Something confusing to new NS2 users.

bandwidth_: bandwidth of the physical layer.
basicRate_: transmission rate for control packets such as RTS, CTS, ACK and Broadcast.
dataRate_: transmission rate for mac layer data packets.
for more:
Visit best website for ns, Kliu's website
http://www.cs.binghamton.edu/~kliu/research/ns2code/note.pdf