MACA-BI (MACA By Invitation) A Receiver Oriented Access Protocol for Wireless Multihop Networks Fabrizio Talucci Mario Gerla Luigi Fratta zy talucciQcs.ucla.edu gerla@cs.ucla.edu f ratta@elet zy .polimi. it Dipartimento di Elettronica Computer Science Department Dipartimento di Elettronica UCLA zyxwv , USA* Politecnico di Milano, Italy Politecnico di Bari, Italy zyxwvut Abstruct- zyxwvutsr A novel wireless MAC protocol named MACA-BI (MACA By Invitation) is in- troduced. MACA-BI is a simplifiedversion of the well known MACA (Multiple Access Collision Avoidance) protocol, which is based on the Re- quest to Send/Clear to Send (RTS/CTS) hand- shake and which has inspired the IEEE 802.11 wireless LAN standard. In MACA-BI, the RTS part of the RTS/CTS handshake is suppressed, leaving only the Clear to Send control message which can be viewed as an “invitation” by the receiver to transmit. This reduction greatly im- proves efficiency when radio turn-around time is significant with respect to packet transmission time. Yet, it preserves the “data” collision free property of MACA. Simulation results for zyxwvu var- ious multihop topologies show that, when traf- fic characteristics are stationary or predictable, MACA-I31 outperforms several known multiple access protocols, especially when “hidden termi- nal” conditions are predominant. I. INTRODUCTION An important component of a wireless network design is the MAC (Medium Access Control) layer. CSMA (Car- rier Sense Multiple Access) was the MAC layer used in the first generation packet radio networks [SI. CSMA prevents collision by sensing the carrier before transmis- sion. A terminal, however, can sense the carrier only within its transmitting range. Transmissions from ter- minals out of range cannot be detected. Thus, in spite of carrer sensing a transmission could still collide at the re- ceiver with another transmission from an “out of range” terminal, often referred to as the “hidden terminal”. The Multiple Access with Collision Avoidance protocol (MACA), proposed by Karn [7], solves the hidden termi- nal problem and outperforms CSMA in a wireless mul- tihop network. Fullmer and Garcia-Luna-Aceves [4] ex- tend MACA by adding carrier sensing. The resulting FAMA-NTR protocol performs almost as well as CSMA in a single-hop wireless network. The same authors pro- pose further improvements (FAMA-PJ [3], CARMA zyxwvu [5]) achieveing even better performance at high loads. In the FAMA-PJ evaluation, an accurate radio model is used to ‘This research was supported in part by a grant from the State of California and Teledyne,under a MICRO program, and in part by an Intel grant. O-7803-3871-5/97/$10.00 zyxwvutsr 0 1997 IEEE 43 5 account for the TX-RX turn-around time (the transition time from transmit to receive state). Their study reveals the impact of the turn-around time on performance. Several modifications of MACA have been proposed which suppress RTS, mostly to transmit multipacket messages or to support real time streams. For exam- ple, to increase the channel utilization for multipacket message transmissions, Fullmer and Garcia-Luna- Aceves propose in [6] to replace all RTS packets but the first with a MORE flag in the header of the data packet. In [4], the same authors propose to use FAMA-NTR in bulk mode to maximizethe throughput. For a multimediaap- plication, Lin and Gerla propose to use RTS/CTS only for the first packet of a real time stream [9]. Subsequent packets are transmitted with a reservation scheme that relies on the periodic nature of the multimedia traffic. Yet, other extensions to MACA have added even more overhead to the RTS/CTS exchange, mostly for error recovery purposes. For example, in [lo] an “invitation minipacket” is introduced to invite the transmitter to re- transmit its last packet, in case it has been lost (Negative Acknowledgment). In another case, the three-way hand- shake is expanded to a five-way handshake (MACAW) with protected ACKs to guarantee transmission integrity in a multihop “nanocell” environment [2]. Unfortu- nately, each additional pass in the handshake contributes one TX-RX turn-around time plus preamble bits (for synchronization), control bits (e.g. source-destination information) and checksum bits. This overhead clearly reduces the useful throughput. Let us focus for a moment on the TX-RX turn-around time in order to appraise its impact on performance. According to the standard proposed in [l], the TX-RX turn-around time should be less than zy 25ps (including radio transients, operating system delays and energy de- tection). Moreover, every transmission should be de- layed by the TX to RX turn-around time (that is, up to 25ps) to give a chance to the previous transmitter to switch to receive mode. This transmit-to-receive tran- sition occurs precisely in the RTS/CTS mechanism of MACA. The higher the channel speed, the higher the turn-around time overhead in terms of bits. Thus, turn- around time will play a key role in future high speed wireless LANs. To reduce, in part, the turn-around overhead, we pro- pose MACA-BI (Multiple Access with Collision Avoid- ance By Invitation), a simplified version of MACA with only a “two-way” handshake. A node ready to transmit, instead of “acquiring” the floor (Floor Acquisition Mul- tiple Access, FAMA) using the RTS (Ready to Transmit)