OCS 2007 QoE parameter

Assess Voice-Related Network Conditions
Several factors contribute to a network’s ability to carry voice. For example, the ITU specifies that from the time speech is uttered, it should reach the listener’s ear within 150 ms for an optimal experience. At 250 ms, the delay is acceptable, but at 400 ms participants will begin to experience awkward start/stop audio collisions, especially during a conference, when speakers are more likely to interrupt one another. To prevent this and other situations, the underlying network should meet the following guidelines:

Factor	Recommended value
Latency	< 50 ms
Jitter	< 30 ms
Packet loss	< 5%
Bandwidth	See below

Measure network latency, jitter, and packet loss during normal network conditions
It is important to verify that baseline network latency, jitter, and packet loss fall below the thresholds that are shown in the previous table. You should measure these factors within each LAN location and between locations on your WAN. The actual test payload should be a UDP stream of approximately 64 byte packets sent every 20 ms. A number of tools are available to perform this test, with varying price and sophistication. Microsoft Network Monitor is one option, but free tools are available on the Internet, and several companies sell other sophisticated network analysis tools like wireshark. With any of these tools, you can measure these network factors in all supported topologies.

Check behavior of firewalls, switches, and packet shapers

Many companies place additional firewalls within the corporate network, which can create issues related to port blocking; however, just because the requisite ports are opened on these internal and perimeter firewalls doesn’t mean that the network path is voice ready. Often these additional network components introduce latency or alter the data flow of the stream. For example, the preferred transport for media flow is UDP, because it is connectionless and therefore more suitable for real-time communication. Most enterprises run their services on TCP, and network administrators may not realize that their infrastructure is giving lower priority to UDP or spreading out successive UDP packets. This type of behavior needs to be checked and corrected.

Perform a Network Audit
Several companies provide services for doing a network audit. A typical audit report will show detailed information about your entire network topology. If your enterprise doesn’t have the tools, resources or expertise to perform the analysis described above, it may be worth enlisting one of these companies to perform a network audit.

QoE and QoS
QoE is effectively a superset of QoS. It brings a broader, end user-centered perspective to ensuring voice and video quality than the narrower, traditional network-centered approach. Because the end user experience can be affected by network factors, Office Communications Server supports targeted QoS by using the differentiated services model (DiffServ). For details and additional resources about differentiated services, see “An Overview of Windows 2000 Quality of Service” at http://technet.microsoft.com/en-us/library/bb742478.aspx.

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