## 6 Results
### Latency
#### 6.2.2 Transmission of One Uplink and One Downlink Message  
Table 5 provides the average latency of successful transmissions (sending an uplink message and receiving a downlink message of same size) for different radio conditions, message sizes and scenarios. The results with DTLS are not shown, as similar values are observed. It is clear from the table that an increase in message size leads to a higher latency. However, note how, in radio condition 0, the message size has a small impact on the latency. As the radio conditions deteriorate, the impact of the message size becomes more prominent. Indeed, message repetition amplifies the difference in transmission time between a short message and a longer one. For example, under radio condition 3 (64 repetitions) and in the \"IP without SCHC\" scenario, the latency for a 50-byte message is 3.3 s, while it more than doubles (6.7 s) for 200 bytes. We clearly see from the table that the latency increases with the deterioration of radio conditions. Note that radio condition 0 and radio condition 1 correspond to the same number of repetitions (only one transmission) but different channel conditions, leading to different radio settings in terms of the transmitted power but most importantly concerning the Modulation and Coding Scheme (MCS). The worse the channel, the more robust the modulation and coding schemes, leading to a slower transmission. Bad radio channels cause more transmission errors, thus triggering HARQ retransmissions, which also introduce delays.  
Another important take away from this table is the high latency in the case of radio condition 4. By examining the logs of the device, we noticed that the messages were not always successfully received for the first transmission, and applicative retransmissions were needed. Retransmissions are only triggered when a timer of several tens of seconds expires, which explains the high latency in this case.  
Comparing the IP and NIDD (without SCHC), NIDD leads to a lower latency for all message sizes and radio conditions. Indeed, as explained in Section 4.4, using NIDD not only reduced the protocol overhead, but it also decreased the message size by avoiding sending IP and UDP headers.  
When using SCHC, the latency is reduced for both NIDD and IP transport. SCHC has a significant impact in reducing the size of the message by compressing the headers  
Figure 10: Impact of SCHC on average DTLS handshake latency, in seconds, with respect to radio conditions (without repetitions and with 32 repetitions).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Condition** & **Size** & **IP** & **IP\_SCHC** & **NIDD** & **NIDD SCHC** \\ \hline \multirow{4}{*}{0} & 50 & 1.5 & 1.5 & 0.6 & 0.6 \\ \cline{2-6}  & 100 & 1.5 & 1.4 & 0.8 & 0.6 \\ \cline{2-6}  & 200 & 1.6 & 1.4 & 1.1 & 0.8 \\ \hline \multirow{4}{*}{1} & 50 & 2.2 & 1.9 & 2.0 & 1.4 \\ \cline{2-6}  & 100 & 2.3 & 2.1 & 2.1 & 1.5 \\ \cline{2-6}  & 200 & 2.7 & 2.0 & 2.3 & 1.7 \\ \hline \multirow{4}{*}{2} & 50 & 3.2 & 2.1 & 2.0 & 1.5 \\ \cline{2-6}  & 100 & 3.6 & 3.1 & 3.2 & 2.5 \\ \cline{2-6}  & 200 & 6.1 & 3.8 & 5.5 & 2.9 \\ \hline \multirow{4}{*}{3} & 50 & 3.3 & 2.1 & 2.2 & 1.6 \\ \cline{2-6}  & 100 & 3.9 & 3.2 & 3.0 & 2.0 \\ \cline{2-6}  & 200 & 6.7 & 4.5 & 5.6 & 4.6 \\ \hline \multirow{4}{*}{4} & 50 & 19.7 & 17 & 15.5 & 14.5 \\ \cline{2-6}  & 100 & 27.3 & 20.9 & 24.1 & 15.3 \\ \cline{2-6}  & 200 & 42.7 & 30.0 & 41.2 & 26.1 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Latency, in seconds, for sending and receiving a message for different radio conditions and differents sizes of messages (no DTLS).  
as well as the DLMS payload, and thus reducing the latency, particularly in bad radio conditions. For example, the latency in the case of 200 bytes is reduced from 42.7 to 30.0 s when using SCHC over IP transport in radio condition 4. Similarly, the latency drops from 41.2 to 26.1 s when using SCHC over NIDD. In addition to the direct impact of reducing the message size on the transmission time, this reduction makes the message less prone to transmission errors.