Wondering what an industry standard is for estimating busy hour concurrent call maximums for a large organisation?
Is there a generally accepted standard ratio for number of users to maximum number of concurrent calls?
E.g. If you have an office with 1000 users (and you do not know existing volumes of calls from the internal PBX cause they don't want to tell you ) and you are going to migrate them to voip should you base a maximum concurrent call estimation using a ratio of 20% of users on a call at once ... should that % be lower or higher??
If someone could point me to some good documentation on the subject it would be greatly appreciated!!
To determine the number of circuits required for an Installation is a bit of an art and does require some known variables to do an accurate calculation. There must be some history or patterns or you are just guessing (which is OK as a starting point too These numbers are not standard from deployment to deployment and depend on things like if there are Call Centres, Outbound Sales Campaigns,acceptable busy signals etc. etc (like you nicely noted).
The Telco's use a rule called the "1 in 10 Trunking Rule" which says that for every 10 phones 1 will be in use at any given time. In a Campus environment like ours, if you take the Call Centres out of the equation the actual use would be more like 1 in 15 or 1 in 20. So you Trunk accordingly.
I'm guessing fro your reference to the PABX group not wanting to share these numbers with you that this is a "competitive" environment. I would ask the customer/client to engage their service provider to do a Traffic Study that will also include busy/hour averages in their current setup. It is probably good to be over-trunked rather than under-trunked, as a starting point especially with an Enterprise environment. You may want to design for Failover in some environments as well.
1 Trunk (in your case channel) per every 10 users, so
100 users = 10 channels
200 users = 20 channels
As you can see this is not an exact science for example;
For our Student Residence we use 5 PRI's for 1000 student phones.
For our Staff we use 4 PRI's for well over 2200 phones.
If you know any of the variables you can use a calculator like the one below to help determine the number of circuits required.
Nice info, Rob! Here's a Cisco doc that suggests that busy hour traffic can be estimated at 15 to 20% of daily traffic. So, if you can get an estimate of total calls for a given day, this might help you to make a good guess.
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