[Cerowrt-devel] treating 2.4ghz as -legacy?
dpreed at reed.com
dpreed at reed.com
Wed Dec 18 10:19:30 EST 2013
Yes - there are significant differences in the physical design of access points that may affect 5 GHz and 2.4 GHz differently. There are also modulation differences, and there may actually be scheduling/protocol differences.
All of these affect connectivity far more than center-frequency will.
1) Antennas. One of the most obvious problems is antenna "aperture". That is a measure of the effective 2-D area of the antenna on the receiving side. A dipole antenna (the cheapest kind, but not the only kind used in access points) is "tuned" by making its length a specific fraction of the wavelength. Thus a 5 GHz antenna of the dipole type has 1/4 the aperture of a dipole antenna for 2.4 GHz. This means that the 5 GHz antenna of the same design can access only 1/4 of the radiated energy at 5 GHz. But that's entirely due to antenna size. If you hold the antenna size constant (which means using a design that is inherently twice as big as a dipole), you will find that range dramatically increases. You can demonstrate this with parabolic reflecting *receive* antennas at the two frequencies. (the aperture can be kept constant by using the same dish diameter). If you look at the antenna elements for 5 and 2.4 in an access pony, you will probably see, if you understand the circuitry, that the 5 GHz antenna has a smaller aperture.
The other problem is antenna directionality for the transmit and receive antennas. Indeed almost all AP antennas have flattened doughnut radiation patterns in free-space. Worse, however, is that indoors, the antenna patterns are shaped by reflectors and absorbers so that the energy is highly variable, and highly dependent on wavelength in the pattern. So 5 GHz and 2.4 GHz signals received at any particular point have highly variable relative energies. In one place the 5 GHz signal might be 10x the energy of a 2.4 GHz signal from the same AP, and in another, 1/10th. The point here is that a "controlled experiment" that starts at a point where 2.4 GHz works OK might find weak 5 GHz, but moving 1 foot to the side will cause 2.4 to be unworkable, whereas 5 works fine. Distances of 1 foot completely change the situation in a diffusive propagation environment.
Fix: get the AP designers to hire smarter antenna designers. Even big companies don't understand the antenna issue - remember the Apple iPhone design with the antenna that did not work if you held the phone at the bottom, but worked fine if you held it at the top? Commercial APs are generally made of the cheapest parts, using the cheapest designs, in the antenna area. And you buy them and use them with no understanding of how antennas actually work. Caveat emptor. And get your antennas evaluated by folks who understand microwave antennas in densely complex propagation environments, not outdoor free-space.
(and don't put your AP in the attic and expect a good signal near the ground.... or in the basement. Physics will make sure that the signal is zero at any ground, so being closer to the ground than the antenna weakens the signal a lot!)
2) Modulation and digitization. Indoor environments are multipath-rich. OFDM, because it reduces the symbol rate, doesn't mind multipath as much as does DSSS. But it does require a wider band and equalization across the band, in order to work well. The problem with 802.11 as a protocol is that the receiver has only a microsecond or so to determine how to equalize the signal from a transmitter, and to apply that equalization. Since the AP is constantly receiving packets from multiple sources, with a high dynamic range, the radios may or may not succeed in equalizing enough. The more bits/sample received, and the more variable the analog gain in the front-end can be adapted, the better the signal can be digitized. Receiver designs are highly variable, and there is no particularly good standard for adjusting the power of transmitters to minimize the dynamic range of signals at the receiver end of a packet transmission. This can be quite different in 5 GHz and 2.4 GHz due to the type of modulation used in the beacon packets sent by APs. Since the endpoints are made by a different designers the PHY layer standards are required to do the job of making the whole system work. Advanced modulation and digitization systems at 5 GHz are potentially better, but may in fact be far more incompatible with each other. I've seen some terrible design choices.
3) Software/Protocol. The most problematic software issue I know of is the idea of using RSSI as if it were meaningful for adaptation of rates, etc. The rate achieved is the best measure of channel capacity, not signal strength! You can get remarkably good performance at lower signal strengths, and poor performance at higher signal strengths - because performance is only weakly affected by signal strength. Even in the Shannon capacity law, inside the log term, the key constraint is the ratio between S+N and N. But that is then reduced by the "log" you take. Far more important is the bandwidth/rate. The larger the bandwidth used to transmit the same rate, the better the performance. This has nothing to do with RSSI. At 5 GHz one could use larger bandwidths and lower the signal rate when there is local noise.
One of the biggest issues at 5 GHz is that due to multipath the "hidden terminal" issue gets worse - and this is a specific issue related to "listen-before-talk" protocols. There are much better ways to deal with hidden terminals than using RSSI to adapt signal strengths or rates on any pairwise link.
Fix: if we could, we should redesign large parts of the 802.11 PHY and packet modulation protocol based on physical properties of the indoor environments where it is use.
There *are* differences between 5 GHz and 2.4 GHz. But they are not due to how far the signals propagate indoors. First order: make sure aperture and antenna patterns are proper, since they are different on the two bands - that is the main reason that the urban legend continues to survive.
Encourage vendors to fix 5 GHz aspects of their products. They should have no excuse, but they scapegoat propagation of the physical energy. That's a lie.
On Tuesday, December 17, 2013 6:43pm, "Stephen Hemminger" <stephen at networkplumber.org> said:
> I concur with Jim.
> My observation is that in our house, upstairs the 5Ghz AP has low signal strength
> reported by the devices, and poor bandwidth.
> Could it be that the radiation pattern of the antenna in WDR3800 laying
> is different for each band. Maybe the 5Ghz band is more of a squashed donut?
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