With all the excitement around SIP and its ability to transform the way we communicate with one another, you may be surprised that the SIP standards body defines only 19 different telephone features. That’s right, only 19. In fact, they have a name, SIPPING-19, and they are the only telephone features that a SIP soft switch is required to support. Specifically, they are:
Music on Hold
Transfer Instant Message
Click to Dial
Message Waiting Indicator
Call Forward Unconditional
Call Forward Busy
Call Forward No Answer
3-Way Conf Party Added
3-Way Conf Party Joins
Incoming Call Screening
Outgoing Call Screening
Now, after being spoiled by the over 700 features on an Avaya H.323 telephone brings to your enterprise you might be asking yourself, “Why in the world would I ever convert to SIP?” In other words, “Why should I upgrade my telephones in order for them to act like 30-year-old technology?” Fear not because not only does an Avaya SIP telephone support the entire SIPPING-19 feature set, it also supports nearly every telephony feature currently supported by your H.323 telephone.
How is that possible? Does that mean that an Avaya SIP telephone uses something other than SIP to support the remaining 680-plus features? Absolutely not and allow me to explain why.
In a previous article I wrote about SUBSCRIBE, NOTIFY, and PUBLISH and how they can be used to build complex SIP solutions. That’s exactly how Avaya supports telephone features beyond the basic 19. The next time you configure an Avaya SIP telephone in System Manager pay attention to the section that lists the event packages that the telephone supports. Specifically, note the one entitled avaya-cm-feature-status. This event package is the secret sauce that allows an Avaya telephone to do more than every other SIP telephone on the market today.
When an Avaya telephone boots, it sends a SIP SUBSCRIBE message for every one of its configured event packages. These event packages direct the Avaya system to empower the Avaya telephone with a particular set of functions. When an Avaya Communication Manager receives a subscription for avaya-cm-feature-status, it knows that this telephone is running Avaya SIP firmware and is able to receive NOTIFY messages that will direct it to perform one of the many extended Avaya telephone features. For example, this is how an Avaya phone supports bridged call appearances, hunt groups, and loudspeaker paging all of which are not part of SIPPING-19.
If you were to put a packet analyzer tool such as Wireshark between a Communication Manager and an Avaya SIP telephone, you would see NOTIFY messages that instruct the telephone’s firmware as to how to process the specific feature in terms of screen display, lamps, and media path. This allows Avaya to remain in full compliance with the SIP specification without having to wait for the IETF to define these extended features.
You Have a Choice
So, when it comes to choosing which handset your enterprise will deploy, the question of SIP over H.323 isn’t one of what features are or are not supported. Rather, it becomes an architectural discussion of Aura, evolution servers, feature servers, and the transformational power of SIP and enterprise cloud computing.
As for me, I like that freedom.
For some deeper follow-ups, please check out these articles: