Basic Of QOS

Without implementing QOS in network it sounds like you are living without soul. The another problem is that network is open to accept more attacks. So with the addition to security QOS is also required in the network. QOS is very effective tool for service provider as well as for enterprise network. In this I am trying to give the brief of QOS model.

Types of QOS Models:-
a) Best Effort
b) DiffServ Model
c) Integrated Service

Prior to DSCP model we were having IP Precedence Value which was in IP header (TOS). It is having 3 bits, it means no more than of 8 combinations. But there after the TOS header was replaced by DS field header. The DS field header consist of 6 bits for DSCP and 2 bits for ECN(Explicit Congestion & Notification).

Best Effort Means no QOS will be used for packets. Packets will be forwarded with the least priority by default. Thats why it is called that on internet the traffic is sent as BE or best effort. Now the question comes how service provider or enterprise can use BE class in the network. It canbe implemented by with the help of MQC. In this we can classify and mark the itraffic which we are looking for & rest of traffic could be put in the BE class.

DiffServ Model consist of Class Selector, Assured Forwarding & Expediated Forwarding. Class selector (CS) is used for the backward compatibility with IP Precedence. Why it is required because the new ip header is using DSCP fields but there are many old ip header exists in the network and they are still on IP Precedence. For this reason Class Selector is used. But now the question comes how 6 bits equal to 3 bits of IP Precendence.

DSCP Class Selector Equivalent Precedence Values
CS0(Default) (000000) 0
CS1 (001000) 1
CS2 (010000) 2
CS3 (011000) 3
CS4 (100000) 4
CS5 (101000) 5
CS6 (110000) 6
CS7 (111000) 7

It is explicity showing that 3,4 & 5 bits are changing and 0,1, & 2 bits are the same. So we can the decimal value of DSCP is given below with respect to IP Precedence

0-7 (000000) 0
8-15 (001000) 1
16-23 (010000) 2
24-31 (011000) 3
32-39 (100000) 4
40-47 (101000) 5
48-55 (110000) 6
56-63 (111000) 7

If the next your router is receiving the values between 32 – 39 and you want to mark the ip precedence value on that packet then use 4 for the same.

DiffServ model is having two more important things besides class selector. One is assured forwarding and another is Expediated Forwarding. As per assured forwarding no more than 4 classes will be used and with in a class three types of drop probabiity is introduced. Lower the number least the drop probability. In expediated forwarding which is also known as EF and having only a one vale i.e 46. The EF class is used for low latency traffic like voip etc & always provide the quaranteed bandwidth. The traffic which falls in EF class will always get priority.

Integrated Model is used for signalling to reserve the bandwidth on per flow basics. RSVP & Admission Control is used in InterServ Model.


regards
shivlu jain

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