[ns] throughput of 802.11b

[andrew.hy]

Hi all,
    I have tried to run 802.11b in ns2, and I use the following setting:
    Mac/802_11 set    SlotTime_            0.000020        ;# 20us
    Mac/802_11 set    SIFS_                0.000010        ;# 10us
    Mac/802_11 set    PreambleLength_      144             ;# 144 bits
    Mac/802_11 set    PLCPHeaderLength_    48              ;# 48 bits
    Mac/802_11 set    PLCPDataRate_        1.0e6           ;# 1 Mbps
    Mac/802_11 set    dataRate_            11.0e6          ;# 11 Mbps
    Mac/802_11 set    basicRate_           1.0e6           ;# 1 Mbps
 
    However, it turns out that the actually achieved throughput of a tcp connection and a cbr connection is around 1.34Mbps and 3Mbps respectively, can anyone tell me why?
    
    Thanks in advance!