I'm going to be facing a scenario shortly where two APs are linked to a single gateway for Internet connectivity thus:

.....W............E.
.......\.........../..
.........\......./....
...........\.../......
............G........

The links WG and EG will be around 17km and the angle described by WG-GE is around 25 degrees.

The decision I'm facing is whether to set G up as a point-to-multipoint, with one card and a sector antenna, or as two point-to-points with two directional antennas and two cards on one router.

The existing EG link has bandwidth of around 10MBps according to a two-way bandwidth test. The Internet connection is through one but potentially two ADSL modems which could each deliver around 6.5MBps down and 650KBps up. Hence each link can carry the maximum the Internet can deliver to it with a bit to spare.

I'm assuming one card could easily carry the combined traffic of both links but would be interested in the views of others as to considerations I may not have, ah, considered.

Hi SweetSunday!

The answer will probably rest with the FCC. Can you make the link without using the 3db-for-1db antenna rule? If so, then either would be ok. If not, then you would be in violation of FCC regs to make the link multipoint.

Thanks SurferTim

My primary concern was whether either approach offered particular advantages or problems when it came to throughput, traffic control or administration.

Fortunately the FCC have no jurisdiction here and I've never heard of a "3dB-for-1dB" antenna rule. Is that just something your bureaucrats have dreamed up to make your life more difficult and expensive and theirs easier and more expensive or is there something practical underlying it that I should know about?

Just my .02cents....

I suppose W & E cannot be changed and must remain as AP's...?? - otherwise it would be a simple change to make.. as in make them stations to the (new) AP G and then you could set it all up on a Sector as an AP...

Either way - I hope you're running a routed network, so you could add 2 x directional antennas from W & E to G and then the 2 networks would operate on their own IP Range...

As an aside - we have had a situation where we had to do the same where G had to remain as a station - and we setup a Sector and "overloaded" the card with 2 x IP addresses... otherwise you might have to bridge the whole thing.

Distance is fine for both 2 and 5GHz - but to answer your question - one card will carry the load.

T

Hi SweetSunday!

Lucky You! Our federal regulations are quite complex.

Max EIRP (effective isotropic radiated power) is 36db (4 watts) but 6db must be antenna gain.

If point-to-multipoint:
transmitter power - transmission line loss + (antenna gain -6) <= 30db

If point-to-point:
transmitter power - transmission line loss + ((antenna gain -6)/3) <=30db

How long do you think it took them to come up with that?!?!

G'day Tanker -

I was careless in my terminology. W is (and E will be) access points for clients local to them but in station mode for the link to G, which is in ap-bridge. (W and E is/will be RB532s with two cards in, running 2.4GHz for the 'local' connections but 5.8GHz for the backhaul to G.)

So I *could* have two cards/antennas at G, running as separate APs for W and E, and I even have room locally at 5.8GHz to run them on different bands.

But on the other hand I could fit a 90' sector antenna at G and make it an AP for W and E on the same network, with NAT at W and E for the networks beyond,

Or I could even set up Virtual AP's at G on the one interface and run G<>E and G<>W as separate networks with one card and one antenna.

If one card can carry the maximum potential simultaneous traffic of both links having two cards and two antennas seems unnecessary cost, and taking up two bands when I don't need to seems somewhat antisocial. Perhaps all things being equal using VAPs on one card and antenna is the way to go. Is there a downside to this way?

Hi SurferTim.

Ah, the bureaucratic mind at work. I expect coming up with that required lots of meetings in plush hotels, required lots and lots of bits of paper they could carry around in important-looking folders and kept lots and lots of them in the illusion of gainful employment, probably for years.

As Confucious said, "Those that can, do. Those that can't do, end up pretending to supervise those who can."

If you have the tower space use two PTP's. You'll get better speeds and more resilience to noise.

If you have the tower space use two PTP's. You'll get better speeds and more resilience to noise.

I have the space but as the links are limited by the DSL's speed anyway (6.5MBps down and 750kBps up) how relevant is wireless speed?

In what way "more resilience to noise"?

If you have the tower space use two PTP's. You'll get better speeds and more resilience to noise.

I have the space but as the links are limited by the DSL's speed anyway (6.5MBps down and 750kBps up) how relevant is wireless speed?

In what way "more resilience to noise"?

Here's a good remark! I see lots of guys trying to squeeze high to very high data speeds over their networks while I also see a lot of guys trying to distribute less then 1Mb Internet access to their clients on their network.
Either there are two different group of users mixed up in this forum which is not even been taken in account by these users themselves or lots of us get blinded by the need for high performance while then they have no use for it!
I think most users on this forum are internet access providing only and can do with low to moderate network speeds. Only the real big players with hundreds or thousand of users, with fibre lines or better and with corporate networks were traffic exists between users within the network need the highest network speeds possible.

I have two 8Mb ADSL line to distribute amongst some 100 users for internet access. I sell 3Mb download to them. In theory I will then have a maximum of 2 x 7,5Mb (that's what I really get) = 15Mb tcp data flow on my network.
So with some extra overheads and reserve I should be fine if all my links (ptp or ptmp) are 24Mbps minimal. My network is actually 36Mb everywhere and the backhauls are even 48Mb in most cases.

With ¨resilience to noise¨ I think JJCinAZ means that with two separate radio channels for each ptp link you would have less problems with noise.
I don't know, I don't think it would make any difference for the AP radio if he has to deal with one station or two.
The other (station)radio's also have to deal with noise. It is the worst one that sets the standard.
In case one station has the worst s/n ratio that link might suffer from that. It will not affect the other link, even if both links are handled by one AP radio.

If that's all the speed you'll be distributing now and in the next 24 months, then go inexpensive route. What I meant by noise resilience (and couldn't elaborate due to virtual keyboard I was on at the time) was that on two PTP links you would likely have more directional antennas when compared to a sector antenna. The directional antennas would be giving you larger gain and listening to less noise.

Hi there SweetSunday....

Here's my take on it...

Keep it Simple...

One 90 deg sector - pickup the 2 incoming links...
Add one IP address for the Sector radio ie xxx.xxx.xxx.1/24
Make W .30 or similar and E .31 (leaves space from 2 to 29 for other stuff - if you ever need it)
NAT it... and set the W & E 0.0.0.0/0 routes on W & E to the .1 Gateway on the Sector.

Add a Queue Tree on G to manage the HTTP, P2P (Bit Torrent and the like - if any) and SMTP mail...

As long as the LOS is good - the distance is not important here - 17 Km is nothing...
Install a good card (RB52H) and run it in the 5 GHz band... not sure if you are FAA "allowed" to use the XR5's...

At 17 Km you should get an air-rate of 54mbps easily...

As Rudy said - what's with all the guys trying to PUSH to whole network when the it is actually tail-circuit "restricted" ..???

We use the exact same structure on our "edge" BSS's where we pick up the 2GHz Band clients and backhaul them into a sector. Not sure what sectors you can get but we use a 17dBi as standard. A 27dBi Grid antenna at W & E should give you a nice robust link.
Consider using Nstreme on the Backhaul Links - just be careful of the Processor Load - but like I said we use plenty of these and they work fine.

Hope this helps.

Tanker