SIP Trunking

Today, SIP trunking is a technology that is much talked-about by vendors and service providers alike. However, enterprises need to understand the technology and service provider capabilities in order to contextualize it against their broader communication goals.

Subha Rama, Industry Analyst, Unified Communications & Collaboration

For a service that was forecast to change the business communications landscape by 2010, SIP trunking is still in the early growth phase. Though almost all the telephony vendors and communications providers have a SIP story to tell, there is substantial confusion among end users on how exactly the technology can be put to use, and what would be its key value propositions in the enterprise context.

SIP trunks are basically used in three broad contexts: 1) as an interconnection option to the PSTN (public switched telephone network); 2) as a SIP port on an enterprise server that offers interconnectivity between different enterprise servers such as telephony, voice mail, presence, etc., and, 3) for SIP interconnectivity between IP-PBXs, replacing traditional TDM (time-division multiplexing) lines. All the signal processing, protocol conversion, transcoding, call routing, quality of service (QoS), authorization, authentication and accounting functions, and switching control to and from gateways are performed by a softswitch or a session border controller (SBC).

In its early days, SIP trunking, though it reduced voice costs by routing more traffic off the PSTN infrastructure, had more or less similar functionalities as TDM, falling short on innovation.  However, the technology has since matured and allows service providers to exploit end-to-end IP to its full potential, by extending it to other applications such as high-definition video/voice, mobility, conferencing, and presence.

Cost Benefits

In a traditional T1 PRI scenario, a single T1 line can accommodate only up to 24 voice channels. Any increase in the number of voice channels can only be in multiples of 24, leading to higher overheads and unused capacity. SIP trunking, on the other hand, supports virtually unlimited incremental or scalable growth, often referred to as N-way (uN-limited) growth. Though the SIP trunk may enter an enterprise network through a dedicated line over a service provider’s network, there is no physical limit to the number of calls that could enter or exit the network at any given time, as SIP trunking allows a customer to oversubscribe the number of voice calls by utilizing advanced compression techniques such as G.711 and G.729 codecs. However, service providers are known to limit the number of calls that may travel over a single trunk for QoS purposes. Some of them allocate 40 percent of the available bandwidth for voice calls and others restrict the number of concurrent voice calls on a single trunk to a maximum of 41, using the call admission control (CAC) features in the IADs.

Connecting UC Islands

Though cost is an important consideration, not all service providers agree that this alone is a differentiator in the market. The value proposition of SIP trunking lies not so much in the cost benefits (especially given that carriers have made their PRI circuit pricing very competitive), but in the applications scalability offered by IP networks in general.  Large enterprises that have a unified communications (UC) roadmap are actively looking to interconnect UC islands by deploying SIP trunks and leverage its multi-site support for voice, data, video and other UC components such as presence, and IM.

SIP Trunking for Call Centers

In a large call center environment, SIP trunking facilitates next-generation communications applications such as automated outbound voice and telemarketing, event broadcast emails and inbound touchtone order fulfillment. In an IP contact center environment, SIP trunks eliminate the need for VoIP gateways and the associated costs for maintenance and IT personnel.

Who Should Opt for SIP Trunking?

The value proposition of SIP trunking differs by size and type of organization and each customer’s unique communication goals. Also, the technology is still evolving, and so are the standards for interoperability. Any SIP trunking migration decision needs to be based on a combination of the following considerations:

  1. A realistic evaluation of how good your current TDM contracts are.
  2. Are you planning to replace some or all of your existing lines? This again depends on any contractual obligations you may have as well as what exactly you want out of SIP trunking. If it is toll savings or least-cost routing, you may as well be content with a few SIP trunks to derive benefits of long distance (LD) and international direct dialing (IDD) costs.
  3. Be prepared for changes. If a gateway is used for the SIP trunk service, it may entail a few changes to your PBX or KTS environments. Also, a gateway/integrated access device (IAD)-based service emulates the PSTN Central Office trunk service, supporting a range of PSTN features. A native SIP trunking service, on the other hand, offers a broader set of IP-based features.
  4. Evaluate the number of simultaneous call paths your business may need in order to estimate bandwidth requirements.
  5. What kinds of QoS or class of service (CoS) requirements do you have? This is a key consideration for evaluating service provider options.
  6. Look for SIP interoperability certifications. Enterprises need to evaluate if their current telephony systems are interoperable with the SIP trunking services offered by the provider.

Which Provider?

Telcos offer different types of SIP trunking services to address the needs of diverse end-user environments. Many offer SIP trunking on dedicated DS1, T1, E1 circuits, while some offer SIP overlay services over fractional T1 or any other DSL/broadband line that the customer may already have. Service providers bundle an IAD to carry analog voice and support the termination of PSTN calls to SIP trunks. The IADs dynamically manage data and voice packets, and prioritizes voice to ensure QoS.  Hosted telephony providers are beginning to offer SIP trunking as an adjunct to their cloud-based services and broader line-replacement strategy. However, many of these providers do not own the network and have to team up with local exchange carriers for interconnect services.

Pure SIP trunking providers, a third category, typically own IP-MPLS network assets, and have a presence across wired and wireless services, which come handy for mobile integration and fixed-mobile convergence services. Many of these providers have a nationwide footprint and offer excellent service levels. As SIP trunking matures and more organizations embrace it to realize the benefits of enriched communications enabled by end-to-end IP-based communications environments, this group would be the most sought after, given that their entire infrastructure in built on an IP  communications foundation.