Please could you help me understand a concept that may be basic to most of you, but for me, I dont fully understand.
When a packet is transmitted from a wireless NIC on a PC to an AP, does the copy of the same packet get transmitted many times in many directions by the wireless NIC or Antenna?.
Please can you be so kind and look at the picture I created.
Is this how it works.
Also, if this is correct, how many copies of the packet would the antenna send?
Many thx indeed to all for your help,
Ken, you're challenging us all to think about things that we often take for granted. Thanks for keeping us on our toes, I know that I've learned quite a bit myself by participating in this discussion!
To answer your first question, Ethernet over copper is not a frequency-based signal. Instead, electricity is used over copper to produce the signal. Generally, placing a positive voltage on the line produces a current in one direction, a binary 1, and placing a negative voltage on the line produces current the other direction, which is a binary 0. This is why Coax and UTP copper have two "cables", because a circuit must be completed between hosts. UTP has 4 pairs, and thus allows for 4 communication streams at once. This is why you can run full duplex using UTP, but with Coax you can only run half-duplex, since there's only one pair of copper (the inner-cable and outer-cable). Since the voltage applied is DC (as opposed to AC), there is no oscillation and no frequency.
I think your definition of a wireless carrier signal is very good. However, DSSS (802.11b) doesn't use a carrier wave, and OFDM (802.11a/g/n) uses carrier waves a bit differently (discussed below). My understanding is that the carrier wave is meant to be a heartbeat of sorts to wireless receivers, keeping them all calibrated and ready to receive more data. However, if an AP were to continually transmit a carrier wave, wireless clients would never be able to talk since the medium would always be busy. I assume (I wasn't able to verify this) that carrier waves can't be used when there are multiple speakers in an environment. But it would be perfect for radio stations and broadcast TV channels.
As for OFDM, all of the above still applies because it only uses carrier waves once it starts talking. Since an OFDM speaker communicates over a multitude of channels at once, each subchannel contains a carrier wave that is modulated to transmit data. This is because the subchannel must remain calibrated when it's not in use so it can be reused at any time. However, once the entire transmission is complete, the transmitter is silent. It doesn't use a "broad" carrier wave.
Something to consider about WiFi is that the start of any WiFi packet contains a series of alternating 0s and 1s for calibration. This is called the preamble. So this takes care of the fact that there is no carrier wave to keep everyone calibrated. Ethernet over copper also uses a preamble to calibrate everyone on the segment. For both WiFi and Ethernet copper, the medium is completely silent if there are no talkers.
I can't speak about fiber, I'm not so knowledgeable about the subject. As discussed earlier, Ethernet over copper doesn't use signals, so it doesn't use carrier waves.
Does that all make sense?
Brilliant discussion and Jeff is one of the best guys you see here on wireless.
The last three pics you posted showing the isotropic radiator, this is the theoretically perfect radiator of which we only have the sun, ie radiating evenly in all directions.
From my degree days as you move from the source you use an inverse square law to degrade the signal strength as you move away from the source.
I will be reading this thread more thouroughly and my view is that if you get your head around this you will be ahead of many engineers putting in WLANs. You need to be able to visualise what the radio is actually doing to do good planning and also see where the main pitfalls are.
There are some good explanations in the CWNA guide (sorry but it is relevant)
Just my two cents
Wow, amazing pictures. Far better than I was able to make :)
I really don't like how text books show how waves get bigger as they expand. It is true that they get bigger, but they don't get stronger. Their amplitude doesn't increase, it actually decreases as it travels over free space. It makes sense, because signal strength decreases as we walk away from an access point or cell tower, and we grumble because we can't get coverage.
There's this concept of a "wave front" that describes, for like of better terms, the front or surface of the wave as it travels outward. This expands as it travels and does indeed get bigger, as they say. However, the amplitude gets smaller as the wave front expands.
Tired of analogies yet? Here's another one - Imagine a kid chewing some gum and starting to blow a bubble. As the bubble expands, the AMOUNT of gum doesn't increase, and yet it's getting bigger. The reason is that it's stretching out. The mass of gum continues to stretch thinner and thinner, causing its structural integrity to continue to deteriorate. So the gum is weaker at any given point despite the gum as a whole being bigger. Eventually, the thing pops and the kid has a mess.
For waves, there isn't a single mass of wave, per se, but there's a single amount of energy that the wave contains. This energy starts off very concentrated and "thick", but in a relatively confined space. As the wave travels outward, the surface or wave front continues to get bigger and bigger, but the energy is spread out over a larger area. So the amplitude is decreasing as the wave gets bigger.
The analogy breaks apart when talking about the bubble bursting. The wave never bursts, as we discussed earlier it simply fades to the background noise of the universe (assuming free-space travel out to infinity).
Strange, huh? As for your diagrams, that's the only thing I found to not be quite accurate. Very good and clearly-displayed info. It's clear you've done a lot of learning over the last few days. Major props for trying to figure this stuff out, I think a lot of wireless engineers don't care about understanding the physics behind the wireless signals. I'm with gstefanick, 5 stars for working so hard at this.
What you're describing is part of the electromagnetic spectrum. See this: http://en.wikipedia.org/wiki/Electromagnetic_spectrum
Those designations are nothing more than titles for groups frequencies. When we talk about radio waves, microwaves, x-rays, gamma-rays, etc... all we're talking about is different frequencies.
Light can be as slow as 1Hz, which would be a 300,000,000 meter wavelength! It can also be as high (and higher) as 300,000,000,000,000 Hz, which would be a 1 micrometer wavelength. The spectrum is simply a range from 1 - infinity for frequencies that light waves can exist at.
So again, when you talk about different groups of frequencies, it's simply for the sake of organization. There's nothing different about any of them other than frequency and wavelength. The groups you list are all subcategories of the larger group called Radio Waves.
I can tell you want to learn. Can you tell I like to talk? :D This is neat stuff, because it goes far beyond the realm of WiFi.
Haha, no need to apologize, it's a confusing topic for sure.
You're almost there. The sine wave will actually go up and down 2.4 billion times (Mega = Million, Giga = Billion), and that's just how many times it goes up and down per second, not per wavelength. A "hertz" (Hz) actually has a unit of "per second", or 1/s, so it's describing how many times something happens per second.
12cm is the length of just one of those 2.4 billion waves. So the 2.4 billion waves extend quite far very quickly! Don't forget that these waves are traveling at the speed of light, which is darn fast. So in a single second's time, one wave is pushed out 2.4 billion x 12cm, or about 300,000,000 meters.
(Interestingly, and not really related to this, if you change the frequency you'll find that the wavelength also changes so that the wave will go the exact same distance in one second - 300,000,000 meters. That's because all light waves, regardless of frequency/wavelength, travels the same speed, which happens to be 300,000,000 meters/second. The calculation you used to derive 12cm, the 300/2400, is actually the formula c = wavelength*frequency, where c = the speed of light, which is again about 300,000,000m/s).
So to clarify my diagrams, think of the sine wave peaks as the black lines on my bottom drawing. Think of the valleys of the sine waves as being an invisible line exactly between the black lines. There is nothing significant about the black lines other than that they represent where the peak of the wave is. It has nothing to do with the thickness/length of the wave. As such, the wavelength is the distance between two black lines, or between two valleys. You can measure the wavelength from any point in the wave as long as you start/stop at the same point in the cycle.
You can't beat new video games all that quickly anymore, that's for sure. I did just beat Mario Galaxy last night on Wii though :) I'm a gamer nerd first, network nerd second, I'm afraid.
Ah, I follow now. I'll respond with a hastily-drawn MS Paint image :) Let me know if it makes sense. Basically, the 12cm is from any point in one cycle to the same point on the next cycle. So if you go from one peak to the next, or one valley to the next, that's the definition of a single wavelength, which is the significance of 12cm.
See also: http://en.wikipedia.org/wiki/Wavelength
And I don't think you want my brain... it's full of mostly-useless information, like how to beat Super Mario Bros. in under 10 minutes :)
Here's a helpful analogy that might be a bit overused, but I think it's still good :)
Imagine a lake with calm water. You pick a stone up and chuck it in, and it creates waves that radiate outward. This represents a very ideal situation for wireless, with no obstacles and no impedence, the waves continue outward pretty far, depending on the size of the stone (the radio power, if you will).
Now imagine you have an obstacle of some kind, let's say a large rock sticking above the surface of the water. If you now chuck a stone in, the waves radiate outward as before. However, when they hit the rock, what happens? You probably already know - the waves kind of bounce off the rock, but they also bend around it. In fact, aside from a small "shadow" directly behind the rock, the waves actually bend around and provide "coverage" behind it. In the end, you'll see those concentric circles extending far beyond the rock, again as long as there was enough power behind the stone dropping into the water.
When wireless waves hit an obstacle, there are difference, but the concepts are largely the same. Some waves will bounce off, and others will bend around it. Some will even go through the object if possible. This is why WiFi waves can extend throughout a building. Waves of this wavelength operate a bit differently than visible light. The color blue, for example, is about 450nm in length. It's not really possible for the color to penetrate opaque objects and refract around obstacles like a 12cm wave is able to do. You can't really think of WiFi waves like visible light.
Finally, that stone dropping in created quite a disturbance, but it was a single force that subsided pretty quickly. Now imagine a large pole that is pumping up and down in the middle of the lake. As it pumps up and down, it creates waves that radiate outward. If it pumps up high and down low, it can create large waves, but if it just barely moves up and down, it can create small ones. Regardless of the size of the wave that's created, it's the same wave that propagates everywhere.
Wireless is a push/pull kind of concept, where electricity is applied to an antenna and it pushes a wireless wave outward. The electricity is cut off and the wave cuts off as well. So whatever radiates outward will always be the same wave, until that wave hits a barrier or obstacle, as described above.
Sorry, that was a bit long. :( Does that help at all, and is that at all what you're looking for?