Don't know why ns2.31 would say otherwise, but according to IEEE 802.11a-1999, which you can access from the IEEE web site, the rates you can achieve with this 5 GHz standard are 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s. (And the standard says that 6, 12, and 24 Mb/s MUST be supported.)

Table 78 shows the modulation and constellation used, which varies from BPSK up to 64-QAM, and the FEC, which can be 1/2, 2/3, or 3/4, to create the listed channel Mb/s capacities.

You have to consider that you are probably looking at the rate of the payload bits on higher layers, and the rates such as 54Mb/s are the rate on the physical layer. All packets have additional physical and MAC layer headers (bits that are not counted for throughput), and there is quite a bit of idle time between the packets used by interframe spacing and backoffs, which lowers the rate. I don't know what the correct theoretical value should be, but as Akhilesh said your values look ok.

Perhaps one important question to ask is what data rate the ns2.31 model was being asked to compute? Was it TCP/IP over 802.11a? It's true that depending on the WiFi mode used, and on upper layer protocols, the net data rate can be no more than roughly half the physical layer data rate, over WiFi.

Yes, i am using TCP over 802.11a, and the data rate is 54 Mbps.

Thank you all for your helping answers, please Nidhi Lal did you simulate 802.11a before and get a higher value? If so please tell me the value.