25-08-2017, 09:32 PM
MAC Protocol for Reliable Multicast over Multi-Hop Wireless Ad Hoc Networks
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Abstract:
Multicast data communication is an efficient communication
scheme, especially inmulti-hop ad hoc networks where themedia
access control (MAC) layer is based on one-hop broadcast from
one source to multiple receivers. Compared to unicast, multicast
over a wireless channel should be able to deal with varying channel
conditions of multiple users and user mobility to provide good
quality to all users. IEEE 802.11 does not support reliable multicast
owing to its inability to exchange request-to-send/clear-to-send
and acknowledgement packets with multiple recipients. Thus, several
MAC layer protocols have been proposed to provide reliable
multicast. However, additional overhead is introduced, as a result,
which degrades the system performance. In this paper, we propose
an efficient wireless multicast MAC protocol with small control
overhead required for reliable multicast in multi-hop wireless
ad hoc networks. We present analytical formulations of the system
throughput and delay associated with the overhead.
INTRODUCTION
Computing and communication anytime, anywhere is a global
trend in today’swireless network technologies.Ubiquitous computing
has been made possible by the advance of wireless communication
technology and the availability of many lightweight,
compact, and portable computing devices. Ad hoc networks
support peer-to-peer communication between active nodes via
multi-hop wireless links. In ad hoc networks, the network topology
is changed by the mobility of the wireless nodes. The ad
hoc networks are self-configurable and do not require any infrastructure
for their operation. This enablesmany applications such
as teleconferencing, disaster relief coordination, and battlefield
operations over wireless networks. These group-oriented and
mission-critical applications require accurate and timely data
delivery [1].
RELATED WORK
IEEE 802.11 wireless local area networks (WLANs) [9] use
multicast transmission. In contrast to unicast transmission, multicast
transmission is unreliable, in the sense thatmulticast packets
are transmitted from the access point (AP) without any ACK
being returned from each receiver. Thus, the transmitted multicast
packets may be lost owing to collisions or errors. IEEE
802.11 also suffers from the hidden node problem, because the
RTS/CTS handshake is not adopted in multicast.
In IEEE 802.11-based ad hoc networks, multicast packets are
broadcast to all the multicast members within one-hop distance
in a single transmission. Packets suffer from increased instances
of the hidden node problem owing to the properties of broadcast
and multicast. The mobility of nodes makes things even more
difficult. In unicast transmissions, MAC can detect the movement
associated with the next hop by retrying several times:
However, this is not possible in the case of multicast forwarding.
The multicast-aware MAC protocol (MMP) [3] was proposed
to address these issues. MMP uses the MAC header to
supportACK-based data delivery. After sending the data packet,
the transmitter waits for the ACK from each destination. ACKs
from the destination nodes are sent in a strictly sequential order
to prevent collisions between ACKs at the transmitter. Overhead
increases as the number of nodes increases owing to the use of
multiple ACKs, thus resulting in throughput degradation. MMP
also suffers from the hidden node problem because of the lack
of the RTS/CTS handshake.
PROPOSED WIRELESS MULTICAST
Multi-Hop Network Environment
The network environment considered here is a multi-hop
wireless ad hoc network. We consider certain multicast routing
protocols from the literature [14]. Multicast routing paths (tree
or mesh) are constructed using multicast routing protocol and
multicast traffic is forwarded to the next hop. In one-hop transmission
over routing paths, a node multicasts a multicast packet
to one-hop member nodes; thus, it is termed a source within onehop
transmission. If the multicast packet needs to be forwarded
to the next hop in the path, one of the members receiving the
multicast packet becomes a source node within the next onehop
range and forwards the packet to the next one-hop member
nodes. This forwarding sequence is defined by the multicast
routing protocol. The transmission mechanism described in this
paper is also applicable to one-hop communication over such a
network environment, because layer-2 protocols deal with onehop
communications. Therefore, if a member node is two-hop
away from source node A in terms of the multicast routing tree,
the node is not a member of source node A within one hop. The
node might be a member of source node B, which is a member
of source node A. That is, when a member node is within
one-hop range of a source, the node is a member of the source
node according to a routing path. Thus, the proposed protocol
provides reliable transmission for each one-hop communication
in multi-hop multicast transmissions.
PERFORMANCE ANALYSIS
System Model
As discussed in the previous section, two types of reliable
multicast are proposed in the IEEE 802.11 MAC layer: The
ACK-based multicast (ABM) and the LBP.We compare the performance
of the proposed method with ABMand LBP. In ABM,
multiple CTSs and ACKs are transmitted in a sequential order
for a packet transmission. In LBP, CTS and ACK are transmitted
only by a leader. Thus, LBP is not only weak against the hidden
node problem but also causes decision error on the transmission
statuses from all member nodes in certain cases. Moreover, the
protocol overhead for selecting a leader increases as the mobility
of nodes increases.
CONCLUSION
Reliable multicast requires a certain level of overhead to
guarantee successful packet transmission. We proposed a reliable
multicast MAC protocol for wireless ad hoc networks. The
MAC layer uses an OFDMA-based acknowledgement for RTS
and data packets for the protocol design. Thus, the overhead required
for reliable multicast is reduced. This reduction in the
overhead enhances system goodput and reliability. The improvement
in system performance is demonstrated by extensive analytical
modelling and numerical results. The proposed method
achieves better goodput performance in reliable multicast as
compared to previous methods. The proposed method also reduces
the packet drop probability in multi-hop wireless ad hoc
networks. Reliable multicast services can be provided in multihop
wireless ad hoc networks using the proposed protocol.