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How does it help avoid frame collisions? Are there any rules for when the back off is allowed, especially in the case of wireless networks?

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    $\begingroup$ What self-study have you done? Backoff is described in standard networking textbooks. What sources have you read, and what specifically didn't you understand? Also, we prefer that you ask only one question per post. "How does MAC layer back-off work?" is too broad. "How does it help avoid collisions?" is probably fine, but you need to show your research/self-study (and exhaust all obvious sources before asking here). "When is back-off allowed?" is also fine, but asking more than one question per post doesn't work well with this site format. Can you edit your question based on this feedback? $\endgroup$ – D.W. Jan 8 '16 at 17:15
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Warning: the following is based on a presentation I created for a graduate course that I took in March 2008 (Chionglo 2008a; 2008b). Thus some ideas or terms may not be up to date.

I will assume that you are referring to the IEEE 802 standard method of organizing networking components because you used the acronym “MAC”.

“How does a MAC layer back-off work?” (2016) is a broad question because there is more than one method for performing the functions at the MAC layer. The IEEE 802.3 MAC is an example of a standard that describes a method for operating at the MAC layer. It has two components: transmit frame component and receive frame component. Transit frame component has three parts: deference process, normal operation, and collision handling procedure. For the collision handling procedure, if the Physical Layer indicates to the MAC layer that a collision has occurred then the IEEE 802.3 MAC layer responds to the indication by

  1. Suspending its transmission.
  2. Sending out a jamming signal.
  3. If the number of attempts to resend the frame is less than 16 then waits an additional period of time before attempting to resend the frame (back-off).
  4. If the number of attempts to resend the frame exceeds 15 then it aborts the transmission and indicates to its client (the Logical Link Control Layer) than an error has occurred.

References

Chionglo, J. F. (2008a). IEEE 802.3 Media Access Control [Graduate course presentation]. FIS 2166 Telecommunications for Information Systems, Faculty of Information [Studies], University of Toronto. Delivered on March 25, 2008.

Chionglo, J. F. (2008b). IEEE 802.3 Media Access Control [Graduate course presentation: text of speech]. FIS 2166 Telecommunications for Information Systems, Faculty of Information [Studies], University of Toronto. Delivered on March 25, 2008.

How does MAC layer back-off work?” (2016). Computer Science Stack Exchange.

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