System Considerations WIRELESS ATM STRUCTURE OVERVIEWS

WIRELESS ATM SWITCHES 338 incoming call as they move around the network. Handoff management must be capable of dynamically reestablishing virtual circuits to new access points without disconnecting communication between a mobile terminal and its peer. A mobility-support ATM switch must guarantee in-sequence and loss- free delivery of ATM cells when it is involved in handoff. Finally, wireless ATM should provide uniformity of end-to-end QoS guarantees. However, providing such guarantees is not easy, due to limited wireless bandwidth, time-varying channel characteristics and terminal mobility. The remainder of this chapter is organized as follows. Section 12.1 outlines various reference configurations for a wireless ATM architecture and a wireless ATM protocol architecture. The wireless ATM protocol architecture is based on incorporation of wireless access and mobility-related functions into the existing ATM protocol stack. Section 12.2 reviews some recent proposals to build wireless ATM systems and related research work. Section 12.3 describes wireless-specific protocol layers. A radio physical layer, a MAC layer, and a DLC layer are summarized in that section. Section 12.4 discusses handoff and rerouting schemes. It also discusses cell routing and cell loss in a crossover switch during handoff. Section 12.5 introduces a mobility-support ATM switch architecture that can avoid cell loss and guar- antee cell sequence during handoff. Performance of the switch is also discussed in that section.

12.1 WIRELESS ATM STRUCTURE OVERVIEWS

12.1.1 System Considerations

Ž . Within the Wireless ATM Working Group WAG of the ATM Forum, various reference configurations for a wireless ATM architecture are dis- w x cussed 8, 9 . In a fixed wireless network scenario, the network components, Ž . switching elements, and end user devices terminals are fixed. The fixed wireless users are not mobile but connect over wireless media. This is the simplest case of wireless access provided to an ATM network, without any mobility support. Examples of this kind of service are fixed wireless LANs and network access or network interconnection via satellite or microwave links. In a mobile ATM network scenario, mobile end user devices communi- cate directly with the fixed network switching elements. The communication channel between the mobile end user devices and the switching elements can be wired or wireless. Mobile wired users change their points of attachment into the network over time, though connections are not maintained during times of movements. In contrast, mobile wireless users can maintain their connections while changing their points of attachment into the network. The next scenario is wireless ad hoc networks where there is no access node available. For example, such a network may consist of several wireless mobile terminals gathered together in a business conferencing environment. WIRELESS ATM STRUCTURE OVERVIEWS 339 The last scenario is ATM networks supporting personal communications Ž . ser®ice PCS access. In this scenario, the end user devices are PCS terminals. This scenario uses the mobility-supporting capabilities of the fixed ATM network to route traffic to the appropriate PCS base station. A PCS-to-ATM Ž . interworking function IWF is required to translate the data stream, which is delivered to the en users via a wireless link or delivered to the ATM network via a mobility-support ATM switch. The IWF resides in the PCS base station Ž . controller BSC , which controls several base stations and manages wireless resources. For the wireless access architecture, two approaches are under considera- tion: integrated access and modular access. An integrated access model incorporates all mobility and radio functions into ATM switches. This ap- proach places more complexity in the switches. In a modular access model, ATM switches are alienated from the radio access mechanisms. ATM switches implement call and connection control and mobility management aspects of wireless ATM, whereas a separate physical entity, known as the access point Ž . AP , implements the radio access functions and deals with all the radio- specific functionality, such as radio MAC and radio resource management functions. This approach reduces the effect of radio and mobility functions on the switches, but requires a new protocol, called access point control Ž . w x protocol APCP 10 , to convey messages between the APs and the ATM switches. Fig. 12.1 A wireless ATM system architecture. WIRELESS ATM SWITCHES 340 Figure 12.1 illustrates a typical wireless ATM architecture. Fixed wired end user devices and media servers are connected to regular ATM switches. In a mobile wireless ATM environment, the wireless ATM system has a geographical cell structure like traditional cellular systems. The whole cover- Ž . age area indoor or outdoor area of the wireless ATM system is divided into Ž . cells microcells or picocells . Each cell has one base station, which is an AP to the wired ATM network. Several base stations are connected to a mobility-support ATM switch. Mobile wireless terminals within a cell com- municate with a base station through the wireless medium tor each the ATM network.

12.1.2 Wireless ATM Protocol