AVR-Chat

I've been working professionally for 3 years now in IEEE 802.15.4 and have these tid-bits, and if you wish, I can elaborate....

ZigBee is not IEEE 802.15.4 and vice-versa. The comparison might be IEEE 802.3 versus IP/ethernet.

ZigBee is one of several network layer protocols used with IEEE 802.15.4. ZigBee is used normally only if you need meshing. For either on-hop networks as peer to peer or point to multipoint, ZigBee is a disadvantage versus simple source/destination addressing, as you would with, say, UDP datagrams or ethernet packets.

The range one can achieve is ordinary RF engineering... most '15.4 modules (don't even think of starting with a chip) have either 2mW or 50mW of power. In some countries, regulations prohibit more than about 10mW of radiated power. MOdules can have on-board antennas with little or no gain, or external antennas with lots of gain, regulations-permitting. 

The receiver sensitivity in these modules, marketing baloney aside, is essentially the same. It's governed by the definition of '15.4, which for the 2.4GHz version, is DSSS 250Kbps before coding and other overhead. A good number is -90dBm at the receiver input. So the range is the usual RF budget: Tx power + antenna gain - distance-dependent path loss at 2.4GHz - additional losses from clutter + antenna gaine versus receiver sensitivy. 

With 60mW modules, with 0dBi gain on one end and 5.6dBi on the other, and elevated 20ft. the line of sight range I achieved was 3/4 mile, with the far end just 4 ft. above ground. WIth (legal) higher gain antennas you can probably get line of sight, do the math- of much more than that, absent sustained interference.

The protocol is CSMA like 802.11, so it can be used in unlicensed bands.

There are perhaps 20 module vendors and 6 chip vendors. 

The most popular for casual use is Digi's XBee series 1 (non-ZigBee).

steve

--- In AVR-Chat@yahoogroups.com, "Chuck Hackett" <egroupscdh@...> wrote:
>
> > From: Mike Payson
> > ....
> > 
> > I'm confused by the power issue... You say you are already providing
> > 48vdc, so why not just step the power down for the Zigbee? Running
> > cable isn't quite as bad since you already have the conduit, but it
> > still seems that running cable would be a hassle, and the cost would
> > be almost identical or even cheaper.
> 
> No problem powering the Zigbees from 48v.  I'm already planning on
> downconverting the power with a switcher regulator to power the nodes (the
> reason for a 48vdc bus rather than ac, etc. is a whole other discussion that
> I won't go into here).
> 
> I only mentioned the 48v power cable to say that there is already a conduit
> to all the (current) node locations.  The point being that it is minimal
> effort to pull the CAT-5 with the power cable and the only added cost is the
> CAT-5 itself which was costing us ~$55 per 1,000 feet.
> 
> From Wikipedia: "Transmission range is between 10 and 75 meters (33 and 246
> feet) and up to 1500 meters for zigbee pro, although it is heavily dependent
> on the particular environment."
> 
> I see on the Sparkfun site that 1mw units are about $23, 2mw $26-$29, pro
> $38-$45.
> 
> (what follows is of the top of my head ...)
> 
> So, assuming two nodes separated by 1,000 feet, it costs me about $28 per
> node with CAT-5, and (since I have to use Pro), let's say $40 per node -
> and, that's assuming that Pro can reach that far in a heavily wooded area.  
> 
> To be fair, since Zigbee can communicate in both directions (east/west on a
> track for instance), let's say 5 nodes, each separated by 1,000 feet, that's
> $220 for cable (4,000 feet @$55/1000) or $44/node.  For Zigbee Pro, that's
> $200 (5 units @$40) or (obviously) $40 per node.  
> 
> Sounds about the same, but: I would have to test to confirm that Pro can
> cover the distance in a heavily wooded area - I know cable works reliably
> that far.  If a Zigbee link doesn't work someone has to diagnose if it's an
> RF problem or not and then track down the source requiring somewhat special
> knowledge and equipment - a cable is a comparatively easy thing to diagnose
> with minimal tools, and knowledge.
> 
> Now, suppose the nodes are clustered in a smaller area (higher density), my
> cable costs per node go down dramatically but the Zigbee costs can (from
> above) only go down to ~$25 per node after the range is small enough for the
> 1 or 2 mw units.
> 
> Another factor:  I'm currently using CAT-5 because it's a "commodity" item
> (LOTS of it is made and available) but I am currently only using one of the
> 4 pair in the cable.  If future use does not surface for the other 3 pairs,
> I could go to a jacketed single twisted pair which would again lower the
> cost for cable, or even, since it's in conduit, I could use single-pair,
> un-jacketed cross-connect wire.
> 
> That said, I still see a use of wireless in my application, for example:
> One of my users is located in a public park and they have a small number of
> signal heads that, due to vandalism, must be put out when needed (let's say
> two days per month) and gathered up at the end of each day.  I can see the
> signal head containing a battery (recharged by just plugging the signal head
> into a charging station at night), a node and Zigbee.  No need to run
> data/power cable in the city park and no vandalism.  I can also envision
> other cases where a network would be a mix of wired and wireless ...
> 
> Oh, one other advantage of Zigbee could be that (if self powered) it is
> inherently protected from lightning, HOWEVER the node is still connected to
> the rails which are still great lightning/ground voltage gradient attractor
> :-)
> 
> Apologies to others not interested in such non-AVR discussions ... Now back
> to our regularly scheduled AVR specific discussion ...
>  
> Cheers,
> 
> Chuck Hackett
> "Good judgment comes from experience, experience comes from bad judgment"
> 7.5" gauge Union Pacific Northern (4-8-4) 844
> http://www.whitetrout.net/Chuck
>