||The architecture of a wireless mobile ad hoc network (MANET) is formed by a cluster of mobile hosts and can be rapidly deployed without any established infrastructure or centralized administration. The channel model of MAC layer can be categorized as the single-channel model or the multi-channel model. In single-channel model, all mobile hosts operate on the single common channel for communication. In multi-channel model, the overall bandwidth is divided into several channels and every mobile host can operate any one or some of these channels for communication. One essential issue of MAC layer is to how to increase channel utilization while avoiding the hidden-terminal and the exposed-terminal problems. Therefore, this dissertation proposes five protocols to increase MAC performance in three major topics, that is, single-channel with power control, multi-channel without location awareness, and multi-channel with location awareness.|
It is well known that using smaller radio transmission power can increase channel reuse. In the first topic, we explore the possibility of combining the concept of power control with the RTS/CTS-based and busy-tone-based protocols to further increase channel utilization in the signle-channel. A sender will use an appropriate power level to transmit its packets so as to increase the possibility of channel reuse. The possibility of using discrete, instead of continuous, power levels is also discussed. Through analysis and simulations, we demonstrate the advantage of our new MAC protocol.
Another approach to relieving the contention/collision problem is to utilize multiple channels. This dissertation also considers the access of multiple channels in a MANET with multi-hop communication behavior. Using multiple channels has several advantages. First, while the maximum throughput of a single-channel MAC protocol is limited by the bandwidth of the channel, the throughput may be increased immediately if a host is allowed to utilize multiple channels. Second, as shown in [6, 54], using multiple channels experiences less normalized propagation delay per channel than its single-channel counterpart, where the normalized propagation delay is defined to be the ratio of the propagation time over the packet transmission time. Therefore, this reduces the probability of collisions. Third, since using a single channel is difficult to support quality of service (QoS), it is easier to do so by using multiple channels .
In the second topic, we propose a new multi-channel MAC protocol DCA(Dynamic Channel Assignment), which is characterized by the following features: (i) it follows an ’’on-demand’’ style to assign channels to mobile hosts, (ii) the number of channels required is independent of the network topology and degree, (iii) it flexibly adapts to host mobility and only exchanges few control messages to achieve channel assignment and medium access, and (iv) no form of clock synchronization is required. Compared to existing protocols, some assign channels to hosts statically (thus a host will occupy a channel even when it has no intention to transmit) [11, 34, 37], some require a number of channels which is a function of the maximum connectivity [11, 23, 34, 37], and some necessitate a clock synchronization among all hosts in the MANET [37, 67]. It is known that using smaller radio transmission power can increase channel reuse and thus channel utilization. It also saves the precious battery energy of portable devices and reduces co-channel interference with other neighbor hosts. Therefore, we combine the above DCA protocol with power control scheme for furthermore improving DCA performance.
Since a MANET should operate in a physical area, it is very natural to exploit location information in such an environment. Therefore, in the third topic, we propose another MAC protocol GRID in the multi-channel system with exploiting position information. Its channel assignment is characterized by two features: (i) it exploits location information by partitioning the physical area into a number of squares called grids, and (ii) it does not need to transmit any message to assign channels to mobile hosts. Several channel assignment schemes have been proposed earlier [23, 28, 37, 54, 67], but none of them explore in the location-aware direction. Based on a RTS/CTS-like reservation mechanism, this medium access protocol does not require any form of clock synchronization among mobile hosts and is also a degree-independent protocol.
In the above GRID protocol, channels are assigned to grids statically. In real world, however, some grids could be very crowded and thus ’’hot,’’ while some could be ’’cold.’’ Apparently, it will be more flexible if channels can be borrowed among grids to resolve the contention in hot spots. This has motivated us to investigate the possibility of dynamically assigning channels to grids. Based on the above protocol and this idea, we further proposed a new protocol, called GRID-B (read as GRID with channel borrowing). We propose four strategies for the sorting: sequential-sender-based borrowing, sequential-receiver-based borrowing, distance-sender-based borrowing, and distance-receiver-based borrowing. The basic idea is that we will assign to each grid a default channel, and a list of channels owned by its neighboring grids from which it may borrow. The purpose is twofold: (i) we dynamically assign channels to mobile hosts so as to take care of the load unbalance problem caused by differences among areas (such as hot and cold spots), and (ii) we sort channels based on mobile hosts’’ current locations so as to exploit larger channel reuse. In GRID, channels are assigned to grids statically, and we find that using a dynamic assignment in GRID-B can further improve the throughput of channels.
|| IETF MANET Working Group, http://www.ietf.org/html.charters/manet-charter.html.|
 Metricom, http://www.metricom.com, Los Gatos, CA.
 IEEE Std 802.11-1997: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, Institute of Electrical and Electronics Engineers, Inc., New York, USA, 1997.
 G. D. et al., Cyberguide: A Mobile Context-Aware Tour Guide,
ACM/Baltzer Wireless Networks, 3(5):421-433, 1997.
 N. Abramson, Developement of the ALOHANET,IEEE Trans. on Information Theory, IT-31:119-123, Mar., 1985.