Clear To Send (RTS)

Clear to Send (CTS) is a control frame employed in the medium access control (MAC) layer protocol IEEE 802.11 RTS/CTS. The protocol uses the concept of Multiple Access with Collision Avoidance (MACA) in wireless networks. The RTS/CTS (Request to Send / Clear to Send) mechanism aims to reduce frame collisions introduced by the hidden terminal problem. CTS frame is sent by the receiver after it gets the RTS frame prior to receiving of the actual data frame.

Working Principle of MACA implementing CTS

The MACA protocol works with the condition that the communicating stations are synchronized and frame sizes and data speed are the same.

Let us consider that a transmitting station STA has data frame to send to a receiving station STB. The operation works as follows:

• Station STA sends a RTS frame to the receiving station.

• On receiving the RTS, station STB replies by sending a CTS frame.

• On receipt of CTS frame, station STA begins transmitting its data frame.

• After successful receipt of the data frame, station STB sends an ACK frame (acknowledgement frame).

Frame Format of CTS

The CTS frame has a simple structure with four essential fields:

• Frame Control − This is a 2-bytes control field that identifies the frame type and contains control information.

• Duration − It is a 2-bytes field that specifies the transmission time required by the data frame, helping other stations avoid interference.

• RA (Receiver Address) − It is a 6-bytes address field containing the MAC address of the original sender (Station A).

• FCS (Frame Check Sequence) − It is a 4-bytes sequence for error detection, typically using cyclic redundancy check (CRC).

Key Benefits

• Hidden Terminal Problem Mitigation − Prevents collisions when stations cannot hear each other but can both reach the access point.

• Channel Reservation − The duration field in CTS alerts nearby stations to avoid transmitting during the reserved time.

• Collision Reduction − Significantly reduces data frame collisions in dense wireless environments.

Conclusion

Clear to Send (CTS) is a crucial component of the IEEE 802.11 RTS/CTS mechanism that helps prevent collisions in wireless networks. By implementing a four-way handshake, it effectively addresses the hidden terminal problem and ensures more reliable data transmission in crowded wireless environments.