Request To Send (RTS)

Request to Send (RTS) 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. RTS frame is sent by the transmitter prior to transmission of the actual data frame.

In wireless networks, the hidden terminal problem occurs when two stations cannot hear each other but can both communicate with a common access point, leading to collisions. The RTS/CTS mechanism solves this by reserving the wireless medium before data transmission.

Working Principle of MACA implementing RTS

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 RTS frame

The RTS frame consists of five fields with a total length of 20 bytes:

• Frame Control − This is a 2-byte control field that identifies the frame type and subtype.

• Duration − It is a 2-byte field that specifies the transmission time required by the data frame and acknowledgment.

• RA (Receiver Address) − It is a 6-byte address field containing the MAC address of the receiving station.

• TA (Transmitter Address) − It is a 6-byte address field containing the MAC address of the transmitting station.

• FCS (Frame Check Sequence) − It is a 4-byte sequence for error detection. Generally cyclic redundancy code (CRC) is used.

Advantages of RTS/CTS Mechanism

• Collision avoidance − Reduces collisions caused by hidden terminal problems in wireless networks.

• Medium reservation − Reserves the wireless medium for a specific duration, preventing interference from other stations.

• Improved throughput − Especially beneficial for large data frames where collision recovery would be costly.

Conclusion

The Request to Send (RTS) mechanism is a crucial component of IEEE 802.11 wireless networks that helps avoid collisions by reserving the medium before data transmission. Combined with CTS, it effectively addresses the hidden terminal problem and improves overall network performance.