Specifications
8 IBM Voice Over Frame Relay Perform Guide
distributed across a normal day and not grouped together at particular peak
hours.
Table 1. Dial Tone Availability (%)
For example, if a particular business has two trunks and makes an average of 3
hours of calls per day over the VoFR network, any single user at any given time
will have a 95% chance that a path to the VoFR network will be available.
However if the same business grew to use the VoFR network an average of 6
hours per day, the percentage would drop to 86% availability. In this case, the
business could upgrade from two to three trunks to get back to an acceptable
level of availability of the voice over data network.
The previous discussion has dealt with the required number of trunks at the
branch office level. The other issue to consider is the number of required trunks at
the headquarters location. The number of headquarters trunks is driven by the
number of branch offices and the number of trunks that each branch office uses.
It is not necessary to deploy a trunk at the headquarters for every remote trunk
that is deployed. Based on normal call patterns, the trunks at the headquarters
location can be shared among all the remote offices. Normally, a headquarters
location will have only a fraction of the total remote trunk count.
The table below assumes that the trunks located at the remote location are being
used in a typical application (rather than an application such as a call center
where the telephones are used extensively. As the table shows, the ratio
increases as the number of remote trunks gets increase. There is also the
expectation that the remote trunks are somewhat equally distributed across
multiple branch office locations.
Table 2. Typical Contention Ratios of Remote/Headquarters Trunks
Number of
trunks
Hours of total calling per 8-hour day
2345678
2 98 95 92 89 86 83 80
3 100999998979594
4 100 100 100 100 99 99 99
Total Number of Remote Trunks at
Multiple Locations
Typical Number of
Headquarters Trunks
Typical Contention
Ratio
222
4 3-4 1.3:1-1:1
6 4 1.5:1
8 5-6 1.6:1-1.8:1
10 5-6 1.7:1-2:1
16 8-9 1.8:1-2:1
24 10-13 1.8:1-2.4:1
32 13-16 2:1-2.5:1
64 24-29 2.2:1-2.7:1