Subject: [manet] Do MANET radios require collision detection?

From: "Scott C. Lemon" <[email-masked]>
Date: Tue, 2 Jul 2002 17:55:31 -0600

Hello again,

I appreciate all of the responses to my other questions/comments ... I'm still digesting and reading and appreciate all of the expertise being shared on this list.  I'm sure that as I follow this conversation more, I'll become more aware of the various topics and issues ...

One area that I wanted to ask a question or two is related to current commercial radios.  It appears that most people are using 802.11b (or maybe even "a") for their MANET work.  I too use 802.11b, however the distance is very limiting.

I have recently been working with a company to develop an alternative radio for my purposes, and am now interested in the radio requirements to work with the software that is currently being used with the 802.11b cards.

My specific question is if most of these MANET protocols and software require collision detection in the radio on transmission?  We almost have completed a prototype of a radio that can be serially or USB attached, that would provide lower bitrates (~115kbps), however much longer distances (1+ miles).  In the current prototype, the radio will just transmit without any collision detection.  Will higher layers in the stack/software perform retries, etc.?  I know that TCP handles this for its streams, however it appears that some of the MANET protocols/software might be UDP or even lower level ...

Again ... this might not be exactly the right question due to my new understanding of some of the MANET software being developed ... am I right to be concerned about this?

Thank you in advance!

Scott C. Lemon
HumanXtensions L.L.C.
http://www.HumanXtensions.com
http://the.Inevitable.Org/anism

   "People often talk about it being the journey, not the destination.
        Consider the possibility that there is no destination ...
                    that in life there is only journey!"

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Wed, 3 Jul 2002 07:38:56 -0600

Hi Scott,
 
I don't know if "require" is the right word, but some sort of collision
management is prudent in wireless nets.  
 
If there were sufficient bandwidth and each terminal had a precise clock,
one could assign a different time slice to each transmitter so that at no
time would two transmitters be on at the same time.  However, when a node is
not using its time slice, it goes unused.  Unless nodes are routinely and
regularly chatty, this wastes lots of bandwidth.  If there are a lot of
nodes, it doesn't leave much for each one.  There is a problem of what to do
if a node joins or leaves the MANET.  It also allows little flexibility in
the event one node has more than the usual amount to say.  Although I've
seen limited use of this approach, I'd say the most effective strategy seems
to be random access, together with collision detection.
 
A prudent thing is to check to see if a transmitter is active in the area
and, if one is detected, wait a random interval before checking again.
Obviously, if another transmitter is already on the air, transmitting will
cause a data collision.  However, in some cases the transmitter cannot
detect the other transmission (this is the hidden terminal problem) and only
the receiving terminal will know a data collision occurred.  The transmitter
learns when it doesn't get its ACK.  However, a transmitter could fail to
get an ACK for other reasons, so figuring out what to do as a result is a
bit of a guessing game.
 
Relying solely on the retransmission of entire packets has a very nasty
effect and really cuts into BW.  A retransmission increases channel
utilization by 100% while adding nothing to throughput.  Also, it is
possible to have waves of retransmissions causing even more data collisions,
leading to more retransmissions and the network gets pretty much choked up.
In short, data collision management is much more effective than letting the
chips fall where they may.
 
Coding, segmentation, error correction, etc. can reduce the impact of an
occasional collision.  RTS/CTS schemes also reduce the probability and
impact of a collision.  In this area, I'd take all the help I could get. 
 
In this working group, the emphasis is on Layer 3 and the upper part of
Layer 2 of the OSI model.  For this reason, the data collision problem is
often not explicitly mentioned.  However, it is a very significant element
of the MANET environment and its impact cannot be ignored.  Furthermore, it
is the sort of problem that is best handled as low in the model as possible
because attempts to manage it at higher levels may aggravate the problem
even when the net is only moderately loaded.
 
John Mullen

-----Original Message-----
From: Scott C. Lemon [mailto:[email-masked]]
Sent: Tuesday, July 02, 2002 5:56 PM
To: [email-masked]
Subject: [manet] Do MANET radios require collision detection?

Hello again,
 
I appreciate all of the responses to my other questions/comments ... I'm
still digesting and reading and appreciate all of the expertise being shared
on this list.  I'm sure that as I follow this conversation more, I'll become
more aware of the various topics and issues ...
 
One area that I wanted to ask a question or two is related to current
commercial radios.  It appears that most people are using 802.11b (or maybe
even "a") for their MANET work.  I too use 802.11b, however the distance is
very limiting.
 
I have recently been working with a company to develop an alternative radio
for my purposes, and am now interested in the radio requirements to work
with the software that is currently being used with the 802.11b cards.
 
My specific question is if most of these MANET protocols and software
require collision detection in the radio on transmission?  We almost have
completed a prototype of a radio that can be serially or USB attached, that
would provide lower bitrates (~115kbps), however much longer distances (1+
miles).  In the current prototype, the radio will just transmit without any
collision detection.  Will higher layers in the stack/software perform
retries, etc.?  I know that TCP handles this for its streams, however it
appears that some of the MANET protocols/software might be UDP or even lower
level ...
 
Again ... this might not be exactly the right question due to my new
understanding of some of the MANET software being developed ... am I right
to be concerned about this?
 
Thank you in advance!

Scott C. Lemon
HumanXtensions L.L.C.
 <http://www.HumanXtensions.com> http://www.HumanXtensions.com
 <http://the.Inevitable.Org/anism> http://the.Inevitable.Org/anism

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Wed, 3 Jul 2002 07:38:56 -0600

Hi Scott,

 

 
 
John Mullen

-----Original Message-----
From: Scott C. Lemon [mailto:[email-masked]]
Sent: Tuesday, July 02, 2002 5:56 PM
To: [email-masked]
Subject: [manet] Do MANET radios require collision detection?

Hello again,

 

 
Thank you in advance!

Scott C. Lemon
HumanXtensions L.L.C.