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Zigbee Networking Basics (35,000ft view)


RF Communication Overview
Wireless communication topologies
Mesh Network Concepts
Unicast
Broadcast
Adura Network Stack
IEEE802.15.4
Zigbee Network Layer
Adura Application Layer

Zigbee RF Channels (*Complete - Charlie)

XPW devices make use of 16 narrow-band RF channels as defined by the IEEE 802.15.4 wireless standard. These channels are in the unlicensed (worldwide) 2.4 GHz band and are spaced 5 MHz apart, with each channel using 2 MHz of bandwidth. The table below shows the center frequency for each channel.

The maximum transmission output power is +18dBm for channels 11-25, and +14dBm for channel 26.

ChannelCenter freq. (GHz)
11 2.405
12 2.410
13 2.415
14 2.420
15 2.425
16 2.430
17 2.435
18 2.440
19 2.445
20 2.450
21 2.455
22 2.460
23 2.465
24 2.470
25 2.475
26 2.480

 

Zigbee Wifi interop(*Complete - Charlie)

XPW and 2.4GHz WiFi both use the 2.4GHz unlicensed ISM band and as a result there is a potential for interference. However, both are based on IEEE standards and they are designed to work together. The amount of data transmitted by XPW devices is extremely low compared to WiFi and it is '''extremely''' unlikely that an XPW network will interfere with a WiFi network. There is a greater possibility that powerful WiFi transmitters could interfere with XPW networks, though it is rarely seen in practice. One way to reduce this possibility is to plan the use of channels to minimize the overlap between the two systems. See the diagram below for the relationship between the relatively wide WiFi channels and the narrower Zigbee/802.15.4 channels.

Recommended Channels
XPW devices come from the factory on channel 15, which is often a good choice as it lies between WiFI channels 1 and 6.  Channels 20 and 25 are also good choices as they fall outside the primary bands of channels 6 and 11.  Channel 26 is outside of most WiFi operation, but XPW devices operate at lower transmit power on channel 26 to comply with FCC regulations.


Zigbee/WiFi channels

White Papers

Many studies have been carried out showing that Zigbee and WiFi can interoperate without issue. Here is one such File:ZigBee_-_WiFi_Coexistence,_Thonet.pdf.

Zigbee PANIDs(*Complete - Charlie)

Zigbee networks can be broken up be physical RF channel, but that only provides for up to 16 independent networks. In addition to the physical channel, networks also make us of a logical network identifier called the Personal Area Network ID (PANID). Devices operating on the same RF channel but with different PANID's will not normally communicate with each other.

The SensorView interface to XPW networks allows the selection of 16 PANID's for a given network. This means that by using both RF channel and PANID, there are 256 unique networks available (assuming all RF channels are acceptable for the site). SensorView PANID's map to the actual Zigbee PANID as shown in the following table.

SV PanIdZigbee PanId
Default 0x2222
1 0x2223
2 0x2224
3 0x2225
4 0x2226
5 0x2227
6 0x2228
7 0x2229
8 0x222A
9 0x222B
10 0x222C
11 0x222D
12 0x222E
13 0x222F
14 0x2230
15 0x2231
16 0x2232

 

Zigbee Device Routing
Device Announce
Latency
Network Saturation: specified and recommended maximum limits(*Complete - Charlie)
XPW networks make use of two basic types of messaging: unicast and broadcast.  In general, unicast messages do not pose any practical limitations on the network.  However, Zigbee networks can support only a limited number of active broadcast messages at any given time.  XPW sensors (e.g., occupancy, photo, switches) make use of broadcast messages so that the entire network can react to a sensor message with minimal latency. The network can support up to a maximum of 30 broadcast messages in any 20 second moving window.  Occupancy sensors typically make up the majority of these messages, as there are usually a much smaller number of photosensors used in a given network.  The devices make every attempt to limit broadcast sensor messages to only those required for proper system operation.  
Occupancy sensor traffic can be reduced on a given local channel by only sending messages that might change the state of devices.  For example, consider two occupancy sensors, one of which is detecting motion, the other not.  If the first sensor sends an occupied message, there is no need for the seconds sensor to send an unoccupied message until the first sensor times out. This of course assumes the sensors are on the same local channel and controlling the same devices.  The traffic on the network will typically increase with the number of channels since traffic cannot be reduced for devices controlling different channels.  

The currently recommended limit for control channels on a network is 50, which includes both local and global channels.  Note that this is not a hard limit, rather a suggested limit for reliable operation.  Note that control channels that only include switches typically contribute very little traffic - switches only send messages when a button is pressed - so it is reasonable not to count switch-only control channels in the 50 channel limit.
 

 

 

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