How many lines and extensions can I connect to my PBX?

There is generally no restriction on the number of lines and extensions that you can connect. However, there are many factors that work together to determine the overall capacity of your system. You can build an expensive and powerful PBX and load it down by using a few heavy features so that it can't handle many calls - or you can build an inexpensive and weak PBX and connect it to lots of extensions that are only used for simple calls. Because of this flexibility it is not easy to make general comments about PBX capacity. In general you will want to determine your needs and build a PBX that fits both your requirements and your budget.

This note will discuss some of the issues involved in managing the capacity and performance of your PBX. While this discussion might seem more technical than most it is important to note that the subject is complicated and many details are not mentioned here.

Basically, regardless of what hardware you start with you will need to figure out how much of a load you will be placing on your equipment given the features and call volume that you will be using. The peak number of concurrent calls is your major concern. You also need to be concerned about any features that you are using like Call Recording (which includes the use of Voice Mail,) as some features can severely limit the number of calls that the server can handle.

The basic information that you will need is the level of system activity during peak usage. Your server might support the installation of SNMP agents to allow you to remotely monitor and graph the system performance. If so you will want to take advantage of this opportunity. You will also need to know the peak number of concurrent conversations. Many PBX software packages offer a dashboard page that you can view to see what's going on at any given moment. Again, though, it's easier to get the information you need if you script an SNMP agent to collect and graph it.

Finally, you need to figure out your peak call-to-extension ratio. In a normal office this might range anywhere from 1 or 2 concurrent calls for every 5 to 10 extensions. For example: if you have 50 extensions you might have an average of anywhere from 5 to 25 concurrent conversations in a typical office. Monitor your usage to determine your averages and your peaks.

Let's put together an example. Let's say you find that:

  • Your peak number of concurrent calls is 10,
  • If you have 50 extensions your call-to-extension ratio is 10:50 or 1 call for every 5 extensions
  • and your system CPU usage during peaks is 20%.

Realistically you don't want to let your system get anywhere near 100% load in any metric you can think of. We're keeping this simple and talking about CPU usage but you need to make sure you don't load any of your pipes or I/O either. So let's say that we accept 80% CPU usage as a maximum. For this example:

10 concurrent calls represents 20% CPU usage – so 80% maximum divided by 20% peak (for 10 concurrent calls) gives us a factor of 4. We can estimate that 4 x 10 = 40 concurrent calls will bring the CPU usage into the 80% range (in reality these figures are not exact for various reasons but they're helpful.) The current call-to-extension ratio suggests that we can connect 10 x 5 x 4 = 200 extensions to the system before the system will start to be overloaded. This can change over time – people might start making more or less use of the system – so it's important to revise this estimate from time to time.

Remember: these numbers are estimated limits. Also, they fluctuate with time. As you get closer to your system limits you need to take steps to ensure that you never see any reduced call quality. This is important because problems with call quality or reachability can have a negative impact on your business.

Finally, you will find that you might add a feature on the system that changes everything. This is particularly true of a customer service queue with the call recording feature enabled.

Therefore, if you see that you are reaching any limits and need to improve system performance, you can start by looking at your setup and configuration. If that doesn't help you can add any required hardware.

BNT Solutions is happy to provide monthly maintenance services for your network and for your PBX. Your agreement with BNT can include performance monitoring and management – please use the Contact Us form or call your representative.