## 3 Results
### Standard Scenario  
The standard scenario may be described as the regular STB utilized at the customer's home. Those devices often maintain good or excellent grades (Table 1). However, example devices report even poor quality (Figure 11). It can be seen that grade (marker X) is correlated with the VSBCT parameter (red dot). When several bitrate changes occur, the grade goes down.  
Figure 10: The device with the frequent assignment of bad grades. After the device reboot, grade 5 appeared.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Grade** & **No. Events** \\ \hline
5 & 6 \\
4 & 6 \\
3 & 45 \\
2 & 317 \\
1 & 2685 \\ \hline \hline \end{tabular} A total of 3059 cluster assignments have been found for that device.  
\end{table}
Table 5: The distribution of cluster assignments for the worst case in the evaluated dataset.  
The bitrate can change every particular number of seconds when a new chunk has to be downloaded. The SCI and SCTI parameters frequently vary between negative and positive values (orange and green dot, respectively).  
Fluctuations are directly related to the bitrate changes caused by network impairments. The STB tries to download the MPEG-DASH chunk with the highest possible bitrate. The lower bitrate cannot be set consistently, similarly to pure OTT applications (YouTube, Netflix). There is no button to decrease the bitrate to omit the bitrate hopping. Hence the telecom operator has to provide the most satisfactory possible network environment.  
Standard scenario #1 assessment is based on the 44 gathered events (Table 6). In the beginning, the STB starts with the 800 kbps playback. Within the next 10 s, the bitrate increased to the maximum for the given channel and reached 6000 kbps. Although the maximum bitrate was achieved, grade 4 was assigned. The quality of the video delivered by 800 kbps is significantly worse than 3000 or 6000 kbps. Therefore, the lower grade was stated. It has to be mentioned that STBs can switch to higher or lower bitrate after at least 5 s due to the MPEG-DASH chunk length. After the morning session, another playback started after 5 p.m. During the hardly 2 h and 30 min, the customer may have witnessed 40 bitrate switches, but the channel was changed only five times. In an excellent scenario, the number of bitrate switches may be equal to channel changes, and the video referenced to bitrate lower than the maximum will not be displayed. For the KinoPolska channel, the 11 bitrate changes appeared in the ca. 4-min time span and fluctuated mainly between 3000 kbps and 6000 kbps.  
Another standard scenario (Figure 12) was assessed based on 14 events (Table 7). In the beginning, the STB played the video with 1255.2 kbps over 16 s, decreasing the grade from 5 to 4. After almost 1 h, the bitrate jumping appeared, and the cluster switched between 3 and 4. At 6:57 PM, the audio/video error occurred, the customer may recognize the black screen and error notification, and the grade suddenly decreased to 3.  
Figure 11: Clustering results for standard scenario #1. _X_-axis–hour and minute timestamp, _Y_-axis–cluster and features used during clustering.  
\begin{table}
\begin{tabular}{r r r r r r r r r r} \hline \hline
**No.** & **Timestamp** & **Channel** & **Bitrate** & **SCI** & **SCI1** & **STCSI** & **VSBCT** & **Cluster** \\ \hline  
After half a second, the STB recovered the signal and started playing the highest possible bitrate. However, the stalling is severe degradation, so the stream recovery does not provide the highest cluster. The grade is an assessment at the moment of occurrence. In future works, constant sampling might be applied to assess the grade at every specified interval. The STBs with the detected stallings should be monitored by the TV operator. Those events might be connected with decreased wireless network quality or signal power.