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SIP trunking vs. VoIP: What's the difference?

SIP trunking and VoIP share some similarities, but they have key differences in origin and functions. How well do you understand these telephony cornerstones?

SIP trunking and voice over IP are the cornerstones of IP telephony. Together, they provide the foundation for...

new capabilities that save money and make employees more productive. Aside from creating more options for voice communication in the workplace, they also give rise to platforms like unified communications (UC). These capabilities are simply not possible using legacy telephone networks or routing calls over the public switched telephone network (PSTN). But it's important to understand the key differences between SIP trunking vs. VoIP.

What is VoIP?

VoIP, or voice over IP, is perhaps the most important acronym in the communications space, as every real-time application used in the workplace today follows in VoIP's footsteps. It was commercialized in 1995 and was a radical breakthrough that enabled real-time transmission of voice over a data network, such as the internet.

Previously, all fixed-line telephony was carried over purpose-built voice networks run by incumbent phone companies. Collectively, these networks comprised the PSTN and, while it provided rock-solid five nines reliability, the incumbents held a monopoly on phone service that became the impetus for VoIP to provide an alternative.

For enterprises, VoIP is often associated with phone systems -- and for good reason. Legacy telephony has long been defined by phone lines connected by a trunking service -- namely T1 lines -- to a premises-based PBX system. Since the 1970s, the PBX has been the standard telephone network architecture in business environments -- at least, until VoIP came along.

What is SIP and its relationship to VoIP?

Session Initiation Protocol represents a set of protocols that facilitate a session running over a data network. A SIP session could be voice-based, in which case, it's analogous to making phone calls over the PSTN.

SIP can support other media modes, such as video and instant messaging. But VoIP is only about voice, so SIP will be one of the protocols used to enable calls over a data network. Just to clarify: Both acronyms have the word protocol, but only SIP is a protocol. With VoIP, voice calls are made over the internet, so it's not a protocol itself. Rather, VoIP is a set of voice technologies that support internet protocols, which enables calls to traverse a data network.

Similarities between SIP trunking and VoIP

It's important to note that VoIP is a phone service -- like dial tone -- to which businesses subscribe, just as they would with a time-division multiplexing (TDM) service for PSTN-based telephony. With the advent of VoIP came the IP PBX, which has now become the successor for premises-based phone systems. Similarly, SIP trunking has emerged as the successor to T1 for the trunking service to support enterprise telephony.

With the relationship between VoIP and SIP explained, the similarities with SIP trunking become clearer. While SIP is one of the protocols used for VoIP, SIP trunking provides the actual connection between a telephony endpoint, such as an IP PBX or a SIP phone, and a data network over which a VoIP call will travel. As such, they are highly complementary, especially for enterprises looking to migrate from legacy phone systems.

Picture of how SIP trunks work
How SIP trunks connect calls to the PSTN and SIP-based endpoints.

Differences between SIP trunking vs. VoIP

SIP trunks differ from PBX trunks in that they carry all forms of media. Being designed for use in a data network, SIP trunks transmit packets, which could carry voice, data or video. This makes them more flexible than PBX trunks, which only support voice, as well as more cost-effective, as voice can be added to this channel, reducing the need for legacy connectivity.

Furthermore, once voice is added to a SIP trunking service, the enterprise can support all communications applications end to end, and that provides the ideal environment for UC. This moves telephony off the voice-only island of TDM and into the integrated world of UC, where it can be more effective in supporting the needs of today's workers.

While these two elements work very well together, the differences of SIP trunking vs. VoIP need to be understood. SIP trunks are the connections that provide voice service to your phone system. Legacy-based PBX trunks only carry voice. They are expensive and inflexible but very reliable.

In contrast, VoIP is one type of voice application that runs over SIP trunks and connects to external networks that support packetized voice. VoIP can also be routed over PBX trunks via a media gateway, but to take full advantage of VoIP's capabilities, businesses will deploy SIP trunks and typically route these calls via a session border controller, and then over a data network like the internet.

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