Line/extention Ratio


#1

I am looking at setting up an Asterisk PBX for a large student housing project. We will have 1 extention for each suite of 4 students, and we will adding aproximatly 20-60 suites a month untill the project of 1200 suites is completed . We will also have about 100 extentions in offices.

All outside call will be handled by a provider who is charging us a flat rate per line (1 line needed for each concurrent call). We would like to have the minimum number of lines needed, but we also want everyone to be able to get an open line when they want one.

We need to come up with a reasonable formula for the ratio of extentions to lines. We can see that a 1:1 ratio would be overkill, but a 10:1 ratio is insufficient. Can anyone with some experience give us a little guidence?


#2

This is a difficult question to answer, as there is no single answer. Are you using POTS lines, PRI, or voip lines? Just curious, makes little difference.

I think what would help is knowing how easily you can change your plan. If it’s a ITSP (voip provider) you should be able to add/delete stuff pretty quickly, if its POTS then you gotta run copper and setup channels (ugh), and PRI could go either way.

What you need will really depend on your usage pattern of it, and students are difficult to predict. It really depends on what you are charging and the type of student. Obviously the biz lines will get more use during the day and the student lines will get more use at night, so it should work out. Your students may call each other mostly (which won’t use channels cuz it goes through the * system only) or they may call home alot… I suggest pick a number within your budget (somewhere between 1:3 and 1:8 i think) and then watch your usage records. Perhaps get an account with a wholesale provider (ie nufone asterlink voxee etc), put $10 in it, and set it up as a failover, so it is dialed when all the trunks are busy. Then watch your CDR records on both the * box and the ITSP, if alot of calls go through the ITSP then you need more trunks, if none ever do and you always have 20 lines free then you are overbuying.

I wish there was a simple number to give, but it really depends on your users.


#3

[quote=“IronHelix”]This is a difficult question to answer, as there is no single answer. Are you using POTS lines, PRI, or voip lines? Just curious, makes little difference.

I think what would help is knowing how easily you can change your plan. If it’s a ITSP (voip provider) you should be able to add/delete stuff pretty quickly, if its POTS then you gotta run copper and setup channels (ugh), and PRI could go either way…

I suggest pick a number within your budget (somewhere between 1:3 and 1:8 i think) and then watch your usage records…

I wish there was a simple number to give, but it really depends on your users.[/quote]

There really isn’t a way to predict the calling patterns of students, since they all have different schedules, relationships, personalities, habits, etc. We will be using VOIP lines and have a 24-72 hr turn around to add lines. So monitoring and adding lines won’t be a big problem.

The tricky part has to do with budgeting. I do have to sell this proposal to the administration after all. Which means that if I base my budget on too low a ratio, there is a chance they won’t approve it, and if I base it on too high a ratio, they might allocate insufficient funds.


#4

so write that into your budget, or make it a two part proposal. IE, you setup one building or maybe the office as a trial phase, then go forward from there.

How much do you pay per channel per month? Might it be cheaper to go with a per-minute provider?


#5

[quote=“IronHelix”]so write that into your budget, or make it a two part proposal. IE, you setup one building or maybe the office as a trial phase, then go forward from there.

How much do you pay per channel per month? Might it be cheaper to go with a per-minute provider?[/quote]
I can’t disclose the exact figure due to an NDA but we will be paying but it’s in the $35-$50 per channel per month range .


#6

thats cool, i hear ya. At the very least, look at the CDR from your existing system (if you have such things), and figure out how many minutes/month you use, then you can get a better idea of pricing.