how many co-location APs can FHSS support? 15 or 12?

ychx

how many co-location APs can FHSS support? 15 or 12?

Why the book shows OFDM 54M can only support one co-location AP? please help me details the co-location AP that a/b/g can support sepreatedly?

JDMurray

FHSS has 79 channels, so the theoretical maximum is 79 co-located APs. Using only 12-15 FHSS APs sounds like a practical maximum for creating a usable (2Mbps) wireless network.

802.11b/g system can't be efficiently co-located because the channels overlap. Although, theoretically, channels 1, 6, and 11 do not overlap, they can bleed over each other causing mutual interference. Your mileage may vary.

802.11a has 12 non-overlapping channels in the 5GHz band, so co-locating up to 12 APs is possible, with eight being more practical.

ychx

but why the third edition book CWNA chart shows:

FHSS 15 co-location with Sych and 12 co-location without Sych. why is different?

Actually my understnding is : 802.1a only have 9 channels not 12. 4 UNII-low, 4 UNII-middle and 1 UNII-high for outdoor.

any opinion?

JDMurray

The more co-located APs the less available data throughput there will be for the wireless networks. The CWNA Study Guide specifies (on pp. 182-184) that 12 FHSS APs is the practical maximum for synchronized FHSS and 15 the practical maximum for unsynchronized FHSS APs. Any more than that in those contexts may cause the data throughput of the networks to be so small as to be nearly unusable.

For 802.11a, there are three frequency bands (low, medium, and high), each containing four channels (pp. 199-201).

keatron

Interesting discussion. It brings to light part of my reasons for loving 802.11a so much. Smaller cells than b and g, high throughput (not the same as signaling rate which is what is usually advertised), and to top it off, b and g (or I should say the 2.4ghz range) is sooooooo crowded these days.

I have to agree with JD. You have only three non-overlapping channels with b/g. With a you have 12 non-overlapping. So you have more room to play with. You'll certainly get more access points in a given amount of space using a, but upside to b/g is more range. So even though b/g will only give you 3 non overlapping channels, because of the range of coverage, you won't need nearly as many access points as you will with a. But if your concern is security and speed, then consider 5 ghz/a has smaller cell sizes (PHY layer security), but higher throughput rates (speed).

JDMurray

The concept of channels 1, 6, and 11 being non-overlapping is only theoretical. It isn't unusual to find channel 11 bleeding down to channel 5, and channel 1 bleeding up to channel 7. This is because the 802.11b/g channel centers are only 5MHz apart. With 802.11a, no channels overlap and the centers are 20MHz apart. (I remember reading a very interesting white paper about this at cwnp.com.)

802.11a appears to have a smaller cell size because higher frequencies require more power to travel the same distance as lower frequencies. If you replace a 100mw 802.11g AP with a 100mw 802.11a AP, the cell will be reduced in size (although this also depends on 2.4/5GHz interference factors too). This can be a problem in P2P wireless links, where moving to 802.11a may force you to boost the transmission power or increase the sensitivity of the receiving systems.