The path between a client of Office Communications Server and the PSTN includes the Mediation Server, the IP/PSTN gateway, and the connection to the PSTN phone itself. For external users of the Office Communicator or Live Meeting client, there is also the edge server deployment in the perimeter network. If PC-to-phone calling is part of a PSTN audio bridge conference, the PSTN bridge can also part of the communication path. Any point in the path can be affected by connectivity or configuration issues that can cause voice quality to degrade. The following table describes PSTN-related voice quality issues.

Table 3.   PSTN-related voice quality issues

Issue Potential Symptoms and Causes

Uncontrolled network hops between the Mediation Server and the IP/PSTN gateway

  • Latency can increase because of the extra distances in this call leg.

  • Losses of RTP packets between the Office Communications Server 2007 Mediation Server and the IP/PSTN gateway may not be recoverable because the gateways have limited processing power and jitter buffer memory.

Incorrect codec settings on the IP/PSTN gateway

  • G.711a-law and G.711mu-law are similar enough that sound may be audible, but distortion can be extremely high.

Outdated IP/PSTN gateway firmware

  • Office Communications Server requires compliance with newer RTP/RTCP, such as support for contributing source CSRC fields in the header. Incorrect parsing by the IP/PSTN gateway can lead to audio artifacts (that is, extraneous noises, such as pops and clicks) or audio loss.

IP/PSTN gateway bugs

  • A gateway that is experiencing a significant load can cause one-way audio issues (that is, on either the transmission being sent or the transmission being received).

Incorrect IP/PSTN gateway routing

  • Errors in routing configuration can cause the audio stream to loop multiple times, which can cause loss of volume.

Other IP/PSTN configuration issues

  • Configuration errors of any IP or PSTN component can cause audio issues, including high or low volume, latency, line echo, and call setup delays.


  • Enabling IPSec between the IP/PSTN gateway and the Mediation Server can cause call setup delays or timeouts.

Dual-tone multi frequency (DTMF) detection

  • The Mediation Server relies on the IP/PSTN gateway, which controls tone detection, to translate dual-tone multifrequency (DTMF) audio tones to RTP packets with encoded DTMF information.

Defective telephone

  • A defective telephone can cause audio distortion, volume issues, buzzing noises from interference, and other audio artifacts (for example, popping and clicking).

Mobile phones

  • Voice quality issues with mobile phone networks can introduce issues that are not related to Office Communications Server.

PSTN audio conference bridges

  • Latency and echo can occur in a conference, particularly if the conference is connected to VoIP or mobile phone endpoints.

See Also