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At first glance, it seems that without the PSTN infrastructure the cost justifications for VoIP are simple. While PSTN usage can be reduced considerably, it cannot be eliminated totally since the PSTN must handle call overflow and any calls not destined to the remote sites. Also, the extra voice over IP traffic may cause an increase in WAN bandwidth. The cost for this extra capacity will have to be subtracted from the savings achieved from reduced PSTN usage. The step-by-step cost justification example below can be used as a model for other network integration projects.
Obtain a busy hour and an overall traffic study from the voice service provider. Most PBXs provide similar statistics. Use the study to determine the number of hours of voice conversation or Erlangs of voice traffic the new integrated voice/data network is to handle during the busiest hour of the average day of the busiest month.
Sample Study
Company XYZ has a Frame-Relay connection in San Jose that is a T-1 physical connection with two 256K CIRs. One 256K CIR is for Houston and the other is for Washington, DC. The average data traffic at San Jose is 150Kbps with a peak of 300Kbps during the busiest hour. XYZ would like to run the voice traffic over the existing frame-relay network if possible with no upgrades of bandwidth, but would upgrade if the costs were justified. The two remote sites have a T-1 physical connection with a 256K CIR. The average data traffic is 75Kbps with a peak of 150Kbps. Company XYZ also has a PBX in San Jose and a PBX in each of its remote sites in Houston and Washington, DC. San Jose has 14 PSTN lines currently for voice traffic between the PBX and the Central Office (CO) switch of the local telephone company. All sites pay $32 per month for PSTN lines.
Call volume to and from the remote sites for San Jose is 600 calls per day with an average call time of three minutes. Grade of service or Blocking Factor is the percent of calls that are busy during the busiest hour of the organization?s day. XYZ is willing to live with 5% busy grade of service. The busiest hour of the day handles 20% of the traffic. No other hour handles more than 12% of the voice traffic. Each remote site gets about the same number of calls from San Jose. Therefore, call volume to and from headquarters and each site is 300 calls per day with an average call time of three minutes. The remote sites can also live with a 5% busy grade of service and the busiest hour of the day handles 20% of the traffic. Like HQ, no other hour handles more than 12%. The two remote sites have 10 trunk lines each for voice traffic. For this example, assume as a worst case that the busy hour of voice traffic is the same hour as it is for data.
Erlangs.
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