Introduction to Grade-of-Service = GoS
The costs of a telephone system can be divided into costs which are dependent upon the number of subscribers and costs that are dependent upon the amount of traffic in the system. The goal when planning a telecommunication system is to adjust the amount of equipment so that variations in the subscriber demand for calls can be satisfied without noticeable inconvenience while the costs of the installations are as small as possible. The equipment must be used as efficiently as possible.
Teletraffic engineering deals with optimisation of the structure of the network and adjustment of the amount of equipment that depends upon the amount of traffic. In the following some fundamental concepts are introduced and some examples are given to show how the traffic behaves in real systems. All examples are from the telecommunication area.
A positive mean holding time, then the carried traffic may become larger than the offered traffic.
Figure 2.11: Histogram for all call attempts repeated within 5 minutes, when the called party is busy.
The following section is based on (Veirø, 2001 [99]). A network operator must decide what services the network should deliver to the end user and the level of service quality that the user should experience. This is true for any telecommunications network, whether it is circuitor packet-switched, wired or wireless, optical or copper-based, and it is independent of the transmission technology applied. Further decisions to be made may include the type and layout of the network infrastructure for supporting the services, and the choice of techniques to be used for handling the information transport. These further decisions may be different, depending on whether the operator is already present in the market, or is starting service from a greenfield situation (i.e. a situation where there is no legacy network in place to consider).
As for the Quality of Service (QoS) concept, it is defined in the ITU-T Recommendation E.800 as: The collective effect of service performance, which determine the degree of satisfaction of a user of the service. The QoS consists of a set of parameters that pertain to the traffic performance of the network, but in addition to this, the QoS also includes a lot of other concepts. They can be summarised as:
• service support performance
• service operability performance
• serveability performance and
• service security performance
The detailed definitions of these terms are given in the E.800. The better service quality an operator chooses to offer to the end user, the better is the chance to win customers and to keep current customers. But a better service quality also means that the network will become more expensive to install and this, normally, also has a bearing to the price of the service. The choice of a particular service quality therefore depends on political decisions by the operator and will not be treated further here.
When the quality decision is in place the planning of the network proper can start. This includes the decision of a transport network technology and its topology as well as reliability aspects in case one or more network elements become malfunctioning. It is also at this stage where the routing strategy has to be determined.
This is the point in time where it is needed to consider the Grade of Service (GoS). This is defined in the ITU-T Recommendation E.600 as: A number of traffic engineering variables to provide a measure of adequacy of a group of ressources under specified conditions. These grade of service variables may be probability of loss, dial tone delay, etc. To this definition the recommendation furthermore supplies the following notes:
• The parameter values assigned for grade of service variables are called grade of service standards.
• The values of grade of service parameters achieved under actual conditions are called grade of service results.
The key point to solve in the determination of the GoS standards is to apportion individual values to each network element in such a way that the target end-to-end QoS is obtained.
Comparison of GoS and QoS
It is not an easy task to find the GoS standards needed to support a certain QoS. This is due to the fact that the GoS and QoS concepts have different viewpoints. While the QoS views the situation from the customer’s point of view, the GoS takes the network point of view.
We illustrate this by the following example:
Example 2.5.1:
Say we want to fix the end to end call blocking probability at 1 % in a telephone network. A customer will interpret this quantity to mean that he will be able to reach his destinations in 99 out of 100 cases on the average. Fixing this design target, the operator apportioned a certain blocking probability to each of the network elements, which a reference call could meet. In order to make sure that the target is met, the network has to be monitored. But this monitoring normally takes place all over the network and it can only be ensured that the network on the average can meet the target values. If we consider a particular access line its GoS target may well be exceeded, but the average for all access lines does indeed meet the target. 2
GoS pertains to parameters that can be verified through network performance (the ability of a network or network portion to provide the functions related to communications between users) and the parameters hold only on average for the network. Even if we restrain ourselves only to consider the part of the QoS that is traffic related, the example illustrates, that even if the GoS target is fulfilled this need not be the case for the QoS.
Special features of QoS
Due to the different views taken by GoS and QoS a solution to take care of the problem has been proposed. This solution is called a service level agreement (SLA). This is really a contract between a user and a network operator. In this contract it is defined what the parameters in question really mean. It is supposed to be done in such a way, that it will be understood in the same manner by the customer and the network operator. Furthermore the SLA defines, what is to happen in case the terms of the contract are violated. Some operators have chosen to issue an SLA for all customer relationships they have (at least in principle), while others only do it for big customers, who know what the terms in the SLA really mean.
