. Introduction 2. Medium Access Control (mac) 2.1. Mac Protocol Structure

Broadband and mobile communications are presently the two major drivers in the telecommunications industry. Asynchronous Transfer Mode (ATM) is considered the most suitable transport technique for the Broadband Integrated Services Digital Network (B-ISDN), due to its ability to flexibly support a wide range of services with Quality-of-Service (QoS) guarantees. These services are categorized in five classes according to their traffic generation rate pattern: Constant Bit Rate (CBR), real-time Variable Bit Rate (rt-VBR), non-real-time Variable Bit Rate (nrt-VBR), Available Bit Rate (ABR) and Unspecified Bit Rate (UBR). On the other hand, wireless communications are enjoying a large growth in the last decade. Wireless Local Area Networks (LANs) in particular, are becoming popular for indoor data communications because of their tetherless feature and increasing transmission speed. The combination of wireless communications and ATM, referred to as " wireless ATM " , aims at providing freedom of mobility with service advantages and QoS guarantees. Wireless ATM is mainly considered for wireless access to a fixed ATM network; in this sense, it is mostly applicable to wireless LANs. A typical wireless ATM network (Figure 1) includes the following main components: • Mobile Terminals (MTs), the end user equipment, which are basically ATM terminals with a radio adapter card for the air interface, • Access Points (APs), the base stations of the cellular environment, which the MTs access to connect to the rest of the network, • an ATM Switch (SW), to support interconnection with the rest of the ATM network, and • a Control Station (CS), attached to the ATM switch, containing mobility specific software, to support mobility related operations, such as handover 1 , which are not supported by the ATM switch. In many proposals, the CS is considered integrated with the ATM switch in one network module, referred to as " Switch Work Station " (SWS). Even though this is the most common architecture, other 3 schemes are possible. For example, APs could be equipped with switching and buffering capabilities, as proposed in [1]. This, in principle, could expedite mobility and call control operations, but could also increase the overall cost of the system significantly, since the APs need to be more complicated, implementing the full signaling ATM stack. The main challenge for wireless ATM is to harmonize the development of broadband wireless systems with B-ISDN/ATM, and offer similar advanced multimedia, multiservice features for the support of time sensitive voice communications, LAN …