Introduction

If you are starting to look at changing over to SIP Trunks to cut your costs, the most important factor will be the number of SIP Trunks you need. After all, one of their main advantages is capacity on demand. This lets you pay for a smaller number of trunks and then pay a small incremental cost to burst up for those busy periods. Let’s look at how you can calculate your SIP trunk requirements.

Traditional trunks are way over provisioned

Before SIP Trunks your only option was to connect physical digital or analogue lines or trunks from a Telco or ILEC to make or receive calls into and out of your PBX. It takes at  least 6 weeks to provision these ‘circuits’ from the Telco, so there was no chance of quickly adding or deleting circuits as and when needed. To account for this, most organizations over-provisioned their trunk circuits and to handle the busiest time of the week; to prevent callers getting busy dialing into or out of the system. But that comes at a cost.

The Cost of No Busy with Traditional Trunks

When you calculate your trunk circuit use over a month, you are paying for circuits that may only be used for 5% of the month and leave your over-provisioned by 50% or more for 95% of the month. What’s more, the only time anyone really thought about this was when installing a new system. Few customers go back or at least go back frequently and review their trunk needs. The Telco isn’t going to advise you that you are paying too much and will make it as difficult as possible to get out of your contract, with many auto-renewing contracts as much as 90 days before expiry.

Existing Trunk Capacity Review

Your existing trunk quantity would often be calculated in of two ways.

1. The first is to use a rule of thumb or ratio of trunks to users, on the basis that your users will be making or receiving an average number of calls. Often this is stated as 1 in 4 or 1 in 5, meaning that for every 4 or every 5 users you would need a trunk. If you had a small system with, say, 2 users, chances are you will need 2 trunks. A 1:1 ratio. The more users, the greater the ability to push the ratio higher.

2. The second is to look at your PBX trunk usage reports and make a determination based on the data contained in the reports.

While there is a more scientific method using something called an Erlang B traffic model, I would seriously doubt anyone used that to determine your trunk needs originally. See the notes below for more on Erlang B and Traffic Theory.

Smaller PBXs

With smaller organizations you will likely have a higher ratio of trunks to users. Smaller PBXs will generally have a simpler setup, less trunk groups and while they may not have as many reports, tend to have graphical reports. The one below is a sample from a smaller PBX.

Larger PBXs

For larger organizations with larger PBXs it will be a little more complex. Some will also have graphical reports but will also provide a lot more detail. You will likely have more trunk groups, a way of bundling several trunks or circuits for a specific purpose or department. Some may be for inbound only, such as DID trunks and so on.

They will offer many different reports to show the health of the system, including things like Attendant Console use, usage by trunk and trunk group, trunks out of use for maintenance and more.

I show a two sample reports from a larger PBX. The first one is showing a report by trunk group. Indicating how many trunks in the group, whether incoming, outgoing or two-way, the busiest hour (1200 meaning noon in the 24-hour clock), total usage in CCS (see the note on CCS at the bottom of the page) and several other measurements.

SIP Trunk baseline

The first thing, in determining your SIP trunk requirements is to gauge your current trunk usage. Your PBX contains data that will help you do that with several built in reports that indicate how often your trunks are completely busy and, if so, for how long. Most PBXs will at least have an ATB or All Trunks Busy report, a Trunk Utilization report, something that will show the busiest period of the week. Some will show you more granular reports, perhaps showing by hour of the day. They may only show for one week, so it may be necessary to pull over several weeks.

Based on that information you can determine an average daily use, which will be the baseline for the number of SIP trunks you need. Then work on how much and how often you are likely to burst or require Capacity on Demand.

SIP Trunks offer Capacity on Demand

SIP Trunks are far more flexible than the fixed circuit analogue or digital trunks of the past. SIP Trunks can offer Capacity on Demand or ‘bursting’ which essentially means that while you may pay for a baseline number of SIP Trunks, at any time you can access a larger capacity for a short period of time – a burst. This increased capacity can provide the extra SIP Trunks needed to handle that spike of demand during the busy period.

Because SIP Trunk providers are much more flexible than traditional Telco’s, you can easily adjust the baseline and capacity on demand to fit your business.

Your SIP Trunk provider will offer you a choice of several Codecs. The Codec converts the voice into data bits. The data bits are grouped into ‘packet’s’ of data of different sizes, depending on the Codec. Which results in an additional data overhead for each Codec. There are complex calculations to determine this, but we have taken a selection of Codecs and provided the total loaded bandwidth for you.

A simple calculation would be to multiply the number of simultaneous calls you think will be made (or the baseline number of SIP Trunks) and multiply by the ‘loaded’ bandwidth below. For example, if you have 8 SIP Trunks and are going to use the G.729 Codec, you would need 8 x 31.2 Kbps or 697 Kbps of bandwidth.

The Mean Opinion Score (MOS) is a subjective review of voice quality when measuring different Codecs. A MOS of 4 is equivalent to traditional trunks and lines, so a MOS close to 4 is very good voice quality.

To sum up (pun intended)

Every business looks for ways to cut costs or find budget for new technology or applications. Meanwhile those same businesses are over spending on their monthly trunk costs. By moving to SIP trunks, you can expect to see 50%+ savings over your traditional trunks.

It really is worth taking a look at your current trunk usage, costs and investigating SIP Trunks.

On note on using Erlangs and Traffic Theory to calculate trunk capacity

Of course there is a far more scientific way to calculate your traffic. Traffic theory was developed in the late ’40’s and the international body that governs telecommunications, the CCITT, gave the unit of measurement used in traffic theory after the Danish mathematician that helped develop the theory, the Erlang.

It does require a deep understanding of traffic theory, decent mathematics skills and is definitely overkill in these circumstances. It was also developed for circuit based switching and SIP trunks leverage packet based switching. While it can handle complex queuing techniques, as might be used in contact centres, it would have to be modified to handle bursting or capacity on demand.

There are links below if you want to check further.

Tips

Tip 1: Check your Telco contract. If it is set to auto-renew, call the Telco and remove that clause. It is a way of locking you in.

Tip 2: Be careful of consultants who come in to review your trunk costs and take a percentage of the savings. They know most organizations oversubscribe so there are good savings in just decreasing the number of trunks, however, they will lock in their percentage for a long time and you could have saved even more by moving to SIP Trunks.

Tip 3: If your business is a heavy inbound or outbound calling organization – perhaps you have a call centre or outbound dialler, you should get your SIP provider to do a more detailed analysis as your standard trunk reports would not be sufficient to cover the complexity of these situations.

Sources:

Erlang Wikipedia: https://en.wikipedia.org/wiki/Erlang_(unit)

Telecom Traffic Theory: https://en.wikipedia.org/wiki/Teletraffic_engineering

Unity SIP Trunking: https://www.unityconnected.com/sip-trunking

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