## 2 Performance Support System
### Definitions, Principles, and Theoretical Foundations
#### 2.1.5 Introducing the Most Important Key Performance Indicators  
As seen in the above figure, several indicators measure the network quality from different aspects. For this purpose, before identifying faults in a network, it is first necessary to thoroughly examine these indicators and the factors that affect them. 1. **SDCCH Congestion**  
Traffic channels in radio networks carry the main network load, while control and signaling channels are used to achieve this goal. These channels are used to control the behavior of mobile phones, to communicate, to control established calls, to facilitate call handover, etc. If subscribers' demands exceed the available resources, congestion occurs in this channel. One solution to this problem is adding more resources and physical capacity to the network resources. Figure 3: The steps of making a call in the GSM network with the network performance measurement indicators. SDCCH establishes success rate  
This indicator shows the percentage of successful requests to take over SDCCH. One of the reasons for a negative value for this index is downlink interference, which occurs when there are too many subscribers' location updates, due to network design issues, and hardware issues. SDCCH drop  
When a network user makes a request to communicate with another subscriber, or when a network user is called by another network subscriber (through an incoming call), there is a need to take over the SDCCH. Some reasons, such as the timing advanced (TA) timer, low strength of the downlink or uplink signal or both, or low quality of the downlink or uplink signal, affect the SDCCH drop. In addition, if the traffic channel is congested, the requested call is returned on the SDCCH. SDCCH means holding time  
This index indicates the required capability of the SDCCH for high-speed access. That is, it measures the mean time the channel is held due to switching between services. The longer this time, the worse the service provided. TCH Congestion  
Congestion in the network means insufficient resources are available to make calls on the network. Congestion in the traffic channel is one of the most critical problems of the radio network, and it strongly affects the quality of the network. Congestion leads to higher subscriber dwell time in the SDCCH. One of the ways to reduce congestion in the network is to use more radio resources. Unfortunately, there are many limitations in terms of economy, lack of space for installing resources, etc. Therefore, it is recommended to use other methods, such as using the \"half rate\" technique. In this method, each period is assigned to two subscribers instead of serving one subscriber. Although the use of half-rate improves the network congestion situation, it reduces the quality of the received signal by the user. Another way to deal with congestion is to share the traffic load of the cell and transfer calls to neighboring cells. Like the half-rate technique, this method has the problem of reducing service quality. This indicator directly affects the user experience and shows the TCH assignment success rate percentage. If there is congestion in the traffic channel, the value of this index decreases. Other factors, such as the amount of coverage, interference, and hardware problems, also reduce the percentage of this index. Call Setup Success Rate  
This indicator, one of the most important key performance indicators used by all mobile phone operators, indicates the successfully launched call rate. There is no single standard method for measuring this index; therefore, every operator can calculate it differently. High values of this attribute indicate good network performance because a high percentage of calls are successfully established. Many reasons can be listed for inappropriate values of this index, including weakness in signal strength, congestion in the traffic and signaling channels, disconnection in the SDCCH, and failure in the appointment of the TCH.