We get the chance to talk to people who are considering SIP trunking for their business every day. One question that comes up all the time is, “How much internet bandwidth do we need to use SIP?” It is an important question because the amount of bandwidth can have an impact on voice quality and general reliability. The good news is that most modern internet connections are sufficient to support more than enough concurrent IP calls for most businesses. Making sure that you are in good shape takes just three steps.
Step 1 – Figure out your maximum number of concurrent calls
Each call takes up some amount of bandwidth, so you need to start by figuring out how many inbound and outbound calls you need to support. This is also important for determining the number of SIP channels you will need, so it is worth spending some time on. If your PBX has reporting functionality, you may be able to analyze the call data records to come up with this number. If not, you can make some assumptions based on the number of employees in the office and the type of work they do. Keep in mind that not everyone is on the phone all the time, so the number of calls for most businesses is not equal to the number of employees. Most companies find that they have about one concurrent call for every 3-4 employees, but this can be quite different for organizations with a call intensive business.
Step 2 – Find out what voice codec your SIP trunking provider uses
Don’t panic. This isn’t as technical as it sounds. (You can learn more about voice codecs here.) What you need to know is that there are several ways that voice signals can be compressed for digital processing. Each compression protocol, or codec, requires a different amount of bandwidth. So, you need to ask your provider which codec they use and how much bandwidth it needs per call. SIP.us uses the G.711 codec, which uses 85kbps of bandwidth per call.
Step 3 – Crunch the numbers
Now all you need to do is multiply the expected number of calls by the bandwidth needed. The result will give you the minimum bandwidth you need for your voice calls. Of course, other data traffic likely be consuming some resources at the same time, so add at least 10% more capacity.
Keep in mind that the amount of bandwidth you actually enjoy can vary from what your provider claims. You want to use the numbers that apply in the real world, so perform a speed test from a computer that is on your internet connection. There are a bunch of free ones online, including http://speedtest.net.
The speed test will report your upload and download speeds. You should use the lower of the two, usually upload, for your calculations. Most tools report speed in Megabits per second (mbps), so you’ll need to multiply that by 1000 to get Kilobits per second (kbps).
Here’s a quick example.
Concurrent calls expected: 5
Per call requirement (if you aren’t using SIP.us, ask your vendor): 85 kbps
Total bandwidth needed: 5 x 85 = 425 kbps
Upload speed: 2.35 mpbs x 1000 = 2350 kbps
Total calls that could be supported: 2350 /85 = 27.64
The connection in this example is more than sufficient to support a 10 person business with 3 concurrent calls expected at any one time.
So that’s it. You just need to gather a bit of information and do a little math to determine if your existing bandwidth is enough. Most customers find that there T1, DSL, cable or metro ethernet connection is more than adequate. Of course, if you need help or want an extra level of confidence before you go live, we’re always happy to chat.