Handoff scheme

Wireless Personal Communication Services (PCS) and broadband networking for delivering multimedia information represent two well-established trends in telecommunications. While technologies for PCS and broadband communications have historically been developed independently, harmonization into a single architectural framework is motivated by an emerging need to extend multimedia services to portable terminals. With the growing acceptance of Asynchronous Transfer Mode (ATM) as the standard for broadband networking, it has become appropriate to consider the feasibility of standard ATM services into next-generation microcellular wireless and PCS scenarios. The use of ATM protocols in both fixed and wireless networks promises the important benefit of seamless multimedia services with end-to-end Quality-of-Service (QoS) control. The wireless ATM (WATM) specification provides an option to existing ATM networks that wish to support terminal mobility and radio access while still retaining backward compatibility with ATM equipments.

The current developments on WATM are mainly based on ATM as the backbone network with a wireless last-hop extension to the mobile host. Mobility functions are implemented into the ATM switches and the Base Stations (BSs). WATM helps to bring multimedia to mobile computers. Compared with the wireless LANs, which have a limitation of bandwidth to support multimedia traffic and slow handoff, the bandwidth of existing mobile phone systems is sufficient for data and voice, but it is still insufficient for real-time multimedia traffic. ATM has more efficient networking technology for integrating services, flexible bandwidth allocation, and service type selection for a range of applications. The current interest and research efforts are intense enough to claim that WATM will continue to be pursued as a research and development topic in the next few years.

There are two major components in WATM networks:

1. A radio access layer providing high-bandwidth wireless transmission with appropriate

Medium Access Control (MAC), Data Link Control (DLC), and so on.

2. A mobile ATM network for interconnection of BSs [Access Points (APs)] with appropriate support of mobility related functions, such as handoff and location management.

We focus on the mobile ATM handoff control required to support Mobile Terminal (MT) migration from one WATM microcell BS to another. The handoff function should ensure that the ongoing connection is rerouted to another AP in a seamless manner. The design goal of the handoff in WATM is to prevent service disruption and degradation during and after the handoff process.

To support wireless users in an ATM network, the main challenges are due to the mobility of the wireless users. If a wireless user moves while communicating with another user or a server in the network, the network may need to transfer the radio link of the user between radio APs in order to provide seamless connectivity to the user. The transfer of a user's radio link is referred to as handoff. During a handoff event, the user's existing connection may need to be rerouted in order to meet delay, QoS or cost criteria, or simply to maintain connectivity between two users, or between a server and wireless users. Rerouting is critical to wireless networks that need to maintain connectivity to a wireless user through multiple, geographically dispersed radio APs. Rerouting must be done quickly to maintain connectivity to the network during a handoff event. In addition, the resulting routes must be optimum. A two-phase interswitch handoff scheme meets the requirement of the rerouting. In the first phase, connections are rapidly rerouted and in the second phase a route optimization procedure is executed. For the two-phase handoff scheme, the first phase is simply implemented by path extension and the second phase is implemented by partial path reestablishment.

We describe the QoS-based rerouting algorithm that is designed to implement two-phase interswitch handoff scheme for WATM networks. We use path extension for each interswitch handoff, and invoke path optimization when the handoff path exceeds the delay constraint or maximum path extension hops constraint. We study three types of path optimization schemes: combined QoS-based, delay-based and hop-based path rerouting schemes.

We use QoS combined path optimization scheme for WATM network. We focus on the problems related to the support of mobility in the WATM network. This scheme determines when to trigger path optimization for the two-phase handoff and how to minimize the service disruption during path optimization.

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