September 4, 2014

SIP Trunk: Equipment Problems Continue

The enterprise is replacing T1/PRI trunks with SIP trunks even though legacy trunks have few problems. Fortunately, SIP trunk technology is relatively mature. But while the enterprise expects that the SIP installation will be as clean as a legacy trunk installation, many problems still exist with SIP deployments.

Don't assume that the SIP trunks problems are the result of poor equipment and software design and implementation. The technologists that install and configure the equipment are much more likely to be the source of SIP trunk implementation problems. 

This Technote is based on the 2014 SIP Survey conducted by The SIP School™. The SIP trunk problems with the providers were discussed in the previous Technote, "SIP Trunks; Provider Problems Continue." This Technote focuses on the PBX and Session Border Controllers (SBC) and Network Address Translator (NAT) problems.

Equipment Problems

One of the questions posed by the survey is "If you've had problems, where have the issues been?"

Problem Distribution (SIP Survey 2014)

The graphic above shows that the survey found that about 20% of the reported problems are associated with the SBC/NAT and 24% associated with the PBX. The SBC/NAT problems have changed little since the 2013 survey. However, the PBX problems have worsened--making you wonder what the equipment vendors are or are not doing to improve their track record. How can the PBX problems increase? 

SBC/NAT 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. The survey asked "If your problems were with your SBC/Edge devices, what were they?"
SBC/Edge Device Problems (SIP Survey 2014)

One way audio remains a problem with only small reduction in occurrence according to the survey. The results imply that the problem is improper configuration. The most likely culprit is the NAT improperly configured for address translation. One of the IP addresses does not match. The one way audio blockage is probably for the incoming part of the call.

Another improper configuration issue is the codec mismatch. Configuring this correctly should be easy if the implementer pays attention to the compatibility requirements. 

SBCs featuring NAT have been on the market for years. They are mature device implementations. The SBC/NAT failure/crash problem does not occur often, but this should not be happening at all. This is especially problematic since a SBC/NAT failure blocks all communications when it fails or crashes.

One of the survey reviewers Hussain Ali of Cisco commented that "It is also critical to configure [Call Admission Control] CAC based on your environment needs. An avalanche of calls, more than the box's capacity or your [Busy Hour Call Attempts] BHCA studies 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."

A surprising response to the survey is that 1 out of 5 of those surveyed did not have a SBC installed so they had no problems. One of the survey commentators, Carl Blume of Oracle, stated "The rise in "No SBC installed" is alarming. These organizations are exposing their communications to DoS attacks, fraud, malware, and lots of other threats. Perhaps our industry needs to do a better job of educating the customer about the risks of connecting their UC directly to a SIP trunk." 

SBC/NAT Checklist

The following checklist of SBC/NAT problems is ordered from the most likely to least likely problem. Use the checklist as part of the SBC/NAT acceptance tests to verify they are correctly configured.

  • One way audio
  • QoS issues due to misconfiguration
  • Codec issues
  • SIP registration failures
  • No audio
  • Call to PSTN blocked
  • Firmware update required 
  • SBC/NAT failure/crash

PBX Problems
The PBX can connect to the SIP trunk directly but most trunk implementations connect the PBX through a SBC to the SIP trunk. So problems with the PBX could be related to connecting to either the provider or the SBC. The survey identified PBX problems, asking "If the problems were found to be with your SIP/ VoIP based PBX, what were they?" 

PBX Problems (SIP Survey 2014)

By far the single biggest problem with the PBX is manual configuration errors. This accounts for 60% of the reported problems. Is it poor training, inaccurate documentation, negligence, or inexperience? It could be one or all of these issues. 

Firmware updates remain a problem. This problem dropped from 40.7% in the 2013 survey to 27.4% in the 2014 survey, a major improvement. SIP registration failures with the trunk provider also dropped from 29.6% to 21.8%. No SIP licenses accounts for 6.6% of the problems. The lack of licenses makes you wonder about the VAR and enterprise and their professionalism. 

PBX Checklist

The following checklist of PBX problems is ordered from the most likely to least likely problem. Use the checklist as part of the PBX to SIP interface acceptance tests to verify that it is correctly configured.

  • Manual configuration errors
  • Codec issues
  • PBX firmware upgrade
  • SIP trunks keep dropping
  • SIP registration failure with ITSP
  • No SIP licenses


Most of the problems described in the Technote can be attributed to ignorance and/or negligence which means more training and experience is necessary to reduce the problems. It may be outdated or incorrect documentation, or even typographical entry errors. If no one is checking the implementer, who is to know? All of these issues could be avoided with installer patience, good training, and independent configuration verification.

This TechNote is brought to you in part due to the generous support of:



I agree with the stats and commentary and have only one suggestion to make and that is to ensure that the PBX provider is reputable and also knows and has experience in working with the SIP service provider of choice. It may also be interesting to gain these statistics from those companies surveyed who did not have any problems to what was done right that alleviated the common problems highlighted in the statistics

Great idea, Shaun! Provide info from users/providers for what was done RIGHT!

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