# SCHC over LoRaWAN Efficiency: Evaluation and Experimental Performance of Packet Fragmentation
## 1 Introduction  
Nowadays, several channel access technologies serve the Internet of Things (IoT). Among them, LoRaWAN, SigFox, and NB-IoT have emerged as technologies designed to deliver high coverage while maintaining a low power consumption. These characteristics are highly valued in IoT because they can support a high density of end devices where periodic battery changes are not feasible or where the available power is limited.  
Low Power Wide Area Network (LPWAN) technologies do not support native IP addressing because the reduced storage space cannot store a full protocol stack or fragment messages at the link layer. As a consequence, it is not possible to connect the end nodes directly to the Internet. Moreover, the lack of IP support prevents the use of standard management protocols and direct access to end devices. One example of the aforementioned problem is the Long Range Wide Area Networks, or LoRaWAN specification. LoRaWAN belongs to the LPWAN. LoRaWAN has a Maximum Transmission Unit (MTU) in theorder of tens of bytes--without link-layer fragmentation--limiting the transmission of standard Internet upper-layer protocols, such as Internet Protocol (IP), Transmission Control Protocol (TCP), User Datagram Protocol (UDP), Hypertext Transfer Protocol (HTTP), and Constrained Application Protocol (CoAP). Furthermore, in the LoRaWAN architecture, each end node cannot be addressed directly from the Internet. Instead, a central module called a network server receives the messages destined for the end nodes and translates between a Uniform Resource Locator (URL) and an identifier of the LoRaWAN end device.  
The Internet Engineering Task Force (IETF) has developed a specification, the Static Context Header Compression and Fragmentation (SCHC) [1], that enables header compression for IPv6, UDP, and CoAP protocols for LPWAN technologies. This compression and fragmentation permit communication through the Internet with an end device directly to the assigned IP address. The LPWAN technology used in this work is LoRaWAN. The specification that defines the operation of SCHC over LoRaWAN is RFC9011 [2].  
The main contribution of this work is to create a model to determine the channel occupancy efficiency of the ACK-on-Error fragmentation mode of the SCHC specification. The model provides information for decision-making regarding which spreading factor to choose based on the efficiency of the channel. The model is based on the transmission periods that occur when the protocol running on top of LoRaWAN (e.g., IP) sends a large message that does not fit in a LoRaWAN packet (i.e., it is larger than the MTU of the access technology). The situation forces the use of the fragmentation mechanism provided by the SCHC standard [1]. We evaluate the transmission periods of a SCHC Fragment when values are modified in the fragmentation sublayer. Besides the model, we provide both experimental and theoretical evaluations of the transmission periods in the upstream channel of a LoRaWAN link, using the ACK-on-Error fragmentation mode from the SCHC specification. This model shows how the channel occupancy efficiency, latency, and spreading factor are inter-related when using SCHC in LoRaWAN: information that allows the end user to make key decisions about which spreading factor to select.  
The remainder of the article is organized as follows: In Section 3, we provide an overview of the SCHC framework and the SCHC over LoRaWAN. Section 4 focuses on the mathematical model proposed to analyze the ACK-on-Error mode channel efficiency performance. Section 5 describes the testbed employed for the experiments and Section 6 discusses the theoretical and experimental evaluations. Section 7 explains how our results can be applied to state-of-the-art LPWAN technologies such as LoRaWAN. Finally, we conclude the paper in Section 8.