# A Systematic Analysis of Narrowband IoT Quality of Service
## 4 Results
### Application Layer Performance: Latency  
In the first application layer measurement, the factors impacting system latency were analyzed under varying coverage situations. The total system latency is the most important QoS criterion for small and infrequently transmitted packets, such as sensor data. Similar to previous evaluations, the signal level was varied in 5 dB steps using the attenuator, and 30 packets were sent per attenuation step and per direction. In a first analysis, the effect of the ECL mechanism on the uplink and downlink latency was considered. For this purpose, packets were grouped by their ECL class and analyzed in terms of latency.  
Figure 11 illustrates the effect of different ECL levels on the total system latency. The measurement confirmed that higher ECLs, and thus more signal repetitions, had an impact on the total system latency. The latency rise between ECL0 and ECL1 was less significant than between ECL1 and ECL2, which indicated a more drastic increase in the repetition number in the latter case. Furthermore, there was a significant difference between the latency in uplink and downlink direction. This could be explained by the allocation of 1 ms subframes over 12 subcarriers in the downlink and 8 ms RUs on a single tone in the uplink, which overcompensated the higher number of repetitions in the downlink direction. In order to obtain a better understanding of the latency contribution of the individual systems, the round-trip-time between the whitelisted server and the MNO backbone (see Figure 5h,g) was measured to be 17.776 ms (N = 30, \(\sigma\) = 0.163 ms), which indicated a latency of about 9 ms. As such, the latency contribution of the Internet link remained below 5% of the total system latency for all use cases.  
The second analysis was concerned with the influence of the RSRP signal level on the total system latency. In addition to transferring regular user data, NB-IoT implements the ability to send exception reports, which are high-priority messages that can be used for reporting an alert condition. The 3 GPP specified a maximum latency of 10 s for waking up the modem and delivering an exception report [1,4].  
Figure 11: The influence of ECL classes on the total system latency. The median latency value is given for each ECL and transmission direction.  
In order to evaluate if exception reports could reduce the system latency, the modem was configured to send 30 exception reports per attenuation step in the uplink direction. For each step, packets were grouped together and assigned to their average signal level, and the latency was analyzed.  
Figure 12 confirms the relationship between the RSRP signal level and latency. In both the uplink and downlink direction, the latency rose significantly below the noise floor. There were two main reasons for this growth: (a) similar to LTE, NB-IoT used MCS to adapt to varying channel conditions; with lower MCS, the coding redundancy was increased, and the TBS decreased, which required more RU per data volume; (b) additionally, ECL2 was employed for signals below the noise floor as discussed in Section 4.2.  
The comparison of the latency of regular user traffic and exception reports showed that there was no latency benefit, and the 3 GPP target of 10 s was still exceeded for signals significantly below the noise floor. Further research is required to identify what was causing this behavior. A possible reason might be that exception reports had not yet been implemented in this NB-IoT network. Overall, the evaluation revealed a significant number of uplink samples with tens of seconds of latency in the uplink direction, which must be considered during application development.