Transmission opportunity (TXOP)

Transmit opportunity (TXOP) is a MAC layer feature used in IEEE 802.11-based wireless local area networks (WLANs). 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 (QoS) as part of Enhanced Distributed Channel Access (EDCA), which provides differentiated access to the wireless medium based on traffic categories.

How TXOP Works

TXOP operates through the following sequence of steps:

• When a station in the WLAN has frames to send, it waits until its 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):

  • Voice (AC_VO) − Highest priority

  • Video (AC_VI) − High priority

  • Best Effort (AC_BE) − Normal priority

  • Background (AC_BK) − Lowest priority

• Once a station acquires the TXOP, it can send frames according to its access category:

  • Background (AC_BK) and Best Effort (AC_BE) − Assigned TXOP value of 0, meaning they can send only one frame per TXOP. These categories must contend for the medium for each frame, similar to Distributed Coordination Function (DCF).

  • Video (AC_VI) − Assigned TXOP value of 4.096ms, allowing multiple frames within this duration.

  • Voice (AC_VO) − Assigned TXOP value of 2.080ms, enabling transmission of multiple frames within this period.

• When the station's TXOP expires, if it has additional frames to send, it must restart the contention procedure.

Access Categories Comparison

Advantages

• Quality of Service − Provides prioritized access for time-sensitive applications like voice and video.

• Improved Throughput − Reduces overhead by allowing multiple frame transmissions within a single contention period.

• Reduced Latency − High-priority traffic experiences shorter delays through preferential channel access.

Conclusion

TXOP is a crucial QoS mechanism in modern Wi-Fi networks that allocates transmission time based on traffic priority. By providing longer transmission opportunities for voice and video while maintaining fairness for lower-priority traffic, TXOP ensures optimal performance for multimedia applications in wireless networks.