A phone can indicate that its traffic is important by marking traffic at layer three.
Now at layer three, we would be using an IP version four header or IP version six header.
In this course, we'll concentrate on IP version four.
In an IP version four header, there's 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.
Now, in the same way as cause, the values are from zero all the way up to seven, so 012 all the way
up to seven.
Voice once again would be marked with a cost value of five.
So a phone as an example, would mark its traffic at layer two and then market at layer three to indicate
how important the traffic is.
The problem with this mechanism is we only have seven classes.
Class zero would be used for best effort.
In other words, you're not going to get a decent quality of service and often six or seven or both
are used for network protocols.
So you are limited in the number of classes that you can use for traffic.
So voice may be using five, video may be using four.
What are you going to use for ERP or business traffic versus HTTP 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 six binary bits and this is known as DSP or differentiated services code
points.
It's the same field in the IP version for 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 six bits are used, it's known as differentiated services code points or DHCP.
Now DHCP is backward compatible to IP precedence.
These values in DHCP are known as class selector values or C values.
So you may see someone talk about this being IP precedence five, but they may also mention this as
class selector five.
It depends whether we're using the old naming convention of IP precedents or or the newer DHCP descriptions.