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The Exposed Terminal Problem
In wireless LANs (wireless local area networks), the exposed terminal problem is a transmission problem that arises when a transmitting station is prevented from sending frames due to interference with another transmitting station. This is prevalent in decentralised systems where there aren’t any entity for controlling transmissions. This occurs when a station is visible from a wireless access point (AP), but not from other stations that communicate with the AP.
Problem Illustration
Suppose that there are four stations labelled STA, STB, STC, and STD, where STB and STC are transmitters while STA and STD are receivers at some slot of time. The stations are in a configuration such that the two receivers STA and STD are out of radio range of each other, but the two transmitters STB and STC are in radio range of each other. This is shown in the following figure −
The above diagram shows that a transmission is going on from STB to STA. STC falsely concludes that the above transmission will cause interference and so stops its transmission attempts to STD. However, the interference would not have occurred since the transmission from STC to STD is out of range of STB. This prevention of transmission is called exposed terminal problem.
Solution
The exposed terminal problem is solved by the MAC (medium access control) layer protocol IEEE 802.11 RTS/CTS, with the condition that the stations are synchronized and frame sizes and data speed are the same. RTS stands for Request to Send and CTS stands for Clear to Send.
A transmitting station sends a RTS frame to the receiving station. The receiving station replies by sending a CTS frame. On receipt of CTS frame, the transmitting station begins transmission.
Any station hearing the RTS is close to the transmitting station and remains silent long enough for the CTS. Any station hearing the CTS is close to the receiving station and remains silent during the data transmission.
In the above example, station STC hears RTS from station STB, but does not hear CTS from station STA. So, it is free to transmit to station STD.
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