Help me interpret these results - Linux - various antenas

If I do a less /proc/net/wireless, I get these results: ralink RT61/Ubuntu linux

Parabolic Antenna 24Dbi

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90 level 207 noise 177

Panel antenna, 12Dbi

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68 level 201 noise 177

Big Tin Cantenna ?Dbi link 52 level 191 noise 161

Small Tin Cantenna ?Dbi link 59 level 193 noise 161

With this last one, I can still download as fast as my provider allows (15KBs), but it sometimes errors out on big downloads. If there is no noise difference between my parabolic and my panel, and the level is so similar (201 vs 207) why is the "link"so much better with my parabolic ?

Also, I have the option to decrease the wireless card speed from 54Mbs right down to 1Mbs. Will this improve reception, and will it affect my downloads in any way (speed or quality wise ?) TIA

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Shadow hath wroth:

Those are RSSI values on a scale of 0 to -255. The values in -dBm are far more useful.

15KBytes/sec = 96Kbits/sec. Is that all your provider allows? Is this a satellite or cellular link? The errors can be anything from interefernce to the ISP throttling your massive download. For example, FAP (fair access policy).

The "noise" number is the signal level when there is no detectable data coming in. That's basically between packets. It's a measure of interference, background noise, thermal noise, and other systems on adjacent channels. In this case, it's really a measure of how directional your antenna is in excluding these sources of crud, er... noise. The narrower antenna beamwidth antenna simply picks up less crud from the sides of the antenna. However, if you perchance aimed the antenna as one of these sources of crud, the noise level indication will increase.

Dropping the speed is normally NOT a function of the wireless card in an infrastructure network. The speed is usually set by the access point. You can change the speed at the client, but the access point still retains control. Some clients allow you to set the maximum speed. When the client and AP exchange capabilities, it will use only those speeds that are allowed up to a maximum. I know of several access points that ignore this "feature" and set the speed to what they consider acceptable.

In ad-hoc mode, which is all clients, the speed is set by the individual clients.

Thruput and connection speed are different animals. In general, your TCP thruput will be half your wireless connnection speed. A

54Mbit/sec "association" will yield about a 25Mbit/sec TCP thruput. UDP is a bit faster. See:

From the table, note that a cutting the speed in half, results in a

1.4 times increase in range.

You certainly will get more range from the slower speeds than the faster. That's because the access point tries to maintain a constant bit error rate. If the error rate climbs, the access point will slow down the connection. Therefore, if the access point is in control of the speed, the signal "quality" is contant at any speed.

This might be worth reading. It's a copy of the Intel Wireless Hotspot Deployment Guide, which is currently missing from the Intel web pile. Don't tell Intel that I'm posting it without their permission:

It covers quite a bit of what you're asking, in addition to much of the basics of wi-fi and how it works. 1.8MBytes.

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Jeff Liebermann

Let it be a secret between me, you and "the Internet users" :)

Will read (downloaded, but I'm too tired atm ... thanks)

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Is there any kind of rough comparison I can make, using these figures ?

From the pdf you thought I might illegally download, I got these figures: At 1Mbps = range 350 feet At 54 Mbps = range 90 feet So I could maybe get pretty good reception with my small can, if I turn down the speed ? It would be as if my provider antenna was

1/4 the distance away ?


Its a Brazilian monopoly. It's that or a 56kbps modem.

Which is why my small tin cantenna has less trash, it's more directional. OK.

I can set my card to 1MB using linux commands, or my RutilT utility. I can even make it pretend it's a 802.11b card.

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I will default it to 1Mbps then....

The provider runs a Zinwell ZWA-G120 with an omni antenna set about 1km from my house. I have a clear sight to it.

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Shadow hath wroth:

Yes, but I would need to get the data sheet on the chipset in your client radio. Many of the manufacturers have RSSI to signal level (in

-dBm) conversion tables or formula. It's a common issue with all wireless products including cellular handsets. For exmple:

I'm lazy. Find the FCC ID on your Ralink RT61 and look up the test report on the FCC ID web pile. That will give you the radio chipset used. Then go to the manufacturers web pile, and get the data sheet on the chip. That should give you a clue as to how to interpret the RSSI or RCPI values.

The figures from the pilphered Intel document are a bit optimistic for indoor wireless range. It's fairly close to what I would expect with really good radios, really nice antennas, no interference, and a perfect line of sight.

Exactly. However, there's no free lunch. 1Mbit/sec uses about 60 times more "air time" as the same amount of data at 54Mbits/sec. That means there's little air time left of other users on the same channel. If they have a tweaked MAC layer (very common) that treats 802.11b signals as interference, the chances of clobbering your ultra long packet time is quite high. 802.11 will fragment the packets to improve the odds, but then your thruput will drop even more. Therefore, if you're going to trade speed for range, I suggest you limit yourself to using 802.11g (OFDM) speeds instead.

Incidentally, I run several systems with the AP's locked to

12Mbits/sec wireless speeds. It's slower, but more reliable (fewer retransmissions). When the internet speed is limited to 1Mbit/sec, it makes no sense to go 54Mbits/sec (25Mbit/sec TCP thruput) when slower will work exactly the same from the users perspective.

Cantennas are not that directional. See:

I would say a -3dB bandwidth of 100 degrees. Compare that with a biquad of 50 degrees, or a 24dBi dish of about 7 degrees.

I would use 802.11b speeds only if desperate. 802.11g is far more resistant to reflections and interference.

Try to guess the gain of the omni antenna by it's length. If there's any coax cable involved, guess the size, length, and attenuation. Then go unto:

and calculate at what speed you can still have at least 10dB SOM (fade margin) at 1km. I've posted numerous walk through on how to do the calcs in this newsgroup. That will tell you how much antenna gain you'll need.

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