Microsoft Lync Server (previously Microsoft Office Communications Server) is an enterprise real-time communications server software, providing the infrastructure for enterprise instant messaging, presence, VoIP, ad hoc and structured conferences (audio, video and web conferencing) and PSTN connectivity through a third-party gateway or SIP trunk. These features are available within an organization, between organizations and with external users on the public internet or standard phones (on the PSTN as well as SIP trunking).
One basic use of Lync Server is instant messaging (IM) and presence within a single organization. This includes support for rich presence information, file transfer and voice and video communication. Lync uses Interactive Connectivity Establishment for NAT traversal and TLS encryption to enable secure voice and video both inside and outside the corporate network.
Lync Server also supports remote users, both corporate users on the Internet (e.g. mobile or home workers) as well as users in partner companies. Lync supports identity federation, enabling interoperability with other corporate IM networks. Federation can be configured either manually (where each partner manually configures the relevant edge servers in the other organization) or automatically (using the appropriate SRV records in the DNS).
Microsoft Lync Server uses Session Initiation Protocol (SIP) for signaling along with the SIMPLE extensions to SIP for IM and presence. Media is transferred using RTP and SRTP. The live meeting client uses Persistent Shared Object Model (PSOM) to download meeting content. The communicator client also uses HTTPS to connect with the web components server to download address books and expand distribution lists. By default, supported combinations include encrypted communications using SIP over TLS and SRTP as well as unencrypted SIP over TCP and RTP. Microsoft has published details of supported configuration for qualified vendors through Unified Communications Open Interoperability Program (UCOIP).
IM is only one portion of the Lync suite. The other major components are VoIP telephony and video conferencing through the desktop communicator client. Remote access is possible using the desktop, mobile and web clients.