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IP Phone Interoperability cc @snom

No man is an island – IP Phone interoperability explored

ip phone interoperability ip phone security lesley hansen on designing an ip phoneIn the fifth article of the series SNOM UK Marketing Manager Lesley Hansen talks about IP Phone interoperability.

Ensuring that you make the most out of your investment is important and is often a consideration as you shop for a new phone system. Budget-conscious business decision makers will want to protect their investment in existing hardware or applications. Forward looking companies plan for the longer term and want to ensure today’s investment remains part of tomorrows solution,. IP phone interoperability is therefore an important issue.

Gateways can be used to help businesses connect a legacy PBX, take the first step towards SIP, or even connect to a Unified Communications(UC) solution. However using a gateway is like involving an interpreter in a conversation. The information will get across but it is slower and more likely to be subject to misunderstandings.#

Optimum performance and simplicity is achieved by selecting products that have been tested and proven to interoperate together.

There is no unique definition of ip phone interoperability because the word has different meanings depending on the context. There are also different shades of interoperability. What can be interoperable in one given system implementation may not work with another, different implementation.

The glossary of telecommunications terms, from NTIA’s ITS defines interoperability as “the ability of systems, units, or forces to provide services to and accept services from other systems, units or forces and to use the services so exchanged to enable them to operate effectively together” and as “the condition achieved among communications-electronics systems or items of communications-electronics equipment when information or services can be exchanged directly and satisfactorily between them and/or their users.”  To me the interesting words here are effectively and satisfactorily.

 The more diverse networks, products and vendors exist, the greater the need to ensure that they can interoperate to ensure that end-to-end communication is possible. At the same time, the more difficult the problem becomes.

So what happened to the standards?

Standards enable interoperability in a multi-vendor, multi-network, multi-service environment. Good standards should leave little room for options and should be universal, produced in consensus with other interested bodies. Of course, this needs time, so a proper balance between quality and speed is needed otherwise a standard that takes too long to produce becomes obsolete.

In a competitive situation not all vendors will choose to comply to all parts of the standards. Moving away from a standard in small ways is what often produces competitive differentiation. It is often non-compliance with standards that secures a long term customer unable to incorporate other vendors’ products into the network. Often, particularly  with larger vendors, a divergence from a standard will over time evolve into a new standard, further confusing business user with a wide array of incompatible standards.

End points in the telephony solution are one of the low cost high volume items in the network.  As such IP Phones are one of the aspects of the IP Telephony solution where standards and interoperability should give the business choice and flexibility without loss of functionality.  Here there is little if any justification for  the use of non-standard based products.  Even standard products however can still have interoperability issues.

Is Interoperability Important to IP Handsets?

There are two possible approaches to IP handsets.  One is to regard the handsets as disposable with a write off period of 12 or 18 months. In this case durability and interoperability are probably both moot subjects as long as the handset functions as well as needed when purchased.  The handset will effectively be written off in the first year of the project and there is no need for it to be interoperable with any other part of the network.

Alternatively there are professional and enterprise handsets where the investment in the handset is recognised as being not only the cost of the hardware but the provisioning and support and maintenance costs. In this case the build quality of the handset is likely to be considerably higher and the life of the handset considerably longer. The Snom 300 series handset for example has a life expectancy in excess of 8 years, a fact that considerably improves the ROI for any IP Telephony project.

If you make the decision to invest in a short life, low cost end points then it is possible using an Audio Lab to have the solution tested with the PBX and IP network to ensure you are not sacrificing voice quality. In a fully equipped Audio Lab you can measure the quality of your VoIP phones and VoIP accessories including wired and wireless headsets, speakerphones and conference audio-devices by utilizing state-of-the-art audio quality measurement equipment and an anechoic chamber facility.

Leading measurement technology combined with the know-how and experience of the audio quality team enables comprehensive subjective and objective testing to determine audio quality parameters to maximize VoIP device potential. The measurement system should use the IP phone specifications published in the latest ETSI and TIA releases.

Establishing IP Handset Interoperability

VoIP systems employ session control and signalling protocols to control the signalling, set-up, and tear-down of calls. They transport audio streams over IP networks using special media delivery protocols that encode voice, audio with audio codecs. Various codecs exist that optimize the media stream based on application requirements and network bandwidth.

So we must look beyond the standardised elements such as session control, signalling and codes when we look for IP Handset interoperability. This is where testing comes in and why most vendors are committed to working with partners to establish and maintain the inter-operability of their products for effective and satisfactory working. Effective and satisfactory implies the need to support the features of the device without any loss of voice quality or service or any degradation to the advertised features of the products.

For example the Microsoft Unified Communications Open Interoperability Programme tests and qualifies devices, infrastructure components, online solutions, services, and solutions provided by third party companies for interoperability with Microsoft Lync Server and clients. Their qualification programs for enterprise telephony services and infrastructure ensure that customers have seamless experiences with setup, support, and use of qualified telephony infrastructure and services with Microsoft’s unified communications software.

Testing IP Handsets

Typically only products that meet rigorous and extensive testing requirements and conform to the specifications and test plans will receive qualification in a vendors interoperability programme. While the specifications are based on industry standards, the programs also define specific requirements for interoperability with third party devices and testing requirements for qualifying interoperability. To qualify as interoperable with third party PBXs or telephony servers IP handsets must meet enterprise-class standards for audio quality, reliability, and scalability. Basic interoperability testing for IP Handset with a PBX would include items such as

  • Call Origination
  • Call Termination (calls are terminated correctly)
  • Call failure handling
  • Hold – Unhold a call
  • DTMF functionality

Additional to these basic interoperability tests the following functions are recommended for IP Handset/PBX interoperability testing:

  • VoiceMail integration
  • attended/unattended transfer
  • Music on Hold
  • Busy lamp field

As a footnote the VoIP industry periodically gets together to test ip phone interoperability. This get together was originally called the SIP bakeoff until a certain bakery products manufacturer threatened legal action. These “test fests” have long since been called SIPITs, details of which can be seen here.

Other posts in our IP phone design week:

How to design an ip phone
How to design an ip phone for voice quality
IP phone design for it departments
IP Phone Security

Check out all our VoIP posts here.

Trefor Davies

By Trefor Davies

Liver of life, father of four, CTO of, writer, poet,

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