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MAC PROTOCOLS WITH QOS
and Chen, 2003; Jain, Das and Nasipuri, 2000;
Tzamaloukas and Garcia-Luna-Aceves, 2001).
MMAC (So and Vaidya, 2004) assumes time
synchronization in the network and time is divided
into fixed-length beacon intervals. Each beacon
interval consists of a fixed-length ATIM (Ad-hoc
Traffic Indication Message) window, followed
by a communication window. During the ATIM
window, each node listens to the same default
channel and negotiates which channel to use for
data communication. After the ATIM window,
nodes that have successfully negotiated channels
with their destinations send out data packets us-
ing 802.11 DCF for congestion avoidance (IEEE
802.11 Working Group, 1997).
Multi-channel MAC protocols in Wireless
Sensor Networks (WSNs) are also studied (Zhou
et al., 2006). Due to the limited size of the data
packets used in WSNs, authors have proposed to
use static frequency assignment to avoid the over-
head of control packets for frequency negotiation.
There are also many TDMA scheduling algorithms
proposed for ad hoc networks (Chlamtac and Kut-
ten, 1985; Chlamtac and Farago, 1994; Bao and
Garcia-Luna-Aceves, 2001; Rajendran, Obraczka
and Garcia-Luna-Aceves, 2003). These algorithms
are mainly designed for sharing a single channel
in the network and providing collision free access.
For example, the TMMAC protocol presented in
(Zhang, Zhou, Huang, Son and Stankovic, 2007)
is one these algorithms that combines TDMA
scheme and channel diversity to improve the
network throughput. It is proved that TMMAC
achieves 84% more aggregate throughput than
MMAC (Zhang, Zhou, Huang, Son and Stankovic,
2007). MMSN (Zhou et al., 2006) is another
MAC protocol that exploits channel diversity
in sensors networks. MMSN omits exchanging
RTS/CTS, because in WSN, the packet is very
small, 30~50Bytes.
In the design of MAC protocols with QoS support,
two basic approaches can be employed. The first
approach is to assign different
priority levels
to
packets (IEEE Std 802.11e, 2004; Ying, Ananda
and Jacob, 2003; Qiang, Jacob, Radhakrishna
Pillai and Prabhakaran, 2002). The major issue
with this approach is how to assign these priori-
ties. This is typically done by defining different
intervals for both the random backoff period and
AIFS (Arbitration Inter Frame Space) period,
such as the EDCA (Enhanced Distributed Chan-
nel Access) function of IEEE 802.11e. In a single
hop environment, EDCA offers better average
delay and throughput than the usual DCF. The
IEEE 802.11s working group plans to extend the
802.11e scheme for the multi-hop wireless mesh
network (Conner, Kruys, Kim and Zuniga, 2006).
The second approach to support QoS is
to
reserve resources
for a particular real-time
traffic flow. For example, each node between
particular source and destination nodes allocates
some dedicated time slots for this flow before
the actual transmission starts. This improves the
end-to-end throughput. However, this reservation
mechanism is much more complex than a priority
mechanism. Typically, it adds signaling overhead
to coordinate the nodes (all nodes between source
and destination must agree in distributed manner
on the reserved resources).
H-MAC PROTOCOL
In this section, we present our H-MAC protocol.
This protocol extends the hybrid multi-hops sche-
ma defined in Z-MAC (Rhee, Warrier, Aia, and
Min, 2005), which combines TDMA and CSMA
according to the contention level. Compared
to Z-MAC, H-MAC uses multi-channel hybrid
schema which guarantees the QoS requirements
for a multi-hop wireless mesh network.
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