Subject: RE: [manet] strong interfering signals

From: "David Holmer" <[email-masked]>
Date: Tue, 24 Feb 2004 21:47:45 -0500

I would say that X would probably not be able to successfully receive
the packet from Z. The reason has to do with the state the MAC protocol
is in when the interference begins. In your example Y has already
started sending to X, so X has detected a valid preamble and start
symbol on the medium. It probably has even decoded the MAC header which
tells it how long the packet is. As a result X may not even be able to
tell that Z is transmitting (and interfering with the packet from Y)
until it tries to verify the frame check sequence and fails. By this
time Z has probably transmitted much of the packet, and the only thing X
would be able to do is carrier sense an ongoing transmission and back
off till it was over. This is because X would have missed the preamble
and start symbol of Z's transmission and thus can't lock its radio in to
receive Z's packet.

One caveat: I think it may be possible that Z's transmission may be
strong enough to cause X to loose synchronization with Y's transmission
and terminate the reception early (this is the cause of truncated
packets). There is some small chance if this happens quickly enough,
that X may have enough time to lock on to Z's preamble before the start
symbol, and then be able to receive Z's transmission. However, I believe
this is very unlikely.

As implemented in NS2, X will not receive Z's packet regardless of the
strength. One slight inaccuracy in the simulation occurs if Z and Y send
their packets at almost the same time to X. In real life, even if the
Y's packet starts to reach X a few microseconds before Z's packet (i.e.
due to propagation delays and timing jitter), Z's packet will still be
successfully received by X because it is so much stronger than the
interfering signal (and thus X can lock on and receive the whole
packet). However as simulated in NS2, neither packet will be received as
Z is treated as colliding with an "ongoing" transmission from Y.

PS. Does anyone else have any comments or experience about the
mechanisms of synchronization loss and its effects on receiving packets?

- David Holmer

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Sundar K
Sent: Saturday, February 21, 2004 5:10 PM
To: [email-masked]
Subject: [manet] strong interfering signals

Hi,

Could anyone clarify on the following scenario in a
MANET:

Assume that a node X is receiving a data packet from
node Y. Assume that in the meantime, node Z (in the
near vicinity of X) initiates transmission to node X.

I understand that with the CSThreshold being below the
RxThreshold in 802.11, such a situation may not be
likely.

However, assuming that a situation as this occurs,
node X would fail be able to recieve Y's packet
successfully. Now, if the signal strength of the
interfering signal from Z is sufficiently strong at X,
would X be able to receive the packet from Z,
successfully ?

thanks in advance,
sundar

_______________________________________________
manet mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/manet

_______________________________________________
manet mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/manet