ZigBee and IEEE 802.15.4

Network topologies and communication models AODV Routing Lecture Overview Technical overview Applications / uses Device Types Protocols Network topologies and communication models AODV Routing Network Technology Richard Anthony The University of Greenwich

The 802 Wireless Space ZigBee is NOT intended to provide high bandwidth service of WLAN ZigBee is NOT intended to provide moderate bandwidth wireless link over short range (as Bluetooth does) ZigBee IS intended to provide LOW bandwidth, robust connectivity at moderate range. ZigBee IS ideally suited to sensing and control applications. Network Technology Richard Anthony The University of Greenwich

General characteristics Overall design goals Low power, low cost, flexible wireless communication technology Range 10 – 100M low power, up to 1000M high power Data rates 20 kilobits/second to 250 kilobits/second Transmission frequency 2.4 GHz in the Industrial, Scientific and Medical (ISM) radio band Transmission power levels 1mW and 100mW Communication structure / Topology Ad-hoc, dynamic Network Technology Richard Anthony The University of Greenwich

IEEE 802.15.4 defines the Physical layer and MAC layer for ZigBee. ZigBee extends IEEE 802.15.4 IEEE 802.15.4 defines the Physical layer and MAC layer for ZigBee. This gives a well-defined, standardised wireless link. ZigBee adds a number of features to facilitate full wireless networks for communities of devices: Device discovery and neighbour associations Device roles Logical grouping (PAN ID) Routing …. Addressing 64-bit IEEE address – Unique address – ‘burned in’ 16-bit Node-ID – Assigned when a node joins a network Network Technology Richard Anthony The University of Greenwich

– problematic for low-resourced sensor nodes ZigBee extends IEEE 802.15.4 Various ZigBee modules available, some e.g. ‘EasyBee’ only cover the wireless transmission and require the higher protocol stack to be implemented in software. – problematic for low-resourced sensor nodes – potentially useful if only a simple point-point link is needed. Network Technology Richard Anthony The University of Greenwich

Determines if a neighbouring device belongs to the network. ZigBee Network layer Performs routing (uses AODV see later) Determines if a neighbouring device belongs to the network. Discovers new neighbours (any role, so includes routers).

ZigBee Application layer (ZigBee system interface to end users). Deals with Addressing ! 64-bit IEEE address – fixed 16-bit Node ID – dynamically assigned Devices within a node identified by Endpoint Identifier (1 – 240) Comprises components added by ZigBee spec (on top of IEEE 802.15.4 radio link spec), includes: ZigBee Device Object (ZDO) responsible for defining role of a device (Coordinator / End device) and for the discovery of new (one-hop) devices and the identification of their offered services. Application objects (application specific data objects). Application support sublayer (APS) offers a well-defined interface and control services. Serves as a bridge between the network layer and the other components of the application layer: manages binding tables – maps devices to the services offered by the discovered devices.

Application domains targeted by ZigBee: Application Profiles released Smart Energy 1.0 Home Automation Health Care RF4CE – Remote Control Telecommunication services Application Profiles in development Smart Energy 2.0 Building Automation Retail Services Network Technology Richard Anthony The University of Greenwich

End Device (E) one or more (FFD or RFD, see below) Device Types A ZigZee network comprises several different ‘types’ of device (assigned roles) Coordinator (C) one End Device (E) one or more (FFD or RFD, see below) Router (R) zero or more (FFD, see below) Classes of devices: Reduced-Functionality device (RFD) Full-Functional device (FFD) FFD nodes can form networks of any type (mesh, star, hybrid). RFD nodes can only connect to a full function node. Network Technology Richard Anthony The University of Greenwich

Device Types - Coordinator The Coordinator is the logical equivalent of an access point in a wireless network, in terms of device connectivity, or a master node in Bluetooth, The role of ‘Coordinator’ is a network-level role (i.e. for establishing the network) and not necessarily an application-level role. The Coordinator can be the data sink for sensing applications, and can issue commands in remote control applications. However, a node does not have to be the Coordinator to carry out such functions. Network Technology Richard Anthony The University of Greenwich

Device Types – End device End devices are the nodes deployed within a given application to perform sensing and actuation. These devices will often be optimised to use low power because will often be placed in low-accessibility positions, or will possibly be attached to mobile objects (consider a trolly in a hospital). Most sensor applications fall natively into the RFD class, with extended networks making use of both FFDs and network coordinators to form bridges and links required by the network topology. Network Technology Richard Anthony The University of Greenwich

A series of hops through several routers may be required. Device Types – Router A Router is a device (class FFD) that will forward messages between end devices and the coordinator. A series of hops through several routers may be required. End nodes may be configured to act as routers: Routers also need to use low amounts of power Routers may be simple sensor devices with low processing resources - therefore need a simple and robust routing strategy: A modified form of AODV routing is used. Network Technology Richard Anthony The University of Greenwich

Network topologies and communication models Simple topologies Wireless link to remote data source Wireless sensing of an environment A router is positioned to extend range ZigBee Coordinator ZigBee Router ZigBee End Node ZigBee End Node and Router Wireless sensing of remote environment Network Technology Richard Anthony The University of Greenwich

Network topologies and communication models Mesh network topologies Routers facilitate a mesh network Multi-path connectivity provides robustness but adds complexity and overheads Generic concept of a mesh connecting the end-devices to the Coordinator Redundant links provide robustness Network Technology Richard Anthony The University of Greenwich

Network topologies and communication models Application-specific topologies Densely sensed area # 1 Densely sensed area # 2 Sparsely sensed area An example where environmental sensing is non-uniform. The application could be, for example, security systems at an airport Network Technology Richard Anthony The University of Greenwich

Network topologies and communication models Topology / connectivity formation Each node maintains Neighbour, Routing and Binding Tables Coordinator is responsible for initialising, maintaining and controlling the network. An End device is said to ‘belong’ to a parent node A parent node can be the coordinator or a router. The End device polls to find a Parent node. The parent node ‘discovers’ the end device by receiving these polls. The parent node adds the end device to a Table Network Technology Richard Anthony The University of Greenwich

ZigBee hardware - Modules ‘EasyBee’ provides the wireless hardware and minimal ZigBee support (need to implement the protocol stack in software) Antenna is a track on the PCB ‘ProBee’ module supports almost entire ZigBee Pro spec. Various antenna options Antenna screws onto here Network Technology Richard Anthony The University of Greenwich

ZigBee hardware – Evaluation Board and Modules (1) ProBee module on ZigBee evaluation board – with ‘chip’ antenna Network Technology Richard Anthony The University of Greenwich

ZigBee hardware – Evaluation Board and Modules (2) ProBee module on ZigBee evaluation board – with Dipole antenna. Network Technology Richard Anthony The University of Greenwich

Very small physical module, no RF amplifier is needed Transmission Power The standard ZigBee transmission power is approximately 1mW. This power level can be driven directly from the ZigBee chips. Advantages include: Very small physical module, no RF amplifier is needed No external antenna is needed, can be on-chip / on circuit board Very low power usage when transmitting. Usable range of about 10m to 100m depending on the environment, antenna, and frequency band. An amplifier can be added, boosting power to approximately 100mW. This power level gives: Far greater range. Better SNR and performance in terms of dealing with obstacles. Shortens battery life or requires replenishable power source. Network Technology Richard Anthony The University of Greenwich

Transmission Power – Range illustration Image from http://www.rfm.com/products/apnotes/wp_zigbeepoweroptions.pdf Network Technology Richard Anthony The University of Greenwich

Application-specific transmission power settings Example – Livestock monitoring on a farm Enclosure area # 2 Enclosure area # 1 Enclosure area # 3 Key Low power (1mW) link used by small collar mounted devices with small battery 100mW link between fixed devices with (large battery, solar or mains power) Network Technology Richard Anthony The University of Greenwich

Protocols – Multiplexing and Separation and coding Channel There are 16 ZigBee channels in the 2.4 GHz band. Each channel requires 5 MHz of signal bandwidth (separation). The 2.4 GHz band Each channel can transmit up to 250 kbit/s. (actual data throughput < max specified bit rate due to packet overhead and processing delays). Coding The radios use direct-sequence spread spectrum coding (DSSS) Offset quadrature phase-shift keying (OQPSK) is used in the 2.4 GHz band (enables transmission of four bits per symbol). Grouping A ‘PAN ID’ enables logical grouping / separation of nodes. - A 16-bit value which is used to uniquely define a PAN.  - Devices identify which networks to join based on their PAN ID. Network Technology Richard Anthony The University of Greenwich

Protocols (Transmission options) Each transmission can be sent secured or unsecured Acknowledged: originator data is confirmed by recipient Unacknowledged: no confirmation Unicast: send to a specific recipient Broadcast: send to all recipients Multichannel: Originator attempts Tx using frequency re-acquisition mechanism Single channel: Originator attempts Tx on the expected channel Network Technology Richard Anthony The University of Greenwich

Protocols (discovery) Device discovery Find other ZigBee devices that can be paired to. Can be attempted repeatedly - for fixed duration, or - until sufficient number of responses received Service discovery Information is exchanged between a pair of devices: Device capabilities: target / controller mains / battery power Vendor information Application information (user-defined description of the device’s functionality – e.g. Entrance foyer camera control) Requested Device type (type of device requested through discovery, e.g. a camera control console might search for PZT cameras) Network Technology Richard Anthony The University of Greenwich

Routing – Ad hoc On-Demand Distance Vector (AODV) Used in ZigBee’s network layer. Achieves robust routing in highly dynamic networks (designed for wireless sensor networks (WSN) and mobile ad hoc networks (MANETs). Reactive - establishes a route to a destination on demand. (Internet routing protocols are proactive; establishment of routing paths is a separate activity to the transmission of packets over those paths. Paths set up in advance of being needed; paths re-established automatically if topology changes.) Establishes a mesh network through the use of broadcast communication. A node broadcasts a route request to all of its neighbours. The neighbours forward broadcast the request to their neighbours, etc. until the destination is reached (this staged broadcast technique is termed ‘flooding’). Destination receives multiple copies and determines the shortest path taken. The route reply is unicast following the lowest cost path back to the source. The source then updates its routing table for the destination address with the next hop in the path and the path cost (number of hops). Network Technology Richard Anthony The University of Greenwich

Routing – AODV in Operation 1 2 3a 3b 4 End Device has data to send to sink, passes message to its parent node. Router broadcasts route request to its neighbours. Neighbouring routers forward broadcast the route request (flood, steps 3a and 3b nearly simultaneous). (Assuming that ‘3a’ message arrives first at the Coordinator) The route reply is sent back – using the route. +Ve points: adaptive, robust -Ve points: latency, overheads Network Technology Richard Anthony The University of Greenwich