[ns] A note on wireless bandwidth

[Robin Poss <robin@ee.cornell.edu>]

There is a "Mac set bandwidth_ 10000000" command that you
can intergrate to your tcl script, and which actually changes the
bandwidth_ variable of the mac class. However, the DSSS_SlotTime
hard-coded in mac-802_11.h:

mac-802_11.h:
/*
* IEEE 802.11 Spec, section 15.3.2
*       - default values for the DSSS PHY MIB
*/
#define DSSS_CWMin                       31
#define DSSS_CWMax                       1023
#define DSSS_SlotTime                    0.000020        // 20us
#define  DSSS_CCATime                    0.000015        // 15us
#define DSSS_RxTxTurnaroundTime          0.000005        // 5us
#define DSSS_SIFSTime                    0.000010        // 10us
#define DSSS_PreambleLength              144             // 144 bits
#define DSSS_PLCPHeaderLength            48              // 48 bits

is  used (see mac-timers.cc, line 227) for
the backoff of the MAC protocol, and I guess the latter won't
work properly if the backoff algorithm don't take into account
the fact that the bandwidth was changed.

So I think one would need to find out which parameters in the
wireless model are related to the bandwidth, and set them to
coherent values, to get a greater bandwidth. Just adjusting the
"bandwidth_" variable won't work.

Should you know what these parameters are, and what values they
should be set to, or which papers/websites I should read to get
that information, I am interested. Someone advised me  the
following paper, which I did not check yet: B. P. Crow, I.
Wadjaja, J. G. Kim, P. T.Sakai, ``IEEE  802.11 Wireless Local
Area Networks,'' IEEE Commm. Mag., vol. 35, no. 9,  Sep. 1997,
pp. 116-126. 

To find it online, go to 
http://ieeexplore.ieee.org/
 -- 
Robin