Determining Trunk Circuit Requirements

There are several trunk traffic engineering steps to calculate the required number of trunk circuits needed to satisfy I/O traffic loads at an acceptable GoS level:

1. Collect and analyze existing trunk traffic data

2. Categorize trunk traffic by groups

3. Determine the number of trunk circuits required to meet traffic loads

   Determine the proper mix of trunk circuit types

Trunk traffic data can be obtained from trunk traffic reports based on CDR data collected by the PBX system. The CDR data is input into a call accounting software program, available from the system manufacturer or third-party software vendor, that generates a variety of billing, internal switch network traffic, and trunk traffic reports. The CDR data does not provide information on calls that were blocked because all trunks were busy. This information is usually available from facility management reports based on optional PBX software programs. Blocked call data is used for determining GoS levels.

Historical trunk traffic data is used to forecast future trunk traffic loads to determine incremental trunk circuit requirements for the following scenarios:

1. Station user growth or contraction

2. Anticipated changing traffic patterns

3. New applications, e.g., centralized VMS

Trunk traffic should be segmented across different types of trunk groups because it is more cost effective to traffic engineer smaller groups of trunk circuits with a common purpose. The first step is to segment trunk traffic into inbound and outbound directions. There are a variety of trunk group types for each traffic flow direction. For example, inbound traffic may be segmented across local telephone carrier CO and DID trunk circuits, dedicated “800” trunk circuits, FX circuits, ISDN PRI trunk circuits, and so on. Outbound trunk circuits are easily segmented into local telephone carrier CO trunk circuits, multiple interexchange carrier trunk circuits used primarily for long distance voice calls, data service trunk circuits, video service trunk circuit circuits, and so on. There is also a variety of private line trunk circuits for PBX networking applications, OPX and other trunk circuits used to support remote station users, and trunk circuits connecting to IVRs and other peripheral systems. Each trunk circuit category can also include several subtrunk groups.

To determine the number of trunk circuits per group trunk type, the traffic load must be calculated. If CDR data reports provide trunk traffic measurements in terms of seconds or minutes, the results must be expressed in terms of hours to determine how many Erlangs of traffic are carried over the trunk circuits to use the trunk traffic tables.

When using the CDR reports to calculate Erlang traffic ratings, it is important to account for call time not tracked by the CDR feature. In addition to the length of a conversation over a trunk circuit, trunk circuit holding time exclusive of talk time includes call set-up (dialing and ringing) time, call termination time, and the time trunk circuits are not available to other callers during busy signal calls and other noncompleted calls (abandons, misdials) that are not recorded and stored by the PBX system’s CDR feature. The missing CDR data time is usually calculated by adding 10 to 15 percent to the length of an average call. For example, if the total number of trunk calls is 100 and the total trunk talk time is 300 minutes, the average call length is 3 minutes. With a 10 percent missing holding time factor, an additional 18 seconds per call (3 minutes, or 180 seconds × 0.1) should be added to the 3-minute average talk time per trunk call. The 10 to 15 percent fudge factor is important and necessary to correctly determine trunk circuit requirements to maintain acceptable GoS levels.