Lisa Pierce

Lisa Pierce | February 02, 2010 |


SIP Trunk Services: The Foundation of Unified Communications

SIP Trunk Services: The Foundation of Unified Communications SIP Trunk services are essential bridges in Unified Communications architectures, but provider offers vary widely. Offerings will continue to improve, but it's critical to make sure you understand what's on offer, and what you're getting.

SIP Trunk services are essential bridges in Unified Communications architectures, but provider offers vary widely. Offerings will continue to improve, but it's critical to make sure you understand what's on offer, and what you're getting.

SIP Trunk services bridge carrier-based IP-services with IP PBXs and other IP telephony systems, thereby interconnecting "islands" of Unified Communications (UC) functionality. These services support features like single number service that tie office, mobile and home office phones together. Simply put, SIP Trunk services act as the glue to support multi-location VOIP, IM, web or IP videoconferencing applications. Compared to many legacy carrier access lines and trunks, SIP trunks are more flexible and efficient, and thus can provide part of the hard savings needed to support a UC business case.

When compared to legacy technology, SIP Trunks are relatively new, applicable standards are evolving, and provider offers vary greatly. Thus prospective customers should issue RFIs and RFPs to find the best providers across a wide range of evaluation criteria, including geographic availability; types of access and services that can interconnect with SIP Trunks; vendor interoperability; available features; resiliency and business continuity; design, ordering and provisioning; service monitoring, management and trouble remediation; SLAs; portal functionality; and prices. Since SIP Trunk services play a vital role in the UC infrastructure, I anticipate providers will make significant improvements in their offers, particularly in reliability and functionality, over the next 2-3 years.

A variety of SIP Trunk service providers exist. For this report I interviewed ten facilities-based providers that U.S. businesses most frequently use for legacy PSTN services today. They include AT&T, Global Crossing, Level 3, Orange Business Services, Qwest, Sprint, Time Warner Telecom, Verizon, XO Communications. Several other providers were invited to participate in this research, but did not provide information in time for inclusion in this report.

Salient SIP Trunk Service Attributes
As readers of this section will come to appreciate, a significant degree of variation exists between providers' SIP Trunk offers and related features and services. Based upon the capabilities in current offers and providers' disclosures of planned enhancements, the current degree of offer diversity will continue and even accelerate, at minimum through 2012. Important service attributes include:

* Geographic service availability. With some exception, virtually all SIP Trunk services offered by these providers conform with their DS-1 access footprints--but some limit local SIP Trunk services to the markets in which they offer local services today. Of the ten service providers who participated in this study and now offer SIP Trunks and related services in the U.S., 40% currently offer or plan to make their SIP Trunk offers and related services available outside the United States. For these providers, the number of countries with SIP Trunk service varies markedly--from a low of 10 to a high of 130.

As enterprises with international connectivity requirements know, the availability of intra-country connectivity for existing voice and data services varies markedly by provider; thus--particularly in the current economic climate in which everyone, including providers, faces limitations in capital resources--it's reasonable to anticipate that SIP Trunk availability will remain within a provider's current service footprint, but not exceed it. But as importantly, it is very prudent to verify that a provider's intra-country SIP Trunk availability meets your company's geographic availability requirements.

* Types of access. All providers provision SIP Trunks over DS-1 interfaces, and they all also support SIP Trunk over Ethernet access (which typically requires at least a DS-1 layer 1 interface). These are the only common threads. Provider-specific exceptions include:

--A distinct minority of providers support SIP Trunks over Fractional T1, broadband or fixed wireless access.

--Eighty percent of providers have minimum requirements for SIP Trunks over DS-1 (for instance, at minimum, 384 kbps).

--Ninety percent also have firm maximum thresholds (eg: no more than 40% of provisioned bandwidth, 41 concurrent calls per trunk, etc.); to conform, enterprises whose site-specific real-time connectivity requirements exceed these limits should expect they will need to acquire additional bandwidth (eg: NX DS-1, DS-3, OC-X), because 80% of suppliers' caps are enforced (eg: provider blocks the next call attempt that exceeds its concurrent call or bandwidth limit).

* Service connectivity: All ten service providers now support or plan to support the interconnection of SIP Trunks with MPLS (to support on-net calls), and the same is true of Ethernet services (particularly, VPLS). Seventy percent provide connectivity between their SIP Trunk and dedicated Internet services. As for legacy voice services:

--All support outbound offnet PSTN connectivity through provisioned gateway services, and all support the termination of calls from the PSTN to SIP Trunks (again, through a provisioning arrangement).

--In all cases, offnet PSTN origination and termination extends to both domestic long distance and local calls.

--Local service SIP Trunk availability mirrors the provider’s local access footprint. Providers usually do not extend SIP Trunks to areas where they are not local access providers (eg, FX lines).

--Twenty percent of providers do not terminate PSTN-based toll free services onto SIP Trunks, and thirty percent do not currently offer IP Toll Free services.

* Vendor Interoperability: No enterprise-class service provider relies exclusively on SIP Forum compatibility tests--too many features are still unspecified. To fill this gap, most, but not all providers have developed their own SIP Trunk specifications. However, to control costs, providers will only proactively perform compatibility tests on recent releases of software that support popular brands of SIP-compatible IP PBXs, phones, etc. This of course limits the potential universe of SIP interoperability--today, only 20% of the providers in this study have demonstrated that they have certified more than a handful of vendor products (AT&T is one such provider).

To help alleviate this bottleneck, service providers often make remote testing capabilities available to customers who want to test interoperability on older software releases or with less popular manufacturers. But in these cases, the customer and vendor must directly work with each other on CPE-service interoperability issues--typically with very little, if any, service provider involvement.

Like SIP functionality itself, as providers introduce additional functionality to their SIP Trunk platforms, their proprietary certification tests also evolve. Thus, it’s entirely possible for a vendor’s particular product and release to have been certified for compatibility for all the capabilities and features on a provider’s 2008 platform, but must re-certify as new provider SIP Trunk functions are introduced, or as a vendor issues its newest software release.


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