Gateway Provisioning

[mbakman]

Is there a rule of thumb or formula to find out how many PSTN lines do you need without getting inbound or outbound busy signals. The phones will replace users' existing phones in a campus environment. All lines will have DID numbers and all calls will have to go in and out of the PRI gateways excluding IP-IP calls. I'm looking at 200 lines.

[dgoodwin]

Well if you are already using a legacy PBX, then why not calculate the current ratio of internal PBX stations to outside trunk channels, and use that as a starting point for the ratio to use with your IP telephony installation. There are various ways you can found out if users are experiencing busy signals.

[mbakman]

We are a centrex customer of Bellsouth so we do not have a PBX on campus. Our current pilot deployment has about 55 phones and 48 lines (2 PRI) and I still get some OutBoundBusyAttempts on my DT-24+. Any idea why?

[rvincent]

you should try to get traffic reports from Bell South for your busy hour. If you can't get any info you could use a standard 6ccs per user with a GoS of .01 and calculate the number of Erlangs required. Then go to a Erland B table to find PSTN lines. Make sure you factor out the station to station traffic. Internal traffic of 20% is a good assumption.

Look at Cisco's ICS7750 Site and Network Design Guide, Chapter 5 for a good explaination of telephone traffic.

[beerygaz]

Have a look at http://www.erlang.com - it's a great site showing calculations for number of lines in a trunk group, etc.