Fair Media Access for Wireless LANs - CiteSeerX

Fair Media Access for Wireless LANs Timucin Ozugura , Mahmoud Naghshinehb, Parviz Kermanib and John A. Copelandc a (Corresponding author)

334156 GA Tech Station, Atlanta, GA 30332 Work: 404-894-2085 Fax: 404-894-0035 Home: 770-951-1209 b IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598 c Communications Systems Center, Georgia Institute of Technology, Atlanta, Georgia 30332 E-mail : fozugur, [email protected] fmahmoud, [email protected]

Abstract This paper addresses load balancing algorithms for the wireless links to solve the fairness problem, where stations are not able to gain access equally to the shared wireless medium according to the current medium access control protocols. The fairness problem occurs mostly because of the existence of hidden stations and the presumption of a non-fully connected wireless network topology. In this paper, pij -persistent carrier sense multiple access based algorithms are proposed in which a fair wireless access for each user is accomplished using a pre-calculated link access probability. Link access probabilities are calculated at the source station in two ways using connection-based and time-based media access methods. Each active user broadcasts information on either the number of logical connections or the average contention time to the stations within the communication reach. This information exchange provides partial understanding of the topology of the network to the stations. Each station reserves a speci c priority for its each link to gain access to the shared medium based on the exchanged information. The proposed algorithms are dynamic and sensitive to the changes in the network topology. They are applicable to all CSMA-based media access control protocols to provide a degree of fairness. Simulation results show that the algorithms result in an order of magnitude performance improvement in terms of network fairness in the wireless network. Keywords: Media access control protocols, load balancing, wireless ad-hoc LANs, CSMA

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1 Introduction A wireless local area network (LAN) is a way to connect portable computers over radio or infrared wireless links in a small area such as an oce or home environment [1, 2]. The medium in a wireless network is a shared resource. Consequently, it is critical that a medium access control (MAC) protocol provides fairness and robustness to the wireless network. The MAC protocols rely on the features of the multiple access protocols. There are several reasons why multiple access is more dicult for wireless LANs than for wired LANs, such as dynamic physical channel characteristics, carrier sensing and hidden station problem. In a wireless LAN in which not all the stations are within transmission range of one another, a station with a packet to send cannot accurately ascertain if its transmission will arrive without collisions at an intended receiver, because it cannot hear the transmissions from other senders that might arrive at the same intended receiver. This is referred to as hidden station problem. Due to the vast challenges, a new class of media access schemes is required to provide fair access to each user. There are many proposed multiple access protocols for wireless LANs, such as carrier sense multiple access (CSMA), polling, and time division multiple access (TDMA). TDMAbased protocols are suitable for infrastructured wireless LANs, where communication is established by the help of a central station, which is connected to a backbone. CSMA-based protocols are generally used to establish a peer-to-peer connectivity in wireless ad-hoc LANs, which presume a non-fully connected network topology. In CSMA-based protocols, stations can not gain access to the medium equally. This is referred to as fairness problem. The current CSMA-based MAC protocols are intended to solve collisions raising because of hidden stations, however they can not solve the fairness problem, which is due to the presumption of a non-fully connected wireless network topology. The goal of this paper is to provide new, ecient, and simple wireless MAC algorithms for balancing the load among the wireless links to solve the fairness problem. Hence, stations can equally share the wireless medium. The addressed solutions for the fairness problem in wireless networks were rst introduced by [3], which are called balanced media access methods. These methods are easy to be implemented in a commercial wireless LAN on top of any CSMA-based MAC protocol. Balanced media access methods are pij -persistent CSMA based algorithms, in which a fair wireless access for each user is accomplished using a pre-calculated link access probability, pij , which represents the link access probability from station i to j. In this paper, we investigate the balancing eect of the proposed methods for the bursty data trac in the wireless LANs in the case of hidden stations. The paper is organized as follows. Section 2 gives the related works in literature, where 2

we provide some background on the media access control protocols. In Section 3 we introduce the window exchange algorithm and balanced media access methods that are intended to solve the fairness problem in wireless networks by balancing the load among the links in a distributed manner. In Section 4, we evaluate the performances of the balanced media access methods using several dierent wireless ad-hoc network con gurations under bursty trac conditions, and describe the periodic window-exchange algorithm method. We then summarize our ndings and future research directions.

2 Related Works The CSMA with collision avoidance (CSMA/CA) protocol is proposed to alleviate the hidden station problem. CSMA/CA with a four-way handshake is used to combat the problem of an indoor fading channels [5, 7, 6]. According to the protocol, when source station attempts to reserve the channel. it sends a request-to-send (RTS) packet to the destination. If the intended destination hears the RTS, it replies with a clear-to-send (CTS) packet. Upon receiving the CTS, the source station sends its data immediately. Any station overhearing an RTS and/or CTS exchange, defers all transmissions until some time after the associated transmission can be nished, [5, 7]. In our system, if a station captures the channel successfully, it transmits multiple data packets, which is called burst transmission. After the transmission of multiple data and their ACK packets, the source station sends an end-of-burst (EOB) packet, and and starts waiting for an end-of-burst-con rmation (EOBC) packet from the intended receiver. Other stations overhearing the RTS/CTS exchange and/or transmission, defer their own transmission until EOB/EOBC exchange, or a certain time period including the burst transmission [4]. The CSMA/CA-based protocols are based on Multiple Access Collision Avoidance (MACA) [7], which has been introduced for single hop datagram service in wireless LANs. Several other MAC protocols have been proposed, which are based on RTS-CTS exchanges, including the MACAW protocol. In MACAW, the MACA protocol is augmented with additional message types and backo and retransmission strategies to improve throughput [6]. The notion of per stream fairness is also proposed where each station keeps separate queues for each stream runs the back-o algorithm independently for each queue. Stations also broadcast the back-o window information. The protocol suggests the transmitter to transmit a data-sending (DS) packet after the receipt of CTS packet and before data transmission. The DS packet is eective in the following scenario. Stations that hear the RTS packet only but not the CTS packet, suer unless the intended destination has received the RTS. 3

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Figure 1: Example of the eect of non-fully connected network topology, (a) station 3 reserves the channel to communicate station 4 by sending a RTS packet, station 4 replies back with a CTS packet, (b) simultaneous communication between station 1 and 2, and stations 4 and 5. In this case, these stations defer their transmission although no successful reservation is achieved. In this protocol, a request-for-request-to-send (RRTS) packet is also introduced. Whenever a station receives an RTS to which it can not respond due to deferral, it contends to send an RRTS packet to the sender of the RTS in the next contention period. The MACAW protocol is not eective if the fairness problem occurs due to simultaneous communications in the network. In this case, stations that can hear both communications, access to the channel with unacceptably long delays.

3 Wireless LAN Topologies The wireless topology has a huge impact on the link throughputs. The performance of other stations either that are not part of the communication, or that can not hear the RTS/CTS exchange, may also be aected. Figure 1 and 2 illustrate how the network topology aects the transmission. Stations can only hear the stations that are connected to them. Referring to Figure 1(a), station 3 reserves the channel by sending an RTS packet to station 4. Station 4 replies back with a CTS packet. Then, station 3 transmits its data, and station 4 replies back with an ACK packet. Station 2 receives the RTS packet since it is within the communication reach of station 3. However, station 2 can not receive the CTS packet because it is not within the communication reach of station 4. Station 1 does not receive neither the RTS nor the CTS packet. Meanwhile, if station 1 has a packet for station 2, station 1 sends its RTS packet to reserve the channel after backing o. Note that medium is idle from the point of view of station 1. Since station 2 can not issue a CTS packet because it is in a 4

Tx:1 ! 2 0.1568

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Link Throughput in Mbps Tx:2 ! 3 Tx:3 ! 2 0.6733 0.6865

Tx:3 ! 4 0.6878

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Table 1: Throughput (Mbps) for the 4-station scenario where link oered load ! 1. Link Throughput in Mbps Tx: 1 ! 2 Tx: 2 ! 1 Tx: 2 ! 3 Tx: 3 ! 2 Tx: 3 ! 4 Tx: 4 ! 3 Tx: 4 ! 5 Tx: 5 ! 4 1.9508 0.1200 0.0821 0.2865 0.2870 0.0871 0.1213 1.9530

Table 2: Throughput (Mbps) for the 5-station scenario where link oered load ! 1. non-participant mode due to the communication between stations 3 and 4, station 1 backs o again after increasing its back-o window size. The data transmission between stations 3 and 4 may end during this time. Since station 1 has backed o with a larger back-o window size, stations 2, 3 or 4 may have higher chance to reserve the channel again rather than station 1. This example shows that a new access method is necessary to provide a fair access chance to each station for each logical connection. The throughput of the 4-station scenario with a fully-loaded system is given in Table 1. The results show that Link12 and Link43 have the lowest throughput, whereas the other links have similar throughput. Note that Linkij represents transmission from station i to station j . According to the table, Link34 is able to transmit 4.38 (=0.6878/0.1568) times more than Link12 . In the 5-station scenario, station 1 is within the communication reach of station 2 only, which is within the communication reach of both stations 1 and 3, and so on. The scenario is illustrated in Figure 1(b). In this scenario, it is possible to have simultaneous communications between stations 1 and 2, and stations 4 and 5. In this case, station 3 switches into a non-participant mode. Since the outer links can capture the channels consecutively, station 3 experiences long delays before gaining access to the channel. The throughput of the 5-station scenario with a fully-loaded system is given in Table 2. The results show that Link54 in the network can transmit 23.79 (1.9530/0.0821) times more than Link23 . Link23 and Link43 have the lowest throughput. The total throughput of the network is 4.89 Mbps which is a result of having two simultaneous communications, where the channel capacity is 4 Mbps. The 6-station scenario using dashed diagonal links is a typical example of a network with non-homogeneous load distribution. The scenario is shown in Figure 2. Stations that are connected 5

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Tx:2 ! 1 1.4491

Link Throughput in Mbps Tx:3 ! 4 Tx:4 ! 3 0.0668 0.0250

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Table 3: Throughput (Mbps) for the 6-station scenario with dashed diagonal links where link oered load ! 1. with dashed diagonal links can hear, but they don't have any load for each other. Stations that are connected with solid links have load in each direction. For example, station 3, which is within the communication reach of stations 2, 4 and 6, has load for only station 4. In the scenario, stations 1 and 6 issue an RTS for station 2 and 5, respectively. Since station 3 is within the communication reach of both stations 2 and 6, it receives the RTS of station 6 and the CTS of station 2. Station 4 receives the RTS and CTS of stations 1 and 5, respectively. As a consequence, stations 3 and 4 experience long delays before gaining access to the channel. The throughput of the 6-station scenario with dashed diagonal links in a fully-loaded system is given in Table 3. Referring to the table, the total network throughput is 5.81 Mbps due to simultaneous communications in the network, where the channel capacity is 4 Mbps. The results also show that Link21 in the network have 57.96 (1.4491/0.025) times more than Link34 . It is clear that the system needs a load balancing algorithm to share the medium equally among the users.

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3.1 Window-exchange Algorithm In the wireless networks, a station with a packet to send starts a back-o timer when the channel is idle. If the station senses any transmission in the media, it stops the back-o timer without resetting it. The back-o timer is restarted when the channel is available again. The station sends the RTS packet when the counter reaches the end of the back-o period. Source station keeps the value of its own last back-o window size, which is used in the last reservation. After each successful reservation, the station decreases its back-o window size BO to BO=2, where BO represents the back-o window size. If the reservation attempt is unsuccessful, the back-o window size BO is increased to 2  BO. Stations generally have high back-o window size due to the hidden stations and topological diculties. The goal of the window-exchange algorithm (WE) is to prevent stations from having high back-o window sizes. According to the algorithm used in our wireless LAN system, the transmitting station inserts the information of the last back-o window size into the RTS packet. Any station receiving this information calculates its new back-o window using

minfcurrent BO; received BOg: The intended receiver inserts the received back-o window information into the CTS packet. Therefore, hidden stations of the source station may also receive the back-o window information.

4 Balanced Media Access Methods The goal of the balanced media access methods is to provide desired fair media access for each station in any wireless network con guration [3]. The methods we introduce are based on pij persistent protocol where station i sends the RTS packet with probability pij to station j after the back-o period, or backs o again with probability 1 ? pij using the same back-o window size. The probability pij is referred to as link access probability. The probability pij is constant in classical p-persistent protocols. Moreover, it is not de ned how to calculate probability pij in dynamic environments. The balanced media access methods describe how to calculate probability pij dynamically in a distributed manner in any wireless medium to balance the load. According to the methods, link access probabilities are calculated at the source station in two ways either with a connection-based or a time-based media access method.

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Figure 3: An example of a wireless network topology illustrating visible and hidden stations, and the link access probabilities according to the connection-based media access method.

4.1 Connection-based Balanced Media Access Method In the connection-based (CB) method, stations calculate link access probabilities for each of their logical links based on the information of the number of connections of themselves and neighbor stations. A logical link represents the link between a station and its visible station. Assume that station A is the source station as shown in Figure 3. A group of stations, Bj , are visible stations of station A. A group of stations, Ck , are the hidden stations of station A. Each Ck is connected to at least one Bj . The rest of the stations in the network are denoted by Dl . Source station A attempts to send its RTS packet to station Bj after the back-o period using a pre-calculated link access probability, pj 1 , or backs o again with probability 1 ? pj using the same back-o window size. Each station broadcasts information on the number of connections to the stations within the communication reach. Referring to Figure 3, station A broadcasts to the neighbor stations (Bj , j = 1; : : : ; 4) that it has 4 logical links. This information exchange can preferably be done when a station discovers a change in the network topology. Stations hearing any discovery process update the number of logical connections, and then broadcast the updated connection information to the other stations within their communication reach. In the following, the computation of link access probabilities using the CB method is described for only station A. The link access probabilities of the other stations can be calculated in the same manner. 1

In this section, we will use the notation pj for the link access probability from source station A to station Bj instead

of pij

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The set of stations that are visible to the source station A is referred to as visible set and is denoted by V. The members of this set correspond to the station labeled as stations Bj (j = 1; : : : ; N ) in Figure 3. Every station Bj in the visible set broadcasts the information on the number of its logical connections, which is denoted by Sj , j = 1; : : : ; N , i.e. Sj is equal to the number of logical connections of station Bj . The set that contains Sj 's of each station Bj is referred to as connection set and denoted by S. Referring to Figure 3, the visible set V and the connection set S are given as V = fBj jB1 ; B2 ; B3 ; B4 g, and S = fSj jS1 = 3; S2 = 1; S3 = 5; S4 = 2g. Source station A keeps track of the values in the connection set S. The number of connections of the source station A is denoted by SA . SA is referred to as connection value. A wireless network has a property of X (1) SA  Sj j 2V The maximum connection value in the connection set S, which is denoted by SAMax , is de ned as n o (2) SAMax = maxj2V Sj Note that one or more stations in the visible set V, may have the maximum value of Sj = SAMax . The link access probability from station A to station Bj is de ned as 8 P > 1 if SA = j 2V Sj > >   > < P A if SA < j 2V Sj and Sj = SAMax ; j 2 V pj = > min 1; SASMax (3) > P > Sj : if SA < j 2V Sj and Sj 6= SAMax ; j 2 V: SAMax According the above equation, the CB method gives higher priority to the link which has the maximum connection value, based on the assumption that the station with the maximum connection value has higher data trac than the other stations in a fully-loaded network. Note that we will show that the fairness problem occurs after approximately 85 percent of total network throughput (see Section 5). The priorities of the other links are proportioned according to the maximum connection value. The rst part of the equation represents the client-server scenario, where station Bj are only connected to source station A, and no hidden station Ck is in the network. The second part of the equation are assigned the highest access probability to station Bj that has Sj = SAMax . The third part is proportioned link access probabilities from source station A to other Bj stations that have Sj < SAMax accordingly based on the maximum connection value.

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4.2 Time-based Balanced Media Access Method In the time-based (TB) method, link access probabilities are calculated based on the average contention period. An average contention period is a time interval between packet arrival to the MAC layer and transmission of the packet to the destination. Note that an average contention period covers collisions, back-o periods, and listening periods, in which another station captures the channel. As we discussed in Section 3, the intended sender would be a non-participant station during the listening period until the channel is idle again. According to the TB method, every station periodically broadcasts a packet to its all logical links. The broadcasted packet carries the information of both the average contention period and a link trac descriptor for the link from source station i to station j . The link trac descriptor is denoted by Lij . Stations update link access probabilities every time they receive new broadcasted packet. The link trac descriptor is de ned as Lij = 1 if source station A had a packet to send to station j in the previous period. Lij is equal to zero if station i has not sent any packet to station j . The link access probability from station i to station j can be calculated as n

o (4) pij = min 1; TTave

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where Tij is an average contention period from station i to station j , is a weight factor of an average contention period, where 2 (0; 1), and Vi is the visible set of station i. Speci cally, the TB method calculates the link access probability of a link by simply dividing its average contention period by the mean value of the contention periods of all neighbor links. As the oered load increases, link throughputs increase up to a point. Then, some links are blocked and their throughputs degrades rapidly, and some of the links become dominant over the others. If the link is blocked, the average contention period of that speci c link increases (numerator in Eq. (5)). Eventually, the contention periods of the neighbor links decreases (Eq. (6)). It results in an increase in the link access probability of the blocked link. In this way, the TB method balances the load among the links by giving a higher priority to a link that is blocked and less priority to a link that is dominant over the others. The weight factor controls the increase rate of the link access probability according to the average contention period. The link access probability for < 1 is always higher than the case where > 1. In this algorithm dierent than the CB method, the link trac descriptor carries the information of the trac demand in the previous period. In this 10

way, a link with no trac is not taken into consideration during the calculation. As a result, the time-based approach gives better results as the trac distribution is dierent among the links. In the next section, we simulate wireless ad-hoc networks with and without the algorithms.

5 Simulation Results In this section, we investigate the performance of the proposed algorithms. The wireless ad-hoc network con gurations used in the simulation are shown in Figure 1 and 2, which cover general wireless ad-hoc LAN topologies, where the presence of hidden stations and the presence of one or more simultaneous communication are taken into consideration. The 6-station scenario, which is given in Figure 2, is simulated in two dierent ways using solid diagonal links or dashed diagonal links, where stations do not generate any load for stations that are connected with dashed links although they are in communication reach of each other. Stations generate load only for whom they are connected with solid links. Also note that stations that have more connections are referred to as inner stations, which have more visible stations. Stations with fewer logical links are referred to as edge stations. The networks are simulated under bursty trac conditions. The bursty trac is modeled using an ON/OFF process, where stations are either generating trac (ON) or not generating (OFF). The amount of time sitting in the OFF or ON state is exponentially distributed, where the mean value of the OFF period is 1.35 sec (approx. 1500 slots), and the mean value of the ON period is 1 sec (approx. 1111 slots), [10]. Note that the slot size is 900 sec, and one slot time is enough to cover the RTS frame and the preamble of the CTS frame. The stations can transmit on both states. The trac generation rate in the ON state is exponentially distributed. The generation rate is 573.73 packets/sec. for G = 1:0, where G is the link oered load, [10]. The link oered load is de ned to be the average number of bits per second passed down to the MAC layer at the source for that speci c link. For example, if the link oered load is 0.1, it means the station generates trac with a mean rate of 57.37 packets/sec for that speci c link during the ON period. In the 4-station scenario, the network has a total oered load of 0.6 for the link oered load of 0.1, since the network has 6 links. In the 5-station scenario, the network has a total oered load of 0.8 for G = 0:1. The additional simulation parameters are given as follows: The wireless channel is capable of transmitting at 4 Mbps. Stations are within 10 meters of each other, giving a maximum propagation delay of approximately 3.33 nsec. The packet length is 2 Kbytes. The back-o window size cannot be more than 128 slots, or less than 8 slots. The transmitting window size is 8 packets. The 11

processing and transmission time of RTS/CTS/EOB/EOBC packets is 1.984 msec, [4]. The sum of the transmission time of an ACK packet and the processing time of a received packet is 872 sec, [4]. A burst transmission (8 packets) takes approximately 45 slots. Since we focus on how stations gain access to the channel, which is directly related with the contention period, we simulate the scenarios in a noise-free setting. Therefore, if a station reserves the channel successfully, it sends exactly 8 packets. We also assume that every station has at most 8 attempts to reserve the channel. If a station is not able to capture the channel after eight attempts, it aborts its transmission. In the time-based media access method, we assume that the stations broadcast the information on the average contention period and the link trac descriptor Lij in every 5K slots, which is approximately 4.5 sec. Increasing the frequency of broadcast will decrease the bandwidth eciency. Simulation run time is one million slots. A fairness index (FI ) is de ned to show the degree of the eect of the algorithm. FI is de ned as the ratio of the maximum link throughput and the minimum link throughput. FI = 1 represents the ideal network condition. It means every link has the same throughput. If FI  1, it means the links can not equally gain access into the medium. In the topologies, some links are similar. The links that have similar throughputs are grouped as follows:

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5.1 Throughput of the 4-station Scenario In this scenario, inner stations dominate the wireless medium due to the reasons we explained in Section 3. Edge stations suer as a consequence. When edge stations suer, the WE method is able to improve the performance of edge stations adequately. The impact of the network topology on the 12

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Figure 4: (a)-(b) Link throughput, (c) fairness index versus link oered load for the 4-station scenario. throughput can be seen as the link oered load increases. The simulation results for ON/OFF trac are given in Figure 7, where all stations generate asynchronous data trac for each of their logical connections with the same intensity. Figure 7(a)-(b) shows the link throughput in terms of oered load, where no algorithm, the WE method, both the CB and WE methods, both the TB and WE methods are implemented. The gures only show the throughputs of non-similar links as stated in Section 4. As the link oered load increases, the link throughput saturates at an oered load of 0.13, which gives a total network throughput of 78 percent under ideal channel conditions. After the link oered load of 0.13, the throughput of Link12 degrades rapidly down to 0.06, where the throughput of Link12 goes over 0.2. Implementation of the algorithms helps Link12 to hold its throughput around a link oered load of 0.15. Figure 7(c) shows the impact of the algorithms on the fairness index. The fairness index of the network by along itself increases up to 4.38 for G ! 1. The algorithms keep the network fairness index stable between 1.12 and 1.6 for G ! 1. The WE method provides the best fairness, where FI = 1:12. Implementing both the CB and WE methods also provides a fair network access. In this case, FI becomes 1.19. Since the CB method gives higher priority to the inner stations, it increases the throughput of the inner stations, and decreases the throughput of the edge stations. Thus, if we implement only the CB method, it worsens the fairness in the network. FI becomes 7.23. As G ! 1, the total throughput of the network with both CB and WE methods is 3.04 Mbps which gives %76 throughput. It is exactly same as the total throughput of the network without any algorithm. Using only the WE increases total throughput slightly, which is %78 (3.12 Mbps).

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5.2 Throughput of the 5-station Scenario In this section, we study the eect of the bursty trac on the 5-station scenario, where edge stations dominate the network, consequently, inner stations can not nd any chance to reserve the channel. Figure 8 depicts the link throughput of one link from a group of similar links, and the fairness index. The original network saturates around 88 percent, which gives a link throughput of 0.11. Clearly, throughputs experienced on Link21 and Link23 decrease considerably as the oered load increases. However, Link12 increases substantially even with a link load about 0.3, which results in a total network load of 2.4. The total network throughput at that speci c point is around 1.24. What is apparent from these results is that there are more than one simultaneous communication in the network, since the total network can not exceed the throughput of 0.88 due to the collisions and overheads in the presence of a single communication. The WE method helps links to hold their throughput above throughput of 0.1. As stated previously, implementing only the CB method 14

decreases the throughput of the edge links. Thus, the throughput of Link12 degrades down to approximately 0.1 with a link oered load of 0.3. Consequently, the throughput of Link32 increases up to approximately 0.27 from 0.19. As seen from the fairness index, the CB method provides more fair access by itself among the other methods as the oered load increases. However, it is worse between an oered load of 0.1 and 0.35. This is due to the fact that the CB method decreases the throughput of Link12 and Link21 . Fortunately, it holds all link throughputs less than approximately 0.27 as the link oered load goes over 0.3, where the throughput of Link12 increases considerably for the other methods. As G ! 1, the network without any algorithm has a fairness index of 23.79, which shows that there is a link in the network that can transmit 23.79 times more than another link. Using only the WE method can not solve the problem, where FI becomes 15.10. Using both the CB and WE methods can improve the performance of the inner stations. In this case, FI becomes 4.07. The fairest network access is achieved by using the TB method for = 2 with the WE method, where FI becomes 3.15. There is an interesting phenomenon such that while the WE algorithm increases the throughputs of the outer links, it decreases the throughputs of the inner links. The impact of the CB method is vice versa. Using both algorithms simultaneously smoothes the impact. In the 5-station scenario, the total throughput of the original network is 4.89 Mbps. Note that the channel capacity is 4 Mbps. The network with the WE method only gives a total throughput of 5.14 Mbps. The network with both the CB and WE methods results in a total throughput of 4.39 Mbps.

5.3 Throughput of the 6-station Scenario with Dashed Diagonal Links The 6-station scenario with dashed diagonal links is a typical example of a network with distributed network load. Simulation results are given in Figure 9. As mentioned previously, the network is likely to have two simultaneous communications. We observe that the network throughput saturates after an oered load of around 0.19, which results in a total throughput of 1.14. The curves indicate that the network will suer more as the oered load increases. As seen from Figure 9(a)-(b), the throughputs of the links between stations 3 and 4 are very low. Figure 9(c) depicts the impact of the algorithms on the fairness index. It is clear that implementing both the TB and WE methods is the only way to combat the eects of poor fair access eectively. The best fairness index appears to be around 2.0 achieved by using both the TB method with = 2 and the WE algorithm. As G ! 1, the network without any algorithm has a very poor fair network access, where FI is 57.96. Using both the CB and WE methods, the network access is improved to a certain limit 15

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Figure 6: (a)-(b) Link throughput, (c) fairness index versus link oered load for the 6-station scenario with dashed diagonal links. where FI is 7.67. The total network throughput without any algorithm is 5.81 Mbps. Using both the TB and WE methods with = 2 decreases the total throughput down to 4.77 Mbps.

5.4 Throughput of the 6-station Scenario with Solid Diagonal Links The 6-station scenario with solid diagonal links is an example of blocked edge stations similar to the 4-station scenario. The network topology has 14 logical links. Hence, the link throughput saturates around a link oered load of 0.06. The simulation results are given in Figure 10. The edge links, Link12 and Link14 , will suer apparently as the oered load increases. The WE algorithm fairly eliminates the problem. However, we observe that implementing both the CB and WE methods provides the most fair access in the network in this con guration. It is more apparent in the fairness index curve plotted in Figure 10(c). Since the fairness index converges to 4.55 for the network without any algorithm implemented, the CB method with the WE algorithm provides a fairness index of around 1.45. As G ! 1, FI becomes 1.71 using only the WE algorithm. As we discussed in the 4-station scenario, using only the CB method worsens the fair access, where FI is 5.92. Using the TB method also leads to the fair network conditions. The algorithms also increases the total network throughput in this scenario. The total network throughput without any algorithm is 3.22 Mbps. The total throughputs achieved by using only the WE method, the TB method for = 1 with the WE method, and both the CB and the WE methods are 3.74 Mbps, 3.73 Mbps and 3.61 Mbps, respectively.

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Figure 7: (a)-(b) Link throughput, (c) fairness index versus link oered load for the 6-station scenario with solid diagonal links.

6 Impact of the Load Distribution The CB method is very eective in a fully-loaded system, where every station has load for each of its logical connections. The TB method is eective in both fully-loaded system and non-homogeneous load distributed system, which is discussed in Section 5.3. However, the TB method does not produce a desired eect unless all stations in the network collaborate. Consider the 4-station scenario in Figure 1(a), and assume that only stations 1 and 3 are active in the network. Station 1 has load for station 2, and station 3 has load for only station 4. The throughput of the network is given in Table 4. Referring to the table, Station 3 is able to send 9.62 times more than station 1. The rst reason is that station 2 is in non-participant mode due to the communication between stations 3 and 4, of which station 1 is not aware. Thus, the channel is idle from the point of view of station 1. It issues consecutive RTS packets because of receiving no responses from station 2. The second reason is that station 3 also wins the contention period even though both stations issue an RTS at the same time. In this case, station 4 receives the RTS of station 3 collision-free, but RTSs collide at station 2. Therefore, station 4 issues a CTS for station 3, but station 2 can not send any CTS due to the RTS collision. The total network throughput also drops to %72:72 from %76. 76 percent throughput is achieved by having the same load intensity over each link (see Section 5.1). In fact, neither the WE algorithm nor the CB method can eliminate the problem. In this scenario, station 1 experiences long delays due to having high back-o window size. Station 3 has an advantage over station 1 even if both stations have the same back-o window size all the time due to the second reason. 17

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Link Throughput in Mbps Tx:1 ! 2 Tx:3 ! 4 0.2739 2.6350 0.3556 2.5622 0.1725 2.7282 0.9150 1.8474 1.0821 1.6501

Fairness Index (FI) 9.62 7.20 15.8 2.01 1.52

Table 4: Throughput (Mbps) for the 4-station scenario with two active stations where link oered load ! 1. The WE algorithm improves the throughput of station 1 up to a point if and only if station 2 tracks the back-o window information of station 3. The achievement is not sucient, where the fairness index drops to approximately 7.20. The reason is that station 1 only receives the back-o information after a successful transmission of its RTS packet. Thus, station 2 inserts the back-o window information of station 3 into its CTS packet. Since the CB method gives less priority to the edge stations, the link access probabilities from station 1 to 2 and from station 3 to 4 are 0.5 and 1, respectively. Therefore, the fairness index rises up to 15.80 by implementing both the CB and WE methods. The throughput is steady around 72.52 percent. The TB method is the only way to improve fair access in the network. If both the TB and WE is applied, the fairness index is enhanced approximately up to 2.019 and 1.52 for = 1 and = 2, respectively. The throughput drops slightly, where 69.06 and 68.31 percent throughput is achieved in each case, respectively. Note that station 2 keeps track of the contention period information of station 3, and broadcasts it to station 1 periodically.

6.1 The Periodic WE algorithm The WE algorithm mostly eliminate the fairness problem if a station that senses the channel idle issues an RTS for a station that in non-participant mode due to other communications. In this case, the suering station should hear an RTS or CTS that carries the back-o information to decrease its high back-o window size. However, as seen in the previous section, station 1 has no chance to receive any back-o information unless station 2 receives an RTS or issues an RTS. Since station 3 has load for only station 4, the only chance for station 1 to receive the information is to transmit an RTS successfully. The problem is solved if station 2 issues a periodic back-o information as well as 18

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Figure 8: Fairness index versus information update frequency for the 4-station scenario with two active stations. inserting the information into RTS and CTS packets. The idea is similar to the TB method. This method is referred to as a periodic window-exchange (WE) algorithm. The frequency of back-o information broadcast has a huge impact on the problem. Figure 11 shows the periodic WE algorithm with and without the TB method versus the back-o information update frequency. Referring to the gure, the periodicity of the TB method is unchanged, which is approximately 4.5 sec. The dashed curves display the upper bound of the fairness index of the network for the given algorithms, which are given in Table 4. The upper bound of the periodic WE algorithm, for example, is the fairness index of the WE method. The fairness index drops to approximately 2.70 if the periodic WE algorithm is used with a frequency of approximately 4.5 msec (50 slots). Implementing both the periodic WE and TB method with = 2 drops the fairness index to approximately 1.22. The total network throughput is unchanged.

7 Conclusions In recent years, CSMA-based MAC protocols have been designed to control the media and to provide a fair and robust wireless network. However, those protocols do not provide a fair network. In this paper, balanced media access methods have been proposed for wireless networks to solve the fairness problem. The proposed methods, which are based on p-persistent CSMA protocols, are generally applicable to all MAC protocols. Two dierent balanced media access methods were introduced: a connection-based and a time-based method. The proposed methods are based on 19

the exchange of information about the number of connections or the average contention period, respectively. Each station is responsible of broadcasting the related information to the stations within its communication reach. Using the received information, each station calculates a link access probability for its individual link. Stations access the medium using the calculated probability, like the p-persistent protocol. In the connection-based method, the information is broadcasted whenever stations realize the change in the network topology. In the time-based method, it is broadcasted in a periodic basis. The connection-based method doesn't have any overhead which is occurred in the time-based method because of the periodic information exchange. The CB method is very eective in a fully-loaded system, where every station has load for each of its logical connections. The TB method is eective in both fully-loaded system and non-homogeneous load distributed system According to the scenarios studied, the best results are given sometimes by the network without any algorithm, sometimes by the window-exchange algorithm, but most often by the balanced media access methods. Although it does not always achieve the fairest access, the connection-based method with the window-exchange algorithm always achieves a very reasonable degree of fairness, which is close to that of the best con guration, if all links have a similar load. It also provides the fairest access in the 6-station scenario with solid diagonal links. The time-based method with the window-exchange algorithm provides the fairest network access in two scenarios out of ve (the 5-station scenario and the 6-station scenario with dashed diagonal links) out of ve. However, it introduces a periodic information exchange. As a result, the balanced media access methods with the window-exchange algorithm signi cantly eliminate the fairness problem that exists in the wireless CSMA-based MAC protocols. The future research work can be to develop a method for a fair demand assignment in CSMA-based wireless networks.

Acknowledgments

Authors would like to thank C. M. Olsen and B. Rezvani from IBM TJ Watson Research Center, NY for their helpful comments.

References [1] R.O. LaMaire, A. Krishna, P. Bhagwat, \Wireless LANs and Mobile Networking : Standards and Future Directions," IEEE Communications Magazine, pp.86-94, Aug. 1996. [2] K.C. Chen, \Medium Access Control of Wireless LANs for Mobile Computing," IEEE Network, vol. 8, no. 5, pp.50-63, Sept./Oct. 1994.

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[3] T. Ozugur et al., \Balanced Media Access Methods for Wireless Networks," Proc. of ACM/IEEE MobiCom'98, pp. 21-32, Oct. 98. [4] I. Millar and M. Naghshineh, \Advanced Infrared (AIR) IrMAC Draft Protocol Speci cation," IrDA draft proposal for Advanced Infrared standard by IBM/HP Version 0.2, July 1997. [5] P802.11, D3-Unapproved Draft 3: Wireless LAN Medium Access Control (MAC) and Physical Speci cations, IEEE, January 1996. [6] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, \MACAW: A Media Access Protocol for Wireless LAN's," Proc. of ACM SIGCOMM'94, pp. 212-224, 1994. [7] P. Karn, \MACA - A New Channel Access Method for Packet Radio," Proc. of ARRL/CRRL Amateur Radio 9th Computer Networking Conference, pp. 134-140, Sept. 22, 1990. [8] C. L. Fullmer and J. J. Garcia-Luna-Aceves, \Floor Acquisition Multiple Access (FAMA) for PacketRadio Networks," Proc. of ACM SIGCOMM'95, ACM, 1995. [9] L. Kleinrock and F. A. Tobagi, \Packet switching in radio channels: Part I - carrier sense multiple access modes and their throughput-delay characteristics," IEEE Trans. on Commun., Vol. COM-23, No. 12, pp. 1400-1416, 1975. [10] B. P. Crow, I. Widjaja, J. G. Kim, and P. T. Sakai, \IEEE 802.11 Wireless Local Area Networks," IEEE Comm. Mag., pp.116-126, Sept. 1997.

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