Tag Archives: CSMA/CA

802.11 Duration/ID Field

I always knew that Duration/ID field is used by CSMA/CA to predict when wireless medium becomes free. However, I was confused by some publications which stated this field is set to the amount of time (in microseconds), required to transmit current frame, wait SIFS and then receive ACK. CWNA Sybex book has finally helped to understand this better.

Even though Duration/ID field tells STA how much time it has to wait before medium becomes free, it is set to the duration of SIFS + ACK. It doesn’t include the time required to transmit current frame. It kind of makes sense – to read the value from the field, STA needs to receive the frame in full and check its FCS before it can set the NAV with legitimate value.

Also, didn’t know ACK/Block ACK frames always have their Duration/ID field set to 0.

Again, makes sense. Transmission completed, all NAVs have to be reset to 0 – medium is ready for the next transmission.

Good to know: Historically this field was defined to identify STA’s association ID within PS-Poll frame (legacy power management) or Duration in any other frame. In reality, legacy power management is not being used and this field is mostly used as a Duration ONLY nowadays. However, name of the field is still Duration/ID.