Subject: Re: multiple channels (dual channels) - a lost posting ...

From: <[email-masked]>
Date: Mon, 17 Dec 2001 14:21:50 -0500 (EST)

Hi Dmitri,

We have just submitted a paper that compares the single-channel MAC with
dual-channel MAC (data + control channels). The study was done in the
context of the DBTMA (Dual Busy Tone Multiple Access) scheme for ad hoc
networks (for DBTMA paper see, for example,
	http://www.ee.cornell.edu/~haas/Publications/toc02.ps
.) The bottom line is that (unfortunately), dual-channel does *not*
improve the overall network capacity. Intuitively, you can see this as
follows.

In the dual-channel case, the "data channel" is used to transmit packets
only, while the "control channel" is used for access collision
resolution. (I.e., it's like reservation are made on the control
channel and then whoever acquires the data channel can transmit
without collision.)

To accomplish this, the system "bandwidth" has to be partitioned into
two channels. Assigning some bandwidth to the control channel reduces
the data rate on the data channel, so the control channel bandwidth should
be minimized. However, if you reduce the control channel too much, it will
not be able to "resolve" access contention fast enough, leaving the
data channel unnecessarily idle and reducing capacity; i.e., the
control channel then becomes the bottleneck.

The "optimal" calculation of the bandwidth partition between the data and
the control channels is easy to calculate. If you do so and if you take
into the consideration that the access to the control channel itself is
random (e.g., ALOHA), then the bottom line is that the dual-channel
case does not yield more overall network capacity - in fact, practically
it will reduce the network capacity.

I will be happy to post the reference to our paper, once it is accepted.

Zygmunt.
===--==-=-==
~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-
Prof. Zygmunt J. Haas                    tel: +1-607-255-3454
Wireless Networks Laboratory		                  
School of Electrical Engineering         fax: +1-607-255-9072 
Cornell University
323 Frank Rhodes Hall			 e-mail: [email-masked]
Ithaca, NY 14853			 
U.S.A                             http://www.ee.cornell.edu/~haas/wnl.html
~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-