We are planning on 3 AP’s @ 900Mhz with Cavity filters and 3 120deg sectors.
Can we use the same Frequency for all 3 AP’s?
Can I use the same SSID for all 3? - We would like to put up a 4th unit at the same Freq as well with an Omni so if 1 sector is out, the clients would switch to the omni on a temp basis until the sector is fixed.
How can I calculate the proper downtilt?
Can other towers (BH)branching from this site use the same SSID as well? We will obviously use a different Freqency.
Each sector must use a different frequency. You must also ensure that it’s non-overlapping. I’m not sure if the same issue exists in the 900MHz band, but at 2.4GHz many people are fooled into thinking there are 11 channels available (13 in Europe), but actually there are only 3 non-overlapping channels, 1, 6 & 11. If you choose any other channel you are overlapping to a greater or lesser extent.
2a. Use the same SSID.
2b. No problem using an Omni as 4th sector as long as you abide with 1 & 2a.
Down tilt is only appropriate to sectors, you can’t down tilt an Omni, but some are manufacturered with in built down tilt. Calculating down tilt depends on whether your transmitter site is higher than all your customers. In general get the specifications for the antenna, e.g. if the vertical coverage is 10 degrees and all you customers are lower than the transmitter, then down tilt by 5 degrees. Other than that you need to resort to Triganometry to calculate the values. In my experience theory often doesn’t meet with reality, it’s better to do real world tests with one person at the antenna and a roving person, you can then test the effects of different levels of down tilt at different distances from the transmitter.
No real limitation on using the same SSID as long as you’re providing the same service from each transmitter, otherwise it makes it difficult for users to differentiate between transmistters providing different features.
If you limit the channel width to a maximum of 5 or10MHz you won’t get the full 11/54Mbps 802.11b/g bandwidth, this requires a channel width of 20MHz. Some sources quote 30MHz, but I think they’re wrong, one site implies 22MHz which is probably nearer the truth. In theory you usually need to sample any signal at twice the rate before encoding to guarantee lossless decoding.
900MHz band defines 4 channels at centre frequencies of 907, 912, 917 & 922MHz, (source Ubiquiti CM9 datasheet http://www.ubnt.com/downloads/sr9datasheet.pdf) this actually means @ 20MHz channel width there is only one non-overlapping channel, e.g. if you choose 907 as your initial channel then the spread @ 20MHz will be 897-917, if the second channel is 922 the spread is 912-932, hence it overlaps the first channel by 5MHz. Whichever initial channel you choose it will overlap with the other 3 to a greater or lesser extent. So you make your choice, more channels less bandwidth per channel or less channels more bandwidth per channel.
Not having seen any hard evidence, but if you extrapolate I would guess the maximum bandwidth at 5 & 10MHz channel widths will be 2/18Mbps and 5.5/36Mbps respectively for 802.11b/g protocols, not too bad for 802.11g, but this makes 802.11b virtually useless.
Don’t forget that WiFi is half duplex, i.e. you can send or receive, but not both at the same time, thus the real throughputs will be something under half the theoretical values.
Based on my testing with the SR9, the card does produce a reliable 3-4Mb link (1/2 Duplex) in 5 Mhz channel spacing and 12 Mbps at 10Mhz spacing.
802.11b is useless in my application. I am trying to maximize NLOS penetration and enabling 802.11b (According to what I have read) puts the card in DSSS mode. The other modes put the card in OFDM mode which is better for multipath and penetration (Am I correct in saying this?). I am looking to provide subscribers with 256Kbps - 1.5Mbps max in rural areas.
So, what I am really worried about is the maximum number of subscribers on a single RB532. Many people have said a maximum of 20-30… I’m hoping 50 and with 3 sectors that gives me 150 per tower site. Now if I understand correctly, if I want to double the subscribers to 300 at the tower, by doubling the routerboards and moving to 60 deg sectors, it sounds like it is not possible in 900Mhz with the SR9 / MT. Correct?
Keep in mind that 3-4Mbps is going to be split over all your users.
If you’re in an area like I am, people will go crazy for a 256k link. 12 people at 256k is 3Mb.
In my experience, DSSS has slightly better range in open areas, OFDM does better in the real world.
I’ve read where a guy had a half mile link with 2 SR9s, like a -70dB signal, dropped to -88dB when he turned the yagi away from the AP. It sounds like channel overlap will be a signifigant problem with the SR9.
What about re-using same frequency by switching polarity, … will this work?
Also to my understanding I have 4 channels with the SR9 @ 10 Mhz but they are overlapping.. correct.. so does it truely cause problems or will the bands “share” effectively even though they overlap?
Based on what I’m hearing, it makes more sense to go Omni and up the dBi on the omni and furthermore put a CPU instead to handle more customers… Anyone with real numbers and service levels using SR9??