TXOP – An Explanation of Transmit Opportunity

Transmit Opportunity or TXOP is the amount of time a station can send frames when it has won contention for the wireless medium.

This is in relation to the EDCA or Enhanced Distributed Channel Access method.

When a station sends QoS data, it must first contend for access to the wireless medium.

The station must perform a Clear Channel Assessment (CCA) and determine the channel is idle.  It must also have it’s Network Allocation Vector (NAV) set to 0.  Then, it must wait for the appropriate InterFrame Spacing, which would normally be the Arbitration InterFrame Space (AIFS).  Then, it would have to wait for the Contention Window (CW) to complete.  The Contention Window has different a duration for each of the 4 Access Categories.  The Access Categories include: voice (AC_VO), video (AC_VI), background (AC_BK), and best effort (AC_BE).

Each of the four Access Categories has a different TXOP.

The AC_BK and AC_BE categories have a TXOP of 0.  This means they can only send 1 frame during their TXOP.  Once that frame it sent, it must contend for the wireless medium again with the CCA, NAV, AIFS, and CW.  In other words, the Background and Best Effort categories must contend for the wireless medium for each and every frame they send.  This would be similar to Distributed Coordinated Function.

The AC_VI has a TXOP of 4.096ms (22.56ms or 16.92ms for 802.11ac devices*).  This means the video category can send as many frames it can within the time it has been granted.  Once its TXOP is up, it must contend for the wireless medium again if it has additional frames to send.

The AC_VO has a TXOP of 2.080ms (11.28ms or 8.46ms for 802.11ac devices*).  This means the voice category can send as many frames it can within the TXOP it has been granted.  Once its TXOP is up, it must contend for the wireless medium again if it has additional frames to send.

 

*See Attached screenshot for default TXOP Limits according to IEEE Std 802.11-2016.

2 Comments

  1. Hi

    Could you explain how the AC_VO TXOP of 3.008ms is calculated to be this number?

    Regards

    • Hi Mahmoud,
      I don’t remember where I got those numbers, but they were incorrect. Not only were they incorrect, I flipped the values for AC_VO and AC_VI. I’ve reviewed the specs and updated the post to reflect the TXOP Limits outlined in the IEEE Std 802.11-2016.
      Thanks,
      Sean

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