- Tue Jul 11, 2006 11:52 pm
#15717
Be sure to distinguish IEEE 802.15.4 from ZigBee. They are different standards. The 15.4 standard defines the MAC and PHY, kind of like 802.11a/b/g. ZigBee is not an IEEE standard, nor is WiFi. Many applications need only 15.4 with fixed addresses or proprietary network layer routing. ZigBee-less.
Re meshing - any vendor who claims to have been ZigBee 1.0 certified (see their website) must implement self-forming/self-reforming meshing as an option.
In 1.0, there is one "coordinator". All network nodes are called either Routers or EndPoints. An endpoint is a node that cannot relay traffic, whereas routers can.
Every node except the coordinator finds a parent by choosing among the neighbors that it can "hear". ZigBee defines how this is done, using broadcast frames and hop counts and so on. It's complex. ZigBee 1.0 uses a variant of the widely used (Internet) protocol called AODV. You can read that RFC. Unlike wired networks where every candidate route from A to B (via intermediate routers) has about the same error rate, wireless does not. So AODV with ZigBee must do route selection not based merely on fewest number of hops, but rather this plus avoiding unreliable candidate links (RF conditions poor).
As to range, like any wireless, it "depends" - on
Transmitter power - modules for sale range from 1mWatt to 30 and 60 and even 100mW. Each doubling of power is 3dB. 1mW = 0dBmW (or dBm as it's abbreviated). So 2mW = +3dBm; 100mW = 20dBm. Each 6dB of more power adds a lot of cost to the radio. Compare prices.
Receiver sensitivity. All the brands are essentially the same. As is the case with WiFi too, due to the 802 standards. ZigBee is so far used only with IEEE 802.15.4 underneath ZigBee as the network layer.
Antenna gain, pattern and where each node is within the othe node's antenna pattern.
RF Path obstructions: Walls of some material type (drywall is about 3dB of loss at 2.4GHz), trees, human bodies, metal, etc.
RF Signal Impairments: interference, reflections, multipath, etc. A very busy WiFi network on/near the chosen freq. for '15.4/ZigBee will of course reduce throughput. But 15.4 is just 2MHz wide whereas WiFi is 20MHz, so 15.4 sees WiFi on nearlyl the same freq as an increase in the noise floor.
15.4 has about a dozen non-overlapping 2MHz channels whereas 802.11 has just 3 non-overlapping 20mHz channels.
Also, since 15.4 uses just 2MHz and 250Kbps, it requires a much lower signal to noise than does WiFi. So it works well with received signals in the -'90s whereas WiFi struggles at lower rates at -80 to -90.
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Now all this said, marketing people like to advertise Line Of Sight range in the absence of all impairments and with just 2dBi gain antennas, or less, the range for 1mW is perhaps 300 ft and 100mW can be perhaps a kM.
It Depends.
Of couse, on 2.4GHz, we have nots of noise and interference so WiFi is a Wireless LAN and 15.4 is a wireless PAN, neither are wireless WANs.
I am very impressed with the indoor range I'm seeing penetrating many walls in an office building for 100+ ft range through say 10 walls.
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There now. You asked what time is it and I told you how to make a watch!
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