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Floor Acquisition Multiple Access (FAMA) for

Packet-Radio Networks

Chane L. Fullmer and J.J. Garcia-Luna-Aceves

Computer Engineering

University of California

Santa Cruz, CA 95064


Abstract| A family of medium access control protocols for single-channel packet radio networks is specified and analyzed. These protocols are based on a new channel access discipline called floor acquisition multiple access (FAMA), which consists of both carrier sensing and a collision-avoidance dialogue between a source and the intended receiver of a packet. Control of the channel (the floor) is assigned to at most one station in the network at any given time, and this station is guaranteed to be able to transmit one or more data packets to different destinations with no collision with transmissions from other stations. The minimum length needed in control packets to acquire the floor is specified as a function of the channel propagation time. The medium access collision avoidance (MACA) protocol proposed by Karn and variants of CSMA based on collision avoidance are shown to be variants of FAMA protocols when control packets last long enough compared to the channel propagation delay. The throughput of FAMA protocols is analyzed and compared with the throughput of non-persistent CSMA. This analysis shows that using carrier sensing as an integral part of the floor acquisition strategy provides the benefits of MACA in the presence of hidden terminals, and can provide a throughput comparable to, or better than, that of non-persistent CSMA when no hidden terminals exist.

I. Introduction

Packet-radio networks based on a single common channel extend packet switching technology into environments with mobile users, can be installed quickly in emergency situations, and are self configurable [12]. As such, they are likely to play an important role in the future of computer communication. The medium access control (MAC) protocol with which packet-radios (or stations) can share a common broadcast channel is essential in a packet-radio network. The CSMA (carrier sense multiple access) protocols [11] have been used in a number of packet-radio networks in the past [12]; these protocols attempt to prevent a station from transmitting simultaneously with other stations within its transmitting range by requiring each station to listen to the channel before transmitting.

The hardware characteristics of single-channel packetradios are such that a packet-radio cannot transmit and listen to the channel simultaneously. Because of this, such

Supported in part by the Office of Naval Research under Grants N00014{92{J{1807 and N00014{94{1{0688

improved protocols as CSMA/CD (collision detection) [15] cannot be used in a single-channel packet-radio network. However, the throughput of CSMA protocols is quite acceptable, except in the case in which multiple transmitters within range of the same receivers cannot sense one another's transmissions [20]. Such hidden terminal" problems degrade the performance of CSMA substantially, because carrier sensing cannot prevent collisions in that case, making the protocol behave essentially as the pure ALOHA protocol [1].

To address the problems of hidden terminals in singlechannel networks, Karn [9] proposed a protocol called MACA (multiple access collision avoidance) which attempts to detect collision at the receiver by establishing a requestresponse dialogue between senders and intended receivers. When a sending station wants to transmit, it sends a requestto-send (RTS) to the receiver, who responds with a clear-tosend (CTS) if it receives the RTS correctly. Several other MAC protocols based on similar RTS-CTS exchanges, or RTSs followed by pauses, have been proposed [2], [7], [18] for single-channel wireless and wireline networks. Interestingly, none of these protocols specify any provision to prevent data packets from colliding with control packets (RTS and CTS packets).

In theory, the throughput of a MAC protocol based on an RTS-CTS dialogue can be improved by increasing the length of the data packet that follows each successful RTS- CTS exchange. However, in practice, different applications may utilize the same channel; therefore, while some applications can benefit from very large data packet sizes (e.g., transfers of video files), others do not (e.g., a telnet session). Furthermore, if the time it takes for a radio to transition from transmit to listen mode can take a few milliseconds, the effective duration of an RTS becomes comparable to the duration of a data packet in a high-speed terrestrial radio channel (e.g., 1Mbps), because the sender of an RTS takes on the order of an average data packet length to be able to receive the CTS it needs, regardless of how long an RTS and CTS lasts. Accordingly, enabling a transmitter to send multiple packets in the clear for each successful RTS-CTS exchange can be used to eliminate the hidden-terminal problems in CSMA while providing better throughput in low-speed or high-speed packet-radio networks than can be achieved by sending a single data packet per successful RTS-CTS exchange.

In this paper, we unify the basic schemes used in many MAC protocols for carrier sensing and collision avoidance into a new channel access discipline that we call FAMA (floor acquisition multiple access). The objective of a FAMA protocol is for a station that has data to send to acquire control of the channel (which we call the floor) before sending any data packet, and to ensure that no data packet collides