802.11b

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 G_t is the transmitter antenna gain, G_r is the receiver antenna gain, d is the distance between the antennas in meters, h_t is the height of the transmitter and h_r 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 P_t 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