Data Centers

Cheat sheet: What you need to know about 802.11ac

The newest wireless networking protocol is 802.11ac, due to be ratified sometime in 2013. Michael Kassner does the research and tells you what you need to know.

[UPDATE 06-26-2013: See below for addition of transmission testing results]

Wi-Fi junkies, people addicted to streaming content, and Ethernet-cable haters are excited. There's a new Wi-Fi protocol in town, and vendors are starting to push products based on the new standard out the door. It seems like a good time to meet 802.11ac, and see what all the excitement's about.

What is 802.11ac?

802.11ac is a brand new, soon-to-be-ratified wireless networking standard under the IEEE 802.11 protocol. 802.11ac is the latest in a long line of protocols that started in 1999:

  • 802.11b provides up to 11 Mb/s per radio in the 2.4 GHz spectrum. (1999)
  • 802.11a provides up to 54 Mb/s per radio in the 5 GHz spectrum. (1999)
  • 802.11g provides up to 54 Mb/s per radio in the 2.4 GHz spectrum (2003).
  • 802.11n provides up to 600 Mb/s per radio in the 2.4 GHz and 5.0 GHz spectrum. (2009)
  • 802.11ac provides up to 1000 Mb/s (multi-station) or 500 Mb/s (single-station) in the 5.0 GHz spectrum. (2013?)

802.11ac is a significant jump in technology and data-carrying capabilities. The following slide compares specifications of the 802.11n (current protocol) specifications with the proposed specs for 802.11ac.

(Slide courtesy of Meru Networks)

What is new and improved with 802.11ac?

For those wanting to delve deeper into the inner workings of 802.11ac, this Cisco white paper should satisfy you. For those not so inclined, here's a short description of each major improvement.

Larger bandwidth channels: Bandwidth channels are part and parcel to spread-spectrum technology. Larger channel sizes are beneficial, because they increase the rate at which data passes between two devices. 802.11n supports 20 MHz and 40 MHz channels. 802.11ac supports 20 MHz channels, 40 MHz channels, 80 MHz channels, and has optional support for 160 MHz channels.

(Slide courtesy of Cisco)

More spatial streams: Spatial streaming is the magic behind MIMO technology, allowing multiple signals to be transmitted simultaneously from one device using different antennas. 802.11n can handle up to four streams where 802.11ac bumps the number up to eight streams.

(Slide courtesy of Aruba)

MU-MIMO: Multi-user MIMO allows a single 802.11ac device to transmit independent data streams to multiple different stations at the same time.

(Slide courtesy of Aruba)

Beamforming: Beamforming is now standard. Nanotechnology allows the antennas and controlling circuitry to focus the transmitted RF signal only where it is needed, unlike the omnidirectional antennas people are used to.

(Slide courtesy of Altera.)

What's to like?

It's been four years since 802.11n was ratified; best guesses have 802.11ac being ratified by the end of 2013. Anticipated improvements are: better software, better radios, better antenna technology, and better packaging.

The improvement that has everyone charged up is the monstrous increase in data throughput. Theoretically, it puts Wi-Fi on par with gigabit wired connections. Even if it doesn't, tested throughput is leaps and bounds above what 802.11b could muster back in 1999.

Another improvement that should be of interest is Multi-User MIMO. Before MU-MIMO, 802.11 radios could only talk to one client at a time. With MU-MIMO, two or more conversations can happen concurrently, reducing latency.

What do experts say about 802.11ac?

There is a lot of guessing going on as to how 802.11ac pre-ratified devices are performing. I don't like to guess, so I contacted Steve Leytus, my Wi-Fi guy who also owns Nuts about Nets, and asked him what he thought:

Regarding 802.11ac, we are testing wireless game consoles for a large company in the Seattle area. We test performance using 20, 40, and 80 MHz channels. During the tests, we stream video data and monitor the rate of packet loss in the presence of RF interference or 802.11 congestion.

802.11ac's primary advantage is support for the 80 MHz-wide channel. And without question, the wider channel can stream more data. But, as with everything, there are trade-offs.

I asked Steve what the trade-offs were:
  • I don't think you'll find 802.11ac clients as standard equipment for computers. So, you need to buy one, connect it to the computer via Ethernet, configure the client, and finally pair the client with the router/access point.
  • Unless your application requires streaming large amounts of data, you probably will not experience a noticeable improvement in performance.
  • The 80 MHz-wide channel is more susceptible to RF interference or congestion from other Wi-Fi channels by virtue of its larger width.
  • The 80 MHz channel eats up four of the available channels in the 5.0 GHz band. Some routers implement DCS (dynamic channel selection) whereby they will jump to a better channel in the presence of RF interference. But if you are using 80 MHz channels your choices for better channels are few or non-existent.

Transmission testing results

[UPDATE] Steve Leytus finally was able to break away from his testing long enough to grab screen shots of the three channel widths. I haven't seen this anywhere else, so I thought I'd pass his explanation and slides along:

The three images are of iperf transmitting from one laptop to another at 20 Mbps; both laptops are connected to the same Buffalo 802.11ac router -- one laptop is connected via Ethernet, and the other is associated wirelessly. The transmission test was repeated three times using channel widths of 20 MHz, 40 MHz, and 80 MHz.

You can clearly see how the width of the spectrum trace increases with channel width. The other thing to notice which might not be so apparent is the power level -- as the channel width increases the power level decreases.

This is expected since the transmit power has to be spread out over a wider frequency range. The implication is that as the channel width increases then the distance the signal can reach probably decreases.

20 MHz

40MHz

80 MHz

What is my concern?

First let's gets some physics out of the way (Courtesy of Attenuation of Microwave Signal and Its Impact on Communication Systems [PDF]):

  • The higher the frequency (5.0 GHz versus 2.4 GHz), the greater the bandwidth which allows more data carrying capacity.
  • Attenuation is the reduction of signal strength during transmission.
  • RF signals are attenuated exponentially over distance.
  • Attenuation is directly proportional to the frequency.

My concern rides on 802.11ac needing to use the 5.0 GHz frequency range in order to get the monster data throughput being advertised. That means -- per the physics above -- users will have to live with a significantly smaller coverage area, something those more familiar with 2.4 GHz devices will not expect.

Final thoughts

I have been checking out the forums and early reactions seem mixed. That might be indicative of the sensitivity of each user's situation and physical surroundings when considering how well 802.11ac devices perform. Other than my coverage-area concern, my only other thought: there will always be a bottleneck. And unless you are one of the fortunate connected to Google Fiber, the Internet connection will be it.

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65 comments
techrector
techrector

I have been working on some project where I need to find out distance between device and wifi router based on its strength. I know there could be scenarios like some obstruction in the way which may hamper our signal strength but first considering the best case scenario. And I also know any such kind of method to calculate such information will vary based on device type/manufacturer.

I am new in this field. So any kind of help would be appreciated. Thanks ~ 

Gar Beals
Gar Beals

If anybody has integrated  802.11AC into their home network, do ISPs (mine is Comcast) require an upgrade to their MODEM? Are there any downstream implications from the router to  devices like DirecTV''s network devices? I know that the standard hasn't had the IEEE holy water sprinkled on it yet but 80211N and it's variations were being sold prior to the Standard gaining approval so maybe there are early adopters who could speak to their experience?

don2406
don2406

We rarely need to shift large files and our internet is limited to 10Mb, but .11ac still sounds very helpful.  When 100 users are logging on and loading up their work at around the same time, the peak load is still very high.  The greater trhoughput and multi-user MIMO could help aleviate the blocking that occurs by reducing the retries which account for a lot of the traffic during these peak periods. 

CSMA always suffers loss of effective capacity as demand increases, but with many simultaneous users even a short peak load can reduce throughput so drastically that it takes a very long time to recover. 

As for 5GHz, I join the others in welcoming any reduction in signal strength with distance.  It will reduce external interference and also allow greater density of APs.  I am sceptical about the relevance of the FSPL in this context (directional antennae and multiple reflection paths), but even if the signal strength is lower, it is the signal to "noise" ratio which is important.  With less interference and lower cosmic noise, higher frequencies can work at lower signal levels.

kasb31-tr
kasb31-tr

I am trying to improve my bandwidth to help with my VoiP application which seems to be choppy lately.  It sounds like the 11ac will help but not sure.  I believe my degradation of bandwidth is due to the additional WiFi gizmos slowly entering my house (IPads, IPods, IPhones, etc). 

Question 1): Aside from upgrading my ISP, will the 11ac help my situation with allocating more (or improved) bandwidth?

Question 2): Also, most of our equipment is not 11ac ready meaning the antennae will not take advantage of the 11ac router.  Do I need to get an adapter for all laptops?

I have researched this question and have received mixed reviews.   

Vorpaladin
Vorpaladin

The only reason I am interested in having LAN data speeds this high in my home is streaming video from a NAS to my TV.  The NAS lives in the office upstairs, far from the TV; I would prefer to not run wires.  But range is a big concern; my "n" 2.4 GHz connection requires a repeater to cover the downstairs area.  Devices that are only about 50 feet from the router can't connect to it reliably due to interference from the house structure and contents.  If I would need more than 1 repeater for "ac" I would have a hard time justifying it, especially since performance is lost on every "hop".

mhammett
mhammett

FYI: The comments section doesn't work as smoothly in Chrome as it does in IE. Might want to address that. All of the speeds mentioned are the OTA (Over The Air) data rates. Real throughput is half of that. WiFi is very much a shared environment. Either one of those conditions means that 802.11ac will *NOT* be faster than gigabit Ethernet in ideal conditions In many conditions, it is likely to be barely above 100 meg Ethernet, due to poor RF conditions. Resolving the poor RF conditions will do far more towards more usable throughput than higher maximum throughput numbers. These high speeds are for enthusiasts, enterprises and service providers. Your average Joe in the home will not benefit from the increased speeds. If you were to remove the increasing modulation complexity, larger channels, etc. and just applied real, working beamforming to 802.11g, that would be sufficient for a significant number of homes. Yes, today's FIOS and Comcast connections would then require an n system to take advantage of the speeds over 25 megabit. I hope to God that the consumer routers come with a 20 MHz channel by default and not 80. Doing otherwise would just further pollute the RF environment. MU-MIMO will be great for enterprises and service providers. The latency over a Wi-Fi connection is negligible, but being able to send data to multiple clients simultaneously will be a great improvement for enterprises and service providers. It would be really nice if the APs had the intelligence to move problem clients to other, reduced chain count connections to mitigate their effect on total system performance. Frequency alone has nothing to do with bandwidth. All else equal, a 20 MHz channel at 500 MHz will pass as much data as a 20 MHz channel at 500 GHz. However, our spectrum allocation model is such that generally higher frequencies have larger amounts of spectrum allocated to them. This is due to as you go higher up in frequency, larger and larger frequency deltas are required for any noticeable change in how that frequency acts. 50 MHz and 5,050 MHz behave VERY differently. 50 MHz will go through just about any material other than Mother Earth herself (well, and bodies of water), while 5,050 MHz can't escape a concrete room or a metal shed or a tree line. 50 GHz and 55 GHz face the same penetration challenges. I believe I have commented elsewhere on here already about how the frequency to attenuation graph isn't smooth. Many things in the atmosphere affect frequencies differently. http://www.phys.hawaii.edu/~anita/new/papers/militaryHandbook/rf_absor.pdf The same thing continues below 10 GHz, but it becomes less and less of a factor.

chitsamatanga
chitsamatanga

Hie, finding it difficult to print your articles!! how do you print them?

WmRobt
WmRobt

My isp connection is 65Mb [and not all that fast] and I have tested connections to specific servers which confirm that; however connecting with a google page for example takes over 3 seconds all too often. Not picking on google same goes for most other [if not all] services I connect. Of course that includes loading the page as well. But I do?t care where the time is spent. It just takes too long and I dont see a faster wifi changing that.

Slayer_
Slayer_

Cause it seems n is still a fair bit slower than normal 100mbit wired connection.

Kim SJ
Kim SJ

Having much experience of trying (and often failing) to make in-home WiFi work at 2.4GHz in the presence of devices like video repeaters, baby monitors, etc, I think there will be a big win for users with such issues. Afaiu, the "play nicely" rules are much better defined in the 5GHz band.

GrizzledGeezer
GrizzledGeezer

...that 802.11ac is a solution for problems that many users aren't likely to have. It's also possible that the great configuration flexibility possible might cause installation and deployment problems. 802.11ac appears to be more-suitable for business than consumer use. Nothing wrong with that.

thomas4442
thomas4442

I didn't get into the nuts and bolts of wifi frequency. You guys took my layman's scenario and 11ac and went in-depth. Thanks! And yes Michael, I use 2.4 and wouldn't get the area with 11ac. For the last few years I've only had to deal with home networks and home users. I graduate soon, and then will likely deviate from my simple explanations. No way I could use the terminology used here for my current clients. I look forward to advancing into the enterprise arena. Thanks again.

kfilius
kfilius

"Good, Fast, Cheap: Pick any two (you can't have all three)" (RFC 1925) I love it.

andrew232006
andrew232006

Being 5GHz only makes me wonder how 802.11ac routers will work with wireless g and n devices. Support for those devices isn't something I'm willing to give up any time soon I'm currently stuck on 20mhz because many of my devices won't work with my router set to 40mhz. So I'm guessing none of my current devices will work on 80mhz or 160mhz.

mrobbins
mrobbins

Thanks for this article. A neat summary of the key points and concerns and well worth the time spent reading it. More like this please TechRepublic.

Henry 3 Dogg
Henry 3 Dogg

"That means — per the physics above — users will have to live with a significantly smaller coverage area, something those more familiar with 2.4 GHz devices will not expect." Except that the beaming gives improved coverage range - also per the physics above. Having just replaced two 802.11n Airport Extremes with a single 802.11ac, I can tell you that the range is better.

Henry 3 Dogg
Henry 3 Dogg

"I don’t think you’ll find 802.11ac clients as standard equipment for computers." Heard of the MacBook Air?

georgeou
georgeou

The 80 MHz quad-width channels are in the 5 GHz range which has anywhere from 12 to 24 independent channels. 2.4 GHz only has 3 independent channels so you have less congestion in 5 GHz using 80 MHz wide channels than using normal 20 MHz wide channels in 2.4 GHz. As for "Attenuation is directly proportional to the frequency", that's only half the equation. The other half of the equation is that a given antenna length is effectively twice as long for 5 GHz as it is for 2.4 GHz because it covers more wavelengths. That nullifies the increased attenuation and you get more or less the same range through air or through most materials. Some materials may block one frequency more than another frequency though most materials block 5 GHz slightly more than they block 2.4 GHz. Some materials like brick or stucco can block 5 GHz substantially more than 2.4 GHz. But more effectively blocking is an asset rather than a liability In the enterprise space because it allows for denser access point deployment and higher capacity. If higher range is needed, you can always use a higher (larger) gain antenna or a directional patch antenna to optimally shape the coverage.

ok1dix
ok1dix

RF signals are attenuated with square of distance NOT exponentially. I wonder if the other informations in the article are also so reliable.

thomas4442
thomas4442

I currently have a home network with AT&T as my ISP. I have 6mb speed. I tested the speed by bouncing a 30mb file off of a server in North Carolina, I'm in S.Ga. I consistently was >upload/download>5.9/5.2. The new modem/router is a 300mb. (WiFi). I have a TPlink wireless usb adapter (150mb) on my office PC in my room. Once I have established my ISP speed, I test the wireless streaming speed like this> Go to YouTube and watch a video (Netflix too), at the bottom of the screen is the video (data) progress bar. It's RED for what's been and being watched. It moves as the video is watched. Just in front of the DOT at the beginning of the RED line is the "Grey area". This tells you how much data is stored in the cache or buffer. If the "Grey area" is staying ahead of and increasing in distance from the DOT, then the wireless>stream>buffer is fine. It was ok at 150mb but not 54mb. 802.11n is better. As for distance, I live in an apartment on the second floor. When I switched to "N" my signal strength doubled outside of my apartment. I needed this as I do PC repair and networking by contract occasionally. Now I can pull up in my parking lot at 30/35 meters or 100ft., and log onto my network in my apartment to retrieve files I may need via laptop without getting out of my car. Switching to "N" made my download speeds on wifi go from good to better, and made access from a distance go from not possible or not worth it to possible. I could remote in also, but my daughter is in college and I can leave files in a personal shared folder for her to access when I'm not home. Just goes to the parking lot! P.S. I have 7 to 10 active wifi networks I can pick up various strengths. Occasionally the wifi signal slows. This is a recent issue since my switch to AT&T Uverse. I resolve the issue by changing channels in the router. It has a "best channel" feature that doesn't work well, haven't figured out why yet!

said.fox
said.fox

The article could be regarded very good introduction about the new protocol. However, what about the covering range area that new protocol may offer to me as a home end user. Nowadays, in our houses we swim over a lake of wifi signals of our neighbours, interference and shortage of coverage inside the house may be an important concern. What I need to know, what's the max. distance, horizontally and vertically (up stares, down stares) that my ac wifi network reach?

Vorpaladin
Vorpaladin

@mhammett I was really wondering about the statement that "physics" says you get more bandwidth at higher frequencies, because I've never seen anything in the physics of EM signals that would indicate such a thing.  I'm glad I'm not the only one who thought "WTF?" when reading that part of the article.

Michael Kassner
Michael Kassner

I am always glad when you add your insight to the mix.

DFagerstrom
DFagerstrom

Running in FireFox works for me--as far as copying text. From clipboard I past to Word 2003 format for later searchability.

Michael Kassner
Michael Kassner

If you would like email me, and I can send you a PDF copy.

andrew232006
andrew232006

My ISP sells me 7, I get 6 on good days and opening google is a few hundred milliseconds at most. Of course it doesn't matter if you have your connection saturated by other devices and programs. If I were you I'd test my internet speed on testmy.net with everything using the internet off to see just how much your ISP is lying with advertised speeds. You may also want to check your router for QoS capability which could be used to give http traffic priority over your other internet traffic.

Michael Kassner
Michael Kassner

I have either a 7 Mb DSL or 20Mb 4G Internet connection and I think they are fast. I wonder if there is something else going on with page loading locally.

Michael Kassner
Michael Kassner

I had not heard that. There was very little specified as changed when it comes to the rules of engagement. Do you have some links you could share? But, you will have several more channels to choose from.

Michael Kassner
Michael Kassner

That 802.11ac is being built for those who shuffle a lot of data around "locally" and have multiple devices that will make use of the MU MIMO.

Michael Kassner
Michael Kassner

It should be interesting to see how 802.11ac unfolds. Congratulations on graduating, and best of luck. We need more knowledgeable and vested people such as yourself helping us.

Michael Kassner
Michael Kassner

It is my understanding that many of the vendors are going to have devices that support both 802.11n and 802.11ac frequency bands.

Michael Kassner
Michael Kassner

I am glad that your situation improved. Just a FYI, many 802.11n devices had beamforming antennas as well. As an amateur radio operator who is familiar with both 2.4 GHz and 5.0GHz (hams have primary license in these frequency ranges): http://en.wikipedia.org/wiki/13-centimeter_band https://en.wikipedia.org/wiki/5_centimeter_band I know it takes significantly more power to go the same distance, "free-space propagation" using the higher frequency. My familiarity is with sat comms and working the ISS. I mentioned in the article, there are improvements in the equipment that may offset some of the attenuation, and could explain why you are having better luck with 802.11ac.

Michael Kassner
Michael Kassner

Steve prefaced his statement with think. And he is the kind of guy that will gladly alter his stance with the information you provided. Thanks for letting us know.

Michael Kassner
Michael Kassner

It has been quite a while since you have commented on TR. Thanks for your input. You bring up a good point, George. It looks like at 80 MHz, 802.11ac will have five possible channels on 5.0 GHz. As for your comment on attenuation, it is not that simple according to my mentor, fellow amateur radio operator, and professor of advanced radio electronics at the University of MN. You have to address receiving and transmitting separately. When it comes to transmitting, a full wavelength antenna for 5.0 GHz will actually be "half" the size, not twice a 2.4 GHz antenna (2.4 GHz wavelength is 125 mm and 5.0 GHz wavelength is 59mm, both approximate as it depends on the actual frequency). And using a double wavelength antenna (size required for 2.4 GHz) will introduce standing wave issues and dramatically reduce efficiency. I have witnessed this many times on my SWR bridge when tuning my antennas. It is also why you will see coils on multi-band antennas. As for receiving, at lower frequencies (HF), there is an advantage to using a longer antenna to harness more RF, but not at microwave frequencies. Besides, strictly 802.11ac devices will not have antenna sized for the 2.4 GHz frequency band. Next to blocking, in this regards, 5.0 GHz is much better than 2.4 GHz according to the professor. It is away from the O-H frequency so water-based objects do not affect 5.0 GHz nearly as much as 2.4 GHz. As for 5.0 GHz scattering more, that is also an advantage, it allows MIMO to work that much better.

Michael Kassner
Michael Kassner

I quoted from this source: "The radio signal gets attenuated exponentially while propagating. From above figure it is clear that keeping transmission distance (d) fixed, the higher frequency signal (f2) gets attenuated significantly than does the lower frequency (f1) signal. It is also observed that when the transmission distance is near, then the selection of frequency (microwave antenna of certain frequency) is not so important because there is not much difference(A1) between the attenuation occurred by higher frequency and lower frequency whereas it is very much important to choose the appropriate frequency (microwave antenna) when the transmission distance is far because there is a significant difference (A2) between the attenuation occurred due to higher and lower frequencies. One should choose the lower frequency microwave antenna to establish the link between the two stations located quite far away from each other." http://www.sanjayagurung.com/allfiles/documents/research_projects/AttenuationOfMicrowaveSignalAndItsImpactOnMobileCommunication.pdf This was my second source: "For example, a 2.4 GHz signal, such as that used by many IEEE devices, will attenuate by approximately 80 dB in the first 100 meters and then by another 6 dB in the second 100 meters. As you can see, the attenuation becomes much less in the second 100 meters than in the first, and this is due to logarithmic attenuation. The following formulas are used to calculate free space path loss in dB: LP = 36.6 + (20 log10(F)) + (20 log10(D))" http://faculty.ccri.edu/jbernardini/JB-Website/ETEK1500/1500Notes/CWNA-ed4-Chapter-2.pdf I have seen the logarithmic formula on several other sites. Maybe you can help me out here. Would this be a better statement: "Free-space propagation of an RF signal is attenuated logarithmically." As to whether that means exponentially or attenuated with the square of the distance, I can't say, that's why I left it to the experts. What is your opinion?

Michael Kassner
Michael Kassner

I assume you are using 2.4 GHz. The coverage you are getting now is likely not possible with 802.11ac due to the 5.0 GHz requirement.

Michael Kassner
Michael Kassner

That kind of information is not available yet. There really isn't that much equipment out there, and I know of no computers that have built-in 802.11ac clients. Also, 802.11ac will be more sensitive to individual physical surroundings and spurious RF signals. Are you able to use Wi-Fi currently?

bowenw
bowenw

It has been my experience over the last 15 years of broadband connections that if you get 2/3 of the speed the ISP claims, you're doing excellently. This is especially true with either 802.x wireless or cable, since both are hubbed, not switched (common collision domain). My present home cable connection (Time-Warner) varies all over the place - sometimes slower than a V.90 modem, sometimes nearly as fast as advertised (7Mb). The DSL I had in California was slower (4Mb) but much more consistent.

Remember, even if you have an OC192 to your equipment your connection will never be faster than the slowest part of the path. 

tackerm
tackerm

I wanted to share my experience with available equipment as of 6/2013. I had to make a decision on a router to replace my old 11bgn router that died. Rather than spend $100 on a good 11n router, I decided instead to get an 11ac router for $175 to avoid replacing the 11n prematurely in the future. For various reasons, I chose the Asus RT-AC66u ac1750 gigabit router, which allows simultaneous connections of 450Mbps at 2.4ghz & 1.3Gbps at 5ghz (theoretically.) At 2.4ghz frequency, you can select channel bandwidth of 20 or 40Mhz. At the 5ghz frequency, you can select 20, 40 or 80Mhz bandwidth. You can also adjust transmission power up to 200mW, depending on local regulations. I wanted to comment that this equipment (& similar competitive offerings) is available now. Also, 11ac PCI & USB 3.0 adapters are available. I bought some 11ac USB dongles for $39 each for guest laptop computers. Can anyone suggest a practical way to test the bandwidth at 80 vs 40, so I can report back? Ping & speedtest.com seem to test my Comcast internet connection more than wireless bandwidth. I apologize in advance if anyone is offended by my naming of vendors or my lack of engineering knowledge.

georgeou
georgeou

My point is that lower range and more blocking is better because it allows for higher density deployments. I've seen a research paper that listed the attenuation for 5 GHz and 2.4 GHz going through various materials and the vast majority of materials block 5 GHz more. But this as a good thing for high density deployments because it allows for more spectrum reuse. If you really want to punch through a wall or reach out further without deploying more access points, you can deploy a higher gain antenna. There was a nice article on ArsTechnica on the massive 5 GHz WiFi deployment at the new 49er’s stadium being built in Santa Clara. There’s really no choice but to go super high density with intentionally low power small and shaped coverage cells if you want to offer high capacity to tens of thousands of people in a stadium. 2.4 GHz is just not going to cut it with only 3 unique 20 MHz channels and only 80 MHz overall. The people building the 49er’s stadium pointed out that by the time they launch in 2014, there will be more generations of iPhone and they will all support 5 GHz. I’m not sure about 802.11ac but at least they’ll support 802.11n.

ok1dix
ok1dix

Well, I should have written it in more details. 1. The RF (especially microwave) signal attenuation is attenuated by a lot of factors, as already stated by others. So the practical result may be very different on various bands. 2. However, when talking about the attenuation by distance following applies: Free-space path loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal. http://en.wikipedia.org/wiki/Free-space_path_loss Don't be confused by logarithms in the equations. They are there just to express the attenuation in dB, but it does not mean that the dependence is logarithmic or even exponential.

mhammett
mhammett

Additional factors affect attenuation such as that frequency's susceptibility to different molecules in the air. For example, 60 GHz has far worse free space loss than 70\80\90 GHz. 60 Ghz is known as the oxygen absorption layer.

martinw
martinw

Doesn't the new macbook air has the 802.11AC client built-in?

charvak.karpe
charvak.karpe

@Michael Kassner  Also, radio signal attenuation is inverse quadratic (drops off with square of distance) because if you think about it, a radio pulse propagates as an expanding sphere.  The surface area of a sphere increases with the square of its radius.  The same principle applies to sound, gravity, electric fields.  I think it's called Gauss's law or something.

You would see an exponential decay if the radio signal goes through a material that does something like absorb 50% of the signal strength every few feet.  That seems plausible.  So, shielding creates exponential losses and distance creates quadratic losses.


But the whole situation is far more complex because of waves bouncing around.  Light obeys the same laws, just is shielded more easily.  Imagine a bright light behind a couch.  In theory, it is attenuated exponentially by the couch so you can't see it.  But in reality, it bounces off walls and you still see it.


charvak.karpe
charvak.karpe

@Michael Kassner  Great question.  x^2 is considered quadratic or polynomial.  2^x is exponential.  It's all about where the variable sits.  And the reason it matters is that 2^x will eventually overtake x^99999 or any other big exponent as x grows, no matter how large you make the exponent.

Michael Kassner
Michael Kassner

First, thanks for explaining. I also have another question, but I am afraid to ask it. Because my math is well, you know. But, here goes: isn't squaring something exponential? I'm wondering if the paper I first sourced had a translation issue.

Michael Kassner
Michael Kassner

And if I remember right, 2.4 GHz is known for O-H (water) absorption layer.

Michael Kassner
Michael Kassner

I prefaced it with that I know of, and what I now know has changed. Do you have one? How does it compare?