sip trunkingAlthough SIP trunking is not complex and can be deployed by organizations with very little networking or telephony experience, there are a few key factors that will determine performance and reliability.  Chief among these is the amount of Internet bandwidth available to carry voice traffic to and from the SIP provider.  So, it is wise to spend a bit of effort figuring out exactly how much bandwidth you will need to support your business.  Fortunately, it is fairly easy to do.

Step 1 – Determine the Number of Concurrent Calls You Must Support

Since each inbound or outbound call requires a certain amount of bandwidth, the first step in calculating your needs is to figure out how many calls are likely to occur at any one point in time.  For most companies, this is not equivalent to the number of employees or extensions because not everyone is on the phone all the time.  Most businesses run about one concurrent call for every three or four employees.  So, if you have a 10 person company, in most cases you can expect 3 – 4 simultaneous calls.  Of course, this changes if your company has a very call intensive business model, such as a call center or an active inside sales team, so you’ll need to factor in both the number of employees and the type of telephone work they do.

In addition, to being an important calculation for determining your bandwidth requirements, each SIP channel supports one concurrent inbound or outbound call, so this assessment will be necessary for determining the number of SIP trunking channels you will need.


Step 2 – Ask Your SIP Trunking Provider about the Codec

The Internet bandwidth necessary to support each SIP trunking call depends on something called the voice codec used by your SIP trunking provider.  You don’t have to know too much about the technicalities of the voice codec (check this out if you’re curious), but you do need to ask your SIP provider which codec they use and how much bandwidth it consumes per call.   SIP.US uses the G.711 voice codec, which consumes 85kbps of bandwidth per call.

Step 3 – Do a Little Math

Simply multiply the expected number of calls by the per call bandwidth requirement given to you by your SIP vendor (in our case 85kbps per call) and you’ll know the minimum amount of bandwidth you require.  Keep in mind that other data traffic will likely be traversing your connection during calls, so leave a margin of at least 10%.  Now all that’s left to do is determine the amount of bandwidth available.

In order to measure Internet bandwidth, a speed test should be run from a computer that utilizes the Internet connection to be tested.  There are many speed tests available online, one example is

You’ll want to use the lower of your upload or download speed (almost always upload) for the final calculation.  Speed is usually presented in Megabits per second (mbps).  To get Kilobits per second (kpbs), multiply mbps by 1000.  Now just subtract the number of kbps required from your bandwidth and you’ll know if it is sufficient.  Here’s an example:

Number of expected concurrent calls for a 10 person business: 3

SIP vendor’sper call bandwidth requirement: 85kbps

3 x 85 = 255kpbs

Upload speed = 2.55mpbs

2.55 x 1000 = 2550kbps

2550/85 = 30

This connection (which happens to be high speed cable) is more than sufficient to support 3 concurrent calls.  In fact, it could support 30.

There you have it, the secret to determining the number of calls your Internet connection can support.  We find that most companies with cable, DSL, T1 or metro Ethernet connections have plenty of bandwidth to support their needs.  Check out this post for a details on a few more considerations that will impact your SIP trunking experience.  As always, feel free to contact us if you have any questions or would like help evaluating your specific needs.


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