Windows by default uses DSCP markings of 40, but that value can be changed in the local system policy on the computers. Take a look at these article for more details: I've run into the same thing when needing to set 46 (0x2E) for traffic traveling over Cisco switches.
Take a look at these articles for more details:
Enabling DSCP Marking in OCS
http://technet.microsoft.com/en-us/library/dd441192(office.13).asp78
http://communicationsserverteam.com/archive/2009/06/04/458.aspx
DSCP & ToS Table
http://www.tucny.com/dscp%26tos
Cisco - Implementing QoS Policies with DSCP
http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml
The other major point is that OCS will only park packets to-from other OCS endpoints, meaning that only traffic between workstations, Front-End servers, and the Mediation server will be marked. Packets leaving the Mediation server externally for a media gateway or over a Direct SIP trunk will NOT be marked at all. In that case you'll need to use the Policy-based QoS feature of Windows Server, applied to the MediationServerSvc.exe process) to make sure outbound packets are stamped. This is by behavior of OCS and I've verified this with the product group.
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS
