SIP Trunking for Dummies: Is My Internet Connection Fast Enough?

Today in our SIP Trunking for Dummies series, we tackle the question of the type of circuits that are suitable for a SIP Trunking.  Although speed is certainly a consideration and will be given full attention here, there are many other elements that go into determining the suitability of a given internet connection for SIP Trunking.


Old analog and digital telephone service was very reliable, but also very expensive with tinny voice quality.   With VoIP, reliability and voice quality are determined by the type and quality of Internet service.  It can be better or worse than the service you are replacing.  If you place an extremely high value on voice quality, VoIP service can be engineered to be superior to analog phone calls. That’s right, VoIP can provide a clearer voice than the old service. This is possible due to a technology called “Wideband” or “HD Audio”.  On the other hand, if your business has a very low volume of calls and you are on a budget, an inexpensive data circuit may be acceptable. 

SIP Trunking on DSL

 The bandwidth required for one phone call is 80K. The slowest DSL connection provides 384K upload speed. Simple math would indicate that the circuit would support four simultaneous calls. However, that leaves no room for any data which is impractical. So, if your company has an older slow speed DSL circuit, VoIP will not work well. 

On the other hand, there is a new type of DSL called ADSL2+, and it provides for faster upload speeds, up to 1MB. This type of bandwidth would allow for several calls while leaving suitable bandwidth for internet access.

SIP trunking on Cable

Cable companies typically provide service that has several MB’s of upload speed, so there is plenty of bandwidth for voice and data.

SIP trunking on T1 

A T1 has 1.44MB of bandwidth in both directions, thus a T1 could handle up to 19 simultaneous VoiP calls.

Quality of Service

Equally if not more important than raw bandwidth is quality of service (QoS).  QoS is the technology used to prioritize voice other data transmission. Used correctly, this ensures quality VoIP calls, even on smaller capacity circuits. QoS can be provided by the data circuit carrier or done through a VoIP gateway device. 

Best Efforts Vs. Guaranteed Circuits

DSL and cable are both services that are provided on a “best efforts” basis by the carrier. This means that the carrier will not guarantee the speed or up time. While both DSL and cable are improving every day, putting VoIP on a best-efforts circuit in turn gives you “best efforts phone quality”. For some businesses that do not heavily rely on their phones to conduct business, DSL or cable is adequate. For critical telephone users, best-efforts probably isn’t good enough.

New Ways of Doing Things

In addition to the usual method of putting VoiP on your data circuit, there are two alternative methods of delivering quality data connections that support VoiP. The first is the simple solution of having two circuits.  One that is dedicated to voice and the other dedicated to data. This alleviates from having all critical services going through one circuit. So, much like the world before VoiP, if data fails, voice keeps working and visa versa. This solution allows for a fast connection such as a cable circuit, and a more reliable, albeit slower circuit for voice. 

The second solution also uses two circuits but adds in a device called a voice gateway. This device will allow the two circuits to do auto-failover.  So, if your primary data circuit is interrupted, data traffic moves to the primary voice circuit. And the same backup capabilities exist if the voice circuit has problems.  This is a beautiful solution as it actually makes VoiP more reliable than the old phone service it replaced.

In summary, internet connection speed, while important is only one of many factors that need to be considered when moving to your new VoIP service. It’s a bit of upfront engineering, but the result is lowered costs, improved features and better voice quality. Welcome to the 21st century of telecommunications.

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