Within the Ethernet standard,
we have a useful flow control feature called the pause frame.
Sometimes one device will send so much traffic to the other
parts of the network can become overwhelmed, causing congestion.
If nothing is done about this, some traffic will eventually be dropped
and overwhelm device can send a pause frame to the original sender.
This asks it to take a break from sending, which allows other devices to catch up.
The goal of this is to prevent frames from being lost.
This might happen if there are devices of different speeds on the network.
A high speed device can easily overwhelm a low speed device
Practically, though, this is common in switches themselves
where they might have many devices sending traffic and can't forward it
out fast enough
Pause Frames use a special
type of Ethernet frame called the Mac control frame.
In this case, Mac means media access control.
It's not the same thing as a Mac address. Anyway,
this frame is used to send network information rather than regular data
in this case.
An opcode of one which means it's a pause frame.
These can be sent to a specific destination address
or to a multicast address.
If it's multicast, all senders
who receive it will pause sending data
inside the frame is the pause duration.
But this isn't measured in seconds or milliseconds.
Instead, it's measured in a unit called a quanta.
A quanta is equal to 512 bit times.
So what's a bit time?
A bit time is the time it takes to put one bit on the network medium.
So a bit time will vary for different interface speeds
Now for the
practical question, do I need to enable pause frames?
Or does it just work in the background?
This is a feature that you can enable and disable
whether it's on or off by default will depend on each vendor and device.
You might have seen settings for flow control on your network interfaces.
This is how you control pause frames.
We can set RX, which enables receiving pause frames,
TX, which enables sending pause frames or both
This is usually set per interface
and only on full duplex interfaces.
Keep in mind that some devices may not support sending pause frames.
This comes down to each vendor and device.
There is a problem with standard pause frames
when a center is asked to pause They will pause all flows.
This is absolutely not ideal
as it may result in interruption of real time traffic like voice and video.
So to address this, pause frames were upgraded
in the 802.1Qbb standard to priority based flow control
This enables pause
frames to be sent on a per class of service queue.
This means that lower priority traffic can be paused
while leaving high priority flows alone.
But you could even have a different pause time per queue,
and that's how pause frames work.
I hope that helps you to manage your networks better.
Take a look at the rest of the videos on this channel,
as I'm sure there is something that will interest you.