Diff Serv Code Points – explained
Looking at the 6 bits used for DSCP:-
The first 3 bits define the PHB (per Hop Behaviour) and the remaining 3 bits define the Drop Precedence.
PHB is then segregated into the following categories:-
The most commonly used PHB is AF (Assured Forwarding). Assured Forwarding is made up of 4 Classes from 1-4 –where 1 is the highest priority. These classes are then further segmented by 3 (low, medium and high) Drop Precedence markings.
This is defined as the likelihood of packets getting dropped when congestion occurs on multiple streams within the same class. There are 3 drop precedence level:-
Low Drop – Low likelihood of packets being dropped
Medium Drop – Medium likelihood of packets being dropped
High Drop – Highest likelihood of packets being dropped
Thus looking at the class and drop precedence combination a complete understanding of DSCP is possible as indicated in the table below.
How the AF Class Label is calculated
The first 3 bits make up the first digit, the next 2 bits make up the second digit, the last digit is ignored by the AF classes.
How the EF Class Label is calculated
All 6 bits are directly converted to decimal (common sense! What a surprise) thus 101111 = 46
08 Thursday Nov 2012
Posted Diferentiaed Services, DiffServ, DSCP, QOSin