WLAN Collision Avoidance via AP-Mediated EOT and NAV Reset
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Solution Overview
Problem
In wireless local area networks (WLANs), the disparity in extended interframe space (EIFS) durations between nodes that decode the MAC portion of a frame and those that do not leads to unfair access to the wireless medium, and the resetting of the network allocation vector (NAV) by clients may not be received by all nodes in the basic service set (BSS), causing inefficiencies and collisions.
Innovation Solution
Implementing a system where clients send an end of transmission (EOT) request to the access point (AP), which responds with an EOT frame that can be received by all stations, ensuring synchronized backoff and resetting the NAV through a CF-End frame response from the AP, matching the BSSID to avoid collisions and ensure all nodes in the BSS reset their NAV.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If nodes start an extended interframe space (EIFS) when MAC header cannot be decoded, then the station allows for transmission of acknowledgment frame from hidden node, but this introduces significant unfairness for nodes which start an EIFS versus those nodes which can decode the MAC portion and continue backoff immediately
Solution Approach 1:
The patent introduces an intermediary mechanism where the access point (AP) sends a CF-End frame to mediate the backoff resumption process. Instead of relying on nodes to independently determine when to resume backoff based on MAC header decoding status, the AP acts as a central coordinator that sends a unified signal to all nodes, ensuring fair and synchronized backoff resumption across the network.
Solution Approach 2:
The system implements feedback through the CF-End frame mechanism. The AP monitors the transmission status and provides feedback to all nodes by sending a CF-End frame when appropriate, allowing nodes to synchronize their backoff resumption based on this feedback signal rather than making independent decisions that could lead to unfairness.
2Ease of operation
If clients reset the network allocation vector (NAV) by transmitting a CF-End frame, then the NAV is reset for that client, but this CF-End frame may not be received by all nodes in the basic service set (BSS) because the client may not be near the center of the BSS
Solution Approach 1:
The access point serves as an intermediary that receives the CF-End frame from the client and then retransmits a CF-End frame to all nodes in the BSS. This mediator approach ensures that the NAV reset signal reaches all nodes regardless of their position in the network, solving the coverage problem where direct client-to-node communication might fail for distant nodes.
Solution Approach 2:
The patent combines the NAV reset function into a centralized AP-controlled mechanism. Instead of individual nodes attempting independent NAV resets that may not reach all network participants, the system merges the reset function through the AP, which has network-wide coverage and can ensure all nodes receive the reset signal simultaneously.
3Reliability
If the extended interframe space (EIFS) duration is based on the lowest mandatory PHY rate, then it provides sufficient time for acknowledgment transmission, but the ACK transmission may be much shorter than the EIFS duration, especially in the 802.11g standard where EIFS is 320 microseconds and an orthogonal frequency division multiplexing (OFDM) ACK transmission plus short interframe space (SIFS) may be as short as 34 microseconds
Solution Approach 1:
The system dynamically adjusts the backoff resumption timing based on actual transmission conditions. Instead of using a fixed EIFS duration that may be excessively long, the AP monitors the actual ACK transmission duration and coordinates backoff resumption accordingly, allowing the system to adapt to varying transmission rates and lengths rather than being constrained by the conservative lowest PHY rate baseline.
Data Source
AI summary
Disclosed herein are various embodiments of collision avoidance systems and methods. One method embodiment, among others, comprises a client sending an end of transmission (EOT) request to an access point (AP), and responsive to the EOT request, the AP responding with an EOT frame.