About Ethernet Flow Control | NEXTY Electronics, an electronic components and semiconductor trading company

Flow control is an important technology for maintaining a balance between the sender and receiver in data communications and preventing data loss and errors. This article provides an easy-to-understand explanation of the most common Ethernet flow control methods, with illustrations.

Why do we need flow control?

What would happen if there was no concept of flow control?
Without flow control, the sender would continue to send data without considering the state of the receiver, resulting in data overflow if the receiver buffer is full, resulting in the loss of important information.

What is the method of flow control in Ethernet?

Ethernet is available in full-duplex and half-duplex communication modes, but this article focuses on the flow control methods used in the currently mainstream full-duplex communication mode.
In full-duplex Ethernet, a special control FRAMES called a "PAUSE FRAMES" is used for flow control.

What is a PAUSE FRAMES?

The PAUSE FRAMES is a control FRAMES used to instruct the other device to pause or resume transmission. The format and the role of each field are explained below.

Destination MAC address (6 bytes): 01-80-C2-00-00-01 (Multicast Address)
Source MAC address (6 bytes): Your own MAC address
Type (2 bytes): 8808h
Operation code (2 bytes): 0001h
Suspension time (2 bytes): 0 to 65535
*About interruption time
1 unit represents 512 bit times.
In the case of 100Mbps communication, 1 unit is equivalent to 5.12 microseconds. In the case of 1Gbps communication, 1 unit is equivalent to 512 nanoseconds.
A device that receives this FRAMES will stop transmitting for the specified duration of the pause time. If the pause time is "0", it will immediately resume transmission.

Flow control using PAUSE FRAMES

By utilizing the PAUSE FRAMES, you can flexibly control the stopping, continuation, and resumption of transmission. Below, we will explain how this works using a concrete example.

Stop sending

When the receiving side sends a PAUSE FRAMES, the sending side stops sending data for a certain period of time.
This prevents the receive buffer from overflowing and ensures stable communication.

Transmission continues to be suspended

If the receiver's buffer continues to exceed the threshold, PAUSE FRAMES are sent repeatedly, causing the sender to stop sending data continuously.
This keeps communication paused until the receiving side can catch up.

Resume sending

After the pause time specified in the PAUSE FRAMES has elapsed, the sender will resume sending data.
This allows for flexible communication control according to the buffer status on the receiving side.

Summary

This article explained the mechanism of PAUSE FRAMES as an Ethernet flow control technology. Please make use of this information when designing or troubleshooting.