Lync Audio Codecs

Lync endpoints use various codecs within the RTP streams but select the appropriate one for each situation. This codec selection is mostly based on the type of call being placed, but endpoint type, server policies, or Call Admission Control settings can force alternative behaviors. These are the types of codecs used:

• RTAudio Wideband—The default codec used between two Lync endpoints in a peer-to-peer call. It produces a high-fidelity audio call using minimal bandwidth.

• RTAudio Narrowband—Used between a Lync endpoint and a Mediation Server when a PSTN call is placed. The PSTN user cannot hear the wideband audio fidelity so there is no need to send a higher-quality stream between the Lync endpoint and the Mediation Server. This can also be used in peer-to-peer Lync calls when bandwidth is restricted.

• G.711—Used between the Mediation Server and a media gateway. The Mediation Server transcodes a Lync endpoint’s RTAudio Narrowband stream and sends it as G.711 to the gateway. If the Media Bypass feature is enabled, a Lync endpoint can send G.711 audio directly to the media gateway, bypassing the Mediation Server transcoding operation.

• G.722—Used for conference calls when the round-trip latency to the A/V conferencing server is less than 25ms. This is a high-quality wideband audio codec.

• Siren—Used for conference calls when the round-trip latency to the A/V conferencing server exceeds 25ms. It is typically used by users joining a conference across a WAN connection or through the A/V Edge server.

The main RTAudio codecs are Microsoft proprietary technology, which means they can be used only by endpoints which license the technology. IP PBX and media gateway vendors typically do not want to incur this additional cost or complexity, so they instead support more common codecs such as G.711. This is why the Mediation Server role exists; it handles the conversion, or transcoding, process by translating an RTAudio stream to G.711 and vice versa, allowing for integration through the media gateways covered in the next section. Another unique feature of the Microsoft media stack is the ability to switch between many of these codecs on the fly.