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LPWAN Working Group                                           JC. Zuniga
Internet-Draft                                                B. Ponsard
Intended status: Informational                                    SIGFOX
Expires: June 7, 2018                                  December 04, 2017


                       SIGFOX System Description
            draft-zuniga-lpwan-sigfox-system-description-04

Abstract

   This document presents an overview of the network architecture and
   system characteristics of a typical SIGFOX Low Power Wide Area
   Network (LPWAN).  It is intended to be used as background information
   by the IETF LPWAN group when defining system requirements of
   different LPWAN technologies that are suitable to support common IP
   services.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 7, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  System Architecture . . . . . . . . . . . . . . . . . . . . .   3
   4.  Radio Spectrum  . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Radio Protocol  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Uplink  . . . . . . . . . . . . . . . . . . . . . . . . .   6
       5.1.1.  Uplink Physical Layer . . . . . . . . . . . . . . . .   6
       5.1.2.  Uplink MAC Layer  . . . . . . . . . . . . . . . . . .   6
     5.2.  Downlink  . . . . . . . . . . . . . . . . . . . . . . . .   7
       5.2.1.  Downlink Physical Layer . . . . . . . . . . . . . . .   7
       5.2.2.  Downlink MAC Layer  . . . . . . . . . . . . . . . . .   7
     5.3.  Synchronization between Uplink and Downlink . . . . . . .   8
   6.  Network Deployment  . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   10. Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document presents an overview of the network architecture and
   system characteristics of a typical SIGFOX LPWAN, which is in line
   with the terminology and specifications defined by ETSI [etsi_unb].
   It is intended to be used as background information by the IETF LPWAN
   group when defining system requirements of different LPWANs that are
   suitable to support common IP services.

   LPWAN technologies are a subset of IoT systems which specifically
   enable long range data transport (e.g. distances up to 50 km in open
   field), are capable to communicate with underground equipment, and
   require minimal power consumption.  Low throughput transmissions
   combined with advanced signal processing techniques provide highly
   effective protection against interference.  LPWAN technologies can
   also cooperate with cellular networks to address use cases where
   redundancy, complementary or alternative connectivity is needed.

   Because of these characteristics, LPWAN systems are particularly well
   adapted for low throughput IoT traffic.  SIGFOX LPWAN autonomous
   battery-operated devices send only a few bytes per day, week or month
   in an asynchronous manner and without the needed for central
   coordination, which allows them to remain on a single battery for up
   to 10-15 years.



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2.  Terminology

   The following terms are used throughout this document:

      Base Station (BS) - A Base Station is a radio hub, relaying
      messages between DEVs and the SC.

      Device Application (DA) - An application running on the DEV or EP.

      Device (DEV) - A Device (aka end-point) is a leaf node of a LPWAN
      that communicates application data between the local device
      application and the network application.

      End Point (EP) - An End Point (aka device) is a leaf node of a
      LPWAN that communicates application data between the local device
      application and the network application.

      Low-Power Wide-Area Network (LPWAN) - A system comprising several
      BSs and millions/billions of DEVs, characterized by the extreme
      low-power consumption, long-range of transmission, and typically
      connected in a star network topology.

      Network Application (NA) - An application running in the network
      and communicating with the device(s).

      Registration Authority (RA) - The Registration Authority is a
      central entity that contains all allocated and authorized Device
      IDs.

      Service Center (SC) - The SIGFOX network has a single service
      centre.  The SC performs the following functions:

      *  DEVs and BSs management

      *  DEV authentication

      *  Application data packets forwarding

      *  Cooperative reception support

3.  System Architecture

   Figure 1 depicts the different elements of the system architecture:








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                +---+
                |DEV| *                    +------+
                +---+   *                  |  RA  |
                          *                +------+
                +---+       *                 |
                |DEV| * * *   *               |
                +---+       *   +----+        |
                              * | BS | \  +--------+
                +---+       *   +----+  \ |        |
        DA -----|DEV| * * *               |   SC   |----- NA
                +---+       *           / |        |
                              * +----+ /  +--------+
                +---+       *   | BS |/
                |DEV| * * *   * +----+
                +---+         *
                            *
                +---+     *
                |DEV| * *
                +---+



                   Figure 1: SIGFOX network architecture

   SIGFOX has a "one-contract one-network" model allowing devices to
   connect in any country, without any need or notion of either roaming
   or handover.

   The architecture consists of a single cloud-based core network, which
   allows global connectivity with minimal impact on the end device and
   radio access network.  The core network elements are the Service
   Center (SC) and the Registration Authority (RA).  The SC is in charge
   of the data connectivity between the Base Stations (BSs) and the
   Internet, as well as the control and management of the BSs and
   Devices.  The RA is in charge of the Device network access
   authorization.

   The radio access network is comprised of several BSs connected
   directly to the SC.  Each BS performs complex L1/L2 functions,
   leaving some L2 and L3 functionalities to the SC.

   The Devices (DEVs) or End Points (EPs) are the objects that
   communicate application data between local device applications (DAs)
   and network applications (NAs).

   Devices can be static or nomadic, as they associate with the SC and
   they do not attach to any specific BS.  Hence, they can communicate




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   with the SC through one or multiple BSs without needing to signal for
   handover or roaming.

   Due to constraints in the complexity of the Device, it is assumed
   that Devices host only one or very few device applications, which
   most of the time communicate each to a single network application at
   a time.

4.  Radio Spectrum

   The coverage of the cell depends on the link budget and on the type
   of deployment (urban, rural, etc.).  The radio interface is compliant
   with the following regulations:

      Spectrum allocation in the USA [fcc_ref],

      Spectrum allocation in Europe [etsi_ref],

      Spectrum allocation in Japan [arib_ref].

   At present, the SIGFOX radio interface is also compliant with the
   local regulations of the following countries: Australia, Brazil,
   Canada, Kenya, Lebanon, Mauritius, Mexico, New Zealand, Oman, Peru,
   Singapore, South Africa, South Korea, and Thailand.

5.  Radio Protocol

   The radio interface is based on Ultra Narrow Band (UNB)
   communications, which allow an increased transmission range by
   spending a limited amount of energy at the device.  Moreover, UNB
   allows a large number of devices to coexist in a given cell without
   significantly increasing the spectrum interference.

   Since the radio protocol is connection-less and optimized for uplink
   communications, the capacity of a SIGFOX base station depends on the
   number of messages generated by devices, and not on the actual number
   of devices.  Likewise, the battery life of devices depends on the
   number of messages generated by the device.  Depending on the use
   case, devices can vary from sending less than one message per device
   per day, to dozens of messages per device per day.

   Both uplink and downlink are supported, although the system is
   optimized for uplink communications.  Due to spectrum optimizations,
   different uplink and downlink frames and time synchronization methods
   are needed.






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5.1.  Uplink

5.1.1.  Uplink Physical Layer

   The main radio characteristics of the UNB uplink transmission are:

   o  Occupied bandwidth: 100 Hz / 600 Hz (depending on the region)

   o  Uplink baud rate: 100 baud / 600 baud (depending on the region)

   o  Modulation scheme: DBPSK

   o  Uplink transmission power: compliant with local regulation

   o  Link budget: 155 dB (or better)

   o  Central frequency accuracy: not relevant, provided there is no
      significant frequency drift within an uplink packet transmission

   For example, in Europe the UNB uplink frequency band is limited to
   868.00 to 868.60 MHz, with a maximum output power of 25 mW and a
   maximum mean transmission time of 1%.

5.1.2.  Uplink MAC Layer

   The format of the uplink frame is the following:



   +--------+--------+--------+------------------+-------------+-----+
   |Preamble|  Frame | Dev ID |     Payload      |Msg Auth Code| FCS |
   |        |  Sync  |        |                  |             |     |
   +--------+--------+--------+------------------+-------------+-----+



                       Figure 2: Uplink Frame Format

   The uplink frame is composed of the following fields:

   o  Preamble: 19 bits

   o  Frame sync and header: 29 bits

   o  Device ID: 32 bits

   o  Payload: 0-96 bits




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   o  Authentication: 16-40 bits

   o  Frame check sequence: 16 bits (CRC)

5.2.  Downlink

5.2.1.  Downlink Physical Layer

   The main radio characteristics of the UNB downlink transmission are:

   o  Occupied bandwidth: 1.5 kHz

   o  Downlink baud rate: 600 baud

   o  Modulation scheme: GFSK

   o  Downlink transmission power: 500 mW / 4W (depending on the region)

   o  Link budget: 153 dB (or better)

   o  Central frequency accuracy: Centre frequency of downlink
      transmission are set by the network according to the corresponding
      uplink transmission

   For example, in Europe the UNB downlink frequency band is limited to
   869.40 to 869.65 MHz, with a maximum output power of 500 mW with 10%
   duty cycle.

5.2.2.  Downlink MAC Layer

   The format of the downlink frame is the following:



   +------------+-----+---------+------------------+-------------+-----+
   |  Preamble  |Frame|   ECC   |     Payload      |Msg Auth Code| FCS |
   |            |Sync |         |                  |             |     |
   +------------+-----+---------+------------------+-------------+-----+



                      Figure 3: Downlink Frame Format

   The downlink frame is composed of the following fields:

   o  Preamble: 91 bits

   o  Frame sync and header: 13 bits



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   o  Error Correcting Code (ECC): 32 bits

   o  Payload: 0-64 bits

   o  Authentication: 16 bits

   o  Frame check sequence: 8 bits (CRC)

5.3.  Synchronization between Uplink and Downlink

   The radio interface is optimized for uplink transmissions, which are
   asynchronous.  Downlink communications are achieved by devices
   querying the network for available data.

   A device willing to receive downlink messages opens a fixed window
   for reception after sending an uplink transmission.  The delay and
   duration of this window have fixed values.  The network transmits the
   downlink message for a given device during the reception window, and
   the network also selects the base station (BS) for transmitting the
   corresponding downlink message.

   Uplink and downlink transmissions are unbalanced due to the
   regulatory constraints on the ISM bands.  Under the strictest
   regulations, the system can allow a maximum of 140 uplink messages
   and 4 downlink messages per device.  These restrictions can be
   slightly relaxed depending on system conditions and the specific
   regulatory domain of operation.

6.  Network Deployment

   As of today, SIGFOX's network has been fully deployed in 17
   countries, with ongoing deployments on 29 other countries, giving in
   total a geography of 2.6 million square kilometers, containing 660
   million people.  The single core network model allows devices to
   connect in any country, without any notion of roaming or handover.

   The vast majority of the current applications are sensor-based,
   requiring solely uplink communications, followed by actuator-based
   applications, which make use of bidirectional (i.e. uplink and
   downlink) communications.

   Similar to other LPWAN technologies, the sectors that currently
   benefit from the low-cost, low-maintenance and long battery life are
   agricultural and environment, public sector (smart cities, education,
   security, etc.), industry, utilities, retail, home and lifestyle,
   health and automotive.





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7.  IANA Considerations

   N/A.

8.  Security Considerations

   Due to the nature of low-complexity devices, it is assumed that
   Devices host only one or very few device applications, which most of
   the time communicate each to a single network application at a time.

   The radio protocol authenticates and ensures the integrity of each
   message.  This is achieved by using a unique device ID and an AES-128
   based message authentication code, ensuring that the message has been
   generated and sent by the device with the ID claimed in the message.

   Application data can be encrypted at the application level or not,
   depending on the criticality of the use case, to provide a balance
   between cost and effort vs. risk.  AES-128 in counter mode is used
   for encryption.  Cryptographic keys are independent for each device.
   These keys are associated with the device ID and separate integrity
   and confidentiality keys are pre-provisioned.  A confidentiality key
   is only provisioned if confidentiality is to be used.

9.  Acknowledgments

   The authors would like to thank Olivier Peyrusse for the useful
   inputs and discussions about ETSI UNB SRD.

10.  Informative References

   [arib_ref]
              "ARIB STD-T108 (Version 1.0): 920MHz-Band Telemeter,
              Telecontrol and data transmission radio equipment.",
              February 2012.

   [etsi_ref]
              "ETSI EN 300-220 (Parts 1 and 2): Electromagnetic
              compatibility and Radio spectrum Matters (ERM); Short
              Range Devices (SRD); Radio equipment to be used in the 25
              MHz to 1 000 MHz frequency range with power levels ranging
              up to 500 mW", May 2016.

   [etsi_unb]
              "ETSI TR 103 435 System Reference document (SRdoc); Short
              Range Devices (SRD); Technical characteristics for Ultra
              Narrow Band (UNB) SRDs operating in the UHF spectrum below
              1 GHz", February 2017.




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   [fcc_ref]  "FCC CFR 47 Part 15.247 Telecommunication Radio Frequency
              Devices - Operation within the bands 902-928 MHz,
              2400-2483.5 MHz, and 5725-5850 MHz.", June 2016.

Authors' Addresses

   Juan Carlos Zuniga
   SIGFOX
   425 rue Jean Rostand
   Labege  31670
   France

   Email: JuanCarlos.Zuniga@sigfox.com
   URI:   http://www.sigfox.com/


   Benoit Ponsard
   SIGFOX
   425 rue Jean Rostand
   Labege  31670
   France

   Email: Benoit.Ponsard@sigfox.com
   URI:   http://www.sigfox.com/



























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