Virtual Clear Channel Assessment for WLAN Throughput
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Solution Overview
Problem
In wireless local area networks (WLANs), especially in mixed environments with high-throughput and legacy devices, conventional methods for resolving contention and avoiding collisions are inefficient due to high overhead in sending protection frames, which slows network throughput and fails to protect 'follow-on' packets from hidden terminals.
Innovation Solution
The implementation of a virtual clear channel assessment (VCCA) mechanism that instructs receivers to indicate the medium as busy for a duration longer than the actual frame transmission, eliminating the need for separate protection frames like RTS/CTS, by embedding NAV information in the CCA mechanism and specifying a virtual CCA period within the physical layer convergence protocol (PLCP) header.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional RTS/CTS protection frames are used for collision avoidance, then hidden terminal protection is achieved, but network overhead increases and throughput decreases
Solution Approach 1:
The patent combines the CCA (Clear Channel Assessment) function with the NAV (Network Allocation Vector) reservation mechanism into a unified virtual CCA approach. Instead of using separate RTS/CTS protection frames, the transmitter embeds a virtual CCA indication in the data frame header that directly reserves the medium for the entire frame exchange sequence, merging channel assessment and reservation functions to eliminate overhead frames while maintaining collision avoidance
Solution Approach 2:
The patent extracts the essential protection function from the conventional RTS/CTS frame exchange and implements it directly within the data frame structure itself. The virtual CCA indication is embedded in the PLCP header or MAC header of the data frame, separating the protection mechanism from the traditional separate reservation frames and integrating it into the existing data transmission flow
2Reliability
If virtual CCA period is extended to protect follow-on packets, then hidden terminal protection improves, but medium access efficiency may be affected
Solution Approach 1:
The patent applies preliminary action by setting the virtual CCA indication to cover the entire frame exchange sequence including follow-on packets before transmission begins. The NAV reservation is established in advance with a duration that encompasses not only the current data frame but also subsequent acknowledgment and retransmission frames, preventing hidden terminals from accessing the medium during the complete exchange sequence
3Productivity
If CCA period is set to actual frame transmission time, then medium utilization is optimized, but hidden terminal collisions are not prevented
Solution Approach 1:
The patent implements preliminary anti-action by proactively extending the CCA indication beyond the actual frame transmission time to cover the entire frame exchange sequence. This preemptive extension prevents hidden terminals from attempting to access the medium during follow-on packets, counteracting the hidden terminal problem before collisions can occur by establishing a reserved period that encompasses all potential transmission activities
Data Source
AI summary
An arrangement avoids contention on a communication medium among devices including at least a transmitter and a receiver. The arrangement involves a first portion configured to instruct a receiver to indicate that the communication medium is busy for a time period substantially longer than an actual frame transmission period being sent from the transmitter to the receiver, and a second portion configured to prohibit the receiver from transmitting on the communication medium during the time period.