The Hidden Terminal Problem

In wireless LANs (wireless local area networks), the hidden terminal problem is a transmission problem that arises when two or more stations who are out of range of each other transmit simultaneously to a common recipient. This is prevalent in decentralized systems where there isn't any central entity for controlling transmissions. This occurs when a station is visible from a wireless access point (AP), but is hidden from other stations that communicate with the AP.

Problem Illustration

Suppose that there are three stations labeled STA, STB, and STC, where STA and STC are transmitting while STB is receiving. The stations are in a configuration such that the two transmitters STA and STC are not in the radio range of each other.

The above diagram shows that station STA starts transmitting to station STB. Since station STC is out of radio range of STA, it perceives that the channel is free and starts transmitting to STB. The frames received by STB are garbled and collision occurs. This situation is known as the hidden terminal problem.

RTS/CTS Solution

The hidden 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.

How RTS/CTS Works

• A transmitting station sends an RTS frame to the receiving station.

• The receiving station replies by sending a CTS frame.

• On receipt of CTS frame, the transmitting station begins data 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 does not hear RTS from station STA, but hears CTS frame from station STB. So, it understands that STB is busy and defers its transmission, thus avoiding collision.

Key Benefits

• Collision avoidance − Prevents simultaneous transmissions to the same receiver

• Channel reservation − RTS/CTS frames reserve the channel for data transmission

• Improved throughput − Reduces frame collisions and retransmissions in wireless networks

Conclusion

The hidden terminal problem occurs when stations outside each other's radio range simultaneously transmit to a common receiver, causing collisions. The IEEE 802.11 RTS/CTS mechanism effectively solves this by using control frames to reserve the wireless channel before data transmission.