asterisk users - Sep 2003 - Traffic Modelling (was IVR only system with scalibility...)

The question was posed:

"incomming calls for 45 or so people that will call in 3 or 4 time each 
day during (approx) normal business hours"

The comment was made (taken out of context):

"The quick math says that 45 people with 4 calls is 180 calls a
day. In a 8 hour day you have 480 minutes. From 480 minutes 1 port could
handle the load if the call was under 2.5 minutes long and everyone
waited till it became available."

Unfortunately as we all know, asking callers to guess when the line is 
free and equally spacing their calls is not terribly realistic (as the 
author of the comment above goes on to imply).

So how does one analyse such a situation? Using statistical traffic 
modelling!

For more information, see http://www.erlang.com/calculator/erlb/

Plug in:
	Busy Hour Traffic: 0.937 Erlangs
	(based on 45 * 4 * 2.5 / 480)

	Acceptable Blocking Factor: 1%
	(we will accept 1 in 100 calls receiving a busy signal)

Result:
	you will need 5 incoming lines.

If you are willing to tolerate (say) 3% of calls receiving a busy 
signal, you can get by with 4 lines etc. and etc.

Hope you find the above useful in planning your Asterisk installation.

Nice goin'!! I will use this for a reference point to establish baseline
numbers of phone lines. The good news is that the equation is not linear (eg 45
people need 5 lines, 100 need 10 lines). So I can double my potential users and
ONLY need 2 more lines (qty 7). The bad news is that I don't 100% think I
will have purely random connections.

On Thu, Sep 04, 2003 at 12:48:58PM -0700, George Pajari
wrote:
> The question was posed:
> 
> "incomming calls for 45 or so people that will call in 3 or 4 time
each
> day during (approx) normal business hours"
> 
> The comment was made (taken out of context):
> 
> "The quick math says that 45 people with 4 calls is 180 calls a
> day. In a 8 hour day you have 480 minutes. From 480 minutes 1 port could
> handle the load if the call was under 2.5 minutes long and everyone
> waited till it became available."
> 
> Unfortunately as we all know, asking callers to guess when the line is 
> free and equally spacing their calls is not terribly realistic (as the 
> author of the comment above goes on to imply).
> 
> So how does one analyse such a situation? Using statistical traffic 
> modelling!
> 
> For more information, see http://www.erlang.com/calculator/erlb/
> 
> Plug in:
> 	Busy Hour Traffic: 0.937 Erlangs
> 	(based on 45 * 4 * 2.5 / 480)
> 
> 	Acceptable Blocking Factor: 1%
> 	(we will accept 1 in 100 calls receiving a busy signal)
> 
> Result:
> 	you will need 5 incoming lines.
> 
> If you are willing to tolerate (say) 3% of calls receiving a busy 
> signal, you can get by with 4 lines etc. and etc.
> 
> Hope you find the above useful in planning your Asterisk installation.
> 
> _______________________________________________
> Asterisk-Users mailing list
> Asterisk-Users@lists.digium.com
> http://lists.digium.com/mailman/listinfo/asterisk-users

For IVR or call center, you will need to the Erlang C model to calculate 
how many IVR lines/agents are required.

Using George's assumptions, 4 lines are required to service the calls 
with a service level of 98.7%, assuming each call should be answered 
within 10 seconds. Average speed of answer (ASA) is 0.8s. Actually 5 
lines would be better, but would bump up the hardware cost since most 
voice cards are available in 4s or 8s.

For comparison:
1 line - ASA=2250s SLA=6.6% (a caller may have to wait up to 2/3 an hour 
for call to be answered!)
2 line - ASA=42.2s SLA=72.1%
3 line - ASA=5.6s SLA=93.2%
5 line - ASA=0.1s SLA=99.8%

Just my $0.02.

George Pajari wrote:
> The question was posed:
>
> "incomming calls for 45 or so people that will call in 3 or 4 time 
> each day during (approx) normal business hours"
>
> The comment was made (taken out of context):
>
> "The quick math says that 45 people with 4 calls is 180 calls a
> day. In a 8 hour day you have 480 minutes. From 480 minutes 1 port could
> handle the load if the call was under 2.5 minutes long and everyone
> waited till it became available."
>
> Unfortunately as we all know, asking callers to guess when the line is 
> free and equally spacing their calls is not terribly realistic (as the 
> author of the comment above goes on to imply).
>
> So how does one analyse such a situation? Using statistical traffic 
> modelling!
>
> For more information, see http://www.erlang.com/calculator/erlb/
>
> Plug in:
>     Busy Hour Traffic: 0.937 Erlangs
>     (based on 45 * 4 * 2.5 / 480)
>
>     Acceptable Blocking Factor: 1%
>     (we will accept 1 in 100 calls receiving a busy signal)
>
> Result:
>     you will need 5 incoming lines.
>
> If you are willing to tolerate (say) 3% of calls receiving a busy 
> signal, you can get by with 4 lines etc. and etc.
>
> Hope you find the above useful in planning your Asterisk installation.
>
> _______________________________________________
> Asterisk-Users mailing list
> Asterisk-Users@lists.digium.com
> http://lists.digium.com/mailman/listinfo/asterisk-users
>