What is SIP Trunking: Explain It
In our quest to demystify the language of internet telephony, we often encounter two foreign terms that are joined at the hip: SIP and trunking. SIP, or Session Initiated Protocol, is frequently touted as the logical choice among those seeking the best unified communication solution for their business. But SIP trunking can be confusing to newcomers, especially since it sounds like a viral dance craze.
In a previous post, we introduced you to SIP and explained how SIP calls and video conferences are started, maintained, and ended using SIP and its game-changing signaling methods. However, business owners typically aren’t thinking about the mechanics of one call. They’re envisioning call centers where their agents are making dozens, if not hundreds, of calls. They want to know how to scale while also controlling costs. SIP trunking can help them achieve these objectives.
Trunking, Then and NowThe terms “trunk” and “trunking” have been used in the telephone world for a long time, and originally referred to the way that a home or business connected to the PSTN, or public switched telephone network. The number of trunk lines determined how many phone calls could be made with the outside world at any given time.
Companies would set up a PBX, or private branch exchange, to help manage the trunk lines since it didn’t make sense to provide each employee with their own dedicated line. For instance, a business with 50 employees might set up 5 trunk lines, knowing that it would be rare for more than 5 outside calls to be occurring simultaneously. The PBX could be programmed so that if the first line was busy, a caller would be routed to the second line and so on.
In the 1990s, telecom companies began offering combined voice and data services, through ISDN (Integrated Services Digital Network) trunks. These were digital networks that were tied to the infrastructure of the PSTN, passing calls and data along the phone lines. The rise of the ISDN brought increased speed and capacity, but there were limits since each ISDN trunk had a fixed number of lines (e.g. the popular T1 trunk held 24 lines).
For companies with a large customer service group or inside sales operations, determining the right number of trunk lines was critical. If you didn’t have enough, customers would receive busy signals or be placed in long queues when it got busy. If you had too many, you would be paying a significant cost for each of those unused lines. For example, a company using ISDN30 trunks who needed 32 channels would still have to install two trunks (giving the business 60 channels instead of the 32 they would actually use), which was costly and wasteful.
SIP TrunkingSIP trunking applies this concept to the world of VoIP, by bridging the gap between the private domain (consisting of everything tied to a company’s PBX) and the public domain (which is the part of the network linked to the PSTN). SIP trunking allows a business to connect to the PSTN using the Internet, through their SIP provider (also known as an ITSP, or Internet Telephony Service Provider). Without SIP trunks, VoIP users would only be able to call other VoIP users.
Here are two high-level examples of how calls to different users would be made by a call center agent named Steve, who works at a company using SIP:
- In call 1, Steve contacts a former supplier in Miami. His call goes through his company’s PBX, which sends it through the company’s SIP provider, which uses a SIP trunk to route it through the PSTN and connect him to his supplier’s phone.
- In call 2, Steve dials his manager, who works in a different office but on the same network. Again, Steve’s call is routed through the PBX, but, using SIP, it goes through his manager’s server and rings his phone, bypassing the PSTN entirely.
Unlike traditional or ISDN trunks, which require hardware and physical installation, SIP trunks are virtual. This means that trunks can be added or removed based on need, with the only limiting factor being available bandwidth.