In IEEE 802.11, TxOP (Transmission Opportunity) is a time duration during which a station (STA) has the right to transmit multiple frames without contention. The method for requesting a TxOP depends on the access category (AC) and coordination function in use.

Methods of Requesting TxOP:

1. Using EDCA (Enhanced Distributed Channel Access) – Contention-Based Access

• Each STA contends for the channel using CSMA/CA.
• Once a station wins contention, it gets a TxOP limit based on its access category (AC) (e.g., voice, video, best effort, background).
• The station can transmit multiple frames within the granted TxOP.
• The duration of TxOP is defined in the EDCA parameter set in the Beacon frame from the AP.

2. Using HCF Controlled Channel Access (HCCA) – Polled Access

• The Hybrid Coordinator (HC) (typically the AP) grants a TxOP to a station.
• The HC uses polling (e.g., in QoS CF-Poll frames) to assign transmission time to stations.
• The station does not contend for the medium but waits for the HC’s grant.

3. Using RTS/CTS Mechanism

• A station may use RTS/CTS exchange to reserve the channel.
• RTS/CTS packets include the duration field, indicating the requested TxOP.
• Other stations update their NAV (Network Allocation Vector) accordingly, preventing collisions.

4. MU-MIMO and OFDMA in 802.11ax (Wi-Fi 6)

• In 802.11ax, the AP schedules multi-user TxOPs.
• The AP assigns RUs (Resource Units) in OFDMA-based transmissions.
• The Trigger Frame (TF) from the AP dictates when and how long a station can transmit.

TxOP Request in Different QoS Mechanisms

Access MechanismWho Requests TxOP?How TxOP is Granted?EDCAStation contends for accessDynamic, based on access categoryHCCAAP/HC assignsPolled-based transmissionRTS/CTSStation sends RTSNAV reservation802.11ax OFDMAAP schedules RU allocationTrigger-based scheduling