SIP and PSTN

Working together

By Stan Baldwin
Updated: February 04, 2011

VoIP communications systems can provide a wonderful array of features and functions within an organization or community. However, much of the world is still talking to each other over land-line PSTN systems. From the earliest days of internet phone calls it was obvious the VoIP and PSTN systems must inter-operate.

Call Process: Behind the Scenes

VoIP to VoIP

When a VoIP telephone user makes a call to someone the call initiates a series of processes which include identifying the intended party, establishing the connection procedure, digitizing the analogue voice signal, encoding the resulting data into a stream of packets which are then shipped out across the internet to the other party. In rough order of their application some of the protocols used to accomplish this are SIP, G7.11, UDP and RTP. At the other end of the call the process is reversed, applying the same protocols to deliver a reassembled stream of data packets to a digital-to-analogue converter. If the receiving party is also using a VoIP handset, the connection is established and the conversation continues until either party hangs up.

VoIP to Land Line

If the called (receiving) party is on a land-line provided by one of the international PSTNs, another step is required. The VoIP call must be converted into something the PSTN system recognizes before it will be forwarded to the intended party. An analogue telephone adapter (ATA) provides the conversion between IP packets and analogue voice signals and manages the call parameters. A PSTN gateway provides ATA functions and usually offers additional management services such as QoS, address translation or billing information.

SIP/VoIP to PSTN termination

“SIP/VoIP to PSTN termination” describes a phone call originated on an internet phone, using Session Initiation Protocol, which connects (terminates) at a land-line phone. The call could be from a single VoIP handset connected to the internet which uses a PSTN gateway service to convert the signals and carry the call to the land-line phone number. Or the configuration can be a LAN, with any number of VoIP phones, which employs a VoIP to PSTN service provider when any of the LAN phones calls a party on a wired network (PSTN). These providers have high-capacity connections in one or more of the national PSTN systems, such as T1 or E1 cables.

Such services organize their equipment to maximize the quality and capacity of the connections between internet and traditional phone systems. One method is to establish multiple entrance points into the PSTN. Having geographically dispersed entrance points (points of presence or POP) allows the service provider to route calls through the closest PSTN entrance or address congestion by using more than a single POP.

The integration of VoIP and PSTN services can permit an organization to “tune” their communication system to match more of their communication needs.

Featured Research
  • How VoIP is Transforming the Healthcare Industry

    The healthcare industry, like many industries, is in the midst of an era of rising costs and an ever increasing pressure to drive down expenses. Now, what if we were to tell you that there was a simple solution to these problems? The answer is VoIP. And to make it sweeter, it allows for your hospital staff to utilize modern mobile devices as resources instead of antiquated phone systems. more

  • [Infographic] 8 Common Pain Points UC Eliminates

    Every company has moments of frustration, it is when these moments become extended periods of inefficiency, or pain points, where we start to see loss in productivity and employee morale. What truly sets a successful business apart from those of its competitors, is how they take these pain points and use them as opportunities to improve upon procedures and systems to eliminate pain points and move beyond what was the status quo. more

  • Go VoIP and Go Green

    You may be looking to switch to VoIP because of the cost benefits that it will bring your company, but did you know that it is also FAR BETTER than traditional phone systems for the environment as well? With environmental impact being at the forefront of both consumer and business minds, it is essential that business decisions are made now based on economic AND ecological impact. more

  • eGuide: Comparing UC Vendors

    Changing your company’s business communications solution is an investment in time and money that will touch everyone in your organization. A successful unified communications (UC) deployment should streamline everyone’s work flow, simplify IT operations and deliver a lowered total cost of operations. Your company deserves nothing less. more

  • Getting More from Your VoIP System

    Too many businesses fall into the trap of setting up their VoIP as a "plug and play" and getting to work. However, we have found that this thinking only leads to businesses failing to get the most out of their VoIP experience. We have put together an in-depth guide that will walk you through 15 easy steps to get more out of your system. more