Meshes

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# 181U Spring 2020
## Wireless Mesh Networks

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# Agenda

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* Mesh Networks
* IEEE 802.15.4
* Zigbee
* Thread

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# IEEE 802.15.4

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A specification for use of license-free radio bands

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# EEEE 802.15.4 Data Frame Format

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* Provides up to 102 Byte data payload capacity
* Data sequence numbering to ensure that packets are tracked
* Frame Check Sequence (FCS) validates error-free data
  - min. 16 Bytes = 128 bits = 0.512 ms @ 250 kbps
  - max. 133 Bytes = 1064 bits = 4.256 ms @ 250 kbps

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#  IEEE 802.15.5 Device Types

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* Full Function Device
  - Talks to several devices
  - Normally powered
  - Can route messages

* Reduced function deice
  - Talks to "parent"
  - May sleep
  - Network edge device

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# Network Topology Models

![](images/zigbeetopologies.jpg# w-50pct fr)

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* Star Network
  - Lowest complexity
  - Limited range
  - Coordinator can be a bottleneck

* Mesh Network
  - Most complex
  - Highest relabilty
  - Fewer bottlenecks

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# Zigbee

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* Based on IEEE 802.15.4, provides the higher
functions up to the application layer for WPAN
  - Mesh topology
  - Short range, 
  - 20 to 250 kbps
  - 2.4 GHz, 915 MHz or 868 MHz
  - Channels 2 MHz wide with Direct Sequence
  - Spread Spectrum media access

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# Zigbee Applications

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# Zigbee is Standardized at all layers

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# Zigbee Network

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There are three logical device types
* Coordinator (one per network)
* Router
* End device
Only the end device is typically battery powered

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# Comparison of Roles at Network Layer

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# Zigbee Application Layer

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* Application layer is defined for interoperation
* Based upon "profiles" for specific types of applications
   - smart lighting
   - smart energy
* Focus is on ensuring "common case" is support relatively seamlessly
* Focus on "certification" that products conform to communication profiles (but not
  that they meet functional requirements)

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# What is Thread

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A low power, secure and Internet-based mesh
networking technology for IoT products.

* Uses IPv6 (6LoWPAN)
* Runs on existing 802.15.4 silicon from multiple providers
* Legacy-free design with updated architecture
  - Designed with a new security architecture to make it
  - simple and secure to add and remove products
  - Supports 250+ products per network
  - Designed for very low power operation

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# Thread Network Topology

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# Network Topology Roles

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# Low Power Operation: "Sleepy Devices"
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* Sleeping devices poll parents for messages
* Sleeping devices not required to check which allows lower power
operation
* Parents hold messages for sleeping devices
* Sleeping device automatically
switches parent connection is lost

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# Secure Commissioning
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* Simple Commissioning
  - User authorizes devices onto the network using smart phone or web
  - Can be done on network if there is a device with a graphical interface
*  DTLS Security session established between new device
and commissioning device to authenticate and provide
credentials
* Once commissioning session is done, device attaches to
network
* MAC security used for all messages
* Application level security is based on end-device
requirements and application layer being used

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# No single point of failure

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* Dynamic Leaders
  - If Leader fails, another Router will become Leader
* Router Promotion
  - Leader can promote Router Eligible devices to Routers to improve
connectivity if required

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# Multiple Border Routers

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* Multiple Border Routers can be used for
off network access
  - Devices operate without Border Router
* What can be a Border Router?
  - Anything with an 802.15.4 radio and another physical layer
  - Home Wi-Fi router
  - Set top box
  - Smart Thermostat (802.15.4 and Wi-Fi)

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# Thread Network

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# The Thread Standard

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* Built on existing IEEE 802.15.4 standard
* Uses 6LoWPAN for L3+ [RFC6282]
  - UDP traffic over modified IPv6
  - Easily to gateway to the cloud or smartphones
* Supports 250+ products per network
* Just a firmware upgrade to existing 802.15.4 radios 
  - Works in the 2.4 GHz ISM band world-wide
* Security using Datagram Transport Layer Security
(DTLS) [RFC6347]
  - Provides integrity, authentication and confidentiality

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# Limitations of 802.15.4 Frames

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* The maximum length of an 802.15.4 frame is 127
octets
* Typical frame overhead of 22 octets leaves 105 octets
* If you add the 21-byte encryption overhead (AES-
CCM-128) then you only have 84 octets for L3+
* For IP-style routing, you’ll need source and
destination addresses and an 8 octet UDP header
   - That is, 76 octets - (size of source + destination
addresses)
   - IPv4 addresses are 4 octets and IPv6 addresses are 16
octets each

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# How Does Thread Fit ?

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Thread defines only to L4

* Applications protocol is
available for customization
* UDP messaging and ACK
* Supports multicast to the
network
  - Potential for mDNS
* Examples include:
  - Zigbee Smart Energy 2.0
  - CoAP and Smart Objects
  - ECHONET Lite
  - Zigbee Cluster Library

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# 6LowPAN

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* IPv6 over Low-power Wireless Personal Area
Networks
  - The 6LoWPAN IETF working group created a
standard that allows IPv6 packets to be sent over
IEEE 802.15.4 networks [RFC6282]
* Encapsulation and header compression that
makes IPv6 packets small enough to transmit
in the 127-byte 802.15.4 frames and still
leave room for payload
* IPv4 is a complex protocol for a router
  - Many fields to look at and variable-length header
* IPv6 header is a fixed length with no checksums
  - Easier for low-resource CPUs to route

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# Summary

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* Acknowledgements -- Slide Material drawn from:
   - "Wireless standards for IoT" by Ermanno Pietrosemoli: http://wireless.ictp.it/school_2019/slides/WirelessstandardsforIoT.pdf
   - "Zigbee: The Standard for IoT " The zigbee alliance
   - "Understanding the Thread Interface of Google's Nest Devices" by Mike Anderson
   - "Developing IoT Devices with Thread"  https://www.threadgroup.org/Portals/0/documents/support/ThreadWebinarJun18final_2596_1.pdf

- Cover image: https://www.threadgroup.org/BUILT-FOR-IOT/Commercial#NetworkTopology