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 carrying 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). An! other 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 23:08:23 -0700 From: cire...@gmail.com To: pnk...@naver.com CC: tinyos-help@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. _______________________________________________ Tinyos-help mailing list Tinyos-help@millennium.berkeley.edu https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help -- Eric B. Decker Senior (over 50 :-) Researcher -- Eric B. Decker Senior (over 50 :-) Researcher _______________________________________________ Tinyos-help mailing list Tinyos-help@millennium.berkeley.edu https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
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