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I'm a newbie on wireless technologies and I'm trying to understand how they work.

One thing I don't understand is this: How come transmissions from different devices don't interfere with each other all the time?

For example, I'm living in a dense metropolitan area. There's a router on my desk, and a laptop connected to it via WiFi. I would bet that in the 100 meter radius surrounding me, there are at least 100 more routers, and at least 200 more devices (laptops or cell phones) that are connected to the aforementioned routers. They are all communicating with each other at the same time. How can my humble laptop and my humble router send messages to each other? When my router send a message, how can my laptop pick it up from all the noise on these frequencies?

This question applies to phone networks as well. How can a phone reliably communicate with its tower when there are 500 phones nearby who are communicating with the same tower? How do they know which data belongs to which phone?

Thanks for satisfying my curiosity!

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Related on Physics: Why don't waves erase out each other when looking onto a wall? –  dmckee Dec 23 '13 at 4:32
    
Related on SU: superuser.com/questions/474280/… –  Passerby Dec 24 '13 at 2:52
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4 Answers

Oh, but they do interfere!

There are several mechanisms in play permitting the sharing of the airwaves by the various radio sources mentioned - the keyword being multiplexing, in its various flavors.

  1. Frequency bands: Different RF devices use different "bands" of frequency, which are typically allocated and governed by the relevant local authorities, e.g. the FCC or the ITU. This is called spectrum allocation, and varies between countries, with some broad overarching trends. The receivers are tuned to receive and amplify only those signals within the band of interest, attenuating the rest of the radio frequencies. This is frequency multiplexing.
    Examples:

    • GPS satellites communicate with civilian GPS handsets on the 1.57542 GHz (L1) and 1.2276 GHz (L2) frequency bands.
    • WiFi / Wireless LAN devices typically use the 2.4 GHz and 5 GHz bands, though a few others are also allocated in certain geographies / purposes.
    • Some RFID devices use the 13.56 MHz band
    • FM radio entertainment channels typically use the 87.5 to 108.0 MHz band (Europe, Africa, India) or variations around that range, e.g. 76 to 90 MHz in Japan.
  2. Frequency channels within bands: Within the above frequency bands, individual transmissions / devices use distinct narrower channels or frequency ranges, often with unused "guard bands" left between them to reduce interference or avoid legacy channels. In addition, mechanisms like dynamic frequency selection (DFS) are used, such as by 5 GHz band WiFi devices, to gracefully and automatically switch channels when interference is observed.
    Thus, from the above 2.4 GHz example above, WiFi devices may be configured for any one of 11 (14 in some countries) channels starting at a center frequency of 2412 MHz, with 5 MHz between adjacent channels, thus 2417, 2422, and so on. Hence if your neighbor's WiFi router interferes appreciably with yours, you can always switch to another channel which doesn't have as much activity.

  3. Spatial diversity: As long as two RF sources are sufficiently separated in geographic terms in relation to the emitted power per device, interference is insignificant. Permissible maximum radio emission power per band is also regulated, and often individually licensed, by spectrum regulatory authorities.
    Thus, even if two BlueTooth headsets in a building were using the same frequency channel, so long as they are physically separated enough given the rather low radio transmission power of each, no RF interference would be noted.

  4. Code division multiplexing - Frequency hopping / spread-spectrum transmission: Certain types of communication devices use dynamically altered frequencies, or even spread-spectrum transmission spanning a range of frequencies, to avoid being jammed by interference. The most familiar such application might be the CDMA cellular service.
    Even when some interference occurs in such techniques, the nature of the mechanism provides sufficient end to end through-put for effective communications to be maintained.
  5. Time division multiplexing: In any given "channel" of communication (and this isn't just RF, it is equally applicable to copper or fiber optics for example) there is a given amount of symbol transmission capacity - at the simplest binary level that may be as many "on" and "off" bits can be transmitted per second, while techniques like Quadrature Phase Shift Keying increase this capacity "density" manifold. Thus, it is simple for transmission equipment to utilize a channel in chunks of time, either with a "drum master" beating time and assigning individual time-slots to each requesting device, or by some form of intelligent anarchy such as collision-detection and re-transmission (e.g. classic Ethernet CSMA-CD).
  6. More exotic methods, such as polarization multiplexing: These are used most commonly in fiber optic communication, but are also widely deployed in point to point radio communications. In this form of channel separation, think of each electromagnetic "beam" being polarized to a specific orientation at transmission. At the remote end, suitably polarized receiving antennas demultiplex or distinguish between the differently polarized signals, thus allowing multiple spatially coincident channels of radio communication.

The above is in no way a comprehensive treatise on how various RF devices can coexist, but it should provide sufficient keywords for further search, if so desired.

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Thanks for the answer. But I'm still not clear on this. Let's take the WiFi example. There are only 11 channels, but there could be a 100 Wifi connections in the same area, so obviously we could expect to have channels that are shared by 10 different connections. How do they manage to know which transmission belongs to whom? Do they use time division multiplexing? If so, how do they know which device occupies which time slots? (I'm curious about what the WiFi protocol does, not what could theoretically be done.) –  Ram Rachum Dec 23 '13 at 10:32
    
The WiFi example involves multiple layers of multiplexing: The channels, as well as space diversity, time division multiplexing, and authentication by the base unit (Access Point, Router, etc). There aren't likely to be 100 WiFi masters within the small range of WiFi, after all. If there are 11 routers in close proximity, let alone 100, something is odd. With 11, each uses a different channel for best throughput. Besides, the WiFi clients authenticate with / connect to distinct SSIDs, so they won't be talking to the wrong router / AP. Each router then acts as a drum master for its clients. –  Anindo Ghosh Dec 23 '13 at 10:55
    
@AnindoGhosh there is about 30~40 home wifi routers my laptop sees at home, and 5 to 6 commercial SSIDs for corporate ran wifi (with multiple routers per SSID). 100 in a particularly crowded building area wouldn't be unrealistic. OP is asking, essentially, how do multiple APs and Clients on 802.11g channel 6 interact without interfering. –  Passerby Dec 24 '13 at 2:24
    
Why wopuld the all be on channel 6? –  Wouter van Ooijen Dec 25 '13 at 8:55
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A number of techniques are used, often in combination.

  • The available frequency spectrum is divided in a large number of bands that can each be transmitted and received independently. This is how radio stations and your WiFi can operate undisturbed by other (nearby) radio stations and WiFi sets. (Frequency division multiplexing)

  • Cell phones aren't called CELL phones for nothing: each cell phone tower covers a small area (it's cell). Neighbor cells don't use the same frequency, but cells at slightly more distance do. Hence a small set of frequencies can cover a vast area without interference. (Spatial division multiplexing)

  • A single cell phone tower can serve a lot of cell phones (and likewise your WiFi set can serve a lot of wireless computers) by talking to each one in sequence. There are countless clever schemes to synchronize such talking. (Time division multiplexing)

  • A cell phone tower can, at the same time and on the same frequency, transmit a different message to a large set of phones, by XORing each message with a key sequence that is unique for the phone, and transmitting the sum of all messages. (Code division multiplexing)

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While it's easier for a master (The wifi AP or a cell tower) to control it's transmissions through careful layout design and communication tricks, how do multiple independent devices (wifi clients, cell phones) do it? Two radio stations on the same frequency will cause crosstalk and/or static if overlapped. –  Passerby Dec 24 '13 at 2:18
    
WiFi is both limited in power/range, does not send all the time, and has a number of channels. –  Wouter van Ooijen Dec 24 '13 at 8:20
    
Wifi is not so limited in power or range that two or ten routers won't overlap, in houses or even worse, apartment buildings. They send quite often depending on the bandwidth in use, and people stick to the same three 1/6/11 channels. –  Passerby Dec 26 '13 at 4:49
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This is a simplistic answer to suit people who describe themselves as newbies in radio

Imagine the radio spectrum to be your hi-fi playing music. If you had a graphic equalizer on it you could do obscene things to the tone of the audio like just enhance stuff at 1kHz - slide the 1kHz control to max and reduce all the others to minimum - this is how a radio tunes itself to one transmission and excluses (largely) all other bands.

A different station might require you only enhancing (say) 500Hz so you slide the 500Hz control to maximum and reduce all the others to minimum - what you hear is just the tones at round about 500Hz.

Radios are allocated bands to transmit on and they have different frequencies so it's fairly easy to just tune in to the station you want.

Wi-fi devices all use different bands of frequencies - there are logical rules when a new device "joins" a wifi router - it gets allocated its own band of frequencies. Same with cell phones etc etc..

You've also got to remember that the power output from a router is intentionally limited so that its range causes limited "crosstalk" to other routers. This is the same for all radio devices like this. There are literally hundreds (maybe thousands) of channels available and if the power for each devices transmission was too high the system would not be possible.

It's a bit like goldilocks really - it's just right given the constraints of average mobility of the device and number of devices in a given "cell".

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There would have to be a whole lot of bands for that to work, no? For example, I live in a city that has 400,000 people in it, and most of them have a cellphone. Does each cellphone have its own band? How many bands are there? –  Ram Rachum Dec 22 '13 at 20:45
    
@RamRachum The power limitations on wifi mean that those connected to one wifi generally don't interfere with those on another wifi router. Same with cellphones - clearly there are limited total bandwidth resources and one particular handset connected to one cell has only limited power to cause potential problems with a phone using the same band on another cell. –  Andy aka Dec 22 '13 at 21:09
    
@UnbanRonMaimon In europe there are 124 seperate RF channels (spaced at 200kHz) given to cell base stations and this largely prevents the problems I've alluded to. Sure TDM is used thereafter but I was giving this answer to a guy who called himself a newbie. –  Andy aka Dec 22 '13 at 21:17
    
You could also think of the different bands as red, green and blue light. The human eye has detectors for each of those three frequency bands. Of course you can also build detectors for infrared, ultraviolet, or some narrow band between green and red (which looks yellow to us no matter how exactly the spectrum is). But when it's dark a single band (e.g. blue) doesn't have much energy so it's better to combine them all. –  maxy Dec 23 '13 at 20:17
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The Inverse R-Squared law comes to the rescue. The intensity of the signal at a distance R from its source is proportional to 1/R^2; the sound or WiFi signal falls off very rapidly as you move away from it.

So consider people conversing at a party. You can hear the person in front of you quite well, and unless the noise level is really high, you can probably converse with them without confusion. You might be distracted by someone speaking loudly a meter or two away; you might need to ask your conversation partner to repeat a few words or a sentence occasionally. But you mostly only hear a low level buzz from people talking in other parts of the room and mostly without much impact on your own conversation.

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