Study on Throughput of Network with Selfish Nodes Based on IEEE 802.15.4 - Advanced Research on Computer Education, Simulation and Modeling - page 412

data structure. Similarly, setting the back off window smaller provides an unfair ad-
vantage by backing off for a shorter interval on average. To design robust and trust-
worthy 802.15.4 protocols, it is necessary to study the selfishness. In this paper we
keep the minimum BE (Backoff Exponent) of the selfish nodes. Then analyze the
result of simulation and find out the impact of selfish nodes on the network.
1.1 Introduction of IEEE 802.15.4 CSMA/CA
WSN is constituted by numerous sensing nodes; every node needs to compete to ac-
cess the channel in the operation of network. The Carrier Sense Multiple Ac-
cess/Collision Avoidance (CAMA/CA) mechanism provides an effective method. In
the mechanism, if one node that wants to send data, it should first listen to the chan-
nel. If the channel is idle and this state last longer than Distributed Inter Frame Space
(DIFS), then the node permit to send Require to Send (RTS) frame. If the channel is
busy, backoff mechanism should be taken to avoid conflicts with other nodes. If one
node starts the back off mechanism, a random number will be generated between [0,
CW]. If the idle state of the channel lasts as long as DIFS, the random number will
subtract one. Only when the random number reduces to zero then the node begin to
send data. The picture of mechanism principle is as follows.
Fig. 1. Data transmission mechanism
In a network of a time running, a source station has some information to send to a
destination station, the source station listens to channel and in this moment, the chan-
nel is idle, the source station sends a RTS to the destination station. On receiving the
RTS from the source station, the destination station replies CTS after a SIFS (Short
Inter-Frame Space). The source station sends data after receive the CTS. Then the
destination station replies an ACK to source station after receiving the data from the
source station. so, the sending process is over .When the source station is sending a
RTS frame, there is another station has data to send to the same destination station,
according to the back off mechanism, the new source stations start back off mecha-
nism to avoid collision. The new source stations begin to send data only when the
random number reduces to zero.