Voice over IP (VoIP)

As internal networks began to grow, it paved the way for Voice over IP (VoIP) based PBXs to emerge. Instead of using traditional analog lines to provide connections between internal users, the VoIP handsets connected to the PBX over the IP protocol, just like a computer or any other device on the network. This allowed voice and data traffic to share a common infrastructure, which cuts down on wiring and management overhead. The disadvantage to VoIP in early implementations was that it relied on shared networks where there were few controls in place and no QoS deployed for the voice traffic. As networks and IP PBX systems matured in later years, more businesses attracted by the lower total cost of ownership and advanced feature sets offered by IP PBX systems began to migrate away from legacy PBXs.

Just as with traditional PBXs, VoIP requires some form of signaling to control the calls. An early form of signaling used for VoIP was H.323, and the Media Gateway Control Protocol (MGCP) also has gained widespread adoption.

The Session Initiation Protocol, or SIP, has also emerged as a standard that many IP PBXs use for signaling, including Lync Server. Lync 2013 and all previous versions of the product use SIP for the internal signaling and for integrations with other PBX vendors because it provides a common framework for controlling calls. Vendors can also implement extensions on top of SIP to provide additional signaling capabilities. That’s not to say that because two systems both use SIP they will be compatible. Often there are differences in the SIP message flow, protocols used, or extensions used that prevent systems from direct integration. Still, the basic concepts of SIP messaging should be understood by any Lync administrator.