Wireless bridge

Patrick Shoemaker shoemakerp at vectordatasystems.com
Thu Jun 18 12:55:28 CDT 2009


Couple of comments:

Regarding ISM spectrum sharing: the 2.4 GHZ band (2400-2500 MHz) and the 
5.8 GHz (5725-5875 MHz) are certainly shared with ISM devices- microwave 
ovens, induction heaters, etc. However, the 5.2 and 5.4 GHz unlicensed 
bands (UNII) are not shared with ISM devices. However, these bands are 
subject to FCC regulations that mandate radar sensing and avoidance. 
This means that if your radios detect the signature of a military radar 
system on their active channel, they will automatically shut down and 
begin a waiting period before switching to another channel. Mandatory 60 
second outage.

There are generally three classes of point-to-point high speed 
unlicensed data radio gear out there today:

1. Wi-fi based gear with some additional hardware and a user interface 
suitable for point-to-point use. Ubiquiti, Tranzeo, HGA, etc. Pretty 
self-explanatory. Sub-1000 range.

2. Gear using a wi-fi chipeset (Atheros, Broadcom, etc.) with a 
proprietary firmware load. Trango, Alvarion, Ligowave, etc. $2000-5000 
range.

3. Gear using a custom designed RF interface. Motorola, Dragonwave, etc.

Given your requirements, I'd encourage you to look at classes 2 and 3. 
Getting any decent amount of reliability from vanilla 802.11 equipment 
is (as you've found) difficult. Gear in categories 2 and 3 from above 
will generally have a built in spectrum analyzer of some sort that will 
be able to see interference not caused by 802.11 devices, performance 
monitoring systems (BER reporting, event logs, etc), SNMP capability, etc.

Definitely choose a system with an integrated antenna. You want a 
directional antenna such as a patch array (panel) integrated with the 
radio. Messing around with RF cabling, connectors, etc. is not necessary 
with what you're trying to do. Minimize the potential points of failure.

Lightning protection is a concern. Most of this gear is PoE powered, so 
you'll have a single cat-5 going to the roof. Make sure it's protected 
with an Ethernet surge suppressor that is properly grounded. Follow the 
radio manufacturer's recommendations here. Your antenna mount must also 
be grounded according to NEC requirements.

The Motorola PTP400 series radio that was recommended is one of the best 
unlicensed point to point radios out there. However, it's been EOL'd and 
replaced by the PTP500. Seems like these are both out of your budget, 
though. As an alternative, you might consider looking at the Trango 
TLink45. This radio uses a proprietary firmware and an Atheros WiFi 
chipset. It has a rudimentary spectrum analyzer, SNMP, ARQ (important), 
and adaptive rate modulation. It also has a dual-polarity software 
switchable antenna. This greatly increases your ability to avoid 
interference. It will run in the 5.3, 5.4, or 5.8 GHz unlicensed bands. 
They retail at about $4000 for a pair, but Trango routinely runs 
specials. They were on special for $1700 per pair in April.

The WISPA list is a great resource for help with projects like this.

Patrick Shoemaker
Vector Data Systems LLC
shoemakerp at vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com

> Message: 6
> Date: Thu, 18 Jun 2009 13:13:17 -0400
> From: Curtis Maurand <cmaurand at xyonet.com>
> Subject: Re: Wireless bridge
> To: Lyndon Nerenberg <lyndon at orthanc.ca>
> Cc: nanog at nanog.org, Peter Boone <NANOG at Aquillar.com>
> Message-ID: <4A3A75AD.8090508 at xyonet.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Lyndon Nerenberg wrote:
>> > On Thu, 2009-06-18 at 11:54 -0400, Peter Boone wrote:
>> >   
>>> >> Oh I know. Luckily it's located in an industrial area just on the
>>> >> outskirts
>>> >> of the city. There isn't a lot of other WiFi (in my opinion); 3-5
>>> >> total
>>> >> SSIDs spread across 2 of the 3 physical channels (1,6,11) depending on
>>> >> which
>>> >> rooftop you measure from. 
>>> >>     
>> >
>> > 2.4 and 5GHz license-free Wifi is license free because the frequencies
>> > are shared with the ISM (Industrial/Scientific/Medical) services. In an
>> > industrial area, competing WiFi is the least of your worries. These
>> > frequencies are also used by industrial grade heating units. Got anyone
>> > in the neighbourhood running a large plastic shrink wrap machine, for
>> > example?
>> >
>> >   
> 
> Motion sensors also run in the 2.4GHz range.
> 
>> > You can't directly detect these other users with a Wifi transceiver.
>> > Depending on the nature of the interference you *might* be able to hear
>> > it directly on a scanner (if you can find one that covers those
>> > frequencies), but you really need a good spectrum analyzer to tell
>> > what's going on.
>> >
>> > Anyway, don't assume the competition for spectrum is only other Wifi
>> > units.
>> >
>> > --lyndon
>> >
>> >
>> >   




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