For a voice service, one would typically set the input Traffic Calculation = Volume Driven (the default). Then the distribution inputs described below are used to take the assumed number of call minutes for a subscriber and works through from annual volume to daily volume to busy hour volume, finally calculating the busy hour traffic in Erlangs. When the calculation is volume driven, the two inputs for data services (Nominal Bandwidth per Connection and Contention Ratio) are ignored.
For a data service, however, one would set the input Traffic Calculation = Peak Driven. Then the number of customers is still specified in terms of number of lines, by setting Connections Unit =
Lines, but the demand is specified based on the nominal bandwidth of the service (e.g., 2Mbits/s), rather than on the volume of traffic. The user therefore first enters Busy Hour Traffic parameters, and then the Traffic per Connection is inferred from the distribution inputs described below.
STEM provides a third option, Independent, which is appropriate for certain media services for which the peak traffic and traffic volume can be estimated, while the day-to-day traffic distribution parameters may be unpredictable or subject to significant uncertainty variation. For example, annual traffic volume may be constrained independently by spending.
Traffic calculation
Determines whether the calculation of busy-hour traffic is
Volume Driven
(for voice services) or Peak Driven (for data services).
The Busy Hour Traffic demand describes the volume of traffic carried during a busy hour. For
voice services
the total Busy Hour Traffic in year n, bn, is calculated from the Annual Traffic per Connection as:
where
cn
= number of Connections at the end of year n
un
= Annual Traffic per Connection in year n
dn
= number of Busy Days per Year in year n
pn
= Proportion of Traffic in the Busy Hour in year n
r
= Annual to Busy Hour Unit Ratio.
When the calculation is Volume Driven
the inputs for Nominal Bandwidth per Connection and Contention Ratio are ignored.
For data services the calculation is Peak Driven: the Busy Hour Traffic is calculated based on the Nominal Bandwidth per Connection and the Contention Ratio. In that case the total Busy Hour Traffic in year
n, bn, is calculated as:
where
cn
= number of Connections at the end of year n
wn
= Nominal Bandwidth per Connection in year n
tn
= Contention Ratio in year n.
When the calculation is Peak Driven
the input for Annual Traffic per Connection is ignored.
Busy days per year
Represents the number of days in a year when there is significant traffic. The default is 250, which is based on 50 working weeks per year and 5 working days per week.
365
would be more appropriate for a domestic service.
Proportion of traffic in the busy hour
Represents the proportion of daily traffic that will be carried by a service in the busy hour. The default is
0.2, which assumes that 20%
of the total daily traffic is carried during the busy hour.
Annual to busy hour unit ratio
A factor used to convert from units of annual traffic to units of busy-hour traffic. The default is
60, which assumes the Annual Traffic Unit is Call Minutes
and the Busy Hour Traffic Unit is Erlangs, on the basis that there are 60 Call Minutes
per hour. This ratio must be changed if different units are used.
-
Divide Annual Traffic per Connection by the Busy Days per Year input to give daily traffic per connection.
-
Multiply by the Proportion of Traffic in the Busy Hour to give the daily traffic per connection falling in the busy hour.
-
Divide by the Annual to Busy Hour Unit Ratio to convert from annual traffic units to busy-hour traffic units.
- Multiply by the number of Connections to give the total Busy Hour Traffic.
For a data service, enter an Annual to Busy-Hour Unit Ratio in accordance with the specified annual and busy-hour traffic units. For example, if the Busy Hour Traffic Unit is Mbit/s and the Annual Traffic Unit is GBytes, the conversion factor is 3600 (going from seconds to hours) divided by 8 (bits to bytes), divided by 1024 (MBytes to GBytes).