtained. For the ratio R, the greater departure from the greater the unfairness. In our simulation, the length of TCP packet is set to bytes. Let w m denote the TCP receiver advertised window size and set it to packets according to the practical settings in most implementations []. Today many TCP receiver implementations send one cumulative ACK for two consecutive packets received (i.e. delayed ACK [][]). Therefore, we also consider the delayed ACK case in our investigation. Denote by b the number of packets that are acknowledged by a received ACK, and we have b =. In the WLAN, the packet loss is assumed to be caused by collision of the mobile stations. Being similar to most of the previous works, we consider the saturated transfer scenario, i.e. the TCP senders are assumed to always have data to transfer. All the simulation points represent averages from repeating runs of simulated seconds by ns- using random seeds independently each time. First we review the unfairness problem in the basic case of single queueing. In the single queueing case, all the TCP packets and ACKs from different flows are mixed in the AP s downlink buffer. The queue limit is set to packets, which is a typical value for commercial IEEE. products []. The simulation results for TCP throughput of multiple downstream and upstream flows are presented in Fig.. As shown in Fig.(a), it is obvious that whenever there is an upstream TCP flow passing through the AP, the throughputs of all the downstream TCP flows are immediately reduced greatly and the up/down throughput ratio R is kept around. When the number of the upstream flows increases, the degradation of the downstream flows becomes outstanding. As shown in Fig.(b), when there are upstream flows existing, the TCP throughput of the downstream flows falls below Kbps and the up/down throughput ratio R rises over. In this single queueing case, the TCP data packets and ACKs share the AP s downlink buffer without differentiation. Since the length of ACK is quite smaller than that of data packet, for the same buffer size more ACKs can be accepted than data packets.

4 T C P T h r o u g h p u t ( K b p s ) 9 data queue limit: (per flow) ACK queue limit: (per flow) T C P T h ro u g h p u t (K b p s ) data queue limit: (per flow) ACK queue limit: (per flow) D D D D D D D D D9 D U (a) downstream flows and upstream flows D D D D D D D D D9D U U U U U (b) downstream flows and upstream flows Fig.. TCP throughput for multiple flows with per-flow queueing and random polling employed at AP. Consequently, the overflow probability of data packet is larger than that of ACK. Furthermore, because the ACK flow is not loss-responsive, it is TCP-unfriendly. Thus when overflows occur at the AP s downlink buffer, only the downstream TCP flows respond to the packet loss by reducing the sending rate. Therefore, ACKs tend to occupy more space of the AP s downlink buffer and accordingly the upstream TCP flows would continue increasing the sending rate and achieve more bandwidth of the WLAN because of the greedy closed loop control nature of TCP. In this way, the upstream/downstream unfairness problem is aggrandized. In this paper we focus our investigation on the case in which per-flow queueing is supported at the AP s downlink buffer. The buffer is statistically partitioned into multiple logical queues. The packets of different TCP flows are divided into different queues so that the buffer overflows of TCP flows are independent and the occupancy interference between data packets and ACKs is avoided. Fig. presents the TCP throughput of multiple downstream and upstream flows with per-flow queueing. For comparability with the single queueing case, the data queue limit is set to for each downstream TCP flow and the ACK queue limit is set to for each upstream TCP flow. The schedule policy of the multiple queues is random polling in order to make the active queues share the same bandwidth. The numerical results show that with per-flow queueing and random polling employed at the AP s downlink buffer, the unfairness between upstream and downstream flows is relatively mitigated compared with the single queueing case. However, the unfairness problem is not totally solved. The data rates of the downstream TCP flows are still penalized by the upstream TCP flows. As shown in the figure, the up/down throughput ratio R is maintained around when the number of upstream TCP flows is increased from to. In practice, the unfairness problem between the up- Routing queue Fig.. buffer. B D high priority Data queue p p p q ACK queue q q Downlink buffer of Accesss Point System model for per-flow queueing at the AP s downlink stream and downstream TCP flows is critical. To understand why, consider for example that in a hot spot when most people connecting to the WLAN are downloading data from a high-speed wired network. Whenever there appears any person who needs to upload files through the same AP, all the downstream flows would be penalized and the downloading rates are shut down. This is not feasible because that usually the download process is prevailing and should be guaranteed for the requirements of most users. IV. MODEL ANALYSIS AND PERFORMANCE EVALUATION In order to have an insight into the TCP unfairness problem over WLAN, we conduct an analytical study of the TCP behavior with per-flow queueing. The system model for per-flow queueing at the AP s downlink buffer is illustrated in Fig.. In the per-flow queueing system, the routing packets generated by routing protocols are enqueued in a particular routing queue which is always treated with the highest priority. Except for the routing

5 Queue Occupancy (in packets) data queue occupancy ACK queue occupancy B D = for data queue = for ACK queue Simulation Time (sec) TCP Congestion Window (in packets) upstream TCP flow downstream TCP flow B D = for data queue = for ACK queue Simulation Time (sec) Fig.. Queue occupancy and TCP window variation ( upstream flows and downstream flows). packets, all the other data packets or ACKs from different TCP flows are divided into different data queues or ACK queues. In the departure process, once the AP knows from MAC that it can transmit a packet, the routing packet would be sent out immediately whenever the routing queue is not empty. Otherwise, all the logical data or ACK queues are polled according to the scheduling policy. For the convenience of analysis, we assume that each data queue is polled with probability p and each ACK queue is polled with probability q. As described in the previous section, the TCP performance depends on the queueing behavior of the AP s downlink buffer. In the per-flow queueing, the losses of data packets and ACKs belonging to different TCP flows are separated and therefore the small-length ACKs would not compete with data packets for the same buffer space. Let B D denote the data queue limit and denote the ACK queue limit respectively. In our analysis, we model the queue of each flow as a queueing system with finite capacity. We assume that only when a packet is transmitted successfully to the WLAN, it can be cleared from the queue. Therefore, the service rate of an individual queue is determined by the transmission rate which can be obtained by the AP from IEEE. MAC layer. Let R w denote the efficient transmission rate of the AP s wireless port, and then the service rates for data queue and ACK queue are obtained as pr w and qr w respectively. Apparently, the arrival rate of data queue is the TCP data rate of the downstream flow R d. The arrival rate of ACK queue is obtained as R u /b since we have assumed b as the number of data packets that are acknowledged by an ACK. When the data queue limit B D is less than the TCP receiver advertised window size w m (B D < w m ), the AP s downlink buffer becomes the bottleneck of data transmission. The numerical results in Fig. show that the congestion window size of the the downstream TCP flow is limited by the data queue limit B D. As TCP responses to loss of each data packet by halving the congestion window, the TCP window size of the downstream flow keeps increasing linearly from B D / to B D periodically. Due to TCP behavior in the congestion avoidance phase, the average utilization rate of the buffer is approached by /. In the congestion avoidance phase, the TCP window size will increase for every round trip time and hence the corresponding duration for the TCP window rising equals B D T d /, where T d represents the average round trip time of the downstream TCP connection. Since we have assumed that the propagation delay in the high-speed wired network is small enough, T d can be approximated by the average queueing delay at the AP s downlink buffer. Therefore, we have T d = B D pr w () Considering the saw-tooth variation of the TCP window, the downstream TCP throughput defined as the ratio of the number of transmitted data packets to the duration is derived as R d = BD B D wdw B D T d / = pr w () Note that if the data queue limit is small enough (e.g. B D = ), the TCP congestion window would be limited to a very small size and there are no enough ACKs which can result in the three duplicate ACKs. In such a case, the Fast Recovery of TCP can not be triggered and the timeouts always occur. Hence the TCP throughput is dominated by the behavior of slow start and the achieved throughput is less than pr w, mainly depending on the TCP congestion window and the retransmission timer. In this paper we do not consider this tiny buffer size case and assume that B D, which is reasonable in practice. When B D w m, the number of TCP packets in flight would be limited by the receiver advertised window w m and hence the queue overflow is avoided. In this way, T d can be approached as the average sojourn time of an

6 Up/Down Throughput Ratio Total throughput (Mbps) Average ratio R Analytical ratio R (ana) Data queue limit B D (in packets) Up/Down Throughput Ratio 9 Average ratio R Analytical ratio R (ana) Total throughput (Mbps) ACK queue limit (in packets) 9 Fig.. The up/down TCP throughput ratio as a function of the data queue limit B D. ( =) M/M/ queueing system: T d = ( ρ D )pr w () where ρ D is the utilization factor defined as the ratio between the arrival rate and the service rate of the downstream flow (R d /pr w ). Without the limit of buffer capacity, the TCP window size would rise to the upper bound w m when the collision errors are recovered by the MAC retransmissions. Therefore, the downstream TCP throughput R d should be R d = w m T d = w m ( R d /pr w )pr w () From Eq.() we have R d = w mpr w w m + pr w () Due to the cumulative acknowledgement nature of TCP, indeed the losses of ACKs have no influence on the congestion window size of the upstream flows. Hence one can conclude that the upstream TCP window size increases until it reaches the TCP receiver advertised window size w m and will keep staying at the size, which has been demonstrated in our simulation (see Fig.). Therefore, the throughput of the upstream TCP flow is expressed as R u = w m T u () where T u represents the average sojourn time of the queueing system. The derivation of ACK s sojourn time T u depends on the length of the ACK queue. When < w m /b, since the loss of ACK does not reduce the sending rate of the upstream TCP flow, ACKs in flight would always fill the ACK queue at the AP s downlink buffer. In this case we have T u = qr w. () Fig.. The up/down TCP throughput ratio as a function of the ACK queue limit. (B D =) When w m /b, the overflow of the ACK queue is avoided because the ACKs in flight has been limited to w m /b by the receiver advertised window. Then based on the same M/M/ queueing analysis as in Eq.(), we have T u () ( ρ A )qr w where ρ A is the utilization factor defined as the ratio between the arrival rate and the service rate of the upstream flow (R u /bqr w ). Combining Eq.()-() we have w m qr w, if < w m /b R u = (9) w m + w m /b qr w. if w m /b Finally, the up/down throughput ratio R is obtained as qw m if < w m /b p R = R u /R d = if w m /b qw m p( + w m /b) () Recalling that in this paper we have assumed w m = and b =. In our simulation, the scheduling policy is assumed to be random polling and hence we have p = q =.. Therefore, the up/down throughput ratio R is approached by R =, if <, if () Considering the scenario of downstream TCP flows and upstream TCP flows, we present the up/down throughput ratio R as a function of the data queue limit B D in Fig. and as a function of the ACK queue limit in Fig., respectively. The computed ratio R is also shown in the figures for comparison. Although

7 Up/Down Throughput Ratio Total throughput (Mbps) Average ratio R Analytical ratio R (ana) TCP Throughput (Kbps) avg throughput (upstream flows) avg throughput (downstream flows) 9 Number of upstream TCP flows 9 Number of upstream TCP flows Fig.. TCP performance with downstream and n upstream TCP flows. (B D =, =) increasing the data queue limit B D helps to raise the TCP congestion window size, it leads to a larger queueing delay on the other hand. Consequently, B D has little impact on the TCP throughput as shown in Fig.. On the other hand, the ACK queue limit is a key factor which affects the fairness between the upstream and downstream flows. When falls below w m /b, the overflow of the ACK queue results in ACK losses for the upstream flows. The losses of ACK do not decrease the sending rate of the TCP, it contrarily reduces the average round trip time of the upstream TCP flow and accordingly the throughput of the upstream flow is increased. Therefore, the greedy closed loop control nature of TCP makes the uploading mobile stations gain more throughput in media contention and finally worsens the unfairness problem between the upstream flows and downstream flows. Results in Fig. show that when is less than, the up/down throughput ratio R is raised up to. Since the data queue and ACK queue are analyzed separately, the analytical results for the up/down throughput ratio R is independent of the number of multiple flows. Fig. plots the throughput ratio as a function of the number of upstream flows. Although the average throughput of upstream and downstream flows both decrease with the increasing number of upstream flows, the up/down throughput ratio R keeps almost invariant at :. V. PROPOSED SOLUTION The previous discussions have shown that with perflow queueing supported at AP, the unfairness between upstream and downstream flows could not be diminished due to the close loop control of TCP. In this section, we are interested in an effective solution that improves fairness between upstream and downstream TCP flows through the AP. Intuitively, based on the analytical model we have derived in Section IV, the fair share of the WLAN among all the upstream and downstream flows could be controlled by a weighted polling strategy employed at the AP s downlink buffer. Our solution is to differentiate the TCP-unfriendly ACK flows from the data flows by allocating different weights for polling. From Eq.() we have: p q = w m R if < w m /b w m R( + w m /b) if w m /b () Consequently, to achieve fairness between upstream and downstream flows, the polling weights p and q need to be determined to guarantee that R =. The per-flow queueing system of the AP s downlink buffer has been illustrated in Fig.. Our solution for the upstream/downstream fairness is described as follows:. When the AP succeeds in contention and is ready to transmit a packet in the WLAN, the downlink buffer first checks if there is any routing packet in the routing queue. The routing packets should be transmitted with the highest priority.. If the routing queue is empty, the AP s downlink buffer uses a weighted polling strategy to choose an active queue for packet transmission. In order to implement our solution, the AP s downlink buffer needs to be aware of the number of the current active queues. If there are m active data queues and n active ACK queues waiting for packet transmission, the polling probability for each data queue and for each ACK queue should be set to p and q respectively, where p and q are determined by p = w m m(w m + ) w m m( + +b b w m) if < w m /b if w m /b ()

8 T C P T h ro u g h p u t (K b p s ) Data queue limit: (per flow) ACK queue limit: (per flow) D D D D D D D D D9 D U (a) downstream flows and upstream flows TCP Throughput (Kbps) Data queue limit: (per flow) ACK queue limit: (per flow) D D D D D D D D D9D U U U U U (b) downstream flows and upstream flows Fig. 9. TCP throughput of the proposed solution for multiple flows. (B D =, =) q = n( + w m ) + w m /b n( + +b b w m) if < w m /b if w m /b (). If the selected queue is empty, the polling procedure goes on choosing the next queue among the left queues according to Eq.() and Eq.(). Until a nonempty queue is selected, the polling procedure ends and the corresponding packet is dequeued and sent out. Up/Down Throughput Ratio Average ratio R Total throughput (Mbps) ACK queue limit (in packets) A. Numerical Results To verify the fairness approach of our solution, we perform simulations with multiple downstream and upstream TCP flows. Fig.9 enumerates the TCP throughput of each downstream and upstream flow with the proposed solution employed at the per-flow queueing system of the AP s downlink buffer. The data queue limit B D is set to for each downstream flow and the ACK queue limit is set to for each upstream flow. The result shows that all the upstream and downstream flows succeed in getting a fair allocation of bandwidth with the proposed solution. The fairness improvement of this solution is obvious when recalling that in the same scenario earlier in Section III, in the per-flow queueing system without special treatment on polling probability, the ratio of the upstream/downstream TCP throughput is kept around : (see Fig.). Fig. presents the throughput ratio achieved by the solution with varying ACK queue limit. The data queue limit B D is fixed as packets. It shows that the : ratio is maintained by adjusting the polling probability of data queues and ACK queues according to the different ACK queue limit as shown in Eq.() and Eq.(). Fig. plots the ratio R and the average TCP Fig.. The up/down TCP throughput ratio achieved by our solution with varying. (B D =) throughput with varying number of upstream flows. The number of downstream flows is set to, the data queue limit B D is set to and the ACK queue limit is set to. The results show that the up/down throughput ratio R stably stays at the fair value :, while the average throughput of the TCP flows degrades with the increasing number of TCP flows. Furthermore, also note that the total throughput is maintained as the number of upstream flows increases. This is explained by the fact that the solution only changes the bandwidth allocation among the different types of flows with little impact on the total capacity. Although in this section we are discussing about the solution to achieve equal fairness between the upstream and downstream TCP flows, it is easy to extend the proposed polling strategy to the weighted fairness cases. Consider a particular hot spot scenario where the major services are information downloading from fixed servers in the wired network to the mobile stations in the WLAN. Hence in order to guarantee the downloading rates for the mobile stations, the AP could adjust the polling parameters p and q according to Eq.() to manipulate

9 Up/Down Throughput Ratio Total throughput (Mbps) Average ratio R 9 Number of upstream TCP flows TCP Throughput (Kbps) avg throughput (upstream flows) avg throughput (downstream flows) 9 Number of upstream TCP flows Fig.. TCP performance of the proposed solution with downstream and n upstream TCP flows. (B D =, =) Up/Down Throughput Ratio Average ratio R Random polling Proposed solution. E- E- E Packet error rate of the wireless channel TCP Throughput Random polling avg throughput (upstream) avg throughput (downstream) Proposed solution avg throughput (upstream) avg throughput (downstream) E- E- E Packet error rate of the wireless channel Fig.. Impact of the random loss on the TCP upstream/downstream fairness ( downstream+ upstream flows. B D =, =). the up/down throughput ratio R at any predefined level. B. The Impact of Channel Errors In our previous analysis and simulation, we have assumed that the channel is error free. However, a lossy channel may result in packet drops due to channel errors. In this case, besides the buffer overflow, the channel error may also cause the packet loss of the TCP flows. Therefore, the impacts of the channel errors on our analysis and the corresponding solution need to be studied. Based on the CSMA/CA mechanism, the IEEE. MAC protocol can not differentiate the dropped packets caused by channel errors from that caused by collisions. Therefore, from the viewpoint of the MAC layer, the channel errors are treated as collisions. In the IEEE.b MAC protocol, the link error recovery shall be attempted by retrying transmissions at the MAC layer [] (generally the short retry limit is set to for non- RTS/CTS transmission and to for RTS/CTS transmission). Furthermore, in the considered hot spot scenario of WLAN, the error rate of the wireless channel is much smaller than that of the channel in cellular networks. Consequently, the impact of the channel errors is mostly shielded by the recovery procedure of the IEEE. MAC protocol. To illustrate the impact of channel errors on the TCP upstream/downstream fairness, we repeat the simulation scenarios of the random polling and the proposed solution with random errors presented at the physical layer. The packet error rates are increased from.% to % for all the downstream TCP flows and upstream TCP flows. The data queue limit and the ACK queue limit are set to. Fig. shows that the accuracy of the analytical model and the fairness improvement brought by the solution are maintained until the packet error rate of the wireless channel exceeds % which corresponds to serious channel conditions. VI. CONCLUSION AND FUTURE WORK In this paper we have investigated the unfairness problem in IEEE. WLAN for TCP flows with per-flow queueing employed at AP. The interactions between the IEEE. MAC protocol and TCP are studied through analysis and simulation. It is found that although the CSMA/CA mechanism tries to provide equal opportunity 9

10 to all the mobile stations in contention, the fairness at the MAC layer can not be maintained at the TCP layer in the presence of both mobile senders and receivers accessing to the wired networks. The impacts of buffer availability and polling strategy of the per-flow queueing system on the TCP fairness are analyzed. Based on the derived analytical model, we extensively propose an efficient solution to be implemented at AP for fairness achievement between upstream and downstream TCP flows. This work is a promising start of the TCP fairness over WLAN and there are still several open issues for us to pursue. Since the analytical model and the corresponding solution are based on the assumption of low error rate of the wireless channel, more works need to be done for the high error rate situations. The unfairness problem of the upstream/downstream flows in the ad-hoc mode of the WLAN is another area that requires more research. Also, the unfairness problem of upstream and downstream TCP flows that have different RTTs needs to be reconsidered in greater details. Furthermore, with the WLAN evolution to IEEE.e where the QOS differentiation is supported by the MAC layer, the weighted fairness problem at the user level should be investigated. [] L. Kleinrock, Queueing System, Volume : THEORY, John Wiley & Sons, 9. REFERENCES [] H. Ahmadi, A. Krishna, R.O. LaMaire, Design issues in wireless LANs, J. High-Speed Networks, vol., pp.-, 99. [] IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. ISO/IEC -:999 (E), Aug., 999. [] K. Tang, and M. Gerla, Fair sharing of MAC under TCP in wireless ad hoc networks, in Proc. of IEEE MMT 99, Oct [] T. Nandagopal, Tae-Eun Kim, X. Gao, V. Bharghavan, Achieving MAC layer fairness in wireless packet networks, in Proc. of the th annual international conference on Mobile computing and networking, August. [] N. H. Vaidya, P. Bahl, S. Gupta, Distributed fair scheduling in a wireless LAN, in Proc. of the th annual international conference on Mobile computing and networking, August. [] Luqing Yang, Winston K.G. Seah, Qinghe Yin, Improving fairness among TCP flows crossing wireless ad hoc and wired networks, in Proc. of the th ACM international symposium on Mobile ad hoc networking & computing,. [] A. Banchs, X. Perez, Distributed weighted fair queuing in. wireless LAN, in Proc. of ICC, vol., pp. -, April. [] S. Pilosof, R. Ramjee, Y. Shavitt, P. Sinha, Understanding TCP fairness over Wireless LAN, in Proc. of INFOCOM, vol., pp.-, April. [9] B. Suter, T.V. Lakshman, D. Stiliadis, A.K. Choudhury, Design considerations for supporting TCP with per-flow queueing, in Proc. of IEEE INFOCOM, vol., pp.99-, March 99. [] UCB/LBNL/VINT Network Simulator - ns (version ). Available at [] J.Padhye, V.Firoiu, D.Towsley, J.Kurose, Modeling TCP Reno performance: a simple model and its empirical validation, IEEE/ACM Trans. Networking, Vol., pp. -, April. [] T.V. Lakshman, U. Madhow, The performance of TCP/IP for networks with high bandwidth-delay products and random loss, IEEE/ACM Trans. Networking, Jun. 99. [] W. Stevens, TCP/IP Illustrated, vol. The Protocols, thed. Reading, MA: Addison-Wesley, 99.