davesims
join:2004-10-09
Roseburg, OR
|  Migrating from FHSS to OFDM
I am considering the purchase of a small ISP that has been in business for many years. The ISP has an existing FHSS wireless network that utilizes the old Breezecom equipment.
I would want to add new equipment to the existing three tower locations. The new equipment would be for new customers and to migrate existing customers from the old FHSS network. I am not sure of the best strategy involved.
Is it conceivable that an OFDM network could coexist with a FHSS network? What would give the best chance of success from a design prospective?
Dave
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superdog
I Need A Drink
Premium,MVM
join:2001-07-13
Lebanon, PA
 ·WaveCrazy.Net
| said by davesims  :Is it conceivable that an OFDM network could coexist with a FHSS network? What would give the best chance of success from a design prospective? Dave Yes, I am living proof that it will work. I have towers right now that have both FHSS and OFDM/DSSS radios working together.
When I started in 2001, the only options for any kind of decent outdoor gear were Breezecom and Raylink, with Raylink being much cheaper. As time went on and things changed, more and more OEM's began selling better outdoor gear for DSSS and then later on OFDM stuff.
At this point, I am phasing out all of my FHSS deployments and replacing them with newer faster 802.11b/g gear. It is cheaper, has better throughput and IMHO, has about as much interference resistance as the older FHSS stuff in most cases. 
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EMC_guy
join:2007-10-13
Sharbot Lake, ON
 ·Rogers Hi-Speed
| reply to davesims
said by davesims :Is it conceivable that an OFDM network could coexist with a FHSS network? What would give the best chance of success from a design prospective? This will give best results:
You can find out the exact FHSS center frequencies for 0 and 1 bits using the emission test reports under FCC ID. Now set your OFDM radio for a narrow channel width (5Mhz or 10Mhz) with center frequency that is 5Mhz away from those FHSS frequencies. |
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davesims
join:2004-10-09
Roseburg, OR
| reply to superdog
superdog, thanks for the input. Did you experiment with physical spacing of the antennas old vs new? If so how far apart did you end up needing to separate them?
Also, did your FHSS network have both backhaul and ptmp using FHSS? If so have you upgraded both to OFDM?
Dave |
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superdog
I Need A Drink
Premium,MVM
join:2001-07-13
Lebanon, PA
·WaveCrazy.Net
| said by davesims :superdog, thanks for the input. Did you experiment with physical spacing of the antennas old vs new? If so how far apart did you end up needing to separate them? I didn't have enough space to change the distance, although the antennas were already at least 5ft apart and that seemed to work just fine. Keep in mind that I only EVER ran 3 sectors on each PoP at once and I would just replace one sector at a time with the AP and then run around and change out all of the CPE's in the following day or two.
I think you will get some very poor results if you try and deploy another 3 120 degree sectors using 802.11b/g and still try and use and/or keep the FHSS units in place. It may work out better if you try and get at LEAST 20ft between each group of antennas AND you use opposite polarity on each group.
said by davesims :Also, did your FHSS network have both backhaul and ptmp using FHSS? If so have you upgraded both to OFDM? Dave My backhauls have always been 802.11A except for one time I used an 802.11b backhaul while using FHSS AP's.
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davesims
join:2004-10-09
Roseburg, OR
| I can't imagine replacing one sector at time and then running around quickly and changing CPE's. How many clients were on a sector? I would think the clients would not be too happy with being down for a day or two.
This has been my delema. I am afraid that I will have to put the new gear to close to the old FHSS gear because of space constraints on the towers.
One thought I have is that it will "work" but the performance of the new gear will be poor while I am converting. In other words if I change a sector at a time and use opposite polarity that possibly the links to the customers will be maintained with poor performance. This because the FHSS hops through the frequencies.
It may also be possible that I could use a small spread, say 5mhz, that doesn't compete with the FHSS.
Dave |
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joshg409
join:2005-05-03
Ottumwa, IA
| reply to davesims
Here is what we do. We connectorize a canopy 2.4 and attach an omni. We do an SA from the canopy, pick a quiet frequency then go into the three Alvarions and exclude the Canopy frequencies from the hopping sets. I know, not the best but it solves the problem of not having to run around to everyone on the sector and change CPE. If you are going from Alvarion to OFDM, you will have to pull new cable. The nice part is the Alvarions only handle about 20-30 clients and you can get three times that on a canopy. So one Canopy versus three AU's to start out. |
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EMC_guy
join:2007-10-13
Sharbot Lake, ON
·Rogers Hi-Speed
edit:
May 8th, @04:54PM
| said by joshg409 :We do an SA from the canopy, pick a quiet frequency then go into the three Alvarions and exclude the Canopy frequencies from the hopping sets. Yes, this is the correct approach.
Sorry, my previous comment is valid for only BFSK radios - Canopy and some SCADA. It should read:
"You can find out the exact BFSK center frequencies for 0 and 1 bits using the emission test reports under FCC ID. Now set your OFDM radio for a narrow channel width (5Mhz or 10Mhz) with center frequency that is 5Mhz away from those BFSK frequencies. "
Older FHSS radios have longer hopping periods - many milliseconds per hop - so it is possible to co-locate after some "trial-and-error" timing coordination.
First make the 802.11 beacon fall in the middle of 2 FHSS hops (by trying different inter-beacon timing (use spectrum analyzer if necessary) - that will make sure beacons do not get clobbered. Now set RTS to a low value both AP and CPE to create shorter 802.11 packets that fit within the FHSS hops. |
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davesims
join:2004-10-09
Roseburg, OR | Thanks guys you all have provided me with some good insight and some validation of my thinking.
Dave |
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superdog
I Need A Drink
Premium,MVM
join:2001-07-13
Lebanon, PA
·WaveCrazy.Net
edit:
May 8th, @05:56PM
| reply to davesims
said by davesims :I can't imagine replacing one sector at time and then running around quickly and changing CPE's. How many clients were on a sector? We had 3 guys doing this at once, and we got it done in 2 days. All of the Raylink CPE's already had CAT5 runs to the radio, so it was just a matter of doing some preconfig on the new radios and then reinstalling an end on the CAT5 at each location. On most swaps, I was in and out in less than 30 minutes.
Oh Yea, about 20 subs per AP.
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