Medium Access Protocol for Wireless Multimedia Networks Bruno Ascenso, Bruno Santos, Rui Rocha, José B. Gerald Instituto Superior Técnico, Av. Rovisco Pais, 1046-001 LISBOA, Portugal Abstract The role of mobile communications in the society is becoming more important every day. The increasing need of a mobile broadband network has driven the intensification of the research efforts in this area. In the wireless LAN field, the 802.11 standard has been a commercial success, making it easy to find products conforming to this standard. But, whenever quality of service is required, the 802.11 fails in supporting flexibly and efficiently real-time requirements. This article describes a wireless MAC algorithm to use with PCNet-Mobile 802.11 cards. Its purpose is to provide access to wireless networks while handling with the quality of service required by the different multimedia applications. Performance results obtained through simulation are presented and discussed. I. Introduction The tremendous commercial success of mobile systems in the last few years along with the recent developments in signal processing and computer performance has boosted the efforts in this area. The need for the development of a wireless mobile broadband network is becoming more important every day. The IEEE 802.11 standard allows the support of both best- effort and continuous media type of services [1], but fails in efficiently providing quality of service (QoS) for different traffic classes in a multi-cell scenario. The need for a suitable MAC protocol is therefore clear. However, the development of a new MAC protocol often requires new functions at the hardware level. This usually implies the development of new interface cards, which is a rather expensive solution. Since some of the existing 802.11 cards are very flexible in what concerns to the medium access controller, one can think in removing the existing firmware and develop a new one capable of flexible QoS provisioning. The data rates supported by these cards are too slow for the most demanding multimedia applications, but are enough to carry voice and compressed low-quality video signals. Guaranteeing QoS at the air interface is not an easy task. The environment is a lot more error prone than other media and it is sensitive to multi-path propagation interference and fading, thus demanding the existence of more complex error control mechanisms [4]. The packet size should be kept small to increase the error immunity. In the fixed network domain, the ATM protocol possesses this characteristic. So it becomes natural to try to use it in the wireless networks [5, 6]. The use of an ATM compatible protocol also brings the advantage of allowing a seamless integration between wireless and ATM wired networks. This paper describes a new MAC protocol, based on ATM transport mechanisms [2, 3], to be used in PCNet-Mobile 802.11 cards. Its purpose is to allow the implementation of wireless multimedia networks, handling different QoS requirements. It tries to fulfill the requirements formulated by IEEE when recommending a standard for wireless LAN’s. The network structure is similar to that of 802.11, with a point coordinator managing the available bandwidth. It uses a TDMA access scheme, scheduled by the access point. The contention has been reduced to what is strictly necessary. Hence, a small contention period exists at the end of the frame, used only to transmit control information. The MAC has been simulated and is now in the implementation stage. This consists in removing the firmware from the cards and downloading the new code. The corresponding device driver has also been developed for Linux. II. Network Architecture In order to guarantee fairness in the wireless access, a structured network topology was chosen, with the access point (BS) being responsible for the control of the bandwidth allocation among the mobile stations (MS). Thus, all traffic between two mobile stations must be transferred through the BS. The BS distinguishes the traffic based on its real-time requirements, implementing four queues with different scheduling priorities, which can be used by four different traffic flows. One possible mapping of these flows are the ATM traffic classes: CBR, VBR, ABR and UBR [7]. This is also the order used to define priorities between the queues in the scheduling. The traffic priority and its QoS requirements are negotiated every time a MS requires a new connection. The QoS parameters are: the minimum required data rate, the peak data rate and the maximum delay. These parameters are used in a different manner according to the priority of the corresponding traffic flow. This usage is summarized in Table I. Table I Usage of QoS parameters CBR VBR ABR UBR Minimum Data Rate Reserved Reserved Reserved Ignored Peak Data Rate Reserved Reserved Ignored Ignored Maximum Delay Assured Assured Ignored Ignored Each mobile station must register itself with the BS before it can communicate through the network. This registration is performed by sending a contention packet with the respective MAC address. If the registration limit has not been reached, the BS chooses the next available identifier and assigns it to the MS. The MS will use this unique identifier to identify