Dear David Rodenas.
Thank you very much for your precious advice.
Sincerely Yours,
Ick-Sung Choi.
-----Original Message-----
From: "David Rodenas"<drod...@hotmail.com>
To: "TinyOS"<tinyos-help@millennium.berkeley.edu>;
Cc:
Sent: 2012-07-25 (수) 15:41:59
Subject: Re: [Tinyos-help] Collision detection at transmitter.
Hi all
I recommend you do some research about Aloha in ISO 18000-7. I have no idea of
this recommended practice of yours, but this is the usual procedure of a
CSMA-CA mechanism. At the best of my knowledge, a node acting as a transmitter
cannot detect a collision, so what this node does is the following:
1) This node is willing to transmit so it listens to the channel a number of
CCA time intevals (e.g., in IEEE 802.15.4 non-beacon mode, is one while in the
beacon mode, this number is two).
2) If the channel is free, it transmits. Otherwise (it detects a transmission
within its coverage range), it delays its own transmission a backoff time
(random time), after which a new attempt of retransmission is conducted.
3) If the channel was free (your case), but your transmission has collided
(e.g., due to hidden nodes), it depends on which type of transmission are you
carryin! g out. For instante, if you are using an unicast transmission with
acknowledgment, the fact that you don't receive the ack from the receiver
(after a particular time interval) tells the transmitter that it was any kind
of fail, and it considers its transmission as a collision. In this case, the
transmitter sets another backoff time (provided that a maximum number of
retransmissions have not been already done). Another example is a broadcast
transmission. In this type, the ack frames are not required, so if the
transmission collided, it depends on your protocol: 1) you broadcast something
and waits for any action to be done, so you detect a possible problem (maybe a
collision); 2) you broadcast something to share some kind of information but
you don't expect anything, so the collision is not detected.
Regads,
David
Date: Tue, 24 Jul 2012 23:08:2! 3 -0700
From: cire...@gmail.com
To: pnk...@naver.com
CC: tiny os-h...@millennium.berkeley.edu
Subject: Re: [Tinyos-help] Collision detection at transmitter.
On Tue, Jul 24, 2012 at 10:35 PM, 최익성 <pnk...@naver.com> wrote:
Dear Eric Decker.
Thank you very much for your kind explanation.
I know that the collision detection is not required in CSMA/CA.
Is there any way to detect the collision at transmitter?
Not that I am aware of. The issue is whether the h/w can can hear itself when
transmitting. I don't think the cc2420 has the capabililtiy and with out
hearself then doing collision detection becomes problematic.
In case of slotted aloha in ISO 18000-7, do we need detect the collision only
at receiver?
No idea. I'm not familar with slotted aloha.
A collision seen at a receiver can show up in various ways. I don't know if
that means that the receiver can reliably determine that a collision has taken
place.
Regardless, I don't see how the receiver detecting a collision can in any
reasonable fashion impact the operation of the transmitter. The transmitter
needs to detect the collision.
If you must do a protocol that requires reliable collision detection, you must
select your radio h/w such that CD is supported.
Thank you very much.
Sincerely Yours,
Ick-Sung Choi.
-----Original Message-----
From: "Eric Decker"<cire...@gmail.com>
To: "최익성"<pnk...@naver.com>;
Cc: <tinyos-help@millennium.berkeley.edu>;
Sent: 2012-07-25 (수) 13:25:52
Subject: Re: [Tinyos-help] Collision detection at transmitter.
On Tue, Jul 24, 2012 at 6:56 PM, 최익성 <pnk...@naver.com> wrote:
Dear tinyos developers.
I have a basic question about collision detection at transmitter.
Its CSMA/CA not CSMA/CD so it is collision avoidance.
In CSMA/CA, transmitter transmits the frame. If there is a collision, the
transmitter tries to transmit the frame after random backoff time.
How can the transmitter detects the collision?
Typically this kind of thing is implemented in h/w. You don't specify what
h/w you are using... (Yes, it would have been helpful if you told us the
specifics of what h/w you are using). So I'm assuming the telosb with the
cc2420 radio.
the cc2420 hardware does Clear Channel Assessment and provides status bits that
tell the driver what is happening. I believe if the channel is busy it is up
to the driver to handle the back off.
I would suggest you take a look at the cc2420 manual (pg 50) and look at the
cc2420 driver where it deals with CCA and possibly the STXONCCA command strobe.
Thank you very much.
Sincerely Yours,
Ick-Sung Choi.
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--
Eric B. Decker
Senior (over 50 :-) Researcher
--
Eric B. Decker
Senior (over 50 :-) Researcher
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