Medium Access Control Layer Data Link Control Layer

WIRELESS ATM SWITCHES 346 connections and their bandwidth requirements. Mobiles assemble a subframe similar to that of the base station. The slotted ALOHA region is used by the mobile terminals to send their bandwidth requirements to the base station.

12.3.2 Medium Access Control Layer

For efficient sharing of the available wireless bandwidth among multiple mobile users, a radio MAC layer is required. One of the key issues of WATM is the choice of an appropriate MAC protocol, which has to incorporate the wired ATM capability of integrating a variety of traffic types with different QoS requirements. In Figure 12.3, the slotted ALOHA section includes control packets for bandwidth request. The control packet includes the number of requested slots and the request type that is used to identify the ATM traffic types. For CBR traffic, the fixed number of slots is allocated at connection setup time. VBR traffic is constantly adapted, while ABR traffic Ž . bandwidth demand is adjusted based on resource management RM cells. UBR traffic is served in a best-effort manner. The standardization process of a MAC protocol for WATM is ongoing Ž . within the Broadband Radio Access Networks BRAN project of ETSI. Ž . w x MAC protocols, such as dynamic slot assignment DSAqq 23 and MAS- w x CARA 24 , are currently under investigation as candidates for the ETSI HIPERLAN Type 2 standard, a developing ETSI standard for WATM. These MAC protocols use a combination of contention-based and con- tention-free access on the physical channel and are based on scheduled TDMA.

12.3.3 Data Link Control Layer

Link-by-link error control is usually omitted in fixed ATM networks because cell corruption due to channel error is extremely rare for reliable media like Ž . copper wire and optical fiber. However, for wireless, bit error rates BERs y 3 w x up to 10 are quite common 26 due to shadowing and other fading effects. To cope with wireless link impairments, a proper error control mechanism is necessary in WATM networks. A DLC layer is necessary to mitigate the effect of radio channel errors before cells are released to an ATM network layer. Depending on the type of service provided, channel quality, capacity and utilization, the DLC layer may implement a variety of means, including FEC and ARQ. w x In 14, 15 , a retransmission-based error control scheme is presented. The proposed error control scheme is applied over a wireless link between a mobile terminal and a base station. The wireless data link control is intended to provide a relatively error-free service to the higher layers by recovering corrupted cells. The error control protocol can be flexibly adapted to differ- ent service classes with varying QoS. The recovery mode for each individual virtual circuit can be set independently during its setup time. As an example, HANDOFF IN WIRELESS ATM