Hidden Terminal Avoidance in Wireless LANs via Conditional NAV

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Solution Overview

Problem

In wireless Local Area Networks (WLANs), especially in mesh networks with long distances between nodes, the 'hidden terminal' problem occurs where nodes within interference range but not transmit range of the transmitter cannot hear CTS frames, leading to potential interference and inefficiencies in channel reservation, particularly when using start-to-finish reservations.

Innovation Solution

The solution involves using control messaging on the control channel to reserve a channel for the duration of a sequence of transmissions (TXOP) with a single reservation on a data channel, delaying the setting of the NAV until the reservation is admitted, thereby eliminating the need for cancellation of unauthorized reservations.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If start-to-finish reservation is used to protect from hidden terminals, then hidden terminal protection is improved, but the complexity of reservation management increases due to cancellation requirements

Engineering Contradiction:

Improvehidden terminal protectionVSAvoidreservation management complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent applies preliminary action by setting the NAV timer only after the CTS frame is successfully received, rather than immediately upon receiving the RTS frame. This preliminary check ensures that the reservation is confirmed before committing the timer resource, eliminating the need for subsequent cancellation operations and simplifying reservation management while maintaining hidden terminal protection.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent extracts the NAV setting operation from the RTS reception process and delays it until after CTS confirmation. By separating these operations and conditionally applying the NAV setting only when necessary (i.e., when CTS is received), the system eliminates unnecessary timer cancellations and reduces management complexity while preserving the reliability of hidden terminal protection.

Inventive Principle:

Principle #2Taking out (Extraction)

2Reliability

If NAV timer is updated upon receiving RTS frame, then channel reservation is established, but interference risk increases when reservation cancellation is needed

Engineering Contradiction:

Improvechannel reservation establishmentVSAvoidinterference risk from premature NAV setting

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent applies preliminary anti-action by preventing the harmful effect of premature NAV setting through conditional logic. The NAV timer is only updated after CTS confirmation, which preemptively prevents the interference risk that would occur if NAV were set based on RTS alone. This conditional approach neutralizes the potential harm before it can manifest.

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The patent converts the potential harm of premature NAV setting into a benefit by using the CTS frame as a confirmation mechanism. The requirement to receive CTS before setting NAV transforms what could be a source of interference into a protective measure, ensuring that NAV is only set when the reservation is genuinely authorized, thereby eliminating interference risks while maintaining reliable channel reservation.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Device complexity

If single NAV timer is used for all reservations, then timer management is simplified, but the ability to track specific reservation sources is lost

Engineering Contradiction:

Improvetimer management simplicityVSAvoidreservation source tracking information

Core Design Contradiction:

Device complexityVSLoss of information

Solution Approach 1:

The patent introduces the CTS frame as an intermediary that carries reservation confirmation information. By requiring CTS reception before NAV setting, the system uses this intermediary frame to convey the authorization status of the reservation. This intermediary mechanism preserves the necessary information about reservation validity without requiring multiple timers, maintaining simplicity while preventing information loss.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS7656854B2Avoiding hidden terminals in wireless local area networks

Publication Date: 2010.02.02 SIGNAL POINT NETWORKS LLC

AI summary

A method, apparatus and computer program product for preventing occurrences of the hidden node problem are presented. A transmission request (CC-RTS) is received from a source node, the CC-RTS frame including a reservation duration value indicating the length of time needed for a transmission reservation. A timer is adjusted for the period other nodes must refrain from transmitting on the channel (NAV) equal to a duration of a transmission request/transmission response (CC-RTS/CC-CTS) handshake. When the CC-RTS has been denied, then a second CC-RTS frame is received having a reservation duration value of zero, and when the CC-RTS is accepted then the NAV is extended to remainder of the reservation duration value of the original CC-RTS frame. The CC-RTS and CC-CTS are sent at a more robust PHY mode than traffic frames.