Exposed Terminal Problem in Wireless LAN

Wireless LAN is used to transmit data from one device to another without using connected medium. There are normally several problems in wireless LAN. One of them is the Exposed terminal Problem. 

Exposed Terminal Problem

In wireless LAN (local area network) communication, the exposed terminal problem is a frequent difficulty. It happens when a wireless node cannot transfer data because another node that is outside its communication range is sending data to another node that is inside it. Throughput and network performance may suffer as a consequence. This happens when a station can be seen by a wireless access point but not by other stations that are connected to the access point.

Let’s assume there are four stations with the names A, B, C, and D, where B and C are transmitters and A and D are receivers. The stations are set up so that the two emitters B and C can hear each other but the two receivers A and D cannot hear each other over radio waves. Transmission from B to A is happening. As a result, C ceases attempting to transmit to D after mistakenly assuming that the above transmission will cause interference. However, since the communication from C to D is outside of B’s range, interference would not have happened. Known as the exposed terminal issue.

How To Prevent Exposed Terminal Problems? 

The use of RTS/CTS (Request to Send/Clear to Send) mechanisms, which can stop two or more nodes from transmitting data at the same time, is the solution to the exposed terminal issue. An RTS message is the first thing a node sends to the intended recipient when it wishes to send data. The sender knows it has a clear route to transmit data unhindered by other nodes if the intended recipient replies with a CTS message. Any station that hears the RTS is near the transmitter and stays silent long enough for the CTS to arrive. During the data transmission, any station that hears the CTS is near to the receiving station and stays silent.

In this case, station C receives RTS from station B but not CTS from station A. To station D, it is therefore open to transmit.

In this way the Exposed terminal problem can be solved by using MAC (medium access control) layer protocol, the RTS stands for Request to Send and CTS stands for Clear to Send.

In Wireless LAN there is another problem which could occur known as Hidden terminal problem, you can also check this out.