DSCP


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

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About Paul B

My name is Paul Bloem and I am employed at Lexel Systems in New Zealand as a Principal Consultant for Unified Communications. I have been working on enterprise voice solutions for over 20 years. My first 10 years were spent working for a Telco in South Africa (Telcom SA). This is where all the groundwork happened as I was exposed to just about every aspect of telecommunication you could imagine. I develop an interest in PBX technologies and eventually became the go-to guy. Next, I had a 10 year run at Siemens South Africa, most of my time there was as a Technical Trainer. During this time VoIP hit the world stage, I had the privilege of introducing VoIP both as H.323 and later SIP across the Siemens HiPath 4000 solution stack. In 2008 I immigrated to New Zealand with my newly attained MCSE, I was ready to go where no PBX Techie had gone before. I was employed to explore OCS 2007 and that was pretty much the beginning of the end for me. I have been working on OCS and Lync ever since. My current role focuses exclusively on Lync and associated technologies.. That includes pre-sales, consulting, architecture and design, training and support. I even get to play in the development space from time to time - focus on play ;-) I was nominated as a Microsoft VTSP for Lync early in 2013 and also awarded Microsoft's MVP award for Lync in 2014.
This entry was posted in Diferentiaed Services, DiffServ, DSCP, QOS. Bookmark the permalink.

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