Traffic engineering goals; E.600 busy hours

Traffic engineering is the use of probability theory and traffic measurements to find how much equipment (trunks, switches, channels, queues) a network needs to carry the expected traffic at an acceptable grade of service and at minimum cost.

Major tasks of traffic engineering

1. Traffic measurement: record calls, holding times and busy-hour traffic on each route and switch.
2. Traffic forecasting: predict future demand from growth of subscribers and services.
3. Modelling: choose the right model (Erlang B, Erlang C, Engset, binomial) for each part of the network.
4. Dimensioning: compute the number of trunks, switching paths, registers and processors needed for a target GoS.
5. Performance evaluation: compute blocking, delay and server utilisation of existing equipment.
6. Routing and overflow planning: plan high-usage and final routes, alternate routing.
7. Monitoring and network management: detect congestion and overload and apply controls.

Goals

• Provide a target grade of service (e.g. GoS 0.01 or 0.02) so most calls succeed.
• Minimum cost: avoid over-provisioning while keeping quality.
• High utilisation of trunks and switching equipment.
• Keep delays (dial tone delay, post-dialling delay) within limits.
• Allow for growth and protect the network from overload.

Busy hour as defined in CCITT Recommendation E.600

Equipment is dimensioned for the busy hour, not the daily average. E.600 defines:

1. Busy hour: the continuous 1-hour period lying wholly within the time interval concerned for which the traffic volume or the number of call attempts is greatest.
2. Peak busy hour: the busy hour of each individual day. It usually changes from day to day.
3. Time-consistent busy hour: the 1-hour period starting at the same time each day for which the average traffic volume or call-attempt count of the exchange or resource group is greatest over the days under consideration.

Traffic
  ^          peak busy hour (day 1)
  |            __
  |      _____/  \____        ____
  |     /             \______/    \
  |____/                           \___
  +----+----+----+----+----+----+----+--> time
      08   10   12   14   16   18   20

In practice, traffic is read in quarter-hour intervals; the four consecutive quarter-hours with the highest total form the busy hour. The time-consistent busy hour is preferred for planning because one fixed hour is used every day.