Dynamic Channel Reservation Based on Mobility in Wireless ATM Networks

0163-6804/99/$10.00 © 1999 IEEE ith the increasing demands for mobile multimedia services, future wireless networks will adopt micro/picocellular architectures in order to provide the higher capacity needed to support broadband services under the limited radio spectrum [1]. Due to its flexible bandwidth allocation, efficient multiplexing of bursty traffic, and provision of a wide range of wireless broadband services, the wireless asynchronous transfer mode (ATM) network is a promising solution for the next-generation wireless communication system. However, handoff will occur frequently in wireless ATM networks because the cell size is much smaller to support higher capacity on the limited radio spectrum. As the handoff rate increases, bandwidth management and traffic control strategy (call admission control, handoff procedure, etc.) become more challenging problems in wireless ATM networks. Handoff is the action of switching a call in progress in order to maintain continuity and the required quality of service (QoS) of the call when a mobile terminal moves from one cell to another [2]. In the wireless environment, we need to consider two additional QoS parameters: dropping probability of handoff calls and blocking probability of new calls from the standpoint of call-level QoS. If the base station (BS) has no idle channels, it may drop the handoff request and cause forced termination of the call in progress. From the subscriber’s point of view, forced termination due to handoff calls is less desirable than blocking of a new call. Therefore, the QoS of handoff calls must be guaranteed while allowing high utilization of wireless channels. Recently, intensive research on channel allocation schemes has been in progress to reduce the handoff dropping probability. Two generic handoff prioritization schemes are queuing handoff requests and reserving a number of channels exclusively for handoff requests. In general, handoff prioritization schemes result in decreased handoff failures and increased call blocking, which, in turn, reduce total admitted traffic. One priority scheme is the handoff queuing scheme (HQS). In HQS, queuing of handoff requests is made possible by the existence of the time interval the mobile terminal spends in the handoff area, where it is physically capable of communicating with both the current and next BSs. The fact that successful handoff can take place anywhere during this interval marks a certain amount of tolerance in the delay for the actual channel assignment to the handoff request. New calls which originate within the cell are blocked if all wireless channels are occupied. New calls are served only when a wireless channel is available and no handoff request exists in the queue. Handoff requests are queued in the new BS if no channel is available in the new cell at the time of arrival. As soon as a channel is available in the new cell, it is offered to the handoff request in the queue. HQS reduces the probability of forced termination at the expense of increased call blocking probability and a decrease in the ratio of carried-toadmitted traffic. The reason is that no new call is granted a wireless channel until the handoff requests in the queue are served [3]. Another priority scheme is the guard channel scheme (GCS), which gives higher priority to handoff calls by assigning them a higher capacity limit to reduce the forced termination probability. GCS shares normal channels for handoff and new calls, and reserves exclusively some fixed number of guard channels for handoff calls. Because GCS is developed under the assumption of stationary call arrivals, it causes decreased total carried traffic and QoS degradation under nonstationary traffic patterns due to fluctuation of mobility. With a small portion of handoff calls, GCS may deviate significantly from the requested QoS of handoff calls. Namely, it results in not only increased blocking probability of new calls, but also inefficient utilization of wireless channels, because only a few handoff calls are able to use the reserved channels exclusively. On the other hand, with a large portion of handoff calls, it is difficult to guarantee the QoS of handoff calls. Thus, an efficient channel reservation scheme should take into account the characteristics of traffic mobility in order to use the limited wireless channel efficiently while satisfying the requested QoS of handoff calls. In this article, we present a dynamic channel reservation scheme (DCRS) based on mobility. The objective of DCRS is to guarantee the required dropping probability of handoff calls while keeping the blocking probability as low as possible. Eventually, the proposed scheme is designed to improve channel utilization while satisfying the QoS of handoff calls. DCRS shares normal channels for new calls and handoff calls, and reserves guard channels for handoff calls. Even though the guard channels are reserved for handoff calls, they can be also used for new calls according to the mobility of calls and status of the network. In order to make new calls use guard channels, the request probability is used to allocate the guard channels to a new call. The request probability of the new call is adaptively determined according to the mobility of calls, total number of channels in a cell, threshold between normal Young Chon Kim, Dong Eun Lee, Bong Ju Lee, Chonbuk National University, Korea