Making 802.11 Take The Longshot 54
Simone from O'Reilly pointed out the continuing developments with Rob Flickenger & Co's efforts with 802.11. This time around they're trying to make a five mile to homes, via a 20.9 mile boost antennae. Fun for the whole wireless family.
Just the thing to wire Australia (Score:1)
I should be able to get say 64K at that range..
The tech has a long way to go..
Re:Discovering America? (Score:1)
Re:How to make an antenna? (Score:1)
Re:Shouldn't be too hard... (Score:1)
Re:FCC rules for 802.11 and fines up to $250,000 (Score:2)
Accually you don't need a $200 installation geek. Your equipment must be either professionally installed, or certified. However the definition of a professional is pretty meaningless. If you are paid to do something you are a professional, which says nothing about your ability to do it right.
Note that you still need to meet regulations. So if you put a 500 watt amp on your signal they will get you, no matter how professional your system is put up. (.5 watts is the biggest amplifer you can buy, and even then the manufactures make a big deal that you cannot run the better antennas with it)
Re:Legality? (Score:4)
The low-power cards pump out around a couple of hundred milliwatts. You can plug them into hi-gain antennae, in which case the ERP in the direction of the beam is increased. Increase that above a certain limit (depends on your Government's regulations) and you go outside the permitted level. Alternatively you can use an amplifier and a lower-gain antenna. Whether you get X decibels of gain from the antenna or the amplifier really doesn't matter, it's the peak energy density coming out that counts.
If you can't remember the math, it may help to know that decibels add: 10dB from the antenna plus 10dB from the amp = 20dB of gain. Decibels are based on logarithms base 10. 3dB = factor of 2 gain, 10dB = factor of 10, 20dB = factor of 100, 30dB = factor of 1000. 200mW into a 20dB antenna is theoretically equivalent to 20W into an omnidirectional atenna (but we call 'em aerials over here, at least if you are as old as I am).
In reality, you would never get the theoretical power, because feeder and mismatch losses in the connectors can easily lose you several dB, much more if you are careless. 10dB loss in connector and feeder would be no surprise. 2.4GHz is where you start to need plumbing and wires get more and more tricky to work with.
Re:boosting airport? (Score:1)
It might be better to look at the pages on how to hack your base station (its much better documented)
Trees in the way? (Score:1)
Coudln't you just use a telescope? You should be able to see the blinking light on the tower.
More to security than just your mail (Score:2)
Legality? (Score:2)
It's not accurate to say 'you can't use an amplifier' nor 'if you don't use an amplifier, whatever you do is legal.'. Both can keep you with the bounds of the ISM rules, or take you outside of it, depending on what you do.
The point is, the right high-gain directional antennae alone can take you outside the technically legal limits of 2.4Ghz ISM. And other situations, you can amplify the signal as long as you stay within the specified limits.
Anyone with some real knowledge of the math involved want to explain this better? (Or correct me?)
Trying.. (Score:2)
I've seen approximately 20 km links using 2.4Ghz ISM band, staying within the limits of the ISM band.
5 miles is no biggie; we were doing that 3 years ago or so with Proxim RangeLAN cards & some good antennae.
READ -- interesting. (Score:2)
For each dB of cable loss or dB of reduced output power, you can add 3dBi of antennae gain, PROVIDED the connection you are making is point to point, and fixed (not mobile). This allows for much longer ptp links than are possible in Canada, for instance, where the rules are simpler (EIRP of 36dBm, period).
So a 20 mile link is probaby quite easily done, provided the connectors and antenna work is top noch (2.4Ghz is sensitive to small errors in cabling, etc).
ALso, and I can't confirm this absolutely, but I know when you get things approved for ISM band in the US, you have to specify the cable length/type and antennae type, and you have to use non-standard antennae connectors (if they aren't fixed or internal). This is because they are techniaclly not supposed to be modified, as if you change any component, you are creating an unapproved transmitter.
Re:Shouldn't be too hard... (Score:1)
Hemos is new to the English language (Score:5)
Re:Shouldn't be too hard... (Score:1)
I think their point is that they want to have cheap, high bandwith, internet access. Of course, this has been done before, even with standard 802.11b equipment.
It's a security breach waiting to happen.
Not quite. It's easy enough to run your own security on top of this link, and block all unauthorized traffic.
Re:Shouldn't be too hard... (Score:1)
rule of thumb calculating distance from db (Score:1)
This is spiffy (Score:1)
[0] I think. It's all 2.4GHz 802.11 gear, but I suppose there could be something magical in the dish.
[1] Which is faster, in both directions, than the T-1 leased by the local school district, even though it's shared and slower on paper. Did I mention the phone company here sucks?
Point to Point VPN (Score:1)
Re:Discovering America? (Score:2)
Actually, for point to point links, the maximum EIRP is 48dbm (total system power), but you must reduce output power by 3db for each 6db added with antenna.
For normal point to multipoint, the EIRP is 36dbm (4W total system power) but the power output of the radio/amplifier cannot exceed 1W (30db).
Re:I already have longer links (Score:1)
The real link. (Score:1)
Re:Wire length (Score:1)
Pretty cose to what you are talking about. (Score:1)
Discovering America? (Score:1)
Add antenna gain, substract some interference and there you go. The only thing you have to care for is your country limitations for transmission power in the 2.4 GHz band (which is the band all these things like Lucent WaveLan, Cisco Aironet or BreezeNet use). It is usually defined as EIRP (Equivalent Isotropic Radiation Power), which means for you the more you gain with the antenna the less power you may feed the antenna..
Re:Shouldn't be too hard... (Score:1)
A relative of mine works for the power company here in Nova Scotia and tells me they're thinking of using 802.11b for distances of up to 100km (62.5mi).
I don't know anything about the technology behind it, but they indicate that if it works, the cost of the project will be under CDN$10,000. The alternative, more established, point to point radio setup costs five times that much. It'll pay for itself in no time when compared to a T1 from the phone company as well.
Not only that, but the ability of 802.11b to dynamically adjust the bandwidth based upon signal strength allows them to avoid the large amount of "spare" signal they normally have to factor into these things to allow for varying conditions.
Seems like a pretty good deal to me!
--- Brent Rockwood, Development Lead
Re:brokent link? n/t (Score:2)
remove the last quote from the URL and you'll be fine.
Wire length (Score:1)
Does anyone know what length the antenna cables can be between the base station and the actual antenna on the roof?
Reason for asking is that I'm looking into building a similar setup for a local group of computers. But the most strategic rooftop is 7 floors up from the appartment. It would be much simpler if I don't have to buy/protect/provide power to a rooftop unit other than just the antenna.
Shouldn't be too hard... (Score:2)
Also, with the questionable security of 802.11, even with WEP enabled, would it really be that good of an idea to have that large of a coverage radius? With technology like this, people wouldn't even have to drive up to the parking lot to gain access to the network, they could do it from their home or someplace far away from their actual target.
It's a security breach waiting to happen.
Re:Shouldn't be too hard... (Score:2)
Re:It only limits effective *radiated* power (Score:1)
Re:How to make an antenna? (Score:1)
I've also been told, by haven't checked, that part of the ISM band overlaps a ham band just below 2400Mhz - so presumably if that is true and you hold the correct class of license, you could operate under that license as long as you meet the general Part 15 requirements.
Re:boosting airport? (Score:1)
IPSec VPN's - no problem! (Score:1)
dBm Budget is the problem, not wire length (Score:1)
In theory, using the lower data rate of 1mbps to maximize the power-per-bit for the best SNR, we should have a receive sensitivity of about -94dBm. With a 30mw radio and 3 dB of cable loss with 24 dBi Conifer dishes we have a best case path loss budget of about 94+15+22+22-3-3 = 147 dBm. That works out to a theoretical max of just over 120 miles best case (no multipath losses, no defraction losses, no atmosperic attenuation, etc). In the real world, it's about 1/2 to 2/3 that distance for a semi stable link at 1 mbps. Similar calcs lead us to shorter distances at 11mbps because the lower power-per-bit drops the receiver sensitivity to 85 dBm - about 45 miles for a best case link, and 20-30 miles for a semi stable link. For rock solid links, immune to most link interference good designers us a minimum link margin of between 10-15 dB - and after doing the math the maximum 11mbps range with this link margin requirement is about 15-17 miles.
If you start with a noisy commercial tower site with lots of broad band noise, you can expect to lose 4-15 dB of receiver sensitivity at the hill top. So where 15-17 miles may have been expected to be a rock solid link, it is instead a semi-stable link subject to minor weather and interference reducing the link performance at times (and possibly completely dropping out for periods).
802.11 performance issues (Score:1)
First 1mbps, 2mbps, 5.5mbps, and 11mbps are raw half duplex modulation data rates. There is considerable discussion the the standard minutes about obtainable performance at various packet lengths. The first problem is the protocol requires sending an 802.11 header at 1mbps followed by the 802.3 data packet and then waiting for an 802.11 ACK packet at 1mbps. So there is a minimum small packet latency of about 2-6ms in a typical transfer between a radio and access point due to the minimum overhead of the wireless link and the MAC/Driver processor latencies.
Cisco and Aironet radios implement the entire 802.11 protocol and handshake in a processor inside the radio card - the MAC (Media Access Controller), so the driver is very simple and takes a single interrupt (or less) per packet. This makes the driver interface similar to a good ethernet interface - and if you look at the B.Reed Airo.c linux driver its basicly the standard shell of an ethernet driver plus some Aironet specific setup, control and error handling. Performance of these cards in generally pretty independent of other I/O in the system.
Lucent and nearly all other radio mfgrs avoid costly firmware development and implement the 802.11 protocol in the host driver. This means that they typically have a complex body of code which is proprietary with an open source wrapper around it. These drivers often take several interrupts per packet to manage the 802.11 protocol and wireless performance is highly variable depending upon host processor performance and interrupt latency (which can be very high with concurrent NE2000 ethernet or IDE disk activity).
Because of the header, flight-time, Ack-packet, and flight-time latencies of around 2ms best case, an 802.11b system pegs at about 500 packets per second with 64 byte packets. This translates to about 64*500*8 = 256kbps max raw data rate with small packets, and nearly half that ftp delivered data rate with small packet sizes. This number is very important, because 1/2 to 1/3 of all TCP packets are this size - the TCP ACK packets. Small packets are also common in a number of streaming UDP protocols used by many multiplayer games and Audio/Video transport protocols. Since most wireless systems operate as wireless bridges, the are limited to an interface MTU of 1500 bytes. Most wireless systems top out at about 500 KBytes per second (4mbps) at this packet size due to the 1mbps header, two flight times, and the 1mbps ACK packet. Using most cards in Adhoc mode, you can use a larger MTU which improves the best case transfer rates by a fair bit.
In a Metropolitan Area Network environment, where there is a single high ground repeater radio the aggregate bandwidth is split between the path up and the path down. There is also some "hidden node" collisions which cause additional retries at a rate highly dependent on the number of concurrent radios, packet size, and other variables. Typical end-to-end link performance over a store and forward single radio repeater is between 120-220 KBytes/sec with most packets being typical 1500 byte TCP ethernet frames and ACK's.
There has been a lot of miss-information about FHSS and DSSS interoperability. In general, a single FHSS system and an 802.11b 11mbps DSSS system will interfer with each other about 33% max - since the FHSS will dwell inside the DSSS channel about that percentage of time. Typical systems have under a 10% duty cycle, so the impact is minor at best. Older 1mbps FHSS may not do the Clear Channel Assessment (CCA) part of the 802.11 protocol, and are a little less friendly since they will barge right in on top of another 802.11 radios transmission. Also 1mbps 802.11 DSSS radios can be impacted a little more because of the longer air time for each data packet - increasing the probability of a collision in the channel. With an 802.11b DSSS radio doing CCA and multiple concurrent FHSS access points on different hopping lists, there becomes the possibility of the FHSS system locking out the single DSSS system. The same applies to 3 or more concurrent DSSS channels active, locking out all of the channels of a single FHSS radio. The primary problem here is in the 802.11 protocol where if a certain number of beacons are not recieved at the expected time, the clients of an access point will disconnect and start hunting for a better access point. This takes several seconds, during which most radios drop all packets they have buffered. Net thruput in this mode drops to less than 1% with 5-30 second packet latecies typical - andhigh packet loss. Essentially unusable with DNS and most TCP protocols.
802.11b system sharing the same channel will typically share the channel fairly using CCA with increased collisions due to higher probability of hidden nodes. Enabling RTS handshaking can greatly minimize the collisions.
Low power cards are NOT couple hundred mw! (Score:2)
There were a few early 802.11 Prism I cards that were 250mw and 500mw produced by minor players in the market. And I believe the same is true of 802.11 FHSS vendors - mostly 100mw.
It's pretty hard to connect a 30mw radio to a 23dBi dish without any cable and connectors losses. Especially given that it generally takes an RG316 jumper to get from the PCMCIA card to some more managable connector for the antenna. Typically lose 1-3 dBi between the radio power amp and the antenna feed element even with short cables and best connectors.
Most people mistake that dBi and dBm are the same, converting 24dBi to dBm we get 24-2.1=21.9dBm EIRP is predicted then to be roughly 13 + 20.8 - 2 = 32.9 dBm for most 30mw radios connected to a Conifer 24dBi dish typical. These leaves a safety margin of 3.1 dBm for radio power, cable and antenna variances, and still remain inside the FCC 36dBm EIRP budget.
Most standard RF cables and connectors do not work well with 802.11b at 2.4GHz. But Times Microwave cable and connectors are excellent - most people use LMR-400 and LMR-600 with TM gold pin connectors for minimum losses. The times cable has the best propagation velocity factor in the industry, which greatly minimizes wave front compression and associated phase distortion in the encoded digital signal.
100' of TMR-600 costs about $165 plus two TM "N" connectors that are $23/ea - for about $220 assembled. The cable and connector loss is about 4.1 to 4.5 dBm. 100' of TMR-400 with connectors is about $130 and 9-10 dBm of loss. Beldon 9813 "LOOKS" almost identical to LMR-400, but has a much slower propagation velocity factor, and often will not work at 11mbps in cable lengths over 25', even though the two have nearly the same RF attenuation at 2.4GHz. A lot of older vendor cables from 1-2mbps 802.11 days are Beldon
ALso beware of center pins plated with other than gold - since the RF at these frequencies will accelerate oxidation which will rapidly change the connector impedance and the cable will "ring" with standing waves if anywhere near a wavelength multiple. Symptoms are good signal strength reported by radios in both directions, but very high retry rates caused by either garbling of the encoded signal and/or the ringing not settling down fast enough to recieve the 802.11 ACK packet.
Similar symptoms are also had when a "passive re-radiator" element is near the beam path at either end - a section of metal near a multiple of the wave length which rings during transmit, and the ringing doesn't die down fast enough to recieve the ACK packet.
Re:boosting airport? (Score:2)
FCC rules for 802.11 and fines up to $250,000 (Score:5)
While it is pretty easy to buy off the self antennas for 2.4GHz on Ebay or mail-order, there are very few vendors who sell certified configurations approved by the FCC for use with specific 802.11 vendors gear. Companies like Winncom and Hyperlink have done the required FCC certifications (which typically cost $15-30K) and can sell kitted systems - radio, cable, antennas to allow self installation of an FCC legal configuration. Otherwise, attempting to save a few hundred bucks is likely to net $250,000 in fines per installation as soon as a competitor (either in the wireless business or your trade) attempts to level the playing field with a complaint to the FCC.
When I was at Networld/Interop last year several of the wireless vendors were quick to note FCC raids in the south brought on by ISP's home brewing illegal configurations that violated the specs.
We have been doing what the O'Reilly boys are trying for several years now, and have a strict policy in our customer base to purchase approved kits from Winncom AND to use professional installation to make sure our network remains legal.
Multipoint links out to 25 miles are not that difficult - but it's also not a piece of cake using commercial hill top radio sites - especially when microwave links, AM/FM/TV broadcast stations, Cell Operators, Paging Operator, 400/800MHz moble repeaters, and other high power transmitters are on or near the tower. These transmitters all product Broad Band Noise (white noise) in their power amplifiers which greatly increase the noise into the 802.11 radio's reciever - often making the receiver deaf to your signals at distances further than a few miles - no amount of filtering will solve this problem, only careful trial and error placement of your antenna will minimize, but not remove the problem. Secondary to this, is the fact that snow, fog, rain, hail at the site will cause reflection/defraction of the broadband noise back into your antenna making the reciever deaf under cetain weather conditions that would not otherwise impair the link.
Also there is a problem with the transmit energy from co-located gear at the site mixing in your amplifier and radio front end, producing harmonic products of your radio's IF frequencies, which go right thru the filters and deafen your radio for as long as the RF carriers are present. We run with 110dB of band pass filter between a 7dBi omni and amplifier/radio to suppress this "Intermod". Reciently one of the PCS cell carriers doubled the number of channels at two sites we share, and seriously disrupted our repeaters for 30-90 minutes at a time - until we identified the source of the problem and added an additional 40dBi of band pass filter. We still see several second hits off the 3rd and 5th harmonics of 400/800Mhz pagers/mobile communications due to the extremely high EIRP they transmit at (several hundred watts). Even though their out of band energy is legal (50-70dB down from their primary carrier) it deafen's our recievers for brief periods due to near-far problems (IE our incoming signal is down around -80dBm - their harmonics are -40 to -10 dBm at the repeaters antenna.
It has taken us 6 weeks of trial and error placement and equipment/cable tuning to make some hill top repeater sites usable. This is not a binary problem where it just plugs and plays after hooking up some wires.
So in short, Metropolitan Area Networks built from off the shelf 802.11 wireless lan gear using repeaters over commercial radio hill tops sites isn't always easy to make links more than 2-3 miles work reliably. Even using this this gear shorter distances across roof tops is often difficult due to diffraction losses caused by the buildings and trees, and other Fresnel zone violations resulting in multipath interference that varies with temperature and weather. On some of our longer (IE weaker) links we have even seen Solar radition interference patterns on radios with west facing dishes as the sun sets at times. And other links installed in the winter, where a tree 90' away, and 40' to the side of the beam, leafed out and disrupted the link. And where standing waves from metal building 400' away at 10 O'clock to the beam, nulled out the beam at certain times of the day probably due to the metal siding warping in the heat and changing the pattern reflected back at the side of the dish antenna.
So do your homework before tring to save a few bucks buy 3rd party 802.11 gear from vendors that have not done the required certification with each radio you have. The several million in fines if you get caught is not a savings. You probably want to get a spectrum analyzer for the 2.4GHz band before you start that has a calibrated dBm vertical scale - like the HP8559s and later series of SA's. You will also need a display which offers "Max Hold", with an 8559A this means an 853A digial display, not a 180 series display. Trying to debug metropolitan area wireless lan's without being able to see the spectrum is nearly impossible - and horribly frustrating.
For more information of microwave path issues, check out:
http://www.tapr.org/tapr/html/ve3jf.dcc97/ve3jf.d
For FCC regs see:
http://www.fcc.gov/oet/info/rules/
Also checkout the fines section elsewhere on the site.
Extracted from current FCC Regs, Part 15
PART 15 - RADIO FREQUENCY DEVICES
Subpart C - Intentional Radiators Section
Section 15.203 Antenna requirement.
An intentional radiator shall be designed to ensure that no antenna other than that furnished by the responsible party shall be used with the device. The use of a permanently attached antenna or of an antenna that uses a unique coupling to the intentional radiator shall be considered sufficient to comply with the provisions of this Section. The manufacturer may design the unit so that a broken antenna can be replaced by the user, but the use of a standard antenna jack or electrical connector is prohibited. [...] Further, this requirement does not apply to intentional radiators that must be professionally installed, such as perimeter protection systems and some field disturbance sensors, or to other intentional radiators which, in accordance with Section 15.31(d), must be measured at the installation site. However, the installer shall be responsible for ensuring that the proper antenna is employed so that the limits in this Part are not exceeded.
Section 15.204 External radio frequency power amplifiers and antenna modifications.
(c) Only the antenna with which an intentional radiator is authorized may be used with the intentional radiator.
Re:Shouldn't be too hard... (Score:4)
Say you live far out from the city, and you can pick between a lowspeed modem or a lot faster radio lan. What would you pick ?
With some encryption, say encrypted webproxy at the ISP, encrypted mail etc, i'd definitly go for it if the alternative was modems. And many things you do online doesnt even require security... i dont really care if someone eavesdrops my slashdot browsing :)
This was done more than a year ago... (Score:1)
Re:This was done more than a year ago... (Score:1)
Re:Shouldn't be too hard... (Score:4)
I want to say that Orielly gives back to the community.
What few people might try and break this would be out numbered by the number of people that would pay for this sort of thing.
The number of people that would be beniftied by this would be great. Schools would have highspeed access, kids who wouldnt have access otherwise.
And yes it is possible to do long range point to point, but then you have to hook it up to each computer.There inlies your real cost, your real reason for having a DMZ on an 802.11 network.
This is a great idea and I forone am going to go out there with my laptop as soon as I get my new lucent 802.11 card.
Are you on the Sfglj [sfgoth.com] (SF-Goth EMail Junkies List) ?
Re:Shouldn't be too hard... THANKS FOR THE LINK!!! (Score:1)
And yes, it runs on Linux....
Shameless Plug for my Web Site:
Wireless LAN Hardware and Systems
Network with a 15 mile radius!
Provide high speed connections to your ISP with out the expensive infrastructure!
Network your campus!
www.techsplanet.com/wlan/ [techsplanet.com]
Hey, at least I didn't use the BLINK tag!
We do this already... (Score:1)
It isn't hard to get a 20 mile point to point link. You can use an off the shelf Cisco BR342 or 352 AP and a 19 DBI antenna. The main problem is radio horizon, you will need to get both antennas off the ground about 50-75'
The systems we build do this sort of thing all the time. We are very careful to not violate the FCC radiated wattage limits, which is why it is important to use pre made plug and play if you are not a network professional with wLAN experience.
We get new toys in all the time. For example, we just got in a micro cellular system that has an effective range of 5 miles. It only cost $1499, and you can link multiple units together without wires to create a micro cellular network. The signal will get relayed till it gets to your servers.
Shameless Plug for my Web Site:
Wireless LAN Hardware and Systems
Network with a 15 mile radius!
Provide high speed connections to your ISP with out the expensive infrastructure!
Network your campus!
www.techsplanet.com/wlan/ [techsplanet.com]
Hey, at least I didn't use the BLINK tag!
Re:It only limits effective *radiated* power (Score:1)
Re:Legality? (Score:2)
In the U.S., the FCC has established EIRP limits on Part 15 spread spectrum applications (which includes 802.11 in all its forms). Other countries have their own limits - generally more strict than the U.S., IIRC
The Part 15 limits on EIRP go something like this:
a) You are allowed up to 1 watt (+30dBm) output from your transmitter.
b) You are allowed up to +6dB antenna gain (over isotropic) with a 1 watt transmitter.
c) For every dB reduction in transmitter output, you are allowed a corresponding dB in antenna gain.
In essence, this means that you are limited to +36dBm EIRP (effective isotropic radiated power). I do not know whether you can subtract transmission line/connector losses from this.
Most 802.11b transmitters are at about 30 mW, or -15 dBm. In other words, you are 15 dB below the maximum allowed transmitter power. So your antenna gain must not exceed 15 + 6 or 21 dB. Judging from the pictures in the article, it looks like they are using dish antennae with probably 23 dB of gain.
Also: The 30 mW transmit power output is an assumption -- Some radios can output 100 mW. This would reduce the legal antenna gain from 21 dB to 16 dB, which would almost certainly push them past the legal limit.
Not that I expect the FBI SWAT team to knock down your door :), but just thought I would point out that the amount of antenna gain you can utilize - and stay legal - is finite.
This topic concerns me because the more that is said in a public forum (such as this story), the more likely the FCC will be to tighten up on 802.11b uses. I have already heard that the FCC is considering making it illegal to manufacture a Part 15 transmitter (which all U.S. 802.11b products are) with an external antenna, for this very reason. I hope that does not come to pass...
brokent link? n/t (Score:1)
link (Score:1)
where they tested... (Score:1)
It's home to eye-popping views of the shoreline and ocean, staggering cliffs and switchbacks, and enough redwood trees to make one think they've stepped onto the forest moon of Endor.
--------------------------------
Re:Wire length (Score:1)
The maximum length of transmission line depends on how much you want to spend. 1 inch diameter coaxial cable would probably have low enough losses to do the job - waveguide would be even better. You'd be looking at 10 bucks a foot for the cable tho - connectors would be on the order of $100 apiece.
It only limits effective *radiated* power (Score:1)
--
spam spam spam spam spam spam
No one expects the Spammish Repetition!
Appx. 50 miles, I think (Score:1)
--
Asymmetrical wireless networking (Score:1)