Multicast for Multirate Wireless LANs

Most research efforts on multicasting in IEEE 802.11 WLANs have focused on improving the
service reliability by integrating ARQ mechanisms into the protocol architecture. In, the
Leader-Based Protocol (LBP) ARQ mechanism has been introduced to provide the multicast
service with some level of reliability. To address the ACK implosion problem, LBP assigns
the role of group leader to the multicast receiver exhibiting the worst signal quality in
the group. The group leader holds the responsibility to acknowledge the multicast packets
on behalf of all the multicast group members, whereas other MTs may issue Negative
Acknowledgement (NACK) frames when they detect errors in the transmission process.

The 802.11MX reliable multicast scheme described in uses an ARQ mechanism supplemented by
a busy tone signal. When an MT associated to a multicast group receives a corrupted packet,
it sends a NACK tone instead of actually transmitting a NACK frame. Upon detecting the
NACK tone, the sender will retransmit the data packet. Since the 802.11MX mechanism does
not need a leader to operate, it performs better than the LBP protocol in terms of both
data throughput and reliability. However, this mechanism is very costly since it requires
a signaling channel to send the NACK frames and busy tones. Moreover, both LBP and
802.11MX schemes do not adapt the multicast PHY rate to the state of receivers.

Very recently, the RAM scheme has been proposed in for reliable multicast delivery.
Similar to the LBP and 802.11MX schemes, the transmitter has first to send a RTS
frame to indicate the beginning of a multicast transmission. However, in RAM the RTS
frame is used by all the multicast receivers to measure the Receiver Signal Strength
(RSS). Then, each multicast receiver has to send a variable length dummy CTS frame
whose length depends on the selected PHY transmission mode. Finally, the transmitter
senses the channel to measure the collision duration and can adapt the PHY rate
transmission of the multicast data frame accordingly. This smart solution is more
practical than 802.11 MX since it does not require a signaling channel but still
requires the use of RTS/CTS mechanism and targets reliable transmission applications.

In SNR-based Auto Rate for Multicast (SARM) is proposed for multimedia streaming
applications. In SARM, multicast receivers measure the SNR of periodically broadcast
beacon frames and transmit back this information to the AP. To minimize feedback
collision, the backoff time to send this feedback increases linearly with the
received SNR value. Then, the AP selects the lowest received SNR to adapt the
PHY rate transmission. The main problem with this approach is that the transmission
mode cannot be adapted for each multicast frame. The multicast PHY rate of SARM is
adapted at each beacon intervals. SARM does not make use of any error recovery
mechanism, such as, data retransmission.

Note that at the exception of RAM and SARM, the mechanisms just described above
only focus on solving the reliability of the multicast service in WLANs. Only
RAM and SARM adapt the PHY transmission rate of the multicast data frames.
In this paper, we define an architecture by integrating the following
facilities: 1) the optimal channel rate adaptation of the multicast service in
IEEE 802.11 WLANs, 2) a more reliable transmission of the multicast data, 3)
the limitation on the overhead required by the signaling mechanism, and
4) the support of heterogeneity of receivers by using different multicast
groups and hierarchical video coding. The definition of the proposed cross layer
architecture is based on the multirate capabilities present in the PHY layer of
IEEE 802.11 WLANs.


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