Home > Exchange and UC > helping OCS not to drop words, applying Voice QoS for OCS on WAN traffic

helping OCS not to drop words, applying Voice QoS for OCS on WAN traffic

Long time I haven’t blogged, I was so busy in some new projects that burned my time.


I will blog about a topic I wanted to blog about it long time ago, applying the correct QoS for OCS 2007 traffic over the WAN, this has been a hot topic (at least for me) as I believe that all of voice traffic requires over WAN links.


I will introduce some QoS terminology in this post, and later will tell you how to apply QoS for OCS traffic.


Quality of Service Models

There are 3 service models:

·         Best Effort · No QoS policies are implemented

·         Integrated Services (IntServ) · Resource Reservation Protocol (RSVP) is used to reserve bandwidth per flow across all nodes in a path, uses the Resource Reservation Protocol (RSVP) to reserve network resources in advance of the data actually traveling across the network. Once the end-to-end bandwidth reservation is in place, the data is transmitted.

·         Differentiated Services (DiffServ) · Packets are individually classified and marked; policy decisions are made independently at each node in a path, DiffServ doesn’t use RSVP, but instead uses hop by hop Behavior or per hop behavior (PHB) to allow each router/hop across the network to examine the packet and decide what service level it should receive.

IP QoS Markings

We currently use 2 QoS marking methods:

·         Precedence · The first three bits of the IP TOS field are evaluated; compatible with Ethernet CoS and MPLS EXP values

·         DSCP · The first six bits of the IP TOS are evaluated to provide more granular classification; backward-compatible with IP Precedence

The following table contains the Precedence Values















Streaming Video



Call Signaling






Bulk Data


The following table lists the DSCP marking values:

Likely to be dropped

AF Class

Drop Probability

DSCP Value


AF Class 1

AF11 (low)

001 01 0

AF12 (medium)

001 10 0

AF13 (high)

001 11 0


AF Class 2

AF21 (low)

010 01 0

AF22 (medium)

010 10 0

AF23 (high)

010 11 0


AF Class 3

AF31 (low)

011 01 0

AF32 (medium)

011 10 0

AF33 (high)

011 11 0

Very High

AF Class 4

AF41 (low)

100 01 0

AF42 (medium)

100 10 0

AF43 (high)

      100 11 0


How to reserve the Bandwidth:

You can use any of the following methods:

·         Policing · Creates an artificial ceiling on the amount of bandwidth that may be consumed; traffic exceeding the cap and be remarked or dropped

·         Shaping · Similar to policing but buffers excess traffic for delayed transmission; makes more efficient use of bandwidth but introduces a delay



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Categories: Exchange and UC
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