By Juan Carlos Zuniga, Sigfox, IETF Internet Area Co-Chair, (edited by Alan J Weissberger)

Introduction:

Amongst the plethora of different Internet of Things (IoT) technologies [see Addendum], Low Power Wide Area Networks (LPWANs) [1] offer mature and well-established solutions for the Industrial Internet of Things (IIoT).

Note 1.  A LPWAN is a type of wireless telecommunication wide area network designed to allow long range communications with low power consumption, low cost interface and a relatively low bit rate for the IIoT.  There are many types of LPWANs.  Some like LTE-M and NB-IoT use licensed spectrum, while others such as Sigfox and LoRaWAN use unlicensed spectrum.

LPWANs enables IoT systems to be designed for use cases that require devices to send small amounts of data periodically over often-remote networks that span many miles and use battery-powered devices that need to last many years.

LPWANs achieve those attributes by having the IoT devices (“things”) send only small packets of information periodically or even infrequently—status updates, reports, etc.—upon waking from an external trigger or at a preprogrammed time interval.

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In order to enable these IIoT connectivity solutions, a common standard is needed to allow the various types of LPWANs to communicate with applications using a common language.  For this to occur, each network must have the ability to connect to the Internet.  However, due to the severely restrictive nature of LPWANs, the abilities of Internet Protocols, specifically IPv6, cannot sufficiently meet the needs of these networks.

To overcome these issues, the Internet Engineering Task Force (IETF) chartered the LPWAN working group (WG) in 2016 to identify common functionality needs across LPWANs and to standardize the protocols that could enable these functionalities across the various networks.

The goal of the IETF LPWAN WG is to converge the diverse LPWAN radio technologies toward a common hourglass model that will provide users with a standard management strategy across networks and enable common Internet-based services to the applications.

To achieve this goal, the IETF LPWAN WG has produced the Static Context Header Compression and Fragmentation (SCHC) [2] specification, an ultralightweight adaptation layer uniquely designed to support the extremely restricted communication resources of LPWAN technologies.

Note 2.  SCHC is expected to become a recognized acronym like several  other IETF protocols (e.g. HTTP, TCP, DHCP, DNS, IP, etc.).  Please see illustration below of SCHC Architecture.

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SCHC will soon be published as a new IETF RFC.  Again, it’s objective is to achieve interoperability across the leading LPWANs, including Sigfox, LoRaWAN, NB-IoT and IEEE 802.15.4w(LPWA) [3].

Note 3.  IEEE 802.15.4w or LPWA

Low Power Wide Area Network (LPWAN) extension to the IEEE Std 802.15.4 LECIM PHY layer to cover network cell radii of typically 10-15km in rural areas and deep in-building penetration in urban areas. It uses the LECIM FSK (Frequency Shift Keying) PHY modulation schemes with extensions to lower bit-rates (e.g. payload bit-rate typically < 30 kb/s). Additionally, it extends the frequency bands to additional sub-GHz unlicensed and licensed frequency bands to cover the market demand. For improved robustness in channels with high levels of interference, it defines mechanisms for the fragmented transmission of Forward Error Correction (FEC) code-words, as well as time and frequency patterns for the transmission of the fragments. Furthermore, it defines lower code rates of the FEC in addition to the K=7 R=1/2 convolutional code. Modifications to the Medium Access Control (MAC) layer, needed to support this PHY extension, are defined.

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Why do LPWANs need their own interoperability standard?

The common characteristics of LPWANs include a power-optimized radio network, a simple star network topology, frame sizes in the order of tens of bytes transmitted a few times per day at ultra-low speeds, and a mostly upstream transmission pattern that allows devices to spend most of their time in sleep mode. These characteristics lead to ultra-long-range networks that allow for connected devices to have an extremely long battery life and be sold at a very low cost, enabling simple and scalable deployments.

LPWANs are especially well-suited for deployments in environments where battery recharging or swapping is not an option and where only a very low rate of data reporting is required. Also, LPWAN networks are fundamentally different than other networks, as they have been designed to handle infrequent message exchanges of payloads as small as approximately 10 bytes.

To manage these very specific constraints, the IETF has developed the SCHC adaptation layer, which is located between the network layer (e.g. IPv6) and the underlying LPWAN radio technology. SCHC comprises two independent sublayers – header compression and fragmentation – which are critical to meeting the specific characteristics of LPWANs.

The SCHC header compression sublayer has been tailored specifically for LPWAN technologies, and it is capable of compressing protocols such as IPv6, UDP and CoAP. It relies on the infrequent variability of LPWAN applications to define static contexts that are known a priori to both protocol end points.

The SCHC fragmentation sublayer, on the other hand, offers a generic approach to provide both data reliability and the capability of transmitting larger payload sizes over the extremely constrained LPWAN packet sizes and the extremely severe message rate limitations. Even though the fragmentation sublayer mechanisms have been designed to transport long IPv6 packets, they can equally be applied to non-IP data messages and payloads, as the functionality can be implemented independent of the header compression.

In order to be fully operational across LPWAN technologies, SCHC has been developed by the IETF under a generic and flexible approach that aims to address the common and unique requirements of these networks. The SCHC specification offers enough flexibility to optimize the parameter settings that need to be used over each LPWAN technology.

The IETF LPWAN WG is now working on the development of different SCHC profiles optimized for each individual LPWAN technology, including Sigfox, LoRaWAN, NB-IoT and IEEE 802.15.4w. Future work also includes definition of data models to represent the static contexts, as well as operation, administration and management (OAM) tools for LPWANs.

Here’s an illustration of the Sigfox SCHC:

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From the early stage IETF Sigfox SCHC profile spec:

The Static Context Header Compression (SCHC) specification describes a header compression scheme and a fragmentation functionality for Low Power Wide Area Network (LPWAN) technologies.
SCHC offers a great level of flexibility that can be tailored for different LPWAN technologies. 
The present (early stage) document provides the optimal parameters and modes of operation when SCHC is implemented over a Sigfox LPWAN.

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Addendum –by Alan J Weissberger

IEEE definition of IoT:

“An IoT system is a network of networks where, typically, a massive number of objects, things, sensors or devices are connected through communications and information infrastructure to provide value-added services via intelligent intelligent data processing processing and management management for different different applications (e.g. smart cities, smart health, smart grid, smart home, smart transportation, and smart shopping).”
— IEEE Internet of Things Journal

IoT communications over LPWANs should be:
 Low cost,
 Low power,
 Long battery life duration,
 High number of connections,
 Low bitrate,
 Long range,
 Low processing capacity,
 Low storage capacity,
 Small size devices,
 Simple network architecture and protocols

Also see IETF draft RFC 8376  LPWAN Overview

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Sigfox Network Characteristics:

 First LPWAN Technology
 The physical layer based on an Ultra-Narrow band wireless modulation
 Proprietary system
 Low throughput ( ~100 bps)
 Low power
 Extended range (up to 50 km)
 140 messages/day/device
 Subscription-based model
 Cloud platform with Sigfox –defined API for server access
 Roaming capability

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References:

https://www.ackl.io/blog/ietf-standardization-working-group-enabling-ip-connectivity-over-lpwan

https://techblog.comsoc.org/2017/10/25/lora-wan-and-sigfox-lead-lpwans-interoperability-via-compression/

https://tools.ietf.org/html/draft-ietf-lpwan-schc-over-sigfox-00