Sizing WAN Links

One objective of the design process is ensuring the network has enough capacity to handle voice traffic. In this post, we’ll do a short example to illustrate the process involved.

Let’s say you’re involved with planning a unified communications deployment for the Hokey Pokey Bottled Water Company. Their headquarters are located in Rochester, NY, and they have a bottling plant in Buffalo, NY. Presently, there are no tie lines between the Key System Unit (KSU) at Buffalo, and the Private Area Branch Exchange (PBX) at Rochester. Each site has its own PSTN service.

Under the new design, a Cisco UCM cluster will be deployed in Rochester to serve the phones at both sites. Each site will continue to have its own PSTN connectivity. However, a new dedicated long-distance trunk is being installed in Rochester, and the company would like to see if Buffalo’s long distance calls could be routed across the WAN to Rochester to take advantage of the lower rates. Furthermore, the company would like to implement Tail End Hop Off (TEHO) in the future.

To analyze this traffic, you need to break down your calls as follows:

Calls which go across the WAN link:

  • Category 1: Internal calls between Rochester and Buffalo
  • Category 2: Local calls from Rochester users to the Buffalo area
  • Category 3: Local calls from Buffalo users to the Rochester area
  • Category 4: Long distance calls from Buffalo users to the Rochester LD trunk

Calls which will continue to go through the local site PSTN:

  • Category 5: Local calls

To determine how many minutes of traffic fall under category 1, you might filter the Rochester PBX Call Detail Records (CDRs) to find all calls to and from the main number of the Buffalo office. If there are DID numbers in Buffalo, don’t forget to include these too.

For the second category, you could use the CDRs from the Rochester PBX and filter them against a list of local exchanges in Buffalo.

However, since the KSU at Buffalo doesn’t keep CDRs, you couldn’t use this approach for category 3. Instead, you’d need to read through the Buffalo phone bills to obtain this data. Hopefully, the phone bill details the calls so you can examine the called number to find out which ones are to Rochester exchanges and which are to other numbers.

While you’re enjoying some up close and cozy time with the Buffalo phone bills, you also need to tally up the long distance calls which will soon be going across to the WAN to the dedicated long distance trunk in Rochester.

In the next post, I’ll show you how to massage all this data and get closer toward determining the amount of WAN bandwidth required to support this traffic.

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