Control message

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

A control frame is an 802.11 frame that facilitates the delivery of other frames over the wireless medium.

A control message is an 802.11 frame that facilitates delivery over the wireless medium. Unlike management messages, which establish communications, or data frames, which carry payloads, control messages coordinate transmission and confirm reception. section 19.1.3

Use these control-message types to understand medium access and delivery reliability:

• RTS (request-to-send) asks for permission to transmit a frame. section 19.1.3
• CTS (clear-to-send) grants permission to transmit after an RTS exchange. section 19.1.3
• Ack (acknowgment) confirms successful frame delivery; without an Ack, the sender retransmits the frame. section 19.1.3

RTS/CTS exchange

A sender transmits RTS, receives CTS, and then sends the data frame. This exchange can reduce collisions, but adds two control frames for each data frame, so modern networks usually avoid RTS/CTS. Ack remains necessary because wireless delivery is less reliable than wired delivery. section 19.1.3

Figure 19.4 RTS, CTS, and Ack messages facilitate smooth communication over the medium. RTS asks for permission to transmit a frame, and CTS grants permission. Ack acknowledges receipt of the frame.

Follow the exchange visually: RTS requests permission, CTS grants it, and Ack confirms receipt. section 19.1.3

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes

Defines three message examples and explains their exchange, overhead, collision effect, and retransmission behavior.

Examples

Uses RTS, CTS, and Ack, including an RTS/CTS exchange followed by data transmission and acknowledgment.

What it assumes

Assumes familiarity with 802.11 frames and distinguishes control messages from management and data messages.

What you can do after

Evaluate wireless exchanges by identifying permission requests, transmission grants, delivery acknowledgments, and retransmission triggers.

How it connects

Connects 802.11 control messages with management messages, data frames, wireless medium access, collisions, and retransmission.

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Related concepts

• messagekind of · A message is a discrete unit of information or data represented, processed, or exchanged in a communication context.
• request-to-sendincludes · A request-to-send message is an 802.11 control message that requests permission to transmit a frame.
• clear-to-sendincludes · A clear-to-send message is an 802.11 control message that grants permission to transmit a frame.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.