Balanced Media Access Methods for Wireless Networks Timucin Ozugura, Mahmoud Naghshinehb,Parviz Kermanib, C. Michael Olsen~, Babak Rezvanib and John A. Copeland” “ Communications Systems Center, Georgia Institute of Technology, Atlanta, GA 30332 b IBM Thomas J. Watson Rwearch Center, Yorktown Heights, NY 10598 &mail : {ozugur, copeland}Qgcatt.gatech.edu {mahmoud, kermani, cmolsen, babak}@watson.ibm.com Abstract Wireless LANs have two configurations: Infiastmctured and ad-hoc wireless LANs. k a typid ad-hoc wireless LAN, The wireless medium is a s=ce shared resource in mobde com- stations estabfish peer-tepeer communication among themsdves puting. Consequently, the media access control (MAC) layer in- independently in their smd area. Note that ad-hoc networks fluences the fairness and robustness of the wireless network. Ac- presume a non-ftiy connected network topology. Wastructured cording to the current MAC protocok, stations are not able to wireless LANs estabfish the communication between stations with gain access equdy to the shared wireless medium. This problem the help of an infrastructure such as a wired or wireless backbone. is conunody known as the fairness problem. The fairness problem The wireless medium is a shared resource. Consequently, occurs mostly bemuse of the existence of hidden stations and the it is critid that a medium access control (MAC) protocol pr~ presumption of a non-Wy connected wird=s network topology. vides fairness and robustn=s to the wirdess network. The MAC This paper addresses solutions to the fairness problem in wireless protocok rdy on the features of the mtitiple access protocok. networks. persistent carrier sense mdtiple access based dg~ There are many proposed mdtiple access protocok for wireless rithms are proposed in which a fair wirel=s access for each user is . . LANs, such as carrier sense multiple access (CSMA), polling, and accomplished using a precsdtiated N acce~ Probabfity, Pij, time division multiple access (TDMA). h this paper, we focus on that represents the Eti access probabtity horn station i to j. Lii CSMA protocok, which is a member of the ALOHA f-y pr~ access probabfities are dcdated at the source station in two tocok. CSMA is designed for radio networks even though it is ways using connection-based and time-based media access meth- rdso successtiy appEed in the wired networks, such as Ether- ods. According to the used methods, each active user broadcasts net. Carrier sensing is not always possible in a wireless medium information on either the number of Iogicd connections or the av- due to the hidden station problem. h a wireless LAN in which erage cent ention time to the stations within the communication not d the stations are within tr-raission range of one another, reach. This information exchange provides partird understand- a station with a packet to send cannot accurately ascertain if its ing of the topology of the network to the stations. Each station transmission win arrive without co~sions at m intended receiver, reserves a specific priority for itse~ to gain access to the shared because it cannot hear the transmission from other senders that medium. It is suggested that the information is exchanged dur- might arrive at the same intended receiver. This is referred to ing the W access discovery procedure for the connection-based as hidden station problem. For example, directed infrared (~) method, and periodidy for the time-based method. Link access media is an environment in which there is a high chance of hid- probabtit ies are modified every time the exchanged information den stations. Figure 1 shows an example of the hidden station is received. The proposed algorithms are dynamic and sensitive problem, where station A is within commtication reach of both to the changes in the network topology. The sdgoritb have been stations B and C. However, station B and C carmot hear each implemented in a specific media access control protocol [1], but other, therefore they are hidden stations for each other. When they are app~mble to d media control protocok. Sim- access station B attempts to reserve the channel accortig to the ~EE dation restits show that the algorithms restit in an order of 802.11 standard, it sends a request-t~send (RTS) packet before magnitude performance improvement in terms of throughput in transmitting the data. Ordy station A receives the RTS packet, a wirel= network. but station C does not. Station A repties to station B with dear- tmsend (CTS) packet. Both stations B and C receive the CTS packet. CTS packet is the ody way for station C to get informed 1 Introduction about channel reservation. E station C does not receive the CTS The emergence of portable terrninrds in work and fiving environ- packet due to the physical obstructions of the k~of-sight, or r- ments is accelerating the introduction of wireless networks, which ceives it in emor, station C may attempt to reserve the channel WU play an important role in the personal communications SY* while station B is tr-mitting its data. It restits in co~sion at terns. A wirdess locsd area network (LAN) is a way to connect station A sdthough station B has reserved the channel succes port able computers over radio or infrared wireless W that are fdy. in a smd area such as an office or home environment. Wireless CSMA with co~sion avoidance (CSMA/CA) is proposed LANs are mu~ flexible and cheaper to instd than wired LANs. to deviate the hidden station problem. CSMA/CA with a four- way handshake is used to combat the problem of an indoor fad- Pemlission to makedigital or hard copies of all or part of this work for ing channek [6]. CSMA/CA is proposed by the ~EE 802.11 per>onalor clmsroom use is granted without fee provided that copies co-ttee. According to CSMA/CA, the channel is reserved by are m~t made or dis~.buted for protit or commercial advantage and that RTS/CTS exchange, and then data transmission is ensured by copies bear this notice and the full citation on the first page. To copy data/ACK exchage. CSMA/CA protocol is b=ed on Mdtiple otherwise. to republish, to post on servers or to redistribute to lists, Access CoMsion Avoidance (MACA) [3]. MACA has been intr~ requires prior spccitic permission and~ora fee. duced for single hop datagram service in wirelex LANs. The NIOBICOkt 9S Dallas Texas USA MACA protocol attempts to detect co~sions at the receiver, CopW.ght ACM 19981-581 13435-,ti98110...$00OO 21 __— —- -. .. . ——
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