Subject: Communication technologies in ad hoc networks

From: Wenjing Lou <[email-masked]>
Date: Wed, 18 Jul 2001 09:14:50 -0400

Hi, all, 

   I am currently working on the routing part of ad hoc network. I studied the
several protocols and have a few questions. I post my questions here and I will
appreciate if some of you could answer my questions.

   1. What communication technologies are used in ad hoc networks? What I mean
is, in the physical layer, are all nodes using a single channle (such as a
frequency band) to communicate, or they use different channels (different
frequency or codes) to communicate? I am wondering, if they are all using a
single channel, then one node's transmitting will block all its neighbors'
transmitting. The efficiency seems to be a problem because there will be much
contention and collision. But if they are all using differnt channels to
communicate, then who is assigning the channel for them? (The channels have to
be locally unique because globally unique channels will cause scalability
problem). I am not aware of a good scheme to dynamically assign locally unique
channels in ad hoc network.  

   2. In the DSR protocol, there is a Promiscuous Mode, allowing a node to
overhear packets passing through its neighbors and thus send gratuitous route
replies. I wonder what is the underlying support for this mode? As mentioned in
1, if all the nodes use a single channel, yes, nodes can overhear each other.
But if they are using different channels, is it possible to overhear?

   Thanks for your time and looking forward to your reply.

   Best regards,

   Wenjing

---

From: Ram Ramanathan <[email-masked]>
Date: Wed, 18 Jul 2001 10:16:49 -0400

> To: <[email-masked]>
> Subject: Communication technologies in ad hoc networks
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> Date: Wed, 18 Jul 2001 09:14:50 -0400
> From: Wenjing Lou <[email-masked]>
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> Hi, all, 
> 

There has been a fair amount of work on this channel assignment problem in
the 80's, and early 90's. Much of the work is cited in the bibliography of 
my paper entitled 

S. Ramanathan, "A unified framework and algorithm for channel assignment in 
wireless networks," Wireless Networks, 5 (1999), pp. 81-94

It is available online at

http://www.ir.bbn.com/projects/dawn/unified.ps

I would caution that much of this work is "thinking", ie, I haven't seen
any commercial products or prototypes doing locally unique channel assignment
in the code, time or frequency domain. A reason for this might be that
commercial physical layer solutions such as 802.11 cards 
that are typically used for prototypes
and experimentation do not support slot synchronization or CDMA. They do
support multiple frequency channels, but I am not sure yo can change those
channels dynamically. (An exception might be the Nokia Wireless Router from
former Rooftop Comms, which uses multiple channels, but I am not sure if
the assingment is "locally unique").

Hope it helps in your research. 

-Ram.