[ns] How do define the link capacity

[Xiaowen Liu]

Hi,

I'm using ns2 to estimate the throughput of a chain of nodes in wireless
networks.  I used 802.11 MAC and set the date rate in WirelessPhy as
2Mbps.  The distance between neighbor nodes is 200 meters.  The
communication range is 250 meters and the interfering range is 550
meters.  For every 4 nodes, the first node and the second node can't
transmit simultaneously becasue the second node can't transmit and
receive at the same time.  The first node and the third node can't
transmit at the same time because the second node cannot correctly hear 
the first node if the third node is transmitting.  Due to the contention
in MAC layer, the throughput could not be greater than 1/3 of the 2Mbps.
That's right if the data rate in WirelessPhy is 2Mbps.  But if I change
the data rate to 200kbps or less, I can get a throughput equal to the data
rate.
That's impossible due to the above analysis.

Now I guess the link capacity is greater than 200k bps.  But I don't know
how to change it.  I use  both the following commands.  But still get 100%
utilization of the link capacity.

Phy/WirelessPhy set Rb_  200000  
LL set bandwidth_ 200000

Anyone can tell me what's the problem?  I really appreciate your help.