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Transmission opportunity (TXOP)
Transmit opportunity (TXOP) is a MAC layer feature used in IEEE 802.11-based wireless local area network (WLAN). TXOP defines the time duration for which a station can send frames after it has gained contention for the transmission medium. By providing this contention-free time period, TXOP aims to increase the throughput of high priority data, such as voice and video.
TXOP is available in Quality of Service as part of Enhanced Distributed Channel Access (EDCA).
Working Principle
TXOP operates in the following sequence of steps −
When a station in the WLAN has frames to send, it waits till it’s Network Allocation Vector (NAV) decrements to 0.
It determines whether the channel is clear by performing Clear Channel Assessment (CCA).
On finding a clear channel, the station waits for a time period equal to Arbitration Inter- Frame Spacing (AIFS).
It waits for completion of the Contention Window (CW). The CW defines different durations for four access categories (AC). The ACs are −
Voice (AC_VO)
Video (AC_VI)
Background (AC_BK)
Best Effort (AC_BE)
Once a station acquires the TXOP, it can send frames depending upon the AC in the following manner −
The stations of background (AC_BK) and best effort (AC_BE) categories are assigned TXOP value of 0, implying that they can send only one frame during their TXOP. In other words, AC_BK and AC_BE categories needs to contend for the wireless medium for each frame. This is similar to the Distributed Coordinated Function (DCF).
The stations sending video (AC_VI) are assigned TXOP value of 4.096ms. This implies that the station can send as many frames it can within this 4.096ms.
The stations sending voice (AC_VO) are assigned TXOP value of 2.080ms, implying that the voice category can send as many frames it can within this time period.
When the station’s TXOP is up, if it has additional frames to send, it should restart the procedure of contending for the channel.
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