>giuseppe4 wrote: I mean: you can put 120 users on a single sector, but it
>depends on the bandwidth you're offering and their quality, right?
Hi. Well, here is my 2c opinion is this:
First of all, Cambium does a much better job, and a much more organized job of managing it's clients than anything else in it's class. The AP does the scheduling, and tells each client when it should listen and talk, and that eliminates 'hidden node' that eliminated 'near far', and that eliminates a ton of problems. Secondly, Cambium's scheduler is an 'Air Fairness' scheduler - so if you have a client who get's spun by the wind for example, and his signal drops from a -60 to a -77 because he's misaligned, Cambium again does a better job by still limiting him to his time slot. That client's performance will suffer, but he won't dominate the Access Point and squeeze everyone else out, which is what can happen on many other products. With other products, if that -77 client is trying to transmit, and if he's set to 1Mbit upload - well, he can still get it, even if that means he's using 40% of the AP's time and impacting everyone else. All these things (and more) translates into being able to scale to many, many more clients than most products can - and to have more predictable performance as you do so. As I mentioned in another thread before, at the last Cambiumn ePMP Training course I was at, there were WISP's there from Alberta, and they had sectors with 60-80 clients on them, and they were very, very pleased with the performance.
That all begin said - YES, if you have all your clients installed at -75 and if you want to give everyone 20 Mbit down and 5 Mbit up, and if everyone streams 90% of their day at their full 20 Mbit - then no, you're not going to fit 80 people doing that all at the same time without something giving somewhere. If everyone is connecting at MCS 10 due to alignment or interference issues - then that will impact the amount of work that can get done per frame. But, if everyone is perfectly aligned, they are all connecting at a -55, they are all MCS15 all the time, and have packages where your typical peak load is within that the AP can do - then you should be good.
Also - the 'Air Fairness' algorithms do a pretty good job of 'fairly' sharing an AP when it's overcapacity too. What I mean by that is - with other products, if the AP is capable of 25 Mbit total capacity, and if you have 20 clients and each is set to a 3 Mbit plan, then bad (and unpredictable) things can happen when that 11 or 15 or 20 of those clients try to use the AP at the same time. The first 5 or 7 client might do fine, but as things approache over capacity, WiFi can start 'out competing' each other in a way. Actually, the clients start 'backing off' when they see contention - so they listen to see if the channel is clear, and if it's not, they back off a random amount of time, wait and then retest to see if the channel is clear again. If it isn't, they will back off for a longer random period, and retest again. That's an OK mechanism on your home router when you probably have 5 devices connected, and when your laptop can hear the tablet in the next room - but in an outdoor network, what that often results in that your full AP will be servicing 10 of it's clients OKishly, and when the 11th or 15th or 23rd client tries to get some air time, it can't get a word in edgewise. YES, there are mechanisms like RTS/CTS which do (kinda) help - but they also lower overall throughput too, so while they are solving one issue, they are creating another issue in the process.
With Cambium, the AP has scheduled all the SM's when they should talk, when they should listen - so a full AP with lots of traffic won't have SM's trying to 'butt into' the conversation. They will have been given a schedule of when their turn is, and your sector will listen for them at that instant. There still can be competitive interference, BUT self-interference or contention at that time slot will be minimized or eliminated in a well designed Cambium network.
And if the AP is simply over capacity, with more data being requested than it can really handle, then Cambium does a reasonably good job of sharing the air fairly - so if you have 40 clients on 5 Mbit plans, and if the sector has 100 Mbit that it can handle, and if all 40 clients are trying to download - then all 40 obviously can't get their 5 Mbit each. But, Cambium ePMP will do a pretty good job of 'fairly' giving everyone 2+ Mbit each of the available 100 Mbit capacity, instead of many other products, where some clients are getting 5 Mbit, and other clients are nearly dead in the water. And - low signal client or far distant clients are still a challenge, BUT they don't single handedly ruin the whole AP's performance. Thier less than ideal situations impact them mainly, and they don't get more than their fair share of air time.
>Is there any suggestion about frequency on two access points in the same direction? I think the best is
>different frequency (you can't reuse in the same direction) and at least 5MHz spacing from other sector, right?
Yes, GPS Syncing and TDMA and having matching Download/Upload Ratios and matching distances and so on - that's all about having matching TX/RX frame lengths and that's all about eliminating self-interference. You see, as a WISP, you deal with two main kinds of interference - competitive interference, and self-interference. Anyone who has done this for a while can tell you two things. A) self-interference is MUCH closer and louder, and B) self-interference is ultimately the only thing you can really hope to control.
Of course, there are ways to mitigate competitive interference by picking the correct frequencies and by using good isolationist, shielding, the correct antennas, etc - but at the end of the day, the competition, and the video surveillance system and the baby monitor are all interfering and you can't do too terribly much about it. But, they are also probably a -78 and they are not right in your beam and so on. However, self-interference can be a -35 and it can be 5 feet away and it IS something that you can 100% control if you design things correctly.
So, if your client is going to be able to hear two sectors (either adjacent sectors, or sectors broadcasting in the same direction) then you're going to want to have as much separation as you can. With GPS Syncing, that can be as little as 5Mhz 'guard band', but if you've got more, you certainly can use more spacing. Without GPS Syncing, the guidelines are to have 2x the guard band as the carrier band - so if you're using 20Mhz channels, you should have 40 Mhz of guard band. Again, that's not a horrible idea even if you are syncing everything properly.
