Gary Audin
Gary Audin is the President of Delphi, Inc. He has more than 40 years of computer, communications and security...
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Gary Audin | June 30, 2014 |


SIP Trunk Equipment Problems Remain

SIP Trunk Equipment Problems Remain SIP trunk problems are usually NOT a technology issue. More often, they stem from mismatches, improper configuration settings, impatient installers, poor training, poor documentation, and modest experience by the installers.

SIP trunk problems are usually NOT a technology issue. More often, they stem from mismatches, improper configuration settings, impatient installers, poor training, poor documentation, and modest experience by the installers.

In most cases, the VAR is the least competent member of the implementation team, according to the 2014 SIP Survey conducted by The SIP School. Providers and equipment vendors fare much better than the VARs in the opinion of nearly 1,000 survey respondents. Problems with the provider are covered in the previous blog, "Provider Issues Still Dog SIP Trunks."

Equipment Problems
The Session Border Controller (SBC) and Network Address Translator (NAT) are the edge devices sitting at the boundary between the PBX and the provider. About 20% of the problems are associated with the SBC/NAT devices, less than for the service provider or the PBX, which account for 24% and 24.5% of the problems, respectively.

The specific nature of the equipment problems is shown in the chart below:


SBC/Edge Device Problems (SIP Survey 2014)

The fact that one way audio remains an issue, almost as bad as in the 2013 survey, implies that the real problem is improper configuration, not a technology issue. This probably has to do with the NAT being improperly configured for address translation.

Codec issues remain high, another improper configuration issue. Getting this right should be easy if the implementer pays attention to the compatibility requirements.

Although SBC failure/crash problems do not occur often, the SBC has been on the market for years so this should not be happening. This is especially important since a SBC failure blocks all communications when it occurs.

One of the survey reviewers, Hussain Ali of Cisco, commented that "It is also critical to configure CAC [Call Admission Control] based on your environment needs. An avalanche of calls, more than the box's capacity or your BHCA [Busy Hour Call Attempts] is a major concern in a SIP environment, and everyone should re-visit CAC configuration to ensure it takes into account the ever changing SIP Trunking environment."

I am surprised that firmware updates continue to be a problem. This is likely to be ascribable to the VAR and/or enterprise IT staff.

PBX Problems
The single biggest problem with the PBX is manual configuration errors. About 60% of the problems can be attributed to this one cause (see chart below).

As with the SBC, firmware updates remain a problem. Fortunately this problem dropped from 40.7% in the 2013 survey to 27.4% in the 2014 survey. SIP registration failures with the ITSP also dropped from 29.6% to 21.8%. The one problem that makes you wonder about the VAR and enterprise is that a lack of SIP licenses accounts for 6.6% of problems.


PBX Problems (SIP Survey 2014)

Equipment Verification Checklists
The two checklists below can be used to verify the successful operation of the equipment. Each list has the problems displayed in descending order of occurrence, most likely to least likely. Use each list to verify that the equipment is configured properly. Test each capability over a period of time like an hour or more with full traffic. Look for anomalies that cannot be explained except by a configuration mismatch, since this is the most likely problem.


The enterprise is replacing T1/PRI trunks with SIP trunks. Since the legacy trunks have few problems, the enterprise expects that the SIP installation will be as clean as a legacy trunk installation.

All five parties - PBX and SBC vendors, trunk provider, VAR, and enterprise - should meet and develop a good working relationship. The equipment vendors and the provider are responsible for their components, not each other's. It is up to the VAR and enterprise IT staff to get the chain of components to work together seamlessly.

Most of the aforementioned problems can be blamed on ignorance, which means more training and experience is required. There could also be outdated or incorrect documentation. It is very likely that there were typographical entry errors. All of these could be avoided with installer patience, good training and independent configuration verification.


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