Telecommunications Lab
(Lehrstuhl für Nachrichtentechnik)
Saarland University, NTF I - 6.2 Computer Science
Campus C6 3, Floor 10
P.O. Box 151150
66041 Saarbrücken
Germany

Multicast MAC Extensions for high rate real-time traffic in Wireless LANs

Reliable multicast in wireless applications is gaining importance with the development in technology. Applications like multicast file transfer, distributed computing, chat, high quality digital TV and whiteboard applications need reliability. However, due to mobility and wireless channel characteristics, obtaining reliability in data transfer is a difficult and challenging task. The current application layer multicast schemes always base on MAC layer broadcast (shown in Figure 1), which is unreliable. IEEE 802.11x does not support reliable multicast due to its inability to exchange RTS/CTS and ACKS with multiple recipients.

Unicast and Broadcast
Figure 1: 802.11x Unicast and Broadcast

This project is to enhance and extend MAC protocols to improve the Quality of Service (QoS) of Multicast with high rate real-time traffic in Wireless LANs. The outline and main tasks of this project are as follows:

  • Reliable MAC Layer Multicast
  • MAC Layer QoS Support for Multicast with high rate real-time traffic
  • Cross Layer Optimization for Multicast with high rate real-time traffic

We explore two main reliable MAC layer multicast schemes in this project. The first one is called Simple Leader Base Protocol (SLBP, shown in Figure 2), which use the leader of the multicast receivers to feedback transmission information. SLBP can provide significant improvement for the leader, and the performance at the other receivers also improves due to the residual effect of MAC layer retransmissions from the sender to the leader. The performance of SLBP depends on the loss correlation between the non-leader receiver and the leader receiver

Main scheme of SLBP
Figure 2: Main scheme of SLBP

The other reliable MAC layer multicast is Beacon driven Leader Based Protocol (BLBP, shown in Figure 3), which uses RTS/CTS/BEACON/DATA/NACK/ACK. The BEACON frame is to announce the sequence number of multicast data and lead the receivers to set timer. On erroneous reception of the packet and have not gotten it yet, the leader does not send an ACK, prompting a retransmission. On erroneous reception (timer timeout) of the packet and have not gotten it yet, the non-leader receivers send negative ACK (NACK), thus destroying the potential ACK from the leader and prompting the sender to retransmit the packet. BLBP provides reliable multicast service for both the leader and non-leader receivers in MAC layer.

Main scheme of BLBP
Figure 3: Main scheme of BLBP

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