The promise of Session Initiation Protocol (SIP) trunking has been to offer IT buyers the potential to reduce their
telecom costs and improve flexible access to the public switched telephone network (PSTN). But the new core driver for businesses considering SIP trunking service adoption is to align it with an enterprise's wide area network (WAN) strategy.
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While the technology is far from ubiquitous, Nemertes Research continues to see growth in SIP trunking service adoption. In 2011, 35% of companies were pursuing SIP trunking adoption, up from 31% in 2010, and almost double the 19% that had deployed it in 2009.
Recently, though, the central driver for SIP trunking service adoption has shifted. While the ability to reduce PSTN access charges continues to drive a great deal of interest, IT leaders interviewed by Nemertes increasingly say that the real value in SIP trunking adoption comes from aligning their PSTN access service with their WAN strategy. The reason is that this approach enables them to take advantage of the ability to centralize PSTN access in the data center, easily failover to backup or disaster recovery sites, and dynamically allocate SIP trunks to deal with seasonal or other drivers of call volume variability.
The benefits of SIP trunking adoption and WAN services
By coupling SIP trunking with WAN services, enterprise architects can leverage several capabilities, including:
Bursting/bandwidth sharing: They can take advantage of multiple connections to the WAN service provider to enable bursts over allocated bandwidth. Some providers essentially allow their customers to pool all access trunk bandwidth, so bursting from one location doesn’t add further cost if the customer can borrow that bandwidth from other locations.
Network simplification: Bundling SIP trunking into WAN access services reduces hundreds or even thousands of time-division multiplexing (TDM) circuits into a much smaller number of physical links.
SIP-based fixed-mobile convergence: SIP trunking service providers are rolling out the capability to route wireless calls from their network directly to their SIP trunking customers, without the need to traverse the PSTN. This offers the potential for significant savings in roaming, per-minute and long-distance cellular costs. In the longer term, these services have the potential to leverage technologies like IP Multimedia Subsystem to integrate on-premises or cloud-based IP telephony platforms with mobile phones for call roaming, forwarding or feature integration between mobile and on-premises services.
The challenges of adding SIP trunking services to your WAN strategy
Even with its advantages, deploying SIP trunking services as part of a WAN strategy isn’t without risk. For most service providers (and their customers), SIP trunking is still a relatively new technology. Its key challenges include:
The need for session border control: The majority of companies implementing SIP trunking use session border controllers (SBCs) at the carrier edge. Justifications include transcoding between dissimilar systems, security, policy management, load balancing between two or more SIP trunking providers, and providing a demarcation line for troubleshooting.
Training: IT executives tell us that it's difficult to find expertise in SIP implementation and management. Their concerns extend beyond hiring their own staff to finding qualified value-added resellers (VARs), consultants and even vendor engineers to help them.
Management tools: IT architects cite management as one of their primary challenges. As a result, we often see companies buying SIP trunking, along with session border control, as a managed service. More than 70% of companies now use managed services for voice, while 30% even turn over the management of session border control to third parties (often the SIP trunking service provider). This approach enables companies to avoid the need to invest in their own tools and training, but it requires buyers to carefully set service-level agreements (SLAs) and keep tabs on whether or not the provider is meeting them.
Centralization and sizing: While centralizing PSTN access at the data center through SIP trunking can reduce costs and improve resiliency, IT architects must carefully size the capacity of their connections, keeping in mind that moving a trunk from TDM to SIP isn’t a 1-to-1 swap. Without compression, SIP trunking requires more bandwidth than TDM thanks to IP's overhead and the necessary underlying data transport. Compression algorithms such as the International Telecommunications Union's ITU G.722 and G.729 can greatly reduce bandwidth requirements but may also lead to a perception of reduced voice quality. Test several different scenarios in the lab before moving forward with general deployment.
Security: Voice over IP (VoIP) architects have not worried much about the potential of outside VoIP attacks on their phone system, thanks to the fact that the PSTN provides a firebreak between their IP PBX and the outside world. With SIP trunking, that paradigm changes. While the risk is still small, it is possible for a hacker to launch an IP-based attack via the SIP trunk to threaten VoIP security and related UC applications. To address the risk, session border controllers or SIP-aware firewalls can provide protection from external threats.
Eavesdropping: Even though TDM voice isn’t encrypted, few people worry about having their phone conversations listened to over the PSTN. The circuit-based nature of the PSTN means that someone would have to identify a specific call path and connect specialized equipment that can capture the call and record it. IP makes this kind of sniffing much easier because a sniffer can easily capture large volumes of call packets. Again, the threat is minimal because the varying nature of IP data flows makes identifying and grabbing both channels of a call difficult outside the local area network (LAN). But if this is a concern, consider enabling encryption on your endpoints.
Analog: IT architects continue to tell us of the pain of implementing analog services like fax, alarm systems and other monitoring services requiring a POTS line. While some SIP trunking providers now support fax, consider reserving a few POTS lines at each location for services requiring analog connections, or leveraging inbound/outbound fax services that eliminate the need for standalone fax machines.
Emergency services: Thanks to growing legal requirements around the world to support emergency services access (e.g., E911 in the United States), SIP trunking service providers are slowly adding support for E911 call routing to their services. These offerings enable the provider to dynamically track extension location and route calls to the proper emergency services location, even if it's outside of the SIP trunking provider’s normal operating area.
In addition, E911 services from companies including Red Sky, 911 Enable and 911ETC can sit between the PBX and SIP trunking services to handle outbound 911 call routing via direct connections to their own call routing services. Over the next several years, the U.S. government's Next Generation 9-1-1 (NG9-1-1)” standards will enable devices to track their own location via a variety of data sources. But NG9-1-1 will only work if SIP trunking service providers support the forwarding of SIP header information to the appropriate emergency services location.
Even given all these considerations, we expect to continue to see growth in SIP trunking adoption. As solutions mature and those responsible for supporting them gain experience, we expect to see higher overall customer satisfaction. The key to a successful deployment is working with your SIP trunking service provider(s) to understand the benefits, how they integrate into your WAN service strategy, and how they can meet ongoing service and support requirements.
→ For more information, view SearchEnterpriseWAN.com's VoIP over WAN tutorial.
About the author: Irwin Lazar is the vice president and service director at Nemertes Research, where he develops and manages research projects, develops cost models, conducts strategic seminars and advises clients. Irwin is responsible for benchmarking the adoption and use of emerging technologies in the enterprise in areas including VoIP, unified communications, video conferencing, social computing, collaboration and advanced network services.
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