# On Applicability of Network Coding Technique for 6 LoWPAN-based Sensor Networks
## 1 Introduction  
In recent years, wireless networks have become present in almost every area of our lives. Growing demand for mobile services provision enforces careful bandwidth allocation to each wireless network, which, as a consequence, may limit its capacity. Moreover, new mobile applications generate higher traffic streams, which can overload the network. Besides, the Internet is a continuously growing network, covering Wireless Sensor Networks (WSNs) and machine to machine communications. This idea refers to the so-called Internet of Things (IoT), where things can communicate while using heterogeneous networks. Most things require an autonomous power supply (mostly small battery), thus they use low power consuming wireless techniques. Traffic streams in such networks can be sent in multihop transmissions, where packets are forwarded hop by hop to a destination. In many cases, the WSNs use IEEE 802.15.4-based sensor motes with a small data rate. However, traffic that is generated by IoT applications can easily overload such networks. Many solutions have appeared in recent years, which can be applied in these networks to increase traffic handling efficiency, and consequently, to decrease the overall traffic volume [1; 2; 3; 4; 5]. One of them is the Network Coding (NC) technique that was introduced by [PERSON] et al., in [6] and was then used in many solutions, discussed for example in [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. It is defined as a \"technique, where a node (nodes) receives information from all the input links, encodes, and sends information to all the output links\" [6]. This technique can be used in telecommunication networks for many purposes, e.g., to reduce the network load, to increase thepackets delivery ratio, to improve the reliability or the security of communication, to improve energy efficiency, etc. The main goal of the NC implementation in WSNs is to increase the network lifetime through decreasing the network load. The reduction of wireless transmissions is crucial for energy saving in sensors motes. The majority of sensors' energy is used by Radio Frequency (RF) transceivers. For example, the CC2420 single-chip used in Tmote Sky motes consumes maximum 23 mA in receive mode and 21 mA in transmit mode (at 0 dB), as compared with 2.4 mA that is used by microcontroller in operating mode and 21 \(\mu\)A in standby mode [24]. In order to achieve this goal, the inter-session NC mechanism (called as NC6 LoWPAN), which is located between Medium Access Control (MAC) and adaptation (6 LoWPAN) layers, is proposed. It can be implemented in WSNs with different MAC protocols without any additional modifications of the protocol stack. Additionally, this enables the implementation of the NC6 LoWPAN in the networks with resource (battery, memory) limitation. The inter-session coding works in enforced coding opportunity, i.e., packets are fractionally delayed in sensor nodes to perform coding. Thus, the network throughput (and consequently energy efficiency) can be improved, even if this leads to increase in the end-to-end packet delay. Moreover, it occurs that slight delay in packets delivery can be acceptable for many services in so called Delay-Tolerant Sensor Networks (DTSN) [25]. Therefore, the network coding technique that is discussed in this paper supplements many solutions for non-6 LoWPAN networks that have been presented so far [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. The main contribution of this paper as compared to the previous work is:  
* the proposal of the relatively simple solution allowing for the encoding of small 6 LoWPAN datagrams (fragments) characterized by compressed headers and which are often changed from hop-to-hop,
* the reduction of the number of signaling messages keeping the NC usefulness for the WSNs, and
* the implementation and verification of the NC proposal in a real 6 LoWPAN-based testbed.