How many SIP Trunks do I need?

Posted by SAS on Mar 20, 2012 11:47:00 AM

When planning your SIP implementation, the most frequently asked question is “How many SIP trunks do I need?”

Whilst a valid question, it needs to be adapted to “How many SIP trunks do I need to operate efficiently?”

Previous restrictions of PSTN and ISDN infrastructure are resolved when migrating to SIP trunks. This means you can finally plan your communications strategy to the correct numbers you need, rather than to the numbers available.

There is no one size fits all solution with SIP. It doesn’t come in set sizes like ISDN2 or ISDN30. When planning for SIP, it’s important to take stock of your current telephony infrastructure, your future plans and any seasonal uplift. Let’s look at how best to work out how many SIP trunks you need.

Site Audit

The first place to start is to assess your current call volumes. It’s important to calculate not just your average call volumes at any one time, but your peak simultaneous call volume. What is the highest amount of calls you concurrently make and receive? This is your baseline figure for the amount of SIP trunks you need.

If you don’t have this figure available, it should be available via your current service provider. You may be able to login to a portal and find this out or give them a call.

ISDN Restriction

If you find your peak maxes out a round number like 30, you may be restricted by your current ISDN lines.

If customers regularly complain that they can’t get through to you, or colleagues often have to wait for a free line to become available when calling out, this is a clear sign that you need to plan for more concurrent SIP calls than currently available.

Legacy ISDN circuits range from 2 concurrent calls to 30 concurrent calls. Often, businesses are forced to stop at 30 due to the expensive requirement of having to install another ISDN30. Whilst costs are saved initially, the cost of not being able to make or receive calls above your restriction goes unreported.

Unpredicted Spiking

Once you have your figure, it is best practice to add overage for unpredicted spikes. If your call data shows your peak concurrent calls at 50, you want to be prepared for when there is a technical issue or you have a special offer that triggers unusual call volumes. Overestimating your usage will ensure your business is proactive rather than reactive when you experience your next spike.

Even if you do spike above the predictions, SIP technology can be pre-configured to burst, giving you additional capacity when required. Different SIP trunk packages are available to tailor your SIP trunks in line with your business needs.

Network Capacity

As you add SIP trunks to your business, you will need to make sure your network is fit for purpose and can handle the additional bandwidth. Best practice is to segregate dedicated bandwidth to your SIP traffic.

Working out how much bandwidth required for SIP trunks is simple. For every 10 concurrent calls you require, this equates to 1Mbps of SIP bandwidth. Each additional 10 concurrent calls requires another 1Mbps So, 100 calls means 10Mbps of dedicated bandwidth, and so on.

It is important to ensure your network has the available capacity - otherwise calls may not be functional. It is also vital to ensure your network is configured to optimise performance. A poor network can result in jitter, latency and packet loss. You can read more about preparing your network for SIP trunks in our SIP trunk installation guide.

Future Growth

With SIP, you have the option to easily configure more trunks at short notice. When taking on more staff or acquiring a new business, it’s likely you will need to add more telephony resource.

Businesses often provide a phone but neglect the phone line and connectivity requirements. This leads to delays in getting the new users up and running. With SIP, you no longer need to wait an extended lead time for new physical telephone lines to be installed. As you add users to your business, SIP trunks can be quickly configured and made available to your telephony estate.

Do bear in mind that any additions to your SIP trunks will have an impact on your bandwidth requirements. It is always best to make SIP changes in parallel with your network.

Seasonal Changes

SIP trunks can easily expand or contract. This means that businesses that scale up or down during seasonal shifts, like Christmas or term time and can swiftly and efficiently turn on additional resource for these peak periods.

During Black Friday sales season, retail businesses often add a raft of temporary staff to field additional calls and keep them on until Christmas to lesser the burden of the Christmas rush. This is the perfect example for SIP to demonstrate its scalability. Your service provider can add additional SIP trunks – in line with your revised concurrent call predictions.

Once the busy period is over, and your temporary staff are released, your SIP setup is scaled down to its original size.

As business requirements change, SIP can change with you. Unlike with legacy PSTN and ISDN lines, each time you add staff, you can scale up accordingly without the expensive investment. Every time you add and remove temporary staff, your SIP trunks can temporarily grow with you – without a minimum term contract and lengthy installation time.

Once you have worked out how many SIP trunks you need – and might need in the future – it’s time to find a SIP provider. Check out our guide to what to look for when selecting a SIP trunk provider for help selecting the best fit for your business.