Wi-Fi Modules in Dense Deployments Where Throughput Collapses
Wi-Fi throughput collapses in dense deployments when preamble misses and conservative energy detection thresholds trigger retry storms and modulation rate decay.
Network Allocation Vector
Meaning
A logical timer maintained by wireless stations tracks the predicted duration of ongoing transmissions to prevent local devices from attempting to access the medium before it becomes free. The use of a network allocation vector acts as a virtual carrier sensing mechanism that complements the physical sensing of the radio environment. When a station hears a frame intended for another device, it reads the duration field in the header and sets its internal timer accordingly.
The station will not attempt to transmit until this timer reaches zero. This process applies to all wifi standards and is a requirement for maintaining order on the channel.
Virtual Sensing
The ability to know when the air will be busy without constantly checking the physical signal level saves power and reduces errors. By using the network allocation vector, a radio can go into a low power state during the transmission of a large data packet from another network member. This is called virtual sensing because the device uses information from a decoded packet rather than raw electromagnetic energy to determine the state of the medium.
This method is more reliable than physical sensing alone because it accounts for the entire time needed for the data and the expected acknowledgment. It ensures that the channel remains reserved for the completion of a specific exchange. This logic is a primary defense against collisions in a busy wireless cell.
Delay Mechanism
The management of the timer directly affects the fairness and the efficiency of the medium access control layer. If the network allocation vector is set to a high value, the station must wait, even if the physical channel appears to be clear. This delay is necessary because the sender might be waiting for a reply that the current station cannot hear.
The timer ensures that the sequence of data and control frames is not interrupted by a third party. If a device ignores this timer, it will cause a collision and force both parties to restart their transmissions. This waiting period is the mechanism that allows for structured communication in a shared space.
It ensures that every station gets a fair chance to transmit once the current reservation expires.
Timing Accuracy
The precision of the internal clock used to decrement the value is a requirement for the proper operation of the protocol. If the network allocation vector is not updated correctly, a station might start its transmission too early or wait too long. Modern wireless chips use high speed oscillators to ensure that these timers are accurate to within a few microseconds.
The standard defines specific rules for how the timer should be updated and when it can be reset. Errors in this logic are a common cause of poor performance and high packet loss in custom radio implementations. Testing these timers is a standard part of the protocol verification for any new wireless module.
The final performance of the network depends on every device maintaining a perfectly synchronized view of the medium occupancy.