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all existing MAC protocols must be improved
or reinvented.
Researchers have started revising the design
of wireless networks MAC protocols, especially
MAC protocols of ad hoc and sensors networks.
The international standard groups are also work-
ing on the specification of new technologies for
wireless mesh networks that includes IEEE 802.16,
802.11s, 802.15.5, and ZigBee. Several researches
issues still exist and need to be solved. In par-
ticular, the interesting research problem related
to the scalability issue of existing IEEE 802.11
networks. The most addressed solution intends
to develop a hybrid MAC protocol that combines
the strength of TDMA and CSMA while offsetting
their (Akyildiz, Wang and Wang, 2005). In the
wireless mesh network, it is important that the
underlying MAC schemes could be able to provide
high bandwidth by exploiting channel diversity
and support QoS requirements. It must have the
capacity of self-organizing, self-configuring, and
self-healing.
In Wireless MAC protocols, using hybrid
schemes outperform random-based and schedule-
based schemes. In case of random-based schemes,
throughput drops significantly when increasing
traffic intensity, number of nodes, or hops in the
network. In addition, random-based schemes can-
not guarantee contention-free transmission. The
one hop packet loss probability increase when the
number of nodes trying to transmit simultaneously
increase. This probability cumulates across mul-
tiple hops. Schedule-based schemes provide for
contention-free transmission slots to each node.
The schedule comprising of these transmission
slots is based on the network traffic and topology.
To derive and propagate the schedule, traffic and
topology information needs to be collected, which
involves network overhead. Thus, the frequent
changes in the network conditions results in high
overheads, and leading to poor performance of
schedule-based schemes.
In this article, we study the problems which
persist at wireless MAC layer in multi-hop wireless
Network. In addition, we propose a new hybrid
MAC scheme, called H-MAC (Hybrid MAC)
for wireless mesh network that combines the
strengths of TDMA and CSMA. H-MAC extends
the hybrid multi-hops scheme defined in Z-MAC
(Rhee, Warrier, Aia, and Min, 2005) to support
channel diversity and QoS requirements for wire-
less mesh network. The main feature of H-MAC
is its adaptability to the level of contention in the
network. In fact, under low contention, H-MAC
behaves like CSMA, and under high contention,
it behaves like TDMA.
H-MAC uses two contention modes: Low
Contention Level (LCL) and High Contention
Level (HCL). It also implements two allocation
algorithms. The first Receiver Based Channel
Assignment Algorithm (RBCA
)
is used for chan-
nel allocation and the second Sender Based Slot
Assignment Algorithm (SBSA) is used for slot
allocation. We have evaluated the performances of
our protocol by comparing it to other used MAC
protocols. In this evaluation, we have used the
ns-2 simulator and we have conducted several
simulation scenarios. The obtained result showed
that H-MAC performs better compared to Z-MAC,
IEEE 802.11 and LCM-MAC.
This article is organized as follows. In the
second section we describe the related works
and discuss the different protocols proposed for
wireless MAC. We present and detail H-MAC
protocol in section 3. In section 4, we present our
simulation and the obtained results. We conclude
our work in section 5.
RELATED WORKS
We classify MAC solutions in three main classes.
The first class is the hybrid protocols that combine
CSMA and TDMA. The second class contains
multi-channel MAC protocols, and the third class
includes MAC protocols with QoS support. In the
next sections, we will outline the strengths and
weaknesses of these classes.
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