When considering SIP trunking solutions, your enterprise has the option to employ a separate SIP access circuit at each location (a "distributed" architecture), or you can consolidate and route all VoIP traffic, including branch site traffic, over just one or a few sites ("centralized"). Last month we looked at the advantages and disadvantage of those options. There are good reasons for each, and the option that's best for you will depend on the nature and needs of your organization. Since the majority of organizations decide to use a centralized model, let's explore that option a little closer.
With centralized SIP, all sites on your network are connected with a private network--typically MPLS-- and PSTN connectivity is provided at only one or a few locations. These centrally-connected sites are often referred to as Enterprise Service Locations (ESLs). In general, the fewer ESLs you have, the greater the savings.
Advantages
Total Cost of Ownership (TCO) savings of 30-50% are widely reported when moving to centralized SIP trunks from TDM, and can be as much as 70% when all inbound/outbound PSTN traffic and Internet traffic is consolidated over one ESL. When you have only one ESL, you eliminate all the other PBXs and session border controllers, as well as all PSTN and Internet access circuits at every site but the ESL. You also gain considerable savings through the simplified management and reduced maintenance costs achieved by eliminating those extraneous elements.
Additionally, because you can run all of your enterprise PSTN and Internet traffic over one unified access circuit instead of many, you can run it far more efficiently and eliminate all the "lost" bandwidth that comes with sizing individual circuits for each service at each site. And then there's the additional benefit of the efficiency increasing as the number of users and time zones increases. Other factors aside, consolidating everything over one ESL will absolutely produce the biggest savings and fastest ROI. But it's not that simple because, like most things, there are trade-offs.
Potential Dangers
The downside, of course, is that a single isolated ESL creates a single point of failure, which is unacceptable for most. Some smaller organizations may decide that if a vendor provides 99.9% uptime, that's only 2.6 business hours a year (10 hours a day * 260 business days per year * 0.1%), and given that most employees have cell phones, the business disruption cost from an occasional outage is less than the cost of provisioning redundant ESLs. They may provision a modest PRI or BBL (broadband loop) for mission critical traffic and 911, but not a fully redundant backup solution at another location.
But even with a strong 99.9% up-time guarantee, if you're a high-volume business that has high-dollar, mission critical traffic 24x365, 99.9% uptime means you may be down nearly 9 hours a year and still be within the vendor's guarantee. And what if it goes down for longer? For most enterprises the risk is unacceptable, so they provision a second, and sometimes a third ESL. Some go the extra distance to also retain a few PRIs with another carrier for back-up. For some, business continuity is so important they provision redundant ESLs with more than one carrier and back-up PRIs.
Redundancy Options
There are a number of ways to achieve varying degrees of reliability, and the "best" solution will be different for everyone, depending on their unique cost vs. benefits appetite. The following loosely summarizes the sliding scale of redundancy options:
The Impact of Geography
Beyond budget and reliability, geography is also an important factor to consider. If you're a large US company with large offices and customer pools on both coasts, it's generally preferable to provision ESLs on each coast. In addition to the reliability protection this affords, there are call routing benefits. For example, if your business has a single ESL in New York, inbound and outbound calls originating and terminating in Los Angeles would have to trombone to NY, then back to LA, potentially creating latency problems. But if you have a second ESL in LA, calls originating and terminating in the West could stay in the West, and East Coast calls could stay in the East. A very large enterprise would likely provision a third ESL somewhere else to provide further reliability and call routing efficiency.
As you can see, even with centralized SIP, there are quite a number of flavors and factors to consider. Your service provider should be able to help guide you through the options and determine the best centralized SIP solution for your organization based on number of locations that need service and where those locations are.
