Diferentiaed Services | Modern Workplace and Collaboration

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:-

Default PHB (Per hop behavior)—which is typically best-effort traffic [000000]
Expedited Forwarding (EF) PHB—dedicated to low-loss, low-latency traffic
Assured Forwarding (AF) PHB—gives assurance of delivery under prescribed conditions
Class Selector PHBs—which maintain backward compatibility with the IP Precedence field.

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.

Drop Precedence

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.

Forwarding Type	Class	Bits (012)	Bits (345)	AF Class Label	Drop Precedence	Use
Assured Forwarding	Class 1	001	010	AF11	Low Drop	Voice Payload
			100	AF12	Medium Drop
			110	AF13	High Drop
	Class 2	010	010	AF21	Low Drop	Video
			100	AF22	Medium Drop
			110	AF23	High Drop
	Class 3	011	010	AF31	Low Drop	Voice Signalling
			100	AF32	Medium Drop
			110	AF33	High Drop
	Class 4	100	010	AF41	Low Drop	General Data
			100	AF42	Medium Drop
			110	AF43	High Drop
Expedited Forwarding		101	111	EF46	N\A	Voice

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

Modern Workplace and Collaboration

~ by Paul B

Category Archives: Diferentiaed Services

DSCP

Share this:

Like this: