DiffServ QoS Model
Out of the three QoS models Differentiated Services (DiffServ) is the most used due to its scalability and ability to classify multiple levels of service. The only real drawbacks of DiffServ is that it doesn’t have an absolute service guarantee like IntServ and its configuration can get quite complex.
DiffServ uses a 6-bit DSCP (Differentiated Services Code Point) to mark packets so they can be treated with different levels of service throughout the network. Basically DSCP is a code used to mark the traffic using the first six bits of the ToS byte inside of a packet. Most people attribute DSCP to a box of crayons and you can mark or color the packet for classification. DSCP looks at those 6-bits in two different sections, the first three which are the priority bits and the second three bits which are the drop preference.
Priority Bits (bits 1 through 3 of the ToS byte)
- 000 = Default
- 001 = Assured Forwarding (AF1)
- 010 = Assured Forwarding (AF2)
- 011 = Assured Forwarding (AF3)
- 100 = Assured Forwarding (AF4)
- 101 = Expedited Forwarding (EF)
Expedited Forwarding is what you will be using for your voice and some of your video traffic. It ensures a minimum departure rate, guarantees bandwidth and polices the bandwidth (the class is not allowed to exceed the guaranteed amount of bandwidth).
Assured Forwarding guarantees bandwidth and allows access to extra bandwidth if available. Each AFK classes uses three DSCP values and is independently forwarded with its own guaranteed bandwidth. The first two bits of the drop preference determines the drop probability of the packet. The lower the drop probability the better.
Drop Probability (bits 4 and 5 of the ToS byte)
- 01 = Low (AF11)
- 10 = Medium (AF12)
- 11 = High (AF13)
Tags: 642-642, ccip, ccvp, diffserv, ipt, QoS, voip
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