Example:

What does it do?

This applet uses standard telecommunications traffic algorithms to compute the number of trunks required in a trunk group for a particular usage. The user provides the amount of usage for the trunk group, and the current number of trunks in the group (optional). The applet will then calculate, for one of three different engineering methods, the number of trunks required for various target grades of service, as well as the current grade of service (if the number of trunks is provided).

Instructions:

1. Enter the usage for the trunk group, in 100 Call Seconds, or CCS's, in the field labeled "Usage in CCS:" This must be a positive integer.
2. OPTIONAL - Enter a number of trunks in the field labeled "Equipped Trunks"
3. Select an engineering method from the pop-up menu. The default is Erlang B.
4. Hit the 'Calculate' button.
5. You must hit the 'Calculate' button after each change of the parameters.

Understanding the Output:

There are actually 4 pieces of information provided, 3 if you didn't enter a number of equipped trunks.

Current GOS - If you entered a number of trunks in the "Equipped Trunks" field, you will have a value under the heading "Current GOS". This is the Grade of Service that would be achieved for a trunk group with the entered number of trunks receiving the entered amount of usage. (See Grades of Service below) In the example above, the current grade of service is P.013. Below the grade of service is the number of trunks you entered, 45 in the example.

Target Grades of Service - Whether you entered a number of trunks or not, you will be presented with the number of trunks required for three target grades of service, namely P.01, P.02, and P.05, given the entered usage. (See Grades of Service below) Under each heading is the minimum number of trunks required for that grade of service. In the example, we see that 40 trunks are required for a P.05 grade of service, 44 trunks are required for a P.02 grade of service, and 46 trunks are required for a P.01 grade of service.

Grades of Service:

A Grade of Service (GOS) is a statistical probability of receiving a busy signal for a given volume of traffic, presented to a given number of trunks. We represent a GOS as a capital P (for probability) followed by a number between 0 and 1. The number is the probability of receiving a busy signal for a call attempt on the trunk group. For example, a P.02 grade of service would indicate a sufficient number of trunks such that no more than 2 busies out of a theoretical 100 call attempts would be incurred during a given hour. A P.01 grade of service indicates only 1 call out of a theoretical 100 attempts would get a busy signal.

What am I doing wrong?

If the applet isn't functioning correctly, first check the status line at the bottom. If there is a problem in the input, it will be displayed in red letters at the bottom of the applet. Here are some things to look out for:

• Make sure the usage and number of trunks are entered as positive whole numbers. No decimal places allowed.
• The maximum allowable usage is 25000 CCS. This would keep almost 700 trunks busy for the entire hour.
• The maximum allowable number of trunks is 700.
• For the Erlang C engineering method, a minimum number of trunks is required for a meaningful probability. If you enter fewer trunks, the status line will indicate an error, and tell you how many trunks are required for the entered usage.