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3SynchronedMAC
Power saving has been the main driving force behind the development of several
protocols that have been recently appeared in the literature (see [11] for a recent
survey). In this context, the largest energy savings are achieved by protocols
whose communications are based on time-division multiple access (TDMA) and
synchronization.
3.1 SA-MAC Protocol Overview
The main aim of the SA-MAC protocol is to synchronize the ON/OFF periods of
senders and receivers. In the following, the protocol operation will be described
by considering a network consisting of a sink node (or base station) responsible
for gathering all the data sensed by all the other nodes. Some of the other nodes
may have, when required, to act as relays enabling the forwarding of the collected
data to the sink station.
The operation of SA-MAC is divided into two phases: 1) the set-up phase
and, 2) the synchronization and data transmission phase. In this paper we will
briefly describe both of these. Other aspects of the protocol operation can be
consulted in [3].
During the set-up phase of the SA-MAC protocol the network nodes exchange
four types of packets, namely discovery packets (DSC), delay packets (DLY) and
acknowledgement packets (ACK and ACKASOC). In the simplest scenario the
set-up phase starts when the base station (e.g., the sink node) announces its
presence as a father node so that all other nodes can start trying to establish a
father-and-child relation. Thus, all nodes that become aware of the presence of
the base station start to broadcast discovery packets (DSC). Upon receiving a
DSC packet, the base station sends a DLY packet to the corresponding node.
The delay packet indicates the time slot that is assigned for transmissions
from the sensor node to the base station. The node acknowledges the DLY packet
with an acknowledgement packet, ACK, and this acknowledgement packet will
be replied to another one from the parent node named ACKASOC. In this way,
the sensor node finishes its association to the base station and then it may
become a father node for other nodes.
In order to illustrate the operation of the SA-MAC protocol in a more complex
scenario, consider a set of nodes consisting of a base station (BS) and nodes Nd1,
Nd2 and Nd3. Let us assume that Nd1 and Nd3 are located within the coverage
area of the BS and that Nd2 is located within the coverage area Nd3, but out of
reach of the BS. Once the BS announces its presence, nodes Nd1 and Nd3 can
start sending DSC packets and collisions may occur at this time. Let us assume
that Nd3 resolves the collision and sends its DSC packet to the base-station and
establishes a father-and-child relation with the BS. At this point in time, Nd2
chooses the Nd3 node as father because it is the nearest one to the BS and if
Nd2 detects that the channel is free, it can start to send its DSC packets. Nd1
detects the end of the association between Nd3 and BS and it sends its DSC,
establishing a father-and-child relation too. Once all the nodes in the first routing
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