Wireless LAN RTS CTS Protocol Exposed Terminal Problem
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Solution Overview
Problem
In wireless LAN systems, the 'exposed terminal' problem occurs where stations outside the signal reach of a communication cannot send data due to incorrect carrier sense, leading to reduced transmission efficiency, and existing solutions like DBTMA are complex and ineffective in harsh environments.
Innovation Solution
A wireless network system that allows terminals outside the signal reach to send data by using a request-to-send and clear-to-send signal protocol, avoiding interference with acknowledgement signals and utilizing an adjacent station relation table to determine signal quality and relative distances, enabling efficient data transmission even in fading environments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If carrier sense method is used to control wireless transmission, then collision between signals can be avoided, but terminals outside signal reach cannot transmit data leading to reduced transmission efficiency
Solution Approach 1:
The patent introduces RTS (Request to Send) and CTS (Clear to Send) signals as intermediary control messages. These signals act as mediators between transmitting and receiving terminals, allowing terminals outside the carrier sense range to coordinate transmissions without causing collisions. The RTS/CTS handshake protocol enables exposed terminals to transmit data by receiving CTS signals that indicate the channel is reserved for them.
Solution Approach 2:
The patent implements preliminary channel reservation through RTS/CTS signaling before actual data transmission. By performing the handshake protocol in advance, terminals can determine whether they are exposed terminals and proceed with transmission during the NAV period without causing collisions, thus improving transmission efficiency while maintaining reliability.
2Productivity
If DBTMA method is used to solve exposed terminal problem, then data transmission from suppressed terminals is enabled, but system complexity increases due to multiple busy tones
Solution Approach 1:
The patent extracts and eliminates the complex busy tone mechanism from the DBTMA method. Instead of using multiple out-of-band busy tones to indicate channel status, the invention uses the existing RTS/CTS control signals already present in wireless LAN systems, thereby reducing system complexity while maintaining the ability to solve the exposed terminal problem.
Solution Approach 2:
The patent makes the RTS/CTS signals serve multiple functions: they not only reserve the channel for the intended receiver but also inform exposed terminals about the channel status. This multi-functionality eliminates the need for separate busy tone signals, reducing system complexity while achieving the same productivity improvement as DBTMA.
3Reliability
If RTS/CTS protocol is used for channel reservation, then collision frequency is reduced, but transmission overhead increases due to additional control signals
Solution Approach 1:
The patent applies RTS/CTS signaling selectively only to terminals that are exposed terminals (those outside the carrier sense range but inside the receiver's range). Terminals that can perform normal carrier sense do not require the full RTS/CTS handshake, thus reducing the overall transmission overhead while maintaining collision reduction benefits where needed.
Data Source
AI summary
A wireless network system including a first terminal for sending a request-to-send signal including information on a medium use period to a second terminal before sending data, receiving a clear-to-send signal from the second terminal, and sending data to the second terminal during the medium use period; the second terminal for receiving the request-to-send signal and sending the clear-to-send signal; a third terminal for receiving the request-to-send signal and transmitting data to a fourth terminal during the medium use period; and the fourth terminal. Thus, it is possible to prevent occurrence of a problem that when it is judged that the wireless channel is being used, transmission of the station is suppressed.