Every year we speak with many potential customers, attend trade shows, and review competitive information about SIP VoIP services, and one thing becomes clear: when deploying a centralized SIP solution, there is a lot of confusion about "bursting". While many carriers offer a centralized SIP product, how they account for sessions across your enterprise and the terms they use are not consistent across the industry, making it doubly difficult for you to compare their respective solutions.
Bursting Before Centralized SIP
If you've managed or purchased corporate data or voice services, you're likely familiar with how bursting works for MPLS or hosted IVR. In the case of IVR, you calculate the peak number of simultaneous inbound callers that will be in the IVR queue (say, 1,000) and then procure that many IVR ports from your carrier. If you later get an unanticipated spike in inbound calls (say, 1,200 total), your carrier might allow this "burst" in one of two ways depending on their policies and the frequency of your bursting:
* They may ignore it (especially if it's not much or frequent or it's within a pre-agreed percentage); or
* They might charge a higher rate or penalty for the 200 extra IVR ports you used. It's like the overdraft protection on your checking account...you have an incentive to try to keep your balance just high enough to cover your recurring needs, but you have the peace of mind that comes from knowing that your checks (or customers) won't get bounced if you have an unexpected spike.
While hosted IVR providers have different treatments and pricing policies for bursting, there is no disagreement or confusion about what bursting means: you commit to a set number of ports and if you unexpectedly spike above that, it's called "bursting". No confusion.
Unfortunately, in the world of centralized SIP, it's not that simple.
Busting SIP Bursting
While the number of carriers deploying SIP Trunks is exploding, not all offer a centralized SIP solution, and among those that do, there are many variations in their offerings, especially in how they handle bursting.
With centralized SIP, most carriers require you to commit to a set number of simultaneous calls ("sessions") for your entire enterprise--similar to the IVR example above. Some will also require you to commit to a fixed number of sessions for each connected site in your SIP network. So far so good, right?
Wrong. This is where it gets confusing, and here's why. Assume you procure 1,000 sessions for your enterprise based on 100 small sites each requiring 10 sessions. Unlike with hosted IVR, very few vendors (if any) allow you to automatically burst above your 1,000 enterprise sessions. If you have a special season or marketing promo coming up, you may be able to negotiate with them to temporarily raise your number of committed enterprise sessions, but it isn't automatic. If you plan for an anticipated spike and manually increase your commit level in advance to accommodate it, it isn't really a "burst" in the traditional sense and many SIP vendors don't call it that...but some do.
So, regardless of what they call it, be sure to ask prospective vendors what's required and how fast they can increase or decrease the number of committed enterprise sessions--if they will do it temporarily, and at what cost.
Either a Lender or Borrower Be!
But what about the session commitments at the small sites? This is where the "bursting" term is most often applied and where the most confusion arises.
Going back to our scenario of 100 sites, each with 10 committed sessions, what happens if you suddenly need 15 sessions at one of them? Some carriers allow you to borrow unused sessions from your other sites, as long as you stay below the committed cap for the enterprise. Some carriers call this "bursting". Some call it "borrowing". And, better still, some don't require site commitments at all, so neither word applies!
Hence, if you're exploring centralized SIP options, make sure you know exactly what each carrier means by "bursting", what they expect you to commit to at the enterprise level and at the site level, and what their policies, pricing, and procedures are for borrowing at the site level and bursting your enterprise commitment.
