Why measure traffic?
Summary: The measurement of traffic in PSTNs allows network operators to determine and maintain the Quality of Service (QoS) and in particular the Grade of service (GoS) that they offer their subscribers. This module is intended to impress upon the reader the significance of traffic measurements. It also dicusses the type of traffic that should be measured, what to do with these measurements, when to make the measurements and the type of errors that can be anticipated when making these measurements.
Why Traffic measurement?
Measurement of traffic within a network allows network managers and analysts to both make day-to-day decisions about operations and to plan for long-term developments [1, 2]. Traffic Measurements are used in many fundamental activities such as [2]: Identification of traffic patterns and trends Calculating the traffic intensity in a specific circuit or group Monitoring the service Dimensioning and managing the network Calculating tariffs Performing forecasting Dimensioning and managing the SS7network Checking the performance of the common channel signalling network The following sections will answer some fundamental questions about Traffic Measurement, such as: What should be measured; when should it be measured; what assumptions are made; and what errors can occur? When should traffic be measured? Traffic Measurements are conducted on a continuous basis and the results compiled into reports used for management decisions on various time scales. Measurements that are taken every few minutes are used for network management (monitoring) and temporary routing, measurements every few hours, days and weeks are used for maintenance purposes and measurements that are taken over months or even years are used for long-term network deployment, upgrading and extensions [2]. To determine normal reference traffic for a network, the ITU recommends that a network traffic analyst must take measurements for the busiest hour of each day for a whole year [2]. The busiest hour is defined as that four consecutive quarter hours whose traffic intensity is the greatest. Measurements taken outside the busy hour can be discarded. The reference intensity of traffic is then calculated by taking the average traffic intensity of the top thirty days in the year [2]. Measurements taken on individual days can be discarded. This will give the normal high traffic intensity in the network, allowing network managers to make long-term strategic decisions [2]. The exemplary graph below shows a plot of the hourly daily traffic:
| Graph 1 |
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What assumptions are made when calculating circuits required?
To perform calculations in circuit-switched networks several assumptions are made [3]: Calls arrivals follow a Poisson distribution Holding times follow a Negative Exponential distribution Blocked calls are lost or overflow There is statistical equilibrium It is useful to remember that the measurements are averages and this process deliberately ignores very short term variations in the traffic, but still allowing for a small but finite loss. The above assumptions are accurate if applied to circuit switched networks however they fail when planning for data traffic, small exchanges and sudden sharp peaks in traffic such as that caused by TV and radio phone-in competitions.
What errors can result from Traffic measurement?
Measurement errors are caused by faults in equipment or constraints on equipment design. The following five errors are examples of some types of errors that can occur [2]:
Traffic Measurement Errors
- Post-processing errors – These errors are generated by operators when analysing data that has been measured
- Pre-processing errors – These errors are generated by computers when compiling data for operators
- Statistical errors – These errors are caused by the averages in traffic measurements and by the fact that measurements are made from discrete samples
- Database errors - These errors exist when errors are generated by faults in the storage of information
- Interpretation errors - These errors occur when analysts misinterpret data
Exercises:
- Is it possible for arriving calls to follow another distribution other than the Poisson?
- Suppose a new value added service has just been introduced by a network service provider. 75% of the customers are attracted to this new service and would like to call in to the network call centers to learn more about the service. What kind of distribution will be exhibited by the traffic?
References
- Penttinen A., Chapter 4 – Traffic Modelling and Measurements, Lecture Notes: S-38.145 - Introduction to Teletraffic Theory, Helsinki University of Technology, Fall 1999.
- Kennedy I., Why Traffic Measurement?, School of Electrical and Information Engineering, University of the Witwatersrand, 2003.
- Kennedy I., What basic data should we measure?, School of Electrical and Information Engineering, University of the Witwatersrand, 2003.
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Last edited by Mookho Tsilo on Feb 17, 2006 6:41 am US/Central.