Specifications

Chapter 2. Design Consideration for a VoFR Network

This chapter deals with several important issues relating to the voice over frame

relay network design.

2.1 Number of Voice Trunks

The number of required trunks is based on the size of the office and the average

number of hours of calling time in the day that the trunks will be used at a

location. This will vary based on the type of business being operated.

Usually, a quick rule of thumb is to apply a subscription ratio of 1:10 (one trunk

per ten telephone lines). This is usually adequate for the average business

environment where calling between offices within the enterprise is required. For

example, a branch office of twenty people in which each individual talks nine

minutes per day to the corporate headquarters yields three hours of average daily

calling hours on the VoFR trunks.

Applications such as customer service centers that are more call intensive may

have a need for greater number of trunks due to the extensive use of the circuits.

Corporate branch offices are usually classified by their size:

• Small branch - up to 25 end users

• Medium branch - 25-150 end users

• Large branch - over 150 users

This means that a small branch will require 2-3 trunks, a medium branch will

require 3-15 trunks and a large branch will require at least 16 trunks. However,

the actual number of trunks that should be deployed should be refined by

calculating the average number of daily calling hours between the locations. Most

small and medium business offices will utilize analog trunks for connection to a

VoFR network. In situations where more than 10-16 trunks are required, it

becomes more cost effective to connect the equipment via a digital carrier such

as T1 or E1. This is a function of the total costs including PBX equipment and the

monthly charges related to direct connection to the PSTN.

In a voice network, one measurement of quality of service is the availability of dial

tone. The target for a voice over data solution should be to maintain a 95%

availability. This means that a given caller should get a dial tone on the VoFR

trunk at least 95% of the time. At percentages less than 95%, the caller is apt to

dial around the VoFR network and use the public network, which defeats the

economic reasons of deploying a voice over data solution. Some advanced PBXs

can be configured to automatically dial around the voice over data equipment if

no trunk is available when the call is placed. At the same time, calls to the PSTN

that could be placed over the VoFR network are blocked.

Table 1 shows the availability of a dial tone as a function of the number of trunks

and the total number of calling hours per day, expressed as a percentage of call

attempts that will receive a dial tone. This is the percentage of the time that a

trunk to the voice over data network will be available to a given user at a given

business location. These figures are derived from standard telecommunications

erlang and trunking formulas. The calculation assumes that the calls are equally