# Evaluation of 6 LoWPAN Generic Header Compression in the Context of a RPL Network
## 6 Discussion  
The simulation study demonstrates relatively high compression ratios for RPL control messages. As a result, the transmitted packets are smaller, which translates into a reduction in the transmission delay. A shorter transmission delay can lead to more traffic accommodation and an increased overall network throughput. It can also reduce the probability of collisions and interference with other devices sharing the same frequency spectrum, which is important in crowded wireless environments. The testbed performance measurements show that the total energy consumption increased for all RPL control packets. The same applies to the total delay. This increase is caused by the execution of the compression algorithm. Improving the implementation of the compression algorithm might decrease the delay and energy consumption. A possible approach is using a faster dictionary-matching algorithm, such as an Aho-Corasick-based algorithm [25]. The content of the dictionary could be modified, but this does not respect the standard. For example, the last 16 bytes of the dictionary are based on heuristics and might not help in compressing RPL packets. Finding better values for the 16 bytes or removing them from the dictionary might improve the performance of the compression algorithm. A specialised compression format for ICMPv6 can yield better results than a generic one. However, this was not the goal of 6 LoWPAN-GHC as this would require a new RFC for every new standard or for every modification of an existing standard. All measurements in Section 5 are performed for an IEEE 802.15.4-based network, operating on the 2.4 GHz Industrial, Scientific and Medical (ISM) band with a data rate of 250 kbit/s. However, the IEEE 802.15.4 standard also defines lower data rates for transmissions on the 868 MHz band: 10 kbit/s, 40 kbit/s and 100 kbit/s. Tables 2 and 3 show  
Figure 6: Power measurements over time of a device processing and transmitting different RPL control packets, with and without 6 LoWPAN-GHC compression. The hatched area is associated with the processing period of the packet, and the dotted area is associated with the transmission period. the calculated impact on the delay and energy consumption for other data rates, respectively. For each data rate, the top bar represents the total delay/total energy for when 6 LoWPAN-GHC is used, with the bottom bar being the total delay/total energy for non-6 LoWPAN-GHC compressed packets. It is important to highlight that the compression delays/energies remain unchanged for the different data rates. The data rates only affect the transmission delay and transmission energy consumption. Table 2 shows that, on average, for data rates lower than 100 kbit/s, the total delay reduces for all the 6 LoWPAN-GHC compressed RPL control packets, satisfying Equation (4). This means that the compression delay introduced by the 6 LoWPAN-GHC mechanism does not nullify the decrease in transmission delay achieved by transmitting less data. Once a high enough data rate is used, the impact of the compression delay is noticeable and increases the total delay. The same conclusion can be drawn for the energy consumption (shown in Table 3). For data rates lower than 100 kbit/s, the total energy consumption is reduced for all the 6 LoWPAN-GHC compressed RPL control packets, satisfying Equation (3). The energy increase needed to perform the 6 LoWPAN-GHC compression is less than the energy gain obtained by transmitting less data. For data rates equal to or higher than 100 kbit/s, the energy spent performing the 6 LoWPAN-GHC compression becomes high enough to have a negative impact on the total energy consumption. If we take, for example, the CC1200 transceiver, which can operate on the 868 MHz frequency band, the transmission power varies between 119 mW and 152 mW. For a DIO, transmitted at 10 kbit/s with the highest transmission power, the total energy consumption is equal to 7.79 mJ with 128 \(\upmu\)J for processing the packet and 7.66 mJ for transmitting the packet.