SIP Trunks Find a Niche

Although not the first VoIP trunking technology available to small and midsize companies, SIP trunking may be the first to have a widespread impact on phone communications for these enterprises.

November 17, 2006

6 Min Read
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When our company first considered SIP trunking as a voice connection to the PSTN, we were advised against it by various VAR representatives, many of whom asked, "Do you want to spend your time running your business or fixing your phones?"

Despite the warnings, we pursued SIP (Session Initiation Protocol) trunking in the hope it could offer benefits and cost savings to a small company like ours. Having used Vonage service to run my business out of my college apartment, I believed SIP-based VoIP service offered tremendous value, scalability and flexibility that could be useful for our expanding business.

There are indeed benefits to be had for small companies, such as fixed cost and easy scalability. But the obstacles to SIP trunking can be formidable. The SIP standard hasn't been around long enough to be universally accepted. And even if a hardware company or VSP (voice service provider) claims it supports SIP, out-of-the-box performance isn't guaranteed. Working through implementation problems can be time-consuming, so it's essential to educate yourself about the potential pitfalls.A Good Fit

SIP trunking, an interoperable SIP-based VoIP connection established between a carrier's voice equipment and a customer's IP PBX, held appeal for our company because of its price, specifically the fixed communication costs available through unlimited voice plans. SIP long-distance costs an average of 50 percent less than conventional long-distance service.

And once a SIP trunking system is set up, adding lines is as simple as asking the VSP for more simultaneous lines to be enabled. If your Internet connection has enough bandwidth, no additional hardware or software modifications are necessary.

Finally, adding a branch office to the SIP trunk is easy. All you need is an IP PBX for the new office, along with a broadband Internet connection. Both main and branch offices can use the same VSP. And you can "peer" branch offices together using SIP, making interoffice communication completely free.

Other benefits of SIP trunking include presence awareness between companies; the potential for improved voice quality; and integrated voice, video and IM between offices.The Sip Standard Dilemma

Conventional telco service delivery standards are well-established. For POTS lines, the protocol is very simple, and compliance is not difficult. Higher-capacity solutions such as T1 PRIs, though technically more complicated, have been around long enough that the communication protocols are observed universally.

Not so for SIP trunks. SIP as a protocol works great, but proper functionality is contingent on compatible implementation on both client and provider sides. To address potential problems, many SIP VSPs provide customers with preapproved equipment or a list of compatible equipment to select from.

Because we didn't fully understand the compatibility concerns, we took the reverse approach: We purchased hardware based on functional requirements, then looked for a compatible VSP. Learn from our mistake: Many service providers claimed they would support our Cisco Call Manager Express IP PBX, but upon establishing service, many of the advanced call features failed to work. Among the five SIP VSPs we tried, only one was able to properly accommodate our IP PBX.

There is one major shortcoming with Cisco's SIP trunking implementation: As of this writing, it can handle SIP transfers and forwards only through a proxy--in this case, a VSP. All transfers and forwards using Cisco's hardware require a customized setup for each customer.

Full VoIP SIP DeploymentClick to enlarge in another window

We gather that Cisco implemented call functionality this way with the assumption that two Cisco CMEs would be connected using SIP, with an administrator in control of both boxes. In other words, though Cisco supports SIP trunking by definition, it does not yet provide complete support for SIP as a connection to the PSTN. The company says it plans to support this SIP trunking functionality in CME in early 2007.

Another SIP trunking initiative under way is the SIP Forum's SIPconnect. Endorsed by companies such as Avaya, Cbeyond and Cisco Systems, SIPconnect aims to tie enterprises to VSPs with an end-to-end IP connection.

The Right Connection

One of the major caveats of any SIP trunking solution is that voice packets must arrive in time and in order. Losing only a few packets quickly garbles a conversation. This usually occurs due to overuse of available bandwidth or too many hops between the IP PBX and the VSP.One way to address this is to use a VSP that also serves as your ISP. Such providers can set up appropriate QOS metrics to ensure that important sales calls take precedence over a co-worker's download of the latest "The Office" episode. One pitfall of using the same provider for voice and Internet access is that you are restricted to the provider's voice and data service offerings, and pricing tends to be more expensive for a higher quality of service.

We opted for a VSP independent of our ISP, as we found our ISP's voice service unsatisfactory. We obtained a dedicated 768-Kbps fractional T1 line that connected us to a Level 3 Communications Tier 2 provider. After some searching, we found Bandwidth. com, which provides SIP trunking service through Level 3 Tier 1 soft switches. After our voice data leaves our ISP's network, it never has to leave Level 3's backbone before connecting to the PSTN--resulting in superb call quality, with very few QoS problems in our three months of experience.

Matt Vlasach is CEO of Pacific Swell Networks. Write to him at [email protected].

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