SIP Trunking; where are we at now? Chasing a myth?

SIP is rapidly becoming the de facto standard protocol for IP telephony. Hang on, did I just answer my opening question? Well not quite, so let me re-phrase that question and see if I can find some insight into the fact that, although SIP is increasingly pervasive, then why is it that ISDN is still the preferred way of connecting our PBX to a carrier?

SIP is a relatively long protocol. Its initial standardization  took place in 1999 and SIP 2.0 was only published in 2.0. SIP was originally developed based on voice applications, but has always been envisioned to be extended beyond that, and indeed, apart from telephony (SIP dialing) it currently supports, video conferencing, streaming media distribution, instant messaging, presence information, file transfers and much much more.

Straight away you can see how versatile SIP is, compared to its alternatives such as ISDN, MGCP, H323, QSIG and Skinny. All these protocols, apart from H323, are traditionally used to exchange telephony capabilities and set up telephony calls. ISDN is fastly becoming old school, yeah sure it is a stable and proven technology, that offers circuit switched connections, using copper infrastructure, but it has some serious limitations. Video and ISDN? Not a hope in hell, unless channel bonding is your thing. Addressing and ISDN? Same as in the days of yester year; phone numbers.  There is no IP in ISDN. ISDN has been around since 1988 and will not get much older.  Increased fibre roll out in many countries, will only speed up this process.  In Australia, the catalyst for  killing ISDN dead, is the NBN roll out. So this pretty much rules out the future of MGCP as well (not that it ever was a real Telco grade protocol anyway).

So you would think that SIP and H323 would go head to head to take over from ISDN. Well not exactly. H323 is on its way out as well. The question is; ‘why?’. H323 has its foundation as a multimedia protocol and SIP has its origin as a VOIP communications protocol. So these protocols are to a large extent similar in their capabilities. I feel the biggest difference and benefit that SIP has over H323 is its extensibility. Sure H323 is somewhat extensible, either in standardized form or vendor specifically, but no where near as substantial as in SIP.

You could argue that there is nothing that SIP already does, H323 cannot do. Then why doesn’t H323 do so?

Globally Telephony providers are obviously slowly starting to market SIP trunking as the alternative over ISDN. A lot of critisism regarding SIP  is aimed at its inter vendor compatibility, which people claim is troublesome and complex. But really, compatibility problems between vendors are inherent to every protocol and SIP is no exception.

You could say that the protocol battle for dominance in the IP Telephony space has been won in favor of SIP, irrespective of one’s belief that it is the best protocol for the job.


Further migration of ISDN to SIP is highly dependable of further fibre network expansion

So, why, if SIP has been around about 16 years, is it that it is still not fully pervasive?  Well, in the PBX space it already is, Cisco use it, Avaya use it and so does Nortel just to name a few. It the PBX connection to the service provider (read PSTN) that is only slowly becoming true IP. The reason why the adaptation to SIP is so slow on this front, is not because SIP is not a good solution, it is because there was no real drive from the providers to make the transition to SIP.

Telephony providers have, in the past, been reluctant to really prominently put SIP forward as a service. Mainly because of fear of cannibalization of their highly profitable ISDN services. And in addition to this, businesses were lacking the technical understanding to consider SIP trunking as a viable option for ISDN replacements, so logically there was very little commercial drive for the providers to expand and invest in SIP Trunking.

Because ISDN usage penetration is so high, it would mean that replacing it with SIP and essentially converging voice and data would mean major investments by these same providers. Of which the biggest investment would be to put glass fiber infrastructure into the ground, as copper is simply not future proof, and I am not even going to argue why that is.  This change and investment in underlying infrastructure will truly converge voice and video, and will thus make SIP more prominent. In Australia the NBN will do just that. Now I am not arguing that the NBN roll out is a panacae for all that is wrong with our current copper infrastructure. What I will argue is that it will have a drastic impact on ISDN, in the way that it will end up as a wayside technology. In the same way as coaxial Ethernet and dialup modems have found their grave; because they were simply superseded. You might still not like NBN, well get used to it.  New residential areas and industrial estates around Australia are fitted out with fiber not copper. This means that even if you order ISDN in these areas, what you are getting is emulated ISDN being presented to you by some provider’s IAD device and you plug it into your ISDN capable PBX. So emulated ISDN over the same fiber strand that goes into your premise and that also carries your data. Bye Bye separate circuit, bye bye redundancy.

So where are we at now? Well SIP communications will continue to grow. How fast this growth will be, depends on the cost model that the providers will use. In other words, until it becomes appealing for organisations with existing ISDN lines to migrate to SIP trunking. Combined with the further roll out of fiber infrastructure.