HP Survivable Branch Communication zl Module powered by Microsoft Lync Planning and Design Guide 2011-02

2-7
Design Considerations
Planning Call Capacity
Your records might already report peak usage times and maximum number of
concurrent calls. If you are planning to route some PSTN calls through the
main office or another site rather than through the SBM, subtract those types
of calls from the maximum. Also consider whether some calls that currently
pass through the PSTN might now become peer-to-peer calls to other users
within the WAN, and subtract those calls. You then have your number for
maximum concurrent local PSTN calls. Purchase telephony cards that provide
at least that number of channels. If possible, you might provide a 10-percent
buffer over that value to provide for unexpected bursts or future growth.
If your records do not report the peak number of concurrent calls, you might
need to calculate the value yourself.
For example, you can create a spreadsheet in which you track your calls, with
a column for each minute in your business day and a row for each call. You
can then mark the times for each call. In Figure 2-1, a call’s time is indicated
by marking the appropriate columns with an X and highlighting the cells. You
can then easily sum the number of concurrent calls at any given minute (for
example, in Microsoft Excel, using an COUNTIF(<columnrange>,“=X”) state-
ment). The highest number is your maximum number of concurrent calls. In
the example, which only shows 22 minutes, the maximum is 17 concurrent
calls.
Again, subtract any calls that you plan to route through a PSTN gateway at the
main office or another site—as well as any calls to users at those sites.
You should create such a spreadsheet for every day of the week to plan for
your true maximum. And, again, you might provide yourself with a 10-percent
buffer.