The received power Pr at a distance d from the transmitter for the two-ray ground reflection model [2] can be expressed as:

          (1)

 

where Gt is the transmitter antenna gain, Gr is the receiver antenna gain, d is the distance between the antennas in meters, ht is the height of the transmitter and hr is the height of the receiver.

The power level at which the packet was received at MAC layer is compared with the receiving threshold (RTX) and the carrier-sense threshold (CTX). If the power level falls below the carrier sense threshold, the packet is discarded as noise. If the received power level is above the carrier sense threshold but below the receive threshold, the packet is marked as a packet in error before being passed to the MAC layer. Otherwise, the packet is simply handed up to the MAC layer.

In NS-2, the default transmitting power Pt is 0.28, and thus the transmission range (TX_range) and physical carrier sensing range (PCS_range) are calculated as 250 meters and 550 meters. However, as discussion in [1; pp.97, 94, 73], the default values of TX_range and PCS_range used in the NS-2 is two to three times higher than the values measured in practice. To accurately model the attenuation of communication radius between antennas close to the ground, in our simulation model, the TX_range is set 40 meters, and the PCS_range, with respect to the transmitting station, is set to 90 meters. Thus the RTX and the CTX can calculated as 1.74293×10-08 and 3.44283×10-09, respectively. The height of antennas is 1.5m above the ground plane operating in the 2.4 GHz band. Table n and Table n lists the parameter values that are used for simulation in the 802.11b/g standard specification .

Table 1. Value of the IEEE 802.11b parameters

SLOT

SIFS

DIFS

PHYhdr

CWmin

CWmax

RTS

CTS

ACK

20 μsec

10μsec

50 μsec

192 bits

32

1024

20

14

14

 

Table 2. Value of the IEEE 802.11g parameters

SLOT

SIFS

DIFS

PHYhdr

CWmin

CWmax

RTS

CTS

ACK

9 μsec

10μsec

28 μsec

192 bits

16

1024

20

14

14