Wifi CSMA/CA DCF - NAV and IFS (Inter frame Spacing)

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Feb 5th, 2009
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Hello all :)


Can I just confirm with all the this is the way the 802.11 CSMA/CA DCF works


So, All stations are listening to other frames on air. For simplicity, can we just use DIFS as the example here.


There are 4 stations on a wireless LAN


a) Let just say that sta1 has just transmitted a frame, and it has, in the frame, a duration field of 60 usec


b) That frame is received by sta2, sta3 and sta4 that indicate the duration and they set their NAV fields in their 802.11 code with a value of 60 usecs


Now they all know when the medium is going to be free correct at what point in time? At the zero out of the NAV? and they all have the same value for NAV? Is this correct?


c) OK, so now do all stations wait for DIFS ?


d) AND, If medium is still clear after DIFS, is it then, that they start their random backoff timer and the fastest one to zero grabs the medium?




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One last point if I may. Is it correct to call the SIFS/DIFS/PIFS operations "Physical carrier-sensing" and NAV operations "virtual carrier-sensing" and that CSMA/CA DCF MUST use both mechanism

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I would really appreciate if someone could clarify these points for me?


I am just learning about 802.11 QoS so need to understand the basics first :)) I read the VoWAN design guide and am not sure I have grasp it.


Many thx indeed for you help as always :))


Ken


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htarra Thu, 02/12/2009 - 16:15
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When aggregation of frames is not possible, 802.11n provides a mechanism to reduce the overhead involved with transmitting a stream of frames to different destinations. This mechanism reduces the interframe space between receiving a frame, typically an acknowledgement frame, and sending a subsequent frame. The 802.11e extension for quality of service added the ability for a single transmitter to send a burst of frames during a single, timed transmit opportunity. During the transmit opportunity, the sender does not need to perform any random backoff between transmissions, separating its frames by the smallest allowable interframe space, the short interframe space (SIFS).


scottmac Sat, 02/14/2009 - 07:26
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Ken:


In your first scenario, you assume that all stations can hear each other, and that none are in contention which not a likely case (plus there is a potential "hidden node" issue).


Another complication is the type of traffic the node has to send. The 802.11 protocol is very specific about what spacing and other parameters are used for a given type of traffic and transmission function, which would also determine the priority to send or capture the channel.


Your last point is correct that both mechanisms are observed.


Check out the CWNA and CWAP books by Planet3 (published by Osborne, ISBN#0-07-225538-2 and ISBN# 0-07-225585-4).


The CWNA book has excellent coverage of the basics to an intermediate level, the CWAP book covers the details down to the bits & chips.


IMO, they are the best all-around instruction and reference books for practical wireless networking.


Good Luck

Scott


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