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PRI utilization

Hi, we currently have 4 PRI's coming into our company and I need to find out if all of these are necessary. Is there a way to determine the usage or is there some kind of formula to figure this out. Also, if we are planning on adding 1 more Channelized T1/PRI port how do I know if we have enough DSP's in the router to handle it? Here is what is currently in our router:

Cisco 2821 (revision 53.51) with 247808K/14336K bytes of memory.

2 Gigabit Ethernet interfaces

96 Serial interfaces

4 Channelized T1/PRI ports

1 Virtual Private Network (VPN) Module

4 Voice FXS interfaces

DRAM configuration is 64 bits wide with parity enabled.

239K bytes of non-volatile configuration memory.

62720K bytes of ATA CompactFlash (Read/Write)

PVDM Slot 0:

64-channel (G.711) Voice/Fax PVDMII DSP SIMM PVDM daughter card

PVDM Slot 1:

64-channel (G.711) Voice/Fax PVDMII DSP SIMM PVDM daughter card

Hardware Revision : 4.0

Thanks,

Linda

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Red

Re: PRI utilization

Hi Linda,

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 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.

You can always engage your service provider to do a Traffic Study that will also include busyhour averages in your current setup. It is probably good to be over-trunked rather than under-trunked, 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

You can also use RTMT to monitor your PRI's and send you email alerts when defined thresholds have been exceeded.

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.

http://personal.telefonica.terra.es/web/vr/erlang/eng/mcerlc.htm

PS. You do have enough DSP's for the new T1/PRI

I hope this helps, there is no way to give you an exact answer here :)

Rob

2 REPLIES
Hall of Fame Super Red

Re: PRI utilization

Hi Linda,

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 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.

You can always engage your service provider to do a Traffic Study that will also include busyhour averages in your current setup. It is probably good to be over-trunked rather than under-trunked, 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

You can also use RTMT to monitor your PRI's and send you email alerts when defined thresholds have been exceeded.

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.

http://personal.telefonica.terra.es/web/vr/erlang/eng/mcerlc.htm

PS. You do have enough DSP's for the new T1/PRI

I hope this helps, there is no way to give you an exact answer here :)

Rob

New Member

Re: PRI utilization

Thanks Rob!!!

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