General Conference (Part zyx A) zyx ADAPT: zyxwvutsr A DYNAMICALLY SELF-ADJUSTING MEDIA ACCESS CONTROL PROTOCOL FOR AD HOC NETWORKS* I. Chlamtac, A. Farag6, A. D. Myers, V. R. Syrotiuk, and G. Ziruba Center for Advanced Telecommunications Systems and Services (CATSS) The University of Texas at Dallas {chlamtac, farago, amyers, syrotiuk, zaruba} @utdallas.edu Abstract This paper presents A Dynamically Adaptive Protocol for Transmission (ADAPT) for ad hoc networks that combines, in a novel way, a collision-free allocation based protocol and a contention based protocol while retaining the advantages of each. At low loads, ADAPT uses its contention mechanism to reclaidreuse bandwidth that would otherwise be wasted by a pure allocation based protocol. At high loads, ADAPT pro- vides bounded delay guarantees by dynamically changing its operation to that of its allocation based protocol, avoiding the fundamental problem of instability associated with pure con- tention based protocols. Thus, ADAPT self-adjusts its behav- ior according to the prevailing network conditions. Both anal- ysis and simulation results demonstrate that the two protocols interact in a positive way, showing that it is possible to combine the advantages of two fundamentally different design philoso- phies without suffering from their drawbacks. Introduction A mobile ad hoc network is a self-organizing system of wire- less nodes that requires no fixed communications infrastruc- ture. In the event any two nodes cannot communicate directly, each node must act as a relay, forwarding packets on the behalf of other nodes. Due to the broadcast nature of a radio channel, overlapping transmissions (collisions) may occur resulting in increased packet loss and delay due to ret-ransmissions. Thus a key issue is determining when nodes are allowed to access the channel (i.e., transmit a packet), a decision made by a Media Access Control (MAC) protocol. Generally, MAC protocols may be broadly classified into two groups based on their strategy for determining access rights. In zyxwvutsrqpon contention protocols, such as Aloha, CSMA, MACA, MACAW, FAMA, and 802.1 1 [l, 3,8,9, 10, 113, nodes com- pete asynchronously to access the shared channel. Some use collision avoidance mechanisms [3, 8, 9, 111, and all ultimate- ly use randomized retransmissions. The primary advantage of this group is that they are mobility transparent, i.e., the pro- tocol does not change its operation as the topology changes. "This work was supported in part by the DOD USARO (Army Research Office) under contract No. DAAG55-97-1-0312, While contention based protocols cannot provide deterministic delay bounds, they are effective at low load when few colli- sions have to be resolved. Their primary disadvantage surfaces at high load, when these protocols spend most of their time re- solving collisions. As a result, the throughput approaches zero resulting in an unstable network. In order to avoid instability, deterministic allocation proto- cols were introduced. These protocols, which include TDMA, variations on spatial reuse TDMA 161, and TSMA[4], assign each node a transmission schedule indicating in which of the synchronized slots the node may transmit. Since there is a guarantee that at least one slot in the schedule will be success- ful (i.e., collision-free), these protocols have bounded delay. Simple TDMA assigns a permanent, unique transmission slot to each node in the network. While TDMA is mobil- ity transparent, its throughput is very low since there is no spatial reuse, i.e., no multiple simultaneous transmissions are allowed even when the transmitting nodes are sufficiently far enough apart such that no collision would occur. Variants of TDMA attempt to increase the spatial reuse factor by dynam- ically computing the transmission schedules. However, such protocols are no longer mobility transparent as the transmis- sion schedules must be recomputed as the network topology changes. Furthermore if the network is highly mobile, these protocols potentially become unstable as the nodes can spend virtually all of their time maintaining their transmission sched- ules. The Time-Spread Multiple-Access (TSMA) family of pro- tocols are mobility transparent and have a relatively high de- gree of spatial reuse. However, the a priori computation of the schedule assumes a fixed upper bound on the maximum zyx degree of the network, i.e., the maximum number of nodes that are in the transmission range of a node (its neighbors). If the de- gree constraint is violated, the guarantees on delay are lost and, consequently, these protocols may also become unstable. This constraint was overcome in threaded-TSMA[S], however, the resulting schedules, hence delay, can be prohibitively long. In this paper, we propose a new MAC protocol that com- bines, in a novel way, an allocation and contention protocol. Moreover, the protocol has a simple method of dynamically Global Telecommunications Conference - Globecom'99 zyxwvutsrqp 0-7803-5796-5/99/$10.00 zyxwvutsrqponml 0 1999 I EEE 11