Effects of Ad Hoc MAC Layer Medium Access MechanismsUnder TCPKen Tang, Mario Correa, Mario GerlaComputer Science DepartmentUniversity of California, Los Angeles{ktang, mcorrea, gerla}@cs.ucla.eduAbstractMobile computing is the way of the future, as evident by such initiatives as Bluetooth, Iceberg and HomeRF.However, for mobile computing to be successful, an obvious layer, the MAC layer, must be efficient in channelaccess and reservation. Therefore, in-depth understanding is needed of the wireless MAC layer if wirelesscomputing is to takeoff. Many random access wireless MAC protocols have been proposed and even standardized.However, there has yet been an attempt to understand why certain designs are used and what makes certainprotocols better than others. In this paper, we survey several popular, contemporary, wireless, random access MACprotocols and determine the effects behind the design choices of these protocols.1. IntroductionMobile computing has proliferated in recent years. Initiatives, such as Bluetooth [7], Iceberg [8] and HomeRF [11],are making wireless networking a reality. Such technologies allow for wireless communications between numerousdevices, such as the PC, PDA, cellular and cordless phones, pagers and other appliances, to be practical andseamless. With such diverse and dense communication environment, controlling access to the wireless mediumplays an integral part in making Bluetooth, Iceberg and HomeRF become reality. Therefore, the success of mobilecomputing technologies will in part hinge on the effectiveness of the wireless MAC layer protocol being deployed.In this paper, we provide insights into the design of contemporary wireless, ad hoc random access MAC layerprotocols. In particular, we focus on the various methods of channel access used to resolve contention on thewireless medium. We isolate individual medium access mechanisms (such as carrier sensing, RTS/CTS controlframes and link-level ACKs) through experiments to determine the effect each mechanism has on the network.In section 2, an overview of several random access wireless MAC layers is introduced. Section 3 discusses ourresearch study. Simulation experiments are described in section 4. Results from our UDP and TCP experimentsunder various MAC protocols are presented in section 5 and 6, respectively. Finally, section 7 concludes the paper.2. Existing Wireless Random Access MAC SchemesThis section presents an overview of several popular random access wireless MAC layer protocols. The protocolsinclude Carrier Sense Multiple Access (CSMA), Multiple Access with Collision Avoidance (MACA), MACAW,Floor Acquisition Multiple Access (FAMA) and IEEE 802.11.CSMA (Carrier Sense Multiple Access) requires carrier sensing before transmission. If the channel is free, thepacket is transmitted immediately. Otherwise, it is rescheduled after a random timeout. The major limitation ofCSMA is the hidden terminal and exposed terminal problem [12]. CSMA was used first in the Packet Radionetwork in the mid 1970's [9].An alternative to this scheme involves channel reservation overhead in order to reduce the number of collisions.This scheme, called Multiple Access with Collision Avoidance (MACA), utilizes control frames, which can beoverheard by stations with the ability to collide with the upcoming transmission [10]. These control frames are offixed size, and also contain the length of the proposed data transmission. The station wanting to transmit initiates aRequest To Send (RTS) control frame, and sets a timer to await the Clear To Send (CTS) control frame from the2intended receiver. Any station overhearing the RTS control frame is within range to collide with the expected CTSframe and must defer transmission for a period of time long enough to ensure delivery of the CTS. If the stationdoes not receive the CTS within a specified timeout interval, it assumes a collision has occurred and initiates abackoff before attempting to retransmit. Any station overhearing the CTS control frame is within range to collidewith the upcoming transmission and is required to defer for the full duration of transmission, as specified in thecontrol frame.MACAW [2] is an enhancement to MACA. MACAW utilizes a MILD backoff to select the retransmission time.Besides using RTS/CTS, three other control frames are introduced (DS, ACK, RRTS). The protocol alsoimplements a backoff copying scheme that allows stations to use the same backoff counters when transmitting to thesame station. Moreover, a multiple stream model is included to foster fairness.Floor Acquisition Multiple Access (FAMA) is an experimental MAC protocol specifically developed for the GlomoDARPA program [5]. It is a descendent of MACA. Like MACA, it uses RTS and CTS exchange to prepare thefloor (medium) for data transmission. However, unlike MACA, it also utilizes carrier sensing.IEEE 802.11 [4] expands on FAMA by adding link-level ACKs in addition to the RTS/CTS control frames.Furthermore, 802.11 abandons the traditional CSMA in favor of CSMA/CA (Collision Avoidance). In 802.11, theDistributed Coordination Function (DCF) represents the basic access method that mobile nodes utilize to share thewireless channel. The scheme incorporates CSMA with Collision Avoidance (CSMA/CA) and acknowledgement(ACK). Optionally, the mobile nodes can make use of the virtual carrier sense mechanism that employs RTS/CTSexchange for channel reservation and fragmentation of packets in situations where the wireless channel experienceshigh bit error rate. CSMA/CA works as follows: a node wishing to transmit senses the channel, and if it is free for atime equal to the DCF InterFrame Space (DIFS), the node transmits. If the channel is busy, the node enters a stateof collision avoidance and backs off from transmitting for a specified interval. In the collision avoidance state, thenode sensing the channel busy will suspend it’s backoff timer, only resuming the backoff countdown when thechannel is again sensed free for a DIFS period. A typical sequence of exchange in 802.11 using the virtual carriersensing mechanism involves the source node first sensing the channel using CSMA/CA. After CSMA/CA isexecuted, the source node transmits a RTS, followed by the destination node responding with a CTS, then with thesource node sending the data frame and ending with the destination node confirming with an ACK to the sourcenode.3. Research GoalsThe various random access schemes for wireless ad hoc networks presented in
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