A phone can indicate that its traffic is important by mocking traffic at least three now at least three
we would be using an IP version for header or IP version 6 header in this course will concentrate on
IP version 4 in an IP version 4 header.
There is a field called type of service.
It consists of eight binary values now in the old days we used to only look at the most significant
three bits.
So these three bits of the header were read by routers and networking devices to determine how important
the traffic is and the same way as cause the valleys are from zero all the way up to 7 so 0 1 to follow
up to 7.
Voice once again would be marked with a cos value of 5 so a phone as an example would markets traffic
at Layer 2 and then market at least 3 to indicate how important the traffic is.
The problem with this mechanism is we only have 7 classes close to zero would be used for best effort.
In other words you are not going to get a decent quality of service and often 6 or 7 or both are used
for network protocols so you are limited in the number of classes that you can use for traffic.
So voice maybe using 5 video maybe using 4 What are you going to use for ERP or business traffic vs.
HDP traffic versus other traffic on your network.
You're running out of classes and in addition the classes weren't standardized so any traffic could
end up in any class so this was adjusted to use 6 binary bits.
And this is known as DCP or differentiated services code points.
It's the same field in the IP version 4 header.
It's just the interpretation of the bits rather than using three bits six bits in the header are read
and marked to determine how important the traffic is this is known as IP precedence.
So when only three bits are used it's known as IP precedence when 6 bits are used.
It's known as differentiated services code points or DCP now DCP is backward compatible to IP precedents.
These values in DCP are known as class selector values or CSR values.
So you may see someone talk about this being IP precedents 5 but they may also mention this as CLOs
select a 5.
It depends whether we're using the old naming convention of IP precedents or or the newer DCP descriptions.