Subject: Re: [manet] Will there be a collision???

From: "Andreas Yankopolus" <[email-masked]>
Date: Tue, 02 Mar 2004 10:20:35 -0500

Jasmina,

Interference occurs on the receiving end, not the transmitting end, so B could transmit to A and C to D simultaneously. Given the transmission ranges in your post, C's transmission would not reach A and B's would not reach D.

Note that B and C would not transmit simultaneously in your scenario when using 802.11 radios. Suppose B transmits first: C would hear B's RTS and stay silent until B is finished with its transmission (this assumes RTS/CTS has been enabled). This is known as the exposed node problem.

Cheers,

Andreas

>>> Jas C <[email-masked]> 03/02/04 10:05AM >>>
Hello everyone,
 
This is related to a doubt raised on "strong interfering signals".
 
I need clarification on the following scenario based on a string topology where any node can only hear its immediate neighbors and a nodes interference range and transmission range are equal.
 
If there are four nodes placed as  A ---- B ---- C ----D , can B transmit to A and C to D at the same time without there being a collision between B and C ? 
 
I understand that this is a typical exposed node problem and parallel transmissions are possible depending on the type of MAC protocol being used. 
 
My concern is if B and C are within each other's ranges wouldn't their transmissions  collide whereby A and D would not receive anything ??
 
I would appreciate any suggestions/pointers to help me clear this doubt.
 
Thanks a lot in advance,
Jasmina

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From: Claude Chaudet <[email-masked]>
Date: Tue, 2 Mar 2004 17:29:10 +0100

Hi,

In fact, considering radio propagation and the signal over noise ratio 
required to
decode a frame, if B and C transmit simultaneously, there is a chance 
that both
transmissions will be jammed (it depends on radio propagation though). 
Using
a simple free space propagation model (attenuation of the signal is 1 
over square of distance)
and a SNR threshold  value of 10, a signal can jam other transmissions 
for which the
receiver is distant of about three times the transmission range. (Of 
course, this is
the worst case in the worst theoretical modeling of attenuation).
In a real-wold situation, interferences at reception can happen or not. 
There is no way
of knowing

Moreover, even without RTS / CTS exchange, IEEE 802.11 performs carrier
sense before transmission and B will prevent from transmitting if C has 
already
started its transmission.

Claude.

Le 2 mars 04, à 16:20, Andreas Yankopolus a écrit :

> Jasmina,
>
> Interference occurs on the receiving end, not the transmitting end, so 
> B could transmit to A and C to D simultaneously. Given the 
> transmission ranges in your post, C's transmission would not reach A 
> and B's would not reach D.
>
> Note that B and C would not transmit simultaneously in your scenario 
> when using 802.11 radios. Suppose B transmits first: C would hear B's 
> RTS and stay silent until B is finished with its transmission (this 
> assumes RTS/CTS has been enabled). This is known as the exposed node 
> problem.
>
> Cheers,
>
> Andreas
>
> I need clarification on the following scenario based on a string 
> topology where any node can only hear its immediate neighbors and a 
> nodes interference range and transmission range are equal.
>
> If there are four nodes placed as  A ---- B ---- C ----D , can B 
> transmit to A and C to D at the same time without there being a 
> collision between B and C ?
>
> I understand that this is a typical exposed node problem and parallel 
> transmissions are possible depending on the type of MAC protocol being 
> used.
>
> My concern is if B and C are within each other's ranges wouldn't their 
> transmissions  collide whereby A and D would not receive anything ??
>
Claude.

---

From: "John Mullen" <[email-masked]>
Date: Tue, 2 Mar 2004 09:49:59 -0700

Hi Claude,

Well, this violates the assumptions of the original question, but is
certainly a valid point.  However, due to stochastic fading,
instantaneous differences in signal strength can easily differ from the
predicted mean value by plus-or-minus 20 or 30 dB.  That is, given the
right(?) set of circumstances, the signal from C could actually be
stronger at A than at B.

Which is one of the things that makes MANETs interesting.

:-)

John Mullen

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Claude Chaudet
Sent: Tuesday, March 02, 2004 9:29 AM
To: Andreas Yankopolus
Cc: <
Subject: Re: [manet] Will there be a collision???

Hi,

In fact, considering radio propagation and the signal over noise ratio 
required to
decode a frame, if B and C transmit simultaneously, there is a chance 
that both
transmissions will be jammed (it depends on radio propagation though). 
Using
a simple free space propagation model (attenuation of the signal is 1 
over square of distance)
and a SNR threshold  value of 10, a signal can jam other transmissions 
for which the
receiver is distant of about three times the transmission range. (Of 
course, this is
the worst case in the worst theoretical modeling of attenuation). In a
real-wold situation, interferences at reception can happen or not. 
There is no way
of knowing

Claude.

Claude.