Lisa Pierce
LISA PIERCEFounder & President, Strategic Networks GroupLisa Pierce is president of Strategic Networks Group, a consultancy dedicated to improving the...
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Lisa Pierce | October 18, 2010 |


Ask the Analyst: SIP Trunk Service Basics

Ask the Analyst: SIP Trunk Service Basics How can you use SIP trunks to save money and increase reliability? Expert Lisa Pierce of Strategic Networks Group gets down to the nitty-gritty of SIP trunk offers and capabilities.

How can you use SIP trunks to save money and increase reliability? Expert Lisa Pierce of Strategic Networks Group gets down to the nitty-gritty of SIP trunk offers and capabilities.

In reviewing a number of the questions I have received from business customers on SIP Trunk Services, it occurred to me that a number of these could be grouped into "basic' versus "advanced" categories. As a result, I'm dedicating this article to questions and answers on SIP Trunk Service basics. If you are just beginning to learn about SIP Trunk services, this article can be especially helpful.

I’m speaking at Interop New York later this week on SIP Trunking services. I'm told that this is the first presentation Interop has ever done on this subject, so it will be interesting to see what types of questions the audience will ask.

SIP Trunk Services and the PSTN
Q. On SIP Trunks, can we choose our own long distance provider or do we have to use the SIP Trunk service provider for offnet/PSTN calls?

A. In most optimized SIP Trunk services architectures in which the customer deploys SIP and VOIP CPE, SIP Trunk services are only procured for sites that serve as offnet PSTN conduits. Thus the same provider must be used for SIP Trunk services and PSTN connectivity. These same sites often will have connectivity to SIP-based cloud services like hosted IP IVR and ACD, hosted IP audioconferencing, web and videoconferencing services, etc. For more information, please see my response about flat networks and UC in the next section (last question in this article).

Q. Can SIP Trunks avoid local call usage billing (in California, for example)?

A. I assume the question pertains to offnet calls. This depends on the carrier. Some providers’ SIP Trunk prices offer the option in which one can buy local outbound service in quantity. Typically there is a predetermined limit, which if exceeded will incur a per-call surcharge. But other providers bill for every offnet call, whether local or LD. If your company makes many local offnet calls, it's important to use the RFI and RFP process to understand providers' pricing structures.

Q. Is the day of rating a call as Intrastate vs Interstate in the SIP world going away, to essentially an all Interstate pricing model?

A. Over the long, long term in the US, yes. The FCC declared all VOIP calls as jurisdictionally interstate. Although some state PUCs are unhappy about this, the FCC's decision still stands. But these are calls that are end-end VOIP. Thus at present, if a call transits the PSTN, it is billed jurisdictionally (e.g., intra vs. inter-state). SIP Trunk offnet calls (placed to or from a PSTN connection, or calls that transit the PSTN in part) are more numerous than most IT Directors assume. Here, having facts at your fingertips is essential. A review of most companies' bills shows 75-80%+ of all calls (intra and inter-company) are currently offnet. As companies migrate to SIP Trunks, this percentage will decline. But at the very least, a company can't control other companies' migration to SIP Trunks, and so it must plan for a substantial percentage of calls to be offnet for years.

Q. What gains does SIP Trunking provide if the customer is a single location and a "typical" data customer with a T3 pipe for existing Internet access and multiple PRIs for PSTN access?

A. A single-site company obviously doesn't need to employ CPE-based SIP Trunk signaling between multiple sites, because it doesn’t have those sites (this also assumes no employee telecommutes, etc.,-an increasingly rare phenomenon.). But a single-site company can still use carrier-provided SIP Trunk Services for (1) connectivity to the PSTN for incoming/outgoing calls using compression schemes not available on most PRIs. (One bank I'm familiar with made its whole business case just on this.) (2) connectivity to carrier-based SIP Trunking services like hosted IP IVR/ACD services, IP audio, web and videoconferencing services, and as an alternative to buying, maintaining and managing an IP PBX, using hosted VOIP/UC/IP centrex services. Longer term, SIP signaling will become the de-facto signaling to support real-time communications. So even though a single site business can defer using SIP Trunks, it can’t avoid SIP Trunks forever. Ultimately, SIP signaling cannibalizes legacy protocols like in-band signaling, ISDN, etc.


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