# Evaluation of 6 LoWPAN Generic Header Compression in the Context of a RPL Network
## 7 Conclusions  
In this article, we investigate the impact of 6 LoWPAN-GHC on various performance parameters for RPL-based networks. We implemented 6 LoWPAN-GHC in the Rust programming language, a high-performance compiled language that can be used on embedded devices. The implementation was added on top of the smoltcp library. By means of simulation, we show that 6 LoWPAN-GHC reduces the total number of transmitted bytes, with compression ratios between 1.14 and 1.44 for different RPL control packets. Reducing the frame size reduces the transmission delay. This is beneficial from a networking point of view. Shorter packets reduce the probability of collisions and interference with other devices sharing the same frequency spectrum. The overall network throughput could also increase as the channel could be accessed more frequently. The IEEE 802.15.4 standard defines multiple data rates: 10 kbit/s, 40 kbit/s, 100 kbit/s and 250 kbit/s. Lowering the data rates allows for an increased transmission range, with the cost of an increased transmission delay. For lower data rates, a 6 LoWPAN-GHC compressed control packet experiences an up to 19% decrease in the total delay compared to a non-6 LoWPAN-GHC compressed packet. For data rates higher than 100 kbit/s, the total delay experienced by a 6 LoWPAN-GHC compressed packet is larger than the total delay experienced by a non-6 LoWPAN-GHC compressed packet. This is due to the fact that the transmission delay at a high data rate is already much shorter and so is its reduction thanks to compression. This reduction is nullified by the extra processing delay, caused by executing the compression algorithm. \begin{table}
\begin{tabular}{l c c} \hline \hline  & 6 LoWPAN-GHC Proc. energy & 6 LoWPAN-GHC Transm. energy \\  & 6 LoWPAN Proc. energy & 6 LoWPAN Transm. energy \\ \hline
**Packet** & **Data Rate** & \\
**Type** & **(kbit/s)** & **Total Energy Consumption (Processing + Transmission Energy)** \\ \hline DIS & 10 & \\  & 40 & \\  & 100 & \\ \hline DIO & 10 & \\  & 40 & \\  & 100 & \\ \hline DAO-ACK & 100 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Total energy consumption for RPL control packets for different data rates. For each row, the total energy consumption for a 6 LoWPAN-GHC compressed packet is shown in the top bar, with the bottom bar being for non-6 LoWPAN-GHC compressed packets. The 6 LoWPAN-GHC compression reduces the transmission energy for lower data rates. A similar conclusion can be drawn for total energy consumption. For lower data rates, e.g., 10 kbit/s, it is reduced by up to 22% for a DIO packet. For DISs, DAOs and DAO-ACKs, the energy is reduced by up to 11%, 29% and 20%, respectively. Measurements on real testbeds show that the total energy consumption per 6 LoWPAN-GHC compressed control packet for a device under study using the 2.4 GHz ISM band with a data rate of 250 kbit/s is larger than the total energy consumption per non-6 LoWPAN-GHC compressed packet. The additional energy spent on performing 6 LoWPAN-GHC compression on RPL control packets is higher than the gain in energy obtained by reducing the transmission time. Note that our conclusions are in the context of a RPL network, where the control packets are compressible with 6 LoWPAN-GHC due to their structure. For data packets, the compression ratio could be smaller, meaning that the total energy consumption and total delay could only increase. In future work, it would be interesting to investigate the impact of the static bytes in the dictionary. In the case of the RPL, they do not seem to have an impact on the compression ratio. We anticipate that having a larger dictionary actually increases the compression delay since there are more bytes to match against the data. Conceptualisation, T.V. ; software, T.V. and D.D. ; validation, D.D. and R.V.G. ; formal analysis, R.V.G., R.D.S. and K.S. ; investigation, T.V. and D.D. ; resources, A.B. and K.S. ; data curation, T.V. ; writing--original draft, T.V. and D.D. ; writing--review, R.V.G., R.D.S., A.B. and K.S. ; visualisation, T.V. and D.D.