(below is from <http://en.allexperts.com/e/i/ie/ieee_802.11e.htm>)
HCCA: The HCCA works a lot like the PCF: the interval between two beacon frames is divided into two periods, the CFP and the CP. During the CFP, the Hybrid Coordinator (HC) controls the access to the medium. During the CP, all stations function in EDCA. The main difference with the PCF is that Traffic Classes (TC) are defined. Also, the HC can coordinate the traffic in any fashion it chooses (not just round-robin). Moreover, the stations give info about the lengths of their queues for each Traffic Class (TC). The HC can use this info to give priority to one station over another. Another difference is that stations are given a TXOP: they may send multiple packets in a row, for a given time period selected by the HC. During the CP, the HC allows stations to send data by sending CF-Poll frames. --------------------------------------------------------------------------- PCF: The original 802.11 MAC defines another coordination function called the Point Coordination Function (PCF): this is available only in "infrastructure" mode, where stations are connected to the network through an Access Point (AP). This mode is optional, and only very few APs or Wi-Fi adapters actually implement it. APs send "beacon" frames at regular intervals (usually every 0.1 second). Between these beacon frames, PCF defines two periods: the Contention Free Period (CFP) and the Contention Period (CP). In CP, the DCF is simply used. In CFP, the AP sends Contention Free-Poll (CF-Poll) packets to each station, one at a time, to give them the right to send a packet. The AP is the coordinator. This allows for a better management of the QoS. Unfortunately, the PCF has limited support and a number of limitations (for example, it does not define classes of traffic). HCCA is generally considered the most advanced (and complex) coordination function. With the HCCA, QoS can be configured with great precision. QoS-enabled stations have the ability to request specific transmission parameters (data rate, jitter, etc.) which should allow advanced applications like VoIP and video streaming to work more effectively on a Wi-Fi network. HCCA support is not mandatory for 802.11e APs. In fact, few (if any) APs currently available are enabled for HCCA. The Wi-Fi Alliance has a forthcoming certification (WMM Scheduled Access) that will allow network integrators to easily distinguish APs that allow HCCA. --------------------------------------------------------------------------- (below is just from my limited knowlege) PA: Power Amplifier --------------------------------------------------------------------------- (below is from <http://www.wirelessdictionary.com/wireless_dictionary_UMTS_LTE_PAPR_Definition.html >) PAPR: Peak to Average Power Ratio; Peak to average power ratio is a comparison of the peak power detected over a period of sample time to the average power level that occurs over the same time period. SC-FDMA has a lower PAPR as compared to other radio channel structures such as OFDMA. --------------------------------------------------------------------------- (below is from <http://portal.acm.org/citation.cfm?id=1069514> and <http://www.gigascale.org/pubs/1333.html >) EVM: In digital radio applications, error-vector-magnitude (EVM) is the primary specification which quantifies the performance of digital modulation implemented in silicon. In production testing of wireless systems, measurement of EVM (a critical spec that is directly related to bit error rate) incurs significant test time due to the large numbers of symbols that need to be transmitted for reasons of accuracy. In our approach, EVM is modeled as a function of the system static non-idealities (IQ mismatch, gain, IIP3 parameters) and dynamic non-idealities (system noise, VCO phase noise). Using a selected subset of the OFDM tones, the static parameters are calculated first. These are then used to facilitate noise estimation using a back-end constellation compensation and noise amplification procedure. --------------------------------------------------------------------------- NOW! with that out of the way, let's all put our guns, arrows, cannons, swords, phallic members and the great and powerful REPLY-ALL keys (<http://www.kirikiri.com/that/reply.gif>) away and THANK the non- member/non-usual poster for his time and information! Lawrence, thanks so much for this posting. A little homework on my/our part using <http://lmgtfy.com/?q=802.11+specs> really clued me in on some things I have to take into account when doing installs and growing my customer base. And the rest of you! It is Sunday morning, you should all still be in your underwear drinking coffee taking a day off! :) Just think of me at 4000 feet building a repeater shed.... as the snow starts to fall and have a great day! ryan On Oct 4, 2009, at 7:10 AM, Mike Hammett wrote: > I'm not sure what HCCA, PA, PAPR, or EVM are, but I don't think that > WISPs > need to. > > > ----- > Mike Hammett > Intelligent Computing Solutions > http://www.ics-il.com > > > > -------------------------------------------------- > From: "Gino Villarini" <[email protected]> > Sent: Sunday, October 04, 2009 6:04 AM > To: "WISPA General List" <[email protected]> > Subject: Re: [WISPA] To G or not to G :-) > >> Lawrence post wasn't too technical at all .... Stuff wisps >> operators or >> at least the RF guy of a wisp should know >> >> >> >> Gino A. Villarini >> [email protected] >> Aeronet Wireless Broadband Corp. >> tel 787.273.4143 fax 787.273.4145 >> >> ________________________________ >> >> From: [email protected] [mailto:wireless- >> [email protected]] On >> Behalf Of Jack Unger >> Sent: Sunday, October 04, 2009 1:40 AM >> To: [email protected]; WISPA General List >> Subject: Re: [WISPA] To G or not to G :-) >> >> >> >> Yep it's too bad that many wireless ISPs have no interest in learning >> about wireless. >> >> Scottie Arnett wrote: >> >> I am reading your response and can not decipher all your algorithms? >> Point that out and I will have a much more understanding of what >> you are >> scientifically trying to say. Most WISPS have absolutely no >> scientific >> background! >> >> John >> >> ---------- Original Message ---------------------------------- >> From: "Lawrence E. Bakst" <[email protected]> <mailto:[email protected] >> > >> >> Reply-To: WISPA General List <[email protected]> >> <mailto:[email protected]> >> Date: Sun, 4 Oct 2009 00:15:45 -0400 >> >> >> >> I think you guys know most of this already, but here is my take >> FWIW. >> >> I'm not a WISP, but I spent 5 years leading the design and >> development of an 802.11[agb] security system. We did our own polling >> solution based on 802.11e HCCA to solve the RTS/hidden node problem. >> >> All things being equal (which they often aren't) 802.11b will >> give you a higher S/N and C/I than 802.11g, because in almost all >> cases >> and especially at higher speeds. 802.11g has to lower the PA power >> because of the PAPR of OFDM and meeting the 802.11g EVM spec. >> >> It is true that 2.4 GHz can be very polluted. We found the noise >> floor to be really awful. You would be surprised by the number of >> "entities" that know they are way over the FCC max power in 2.4 >> GHz, but >> I digress. We once measured over 300 PHY errors a second on an >> "unused" >> 2.4 GHz channel. The number went down to 150 PHY errors a second >> inside >> an FCC chamber, if you can believe that. >> >> Having said all that we didn't use 802.11b at all because it's >> data rates are too low for video. >> >> Also while we supported 2.4 GHz, we mostly deployed at 5.8 GHz >> ISM because of the increased power available there and the >> pollution was >> much less, but that maybe different now. >> >> For 802.11[ag] mutlipoint, the sweet spot speed wise is 18-36 >> Mbps. It's very hard to keep a multipoint system at 48 or 54 Mbps >> because you need a great deal of link margin and with all cards you >> loose power as the speed increases to maintain PAPR/EVM. For point to >> point with direction antenna relief you can often maintain 48 or 54. >> >> Antennae make a big difference, as others have noted horizontal >> polarization is usually best and make the beam as narrow as you can >> afford because it raises the effective gain. However, if you are in >> an >> area where everyone else is horizontal it can make sense to try >> vertical. With some of the antennae we used that was as simple as >> rotating the antenna 90 deg at both ends. >> >> Watch out for crappy antennae, cheap cable, bad connectors, and >> so on. That can often cost you a few dB. In the product I designed I >> spent more time then I care to admit trying to make a very tough loss >> budget that I set out as a goal. >> >> There is no substitute for link margin, you can never really >> have enough. >> >> I can confirm that our sweeps with a spectrum analyzer show lots >> of opportunity to use 5 and 10 MHz channels, as others have also >> noted. >> For WISPs it would be "nice" if chip vendors designed the radios so >> that >> you could set the channel bandwidth from 5-40 MHz in 1 MHz >> increments. >> It can be done but probably won't be, although maybe the Microsoft >> WhiteFI stuff force the chip vendors to do it. In WiMax and LTE >> they are >> already doing some things close to this. Still 5, 10, and 20 isn't >> bad >> and probably hits the sweet spot or 80/20 rule. >> >> One of the down sides of fitting a 5 or 10 MHz channel in a >> sweet spot is that it can change at any time. >> >> Best, >> >> leb >> >> At 9:58 AM -0500 10/1/09, Jason Hensley wrote: >> >> >> In 2.4 land, if you have a lot of noise, which protocol >> is better - B or G? >> Is it better to run an AP as locked into one mode or is >> it OK to do a mix? >> >> Max I want off of 2.4 customers is 3meg so not that >> worried about the extra >> speed that G will provide, but, I would like to know >> which is more stable? >> I've always thought that B was more stable overall but >> just provided less >> bandwidth. I've gotten some info that may counter that. >> What's the >> real-world experience with folks in a high-noise >> environment, combined with >> a higher useage AP? >> >> I've got an AP that we've run in B mode only for a >> while. We've started >> having problems with it - speeds go from 3meg at the >> customer to 200k and >> fluctuate constantly. We've worked with RTS, ACK >> timeouts, etc etc and >> nothing seems to have improved the stability. For >> testing purposes we put >> up another AP right next to the one we're having trouble >> with. Switched two >> of our gaming clients to that one (setup as G mode only) >> and they seem to be >> doing better, but not quite as good as we feel they >> could be. This is on >> Deliberant AP's (Duos). The backhaul part of it is not >> the issue - we can >> pull close to 15meg back to our office when cabled into >> the AP. We have >> other Deliberant APs that are running MANY more clients >> than this one so we >> know it's not limitations of the equipment. AP is on >> top of a water tower. >> Have taken all clients off and brought them back on one >> by one and it did >> not reveal anything significant. With just one customer >> on the AP started >> acting up again. Swapped radios in the AP thinking we >> could have one going >> bad and still no luck. >> >> 2.4 antennas are H-pol. We have a ton of noise in the >> area, but we've been >> through basically every channel and it did not help >> either. Other AP's in >> the vicinity are performing fine. Thought of the >> multipath issue so we >> raised our test AP up a little higher than the other >> one. As I said, the >> test AP seems to be better, but next to it on top of the >> tower we can get >> around 8 or 9 meg down (locked into G mode), but at the >> CPE's we're still >> barely getting 2.5-2.8meg. >> >> Any thoughts? We changed everything we can. The new >> "test" AP has a 9db >> antenna compared to the 13db on the "production" AP. >> Other than that, they >> are identical as far as equipment goes. >> >> So, back to the subject question though, what's >> real-world experience with >> G-only mode in the field? >> >> >> >> >> >> ------------------------------------------------------------------------ >> -------- >> WISPA Wants You! Join today! >> http://signup.wispa.org/ >> >> ------------------------------------------------------------------------ >> -------- >> >> WISPA Wireless List: [email protected] >> >> Subscribe/Unsubscribe: >> http://lists.wispa.org/mailman/listinfo/wireless >> >> Archives: http://lists.wispa.org/pipermail/wireless/ >> >> >> >> -- >> [email protected] >> >> >> >> >> ------------------------------------------------------------------------ >> -------- >> WISPA Wants You! Join today! >> http://signup.wispa.org/ >> >> ------------------------------------------------------------------------ >> -------- >> >> WISPA Wireless List: [email protected] >> >> Subscribe/Unsubscribe: >> http://lists.wispa.org/mailman/listinfo/wireless >> >> Archives: http://lists.wispa.org/pipermail/wireless/ >> --- >> [This E-mail scanned for viruses by Declude Virus] >> >> >> >> >> >> Wireless High Speed Broadband service from Info-Ed, Inc. as low as >> $30.00/mth. >> Check out www.info-ed.com/wireless.html for information. >> >> >> ------------------------------------------------------------------------ >> -------- >> WISPA Wants You! Join today! >> http://signup.wispa.org/ >> ------------------------------------------------------------------------ >> -------- >> >> WISPA Wireless List: [email protected] >> >> Subscribe/Unsubscribe: >> http://lists.wispa.org/mailman/listinfo/wireless >> >> Archives: http://lists.wispa.org/pipermail/wireless/ >> >> >> >> >> >> >> >> >> -- >> Jack Unger - President, Ask-Wi.Com, Inc. >> Author - "Deploying License-Free Wireless WANs" >> Serving the Broadband Wireless Industry Since 1993 >> www.ask-wi.com 818-227-4220 [email protected] >> >> >> >> >> >> >> >> -------------------------------------------------------------------------------- >> WISPA Wants You! 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