Gugrajah_Yuvaan_ Ramesh_2003.pdf
Gugrajah_Yuvaan_ Ramesh_2003.pdf
Gugrajah_Yuvaan_ Ramesh_2003.pdf
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Simulation ofa Load Balancing Routing Protocol<br />
3.3. Medium Access Control<br />
Chapter 3<br />
In order to model the contention of nodes for the wireless medium, the Distributed<br />
Coordination Function (DCF) of the IEEE 802.11 Medium Access Control (MAC)<br />
[IEEE99] was implemented. Both physical carrier sensing and virtual carrier sensing<br />
are used by the DCF. Physical carrier sensing uses Carrier Sense Multiple Access /<br />
Collision Avoidance (CSMAlCA). Nodes wanting to transmit first check the channel<br />
to ensure that the channel is idle. Once the node has determined that the medium has<br />
been idle for a minimum time period, known as the DCF Inter-Frame Spacing<br />
(DIFS), it determines a random back-off period by setting an internal timer. When<br />
the timer reaches zero, the node may begin transmission. However, if the channel is<br />
seized by another node before the timer reaches zero, the timer setting is retained at<br />
the decremented value for subsequent transmission.<br />
Instead of using physical carrier sensing only, virtual carrier sensing was<br />
implemented in the simulator to reduce the probability of collisions due to hidden<br />
terminals. The hidden terminal problem is demonstrated in Figure 3-3. If node A is<br />
transmitting to node B, node C may not be in range of the transmission and therefore<br />
will not be aware of the transmission from node A. Node C will therefore assume<br />
that the medium is free and will begin transmitting to node B. This will result in a<br />
collision at node B. Virtual carrier sensing therefore allows nodes to reserve the<br />
medium for a specified period of time through the use of Request to Send (RTS) /<br />
Clear to Send (CTS) transmissions to avoid the hidden terminal problem. Referring<br />
again to Figure 3-3, when node A wants to transmit to node B, it first sends an RTS<br />
packet. The RTS packet includes the receiver address and duration required to be<br />
reserved. Physical carrier sensing is used before transmitting the RTS. Once the<br />
packet is received by node B, node B replies with a CTS that also includes the<br />
duration of the reservation by node A contained in the RTS. This allows node C to be<br />
informed of the ensuing transmission even though it is not within range of node A.<br />
All correctly received unicast packets are followed by the transmission of an ACK.<br />
Broadcast packets however are not preceded by an RTS/CTS combination and are<br />
3-8