We develop and analyze an elegant, opportunistic medium access control (MAC) protocol based on the proposed MAC standard for wireless local area networks (WLAN) - IEEE 802.11. Our adaptation of 802.11 is called CATER (Code Adapts To Enhance Reliability) and allows communicating stations to reconfigure their transceivers to use a longer pseudo-noise (PN) code when retransmissions are unsuc-cessful over a degraded channel. Results show that our protocol continues to function, permitting up to 14 percent normalized aggregate throughput, at times when IEEE 802.11 fails. In addition, throughput experiences only a small decrease due to protocol overhead during periods when stations experience a good channel with few bit errors.

In Personal Communications Services (PCS) networks, subscribers' current locations are usually maintained using a two-level hierarchical strategy with two types of databases, specifically, VLRs and HLRs. Location records might be lost due to the malfunction of mobility databases. GSM uses aggressive restoration to restore the HLR data from the VLRs. However, previous work on aggressive restoration is based on periodic checkpointing and might not be feasible in general. This paper proposes a novel aggressive approach, based on aperiodic checkpointing, to back up HLRs and VLRs. The aperiodic checkpointing scheme could use the number of uncheckpointed location records as the threshold value to trigger a backup process. Performance analysis shows that aperiodic checkpointing is more feasible than periodic checkpointing for aggressive restoration.

Handoff support is one of the key elements in cellular Personal Communication Systems (PCS). Traditional approaches hide handoff support from the subscriber. However, the main difficulty in handoff support stems from terminal mobility which can only be controlled by the subscriber, who may again have different requirements of mobility support under different environments. Therefore, we suggest that the subscriber should participate in handoff support in the following manner: first, the subscriber is encouraged to declare the requirement of mobility support at call setup time; second, when a handoff cannot be supported, the subscriber should be informed in advance so that (s)he can decide whether or not to control movement since a subscriber may sacrifice mobility for maintaining communication in progress. This approach can reduce call dropping rate and improve resource utilization. We will describe this approach and propose a service classification for mobility support in this paper.