Wireless Access

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Regular Contributor I

CSMA/CA doubt

Hi there,

 

I am new in the Wireless world and I have a small doubt about the CSMA/CA process. I know all the steps about of hearing if the medium is free, running the backoff algorithm and so forth. My doubt is the following, when a client eventually get access to the medium and transmit its data, do it transmit only one frame, two frames or all the data it has buffered?

 

Regards,
Julián

Chief Airhead

Re: CSMA/CA doubt

@Ejohnson or @clukas might be able to help

Sean Rynearson | Chief Airhead
Aruba, a Hewlett Packard Enterprise Company
Regular Contributor I

Re: CSMA/CA doubt

Hi there,

 

Does anyone know what the standard says about that?

 

Regards,

Julián

Re: CSMA/CA doubt

Occasional Contributor I

Re: CSMA/CA doubt

Hi Michael-dc,

 

I had already read the post you show but it doesn't answer my question. I already read, participated and asked the same question in that post, but the question is still open.

 

Regards,

Julián

Regular Contributor I

Re: CSMA/CA doubt

Hi everyone,

 

I found this about one of the enhancements of 802.11n:

 

Block acknowledgment —Normally, 802.11 requires that each frame of data transmitted must be acknowledged by the recipient. If a frame goes unacknowledged, the transmitter can assume that the frame was lost and needs to be resent. The overhead of having acknowledgment messages interleaved with every transmitted frame is inefficient; it uses up airtime on the shared media.
With 802.11n, data frames can be transmitted in one burst. Only one acknowledgment is expected from the recipient after the burst is complete. More airtime can be spent sending data, increasing the overall throughput.

 

Therefore I think with the standards prior to 802.11n, the stations sent only one frame when they got the medium. From 802.11n on, this changed and they can transmit more than one frame.

 

Regards,

Julián

Occasional Contributor II

Re: CSMA/CA doubt

Its depends, however with WMM enabled clients they will receive an allotted amount of time to send frames.  This is known as transmit opportunity or TXOP. During the TXOP, multiple frames can be sent using Short

Interframe Spacing SIFS between each packet (protects the medium from others). There are default TXOP limit values for each QoS access category, but they can be changed on the AP.

Regular Contributor I

Re: CSMA/CA doubt

Hi tritterbush,

 

I think you are right but when you talked about WMM you are addressing a specific situation. I think with WMM enabled stations will receive an allotted amount of time to send many frames because the applications are delay sensitive. But I am talking if in a general situation a stations can send many frames once it gets the medium. In addition, I think WMM didn't exist prior to 802.11n though I am not sure.

 

Regards,

Julián

Occasional Contributor II

Re: CSMA/CA doubt

Wifi Alliance required WMM a subset of 802.11e for 802.11n certification.


The 802.11 standard says one frame per contention windows, QoS changes the random backoff period which changes the contention window.
Higher QoS value, lower random backoff, better chance at getting access to the medium.

Regular Contributor I

Re: CSMA/CA doubt

Hi tritterbush,

 

Clear, many thanks :)

 

Best regards,

Julián

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