RandomAccessMACforEfficient Broadcast Support i nAdHocNetwork s

importantpart of oureverydaylifestyle. Onemajo rareathat will have anenormous impact ontheperformance of wireless ad hocnetworksisthemediumaccesscontrol(MAC)lay er.

Random Access MAC for Efficient Broadcast Support i

n Ad Hoc Network


Ken Tang, Mario Gerla

Computer Science Department

University of California, Los Angeles

{ktang, gerla}@cs.ucla.edu

Abstract - Wireless communications are becoming an important part of our everyday lifestyle. One major area that will have an enormous impact on the performance of wireless ad hoc networks is the medium access control (MAC) layer. Current random access MAC protocols for ad hoc networks support reliable unicast but not reliable broadcastIn . this paper, we proposed a random access MAC protocol, Broadcast Support Multiple Access (BSMA), which improves broadcast reliability in ad hoc networks.


Wireless technology has rapidly expanded in recent years and will continue to grow in years to come. Initiatives such as Bluetooth [4], Iceberg [5] and HomeRF [11] are making wireless networking a reality. Such technologies allow for wireless communications between numerous devices, such as the PCs, PDAs, cellular and cordless phones, pagersand other appliances, to be practical and seamless.

Many random access protocols exist in ad hoc networks that facilitate wireless ad hoc communication. Som of e these protocols include Carrier Sense Multiple Access (CSMA) [6], Multiple Access with Collision Avoidance (MACA) [7], MACAW [1], Floor Acquisition Multiple Access (FAMA) [3] and IEEE 802.11 [2]. However, none of these protocols are designed to support the reliable broadcasting odata. f For example, 802.11 uses collision avoidance along with RTS/CTS/ACK control frames to transmit unicast packets in order to combat hidden and exposed terminals. For broadcast packets that are to be received by all neighbors ofthe source node, no control frames are used. Therefore, broadcast packets are sent blindly without consideration of hidden and exposed terminals and channel noise.


In this paper, we propose an ew protocol, Broadcast Support Multiple Access (BSMA), which facilitates the transmission of broadcast packets and therefore supports multicast protocols such as ODMRP [8][10] as they heavily rely on broadcasting packets to achieve multicast. Our protocol utilizes some of the features of 802.11. Therefore, an overview of 802.11 is in order.

IEEE 802.11 utilizes a collision avoidance scheme along with RTS/CTS/ACK control frames to transmit unicast packets. In 802.11, the Distributed Coordination Function (DCF) represents the basic access method that mobile nodes utilize to share the wireless 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 mechanismthat employs RTS/CTS exchange for channel reservation and fragmentation of packets in situations where the wireless channel experiences high bit error rate. CSMA/CA works as follows: node a wishing to transmit senses the channel, and if it is free for a time equal to the DCF InterFrame Space (DIFS), the node transmits. If the channel is busythe , node enters a state of collision avoidance and backs offfrom transmitting for a specified interval. In the collision avoidance state, the node sensing the channel busy will suspend its backoff timer, only resuming the backoff countdown when the channel is again sensed free fora DIFS period. A typical sequence of exchanges in 802.11 using the virtual carrier sensing mechanism involves the source node first sensing the channel using CSMA/CA. After CSMA/CA is executed, the source node transmits RTS, followed by the destination node responding with CTS, then with thesource node sending the data frame and ending with the destination node confirming with an ACK to the source node. An nodes y receiving RTS, CTS or data frame that does not belong to it will yield long enough for the source and destination nodes to complete the data exchange. For broadcast packets,802.11 nodes simply execute collision avoidance and then transmit the data frame.

Fig. 1 outlines the steps of the BSMA protocol.

1. Collision avoidance phase. 2. Source sends RTS to all neighbors and sets timerto

WAIT_FOR_CTS. 3. Neighbors of source, upon receiving RTS, send CTS

if not in YIELD state and set timer to WAIT_FOR_DATA. 4. If source receives CTS, send DATA and set timer to

WAIT_FOR_NAK. Else, if no CTS and WAIT_FOR_CTS timer expires, back off and go to step Nodes 1. that are not involved in the broadcast exchange, upon receiving CTS, sets their state to

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