Today, we’d like to get back to a few basics about the relationship between the public-switched telephone network and voice over IP.

The PSTN includes a signaling system, a series of central offices and a distribution network. The PSTN employs a packet-based network called Signaling System 7 (SS7) or Common Channel Signaling System 7 (CCSS7) to determine the best call route, connect the callers and control calls. Private voice network systems like PBX and key systems work with the PSTN to create a hybrid public/private network.

Using IP to signal and transport voice brings several fundamental shifts to traditional voice communications. In the legacy PSTN environment, unused bandwidth cannot be shared; using packetized transmission (like an IP packet) for voice shares unused bandwidth and allows for greater efficiency, thereby reducing cost. IP is the packet protocol of choice for voice because the overall volume of users’ WAN traffic is dominated by IP.

In the PSTN, voice network features are delivered to a user on a static pair of copper wires to a static local central office switch or PBX. VoIP allows the traditionally switched services to be delivered to a user anywhere the user is connected.

The three most common ways to deploy private VoIP include the use of VoIP gateways, VoIP-enabled routers or an IP-PBX.

VoIP gateways represent one of the easiest ways to deploy VoIP. A gateway transforms SS7 signaling and traditional voice transmissions into IP-based signaling and transmission techniques. By installing a gateway, a business can connect to an IP or other data network and a TDM network simultaneously.

VoIP-enabling routers means adding a gateway function to a router. Routers can be upgraded to include the gateway and voice-specific features.

IP-PBX and IP-enabled PBX deployments are similar in that they start with PBX features and include a gateway function.

Steve and Larry have co-authored a technology backgrounder about basic telephony and VoIP. If you’d like to read more about the basics or see a presentation featuring Steve and Larry, please see the links below.

Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Larry Hettick is a principal analyst at Current Analysis.