ZigBee White Paper 1

Transcription

1 ZigBee White Paper 1

2 Table of Contents Scope 3 Why ZigBee 3 What is ZigBee? 3 Technical Details 4 PAN ID 5 Channel/Frequency 6 Coexistence with Other Wireless Technologies 7 Interoperability 8 2

3 Scope This document answers the basic question of What is ZigBee? and why this technology has been chosen by AMX. The purpose of this document is to provide information to AMX customers, IT professionals, and persons installing and configuring AMX ZigBee products. Why ZigBee AMX researched and evaluated several technologies prior to adopting ZigBee as our low-end wireless solution. The ultimate driver of the decision was the requirements we set for the technology. The significant requirements are (in no particular order): Standardized solution Interoperability Low-power Reliability Low-cost Co-existence Security The choice of a standardized solution that is interoperable is paramount to the strategic direction of AMX using standards based solutions. The technologies considered by AMX include Bluetooth, , Z-Wave, and ZigBee. All proprietary solutions (including our own) are ranked very low due to the lack of standardization and interoperability. What is ZigBee? ZigBee is an alliance of almost 200 companies which contribute to and/or utilize the ZigBee specifications. AMX is a member of the ZigBee Alliance. These companies include well-known companies such as Honeywell, Motorola, Philips, Samsung, Siemens, and Texas Instruments. All of these companies are working together to define a set of standards for interoperability for a range of markets that currently include: Commercial Building Automation, Home Automation, and Industrial Automation. 3

4 Technical Details ZigBee is built on top of the IEEE standard. It operates in the industrial, scientific, and medical (ISM) radio bands; 868 MHz in Europe, 915 MHz in the US, and 2.4 GHz in most countries world-wide. The 2.4 GHz version of ZigBee has become the de-facto standard of ZigBee networks, and has been implemented in AMX ZigBee product offerings. IEEE General Characteristics: GHz 250Kbps bit rate Direct Sequence Spread Spectrum (DSSS) 16 non-overlapping channels (5 MHz separation) 2 MHz channel bandwidth O-QPSK Modulation Receiver sensitivity -85 dbm or better Transmit power +6 db AES Encryption IEEE specifies a Wireless Medium Access Control (MAC) and Physical Layer (PHY) for a Low-Rate Wireless Personal Area Network (LR-WPAN). Wireless personal area networks (WPANs) are used to convey information over short distances with little or no infrastructure. The IEEE specification provides some mesh network functionality but not enough to completely implement a mesh network. The ZigBee specification fills in the gap left by to create a true mesh network. A mesh network is a network topology where nodes communicate to and through neighboring nodes. They are generally characterized as being self-healing because if one route becomes unusable, the network will find another route to deliver the message. The graphic below depicts a typical star network, such as , and a mesh network topology: 4

5 Notice that both of these networks have a Network coordinator whose function is to administrate the network. Administration consists primarily of forming the network and allowing/dis-allowing nodes to join. Once the network is formed and nodes have joined, the network coordinator is essentially just another node in the network. Every network has a unique 16-bit identifier called a PAN ID (Personal Area Network Identifier) and all nodes in the network are assigned to the same PAN ID. For a star network, whenever a node wants to send a message to another node it sends the message to the hub (node F) of the star and it resends the message to the destination. The hub node is a potential point of failure in a star network and also could limit overall network performance by becoming a bottleneck to communication. In a mesh network, the sender of a message can either send it directly to the destination node or can send the message to a neighbor who will send it along its path. The multiple communication paths of a mesh network create redundant paths such that if one path is unavailable, another path can be taken. Since the NetLinx system is a master-slave arrangement, a ZigBee gateway is used to transfer messages from Ethernet to wireless nodes and vise-versa. Wireless nodes exclusively communicate with the NetLinx master via other wireless nodes and the ZigBee gateway. Depicted below is a typical NetLinx system with ZigBee devices: Considering the depiction above, suppose when node C joins the network it finds that its best route to the gateway is through the repeater F. It determined that F was its best route by using a measure called Link Quality Indication (LQI) that is calculated by the node. Suppose that F became unplugged or an obstacle was place between C and F such that communication between C and F wasn t possible. Node C would discover that its route through node F had failed and begin searching for a new route. It would find routes through nodes B and D. Using LQI calculations for both routes, B and D, it would choose the best route. Later, when node F communication was possible again, node C will revert to using the route through F. PAN ID The PAN ID of a ZigBee network is analogous to the SSID of an network. The PAN ID is simply a 16-bit number that is assigned to each node allowing up to coexisting networks. It is possible to have more than one ZigBee network operating at the same frequency simultaneously, although it s not advisable due to RF interference between the networks. 5

6 Channel/Frequency The available channels (frequency) selections in a ZigBee network (for 2.4 GHz) are numbered All nodes on a single ZigBee network must be on the same frequency and this frequency is set by the coordinator (ZigBee gateway). Each node scans all frequencies in search of its own PAN ID, and once found it attempts to join that network. The following table shows the channel number and center frequency of each ZigBee channel: ZigBee Channel Frequency (GHz)

7 Coexistence with Other Wireless Technologies It is often desirable to have an network in the same space as a ZigBee network. As long as the channel assignments of both networks are done properly they can co-exist with no interference from each other. In a maximal configuration, three networks and four ZigBee network can co-exist without overlapping frequency usage. Maximum spectral usage is obtained in the US by using channels 1, 6, and 11 and ZigBee channels 15, 20, 25, and 26. See graphic below: For Europe, use channels 1,7, and 13 and ZigBee channels 15, 16, 21, and 22. See graphic below: 7

8 Interoperability The organization of ZigBee specifications can be viewed as an upside-down pyramid: Application Interface Layer Clusters Time Alarms Level Switch ON/OFF Scenes Groups Device Profiles Intruder Alarm Closure Devices Generic Devices HVAC Lighting Stack Profiles Other Stacks Home Controls Stack Zigbee V1.0 Spec IEEE The IEEE specification is the foundation document at the lowest layer with the ZigBee v1.0 specification adding the appropriate protocol layers to meet the goals of interoperability. Built on top of the ZigBee spec are Stack Profiles that define the specific parameters of a ZigBee stack that is targeted toward a specific vertical market (e.g. Home Controls). Device profiles define a set of interfaces for a variety of device classes (e.g. lighting) to support interoperability within a stack profile. The device profiles are based upon a Cluster or multiple clusters. Clusters can generally be viewed as atomic (or lowest common) functionality that provides for basic services. For example, a manufacture wishing to make a lighting device for the home market will use the Lighting device profile which indicates that the ON/OFF cluster is required for the lighting device. Further, a lighting device may optionally include a Level, Scene, and/or Group cluster(s). This 8

9 model supports basic interoperability (ON/OFF cluster) and allows the manufacture to create differentiation by incorporating whatever features (clusters) they like within the scope of the Lighting device profile. The specifications also allow a manufacture to add functionality beyond the scope of the Lighting device profile that is manufacture specific. This allows a manufacture to add a custom feature they like but at the cost of interoperability with other manufactures product for that custom feature. One of the big challenges with the ZigBee v1.0 specification is that it provides for a number of parameters that can be set to optimize the performance of the network for a particular application. These settings are agreed to by the appropriate working group within the ZigBee Alliance and are part of the Stack Profile. For example, the cluster-tree mesh networking parameters of nwkmaxdepth and nwkmaxchildren set the maximum number of hops to an end device and the maximum number of children a router can have, respectively. The use of cluster-tree routing and these network parameter settings are specific to each stack profile and must be adhered to for interoperability. The best settings for these parameters are highly dependent upon the target installation for the products in the network. The working groups consider a handful of market use cases and determine optimal settings for these values AMX LLC 3000 Research Drive Richardson Texas, Phone