Teletraffic engineering
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Teletraffic engineering is the application of traffic engineering theory to telecommunications. Teletraffic engineers use their basic knowledge of statistics including Queueing theory, the nature of traffic, their practical models, their measurements and simulations to make predictions and to plan telecommunication networks at minimum total cost. These tools and basic knowledge help provide reliable service at lower cost. Because the approach is so different to different networks, the networks are handled separately here: the PSTN, broadband networks, mobile networks, and networks where the possibility of traffic being heavy is more frequent than anticipated.
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[edit] Introduction
Traffic engineering uses statistical techniques such as queuing theory to predict and engineer the behaviour of telecommunications networks such as telephone networks or the Internet.
The field was created by the work of A. K. Erlang in whose honour the unit of telecommunications traffic intensity, the Erlang, is named. The derived unit of traffic volume also incorporates his name. His Erlang distributions are still in common use in telephone traffic engineering.
The crucial observation in traffic engineering is that in large systems the law of large numbers can be used to make the aggregate properties of a system over a long period of time much more predictable than the behaviour of individual parts of the system.
The queueing theory originally developed for circuit-switched networks is applicable to packet-switched networks.
The most notable difference between these sub-fields is that packet-switched data traffic is self-similar. This is a consequence of the calls being between computers, and not people.
[edit] Teletraffic in PSTN architectures
Teletraffic theory was first developed by Agner Erlang for circuit-switched architectures such as the PSTN. As such, the basics of teletraffic theory is best introduced by examining teletraffic concepts as they relate to PSTNs.
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. The QoS of a network must be maintained or else operators will lose subscribers. The performance of a network depends on whether all origin-destination pairs are receiving a satisfactory service.
Networks are handled as:
- loss systems where calls that cannot be handled are given equipment busy tone or
- queuing systems where calls that cannot be handled immediately are queued.
Congestion is defined as the situation when exchanges or circuit groups are inundated with calls and are unable to serve all the subscribers. Special attention must be given to ensure that such high loss situations do not arise. To help determine the probability of congestion occurring, operators should use the Erlang Equations or the Engset calculation.
Exchanges in the PSTN make use of Trunking concepts to help minimize the cost of the equipment to the operator. Modern switches generally have full availability and do not make use of Grading concepts.
Overflow systems make use of alternative routing circuit groups or paths to transfer excess traffic and thereby reduce the possibility of congestion.
Queueing systems used in telephone networks have been studied as a science. See queuing theory. For example subscribers are queued until they can be served. If subscribers are made to wait too long, they may lose patience and default from the queue, resulting in no service being provided.
A very important component in PSTNs is the SS7 Network used to route signalling traffic. As a supporting network, it carries all the signaling messages necessary to set up, break down or provide extra services. The signaling enables the PSTN control the manner in which traffic is routed from one location to another.
Transmission and switching of calls is performed using the principle of Time-Division Multiplexing (TDM). TDM allows multiple calls to be transmitted along the same physical path, reducing the cost of infrastructure.
A good example of the use of teletraffic theory in practice is in the design and management of a call center. Call centers use teletraffic theory to increase the efficiency of their services and overall profitability through calculating how many operators are really needed at each time of the day.
[edit] Teletraffic engineering in broadband networks
- Main article: Teletraffic engineering in broadband networks
Teletraffic Engineering is a well-understood discipline in the traditional voice network, where traffic patterns are established, growth rates can be predicted, and vast amounts of detailed historical data are available for analysis. However, in modern Broadband Networks, the teletraffic engineering methodologies used for voice networks are inappropriate . Various aspects relating to teletraffic engineering in broadband networks are discussed in this article.
[edit] Mobile traffic
For mobile networks, this article looks at service areas, service provision and service quality:
[edit] Long-tail traffic
Of great importance is the possibility that extremely infrequent occurrences are more likely than anticipated. The reason is that the network might have to withstand the unanticipated traffic.
[edit] Teletraffic economics and forecasting
As mentioned in the introduction, the purpose of teletraffic theory is to reduce cost in telecommunications networks. An important tool in achieving this goal is forecasting. Forecasting allows network operators to calculate the potential cost of a new network / service for a given GoS during the planning and design stage, thereby ensuring that costs are kept to a minimum.
An important method used in forecasting is simulation, which is described as the most common quantitative modelling technique in use today. An important reason for this is that computing power has become far more accessible, making Simulation the preferred analytical method for problems that are not easily solved mathematically.
As in any business environment, network operators must charge tariffs for their services. These charges must be balanced with the supplied QoS. When operators supply services internationally, this is described as trade in services and is governed by the General Agreement on Trade in Services (GATS).
[edit] References
- "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)
[edit] See also
- ATM
- Busy hour call attempts
- Erlang unit
- Clos network
- TCP/IP
- MPLS
- Predictive dialer
- Routing
- RSVP - Traffic Engineering
- Traffic mix
