Y. K. XIAO ET AL. 357
where t is the transmitted signal power. and
are the antenna gains of the transmitter and the receiver
respectively. L() is the system loss. t and r are
the heights of the transmit and receive antennas respec-
tively. We assume that each node is equipped with the
same wireless radio, therefore is only dependant
on the distance of the receiver to two senders. For exam-
ple, the at node 1 in Figure 1 is
P
CI
tGtG
1Lh h
CINR
NR
4
01
131
23 16 10
1
Pdd
CINR CPThresh
Pd
(2)
where and is the received power of signal and
interference at node 1 transmitted by node 0 and 3 re-
spectively. According to the collision detection mecha-
nism, if node 1 is receiving a Data from 0, a new arriving
RTS frame from node 3 can only defer the channel ac-
cess of 1, but does not collide with the TCP-Data from 0.
However, the default value of RTS retry count (RRC) is
so small that node 3 reports a link breakage to its upper
layer too early which causes a serial of problems men-
tioned above. Therefore, the deference occurring at node
1 does not have an effect on node 0, and the latter starts a
new transmission as soon as it receives the ACK frame.
An intuition here is that a large value of RRC should be
able to effectively suppress the channel access of node 0
and 1 and at the same time give a high opportunity to
node 3 to acquire the channel successfully after a MAC
layer transmission of the first session finishes.
01P31P
4. Collision Detection Mechanism-Based
Scheme
The CDMB-MAC scheme can be summarized as follows:
When the CINR value is greater than the CPTh resh value,
a node transmits with a probability p if the channe l is idle
for a period of time equal to a DIFS. With a probability q
= 1-p, it backoffs with a fixed time interval T. When the
backoff timer timeouts, and if the channel is also idle, it
either transmits or defers again with the same probabili-
ties and . This process is repeated until either the
frame has been transmitted or the maximum RTS retry
count RRC is reached. In the latter case, the MAC layer
of the unluck y node will drop its transmitti ng packet and
report a link breakage to its upper layer. If the node ini-
tially senses the channel busy, it waits until the medium
becomes idle without interruption for a DIFS, then ap-
plies the above algorithm. Note that when a node re-
transmits an RTS frame, the backoff window size is still
equal to T. For CDMB-MAC, three important parameters
need be designed carefully, i.e., maximum RTS retry
count RRC, backoff window size T and transmission
probability . According to the simulation results, we
set as 200, T as the minimum contention window
of IEEE 802.11 MAC protocol, that is (31
slot time) [7], and as 0.4.
p
RR
q
p
C
min
TCW
p
d2
For CDMB-MAC, each node determines its RTS
transmission or waits only according to its own channel
state. This is a main advantage over the traditional p-
persistent CSMA algorithm in wireline networks, be-
cause CDMB-MAC does not require to divide time into
discrete intervals and thus does not need a synchroniza-
tion scheme to make nodes agree on slot boundaries.
5. Performance Evaluation
The goal of the following simulation is to evaluate the
performance of the CDMB-MAC scheme in various
scenarios. The maximum window size of TCP, window,
is equal to 8 in the next simulations.
5.1. Classic Topology
1) Case 1: mdd 20031
: In this scenario, the
distance between node 0 and 3 is 600 m and is greater
than the carrier sense range, therefore the two nodes are
fully independent. The main problem with IEEE 802.11
is the serious long-term unfairness and incompatibility
problems as shown in Figure 2.
Figure 3 is TCP throughput of the two flows in the
CDMB-MAC-based network. In the 300 s lifetime of the
two flows, there is not one time when the throughput
reaches zero. The aggregate throughput in Figure 2
(752.522 Kbp s) is greater than that in Figure 3 (680.042
Kbps), however the stability and short-term fairness per-
formance of the latter is far better than that of the former.
And two simultaneous TCP traffics can coexist in the
0100 200 300
0
100
200
300
400
500
Throughput (Kbps)
Time ( s )
From 0 to 1
0100 200300
0
100
200
300
400
500
Throughput(Kbps)
Time( s)
From 3 to 2
Figure 3. Throughput of two TCP flow s in the CDMB-MAC
network, d1 =d2 =d3= 200m.
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