Hybrid And Adaptive Mac Protocol

Wang and Hamdi propose a MAC protocol HAMAC, which, integrates fixed assignment TDMA protocols, reservation-based protocols, and contention-based protocols into a wireless network, simultaneously and efficiently supporting various classes of traffic such as CBR, VBR, and ABR traffic. The HAMAC protocol uses a preservation slot technique to minimize the packet contention overhead in PRMA protocols, while retaining most isochronous service features of TDMA protocols to serve voice and CBR traffic streams.

The HAMAC protocol uses a super frame that is divided into two frames, the downlink frame and the uplink frame. The length of the frames can vary depending on the bandwidth demand. The downlink frame is used by the BS to broadcast the frame configuration information, the connection setup, the allocation information, the request information, and the data to all mobile devices. The information and the data can be broadcast using a single burst because only the BS controls the downlink. Mobile devices can filter out irrelevant information upon receiving them. The first segment of the downlink frame is used for control signaling needed for the frame configuration to be known by all mobile devices before starting the reception and the transmission.

In the HAMAC protocol, the uplink frame consists of three segments. The first segment is used by the mobile devices to upload the CBR data using a TDMA round-robin scheme. There are two types of slots in this segment: the preservation slot and the normal slot. The preservation slot is used to preserve the position for a CBR connection when it is in a silent state. The length of the preservation slot should be as short as possible. During the transmission of the preservation slot, all mobile devices in the same cell should have enough time to recognize the existence of preservation slot or the existence of silent CBR connection. The preservation slot is not useful for the BS, and it is discarded by the BS and does not appear in the downlink frame.

When the preservation slot of a CBR connection is present, the remaining bandwidth of the connection is free. When the CBR connection becomes active again, the preservation slot is replaced by the normal slots and the allocated bandwidth for the connection cannot be used by the other connections and mobile devices. The HAMAC protocol avoids the reservation operation before the transmission of an active talk spurt, and the BS is not aware of the state transition of the CBR connection. As a result, there is no need to make the presence or absence of the preservation slot known to mobile devices using a downlink frame. The preservation slot can appear or disappear without any notification.

The HAMAC protocol uses the continuous bit to compress the header information of consecutive slots when they belong to the same traffic source. In the continuous bit technique, the position of the slots allocated to the connections can float in the uplink frame, rather than having the slots allocated to a connection being assigned to a fixed location. In HAMAC protocol, the location of the slots allocated to the connection, defined as an access point, is assigned as the function of the number of continuous bits rather than the absolute position relative to the beginning of the super frame. As a result, the whole frame is used efficiently without any unusable fragments left. The location should be adjusted once a CBR connection is dropped, or a new CBR connection is established.

The second segment of the frame in HAMAC protocol is used to carry bursty data packets, which have to be reserved and allocated by the BS scheduler. Bursty data traffic occur in large volumes; thus this segment-frame contains only the normal slots.

The third segment of the frame contains the contention slots only. The contention slots are small minislots to reduce the overhead caused by collisions. These slots are contended for under the control of a permission probability with respect to different types of packets. Reservation packets and control packets are more important since they may affect the performance of the second segment access or they may be network-management packets that need to be served as fast as possible. Hence, they are assigned a higher permission probability. The ABR data packets should not significantly affect the system performance, and they are given relatively low permission probability to contend for the minislots. To ensure that there is always a chance for reservation packets and control packets to transmit, the minimum length is set for the third segment frame.

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