Assume you are teaching a class with thirty students, and every one of them needs help with their homework, what would you do? You’d probably schedule a time slot for each student to come in and talk to you one on one (assuming they all had different problems and there was no overlap in your tutoring).
Fast forward to your wireless access point. You have perhaps heard all the rhetoric about 3.5 gigaherts, or 5.3 megahertz ?
Unfortunately, the word frequency is tossed around in tech buzzword circles the same way car companies and their marketing arms talk about engine sizes. I have no idea what 2.5 Liter Engine is, it might sound cool and it might be better than a 2 liter engine, but in reality I don’t know how to compare the two numbers. So to answer our original question, we first need a little background on frequencies to get beyond the marketing speak.
A good example of a frequency, that is also easy to visualize, are ripples on pond. When you drop a rock in the water, ripples propagate out in all directions. Now imagine if you stood in the water, thigh deep across the pond, and the ripples hit your leg once each second. The frequency of the ripples in the water would be 1 hertz, or one peak per second. With access points, there are similar ripples that we call radio waves. Although you can’t see them, like the ripples on the water, they are essentially the same thing. Little peaks and values of electromagnetic waves going up and down and hitting the antenna of the wireless device in your computer or Iphone. So when a marketing person tells you their AP is 2.4 Gigahertz, that means those little ripples coming out of it are hitting your head, and everything else around them, 2.4 billion times each second. That is quite a few ripples per second.
Now in order to transmit a bit of data, the AP actually stops and starts transmitting ripples. One moment it is sending out 2.4 billion ripples pdf second the next moment it is not. Now this is where it gets a bit weird, at least for me. The 2.4 billion ripples a second really have no meaning as far as data transmission by themselves; what the AP does is set up a schedule of time slots, let’s say 10 million time slots a second, where it is either transmitting ripples, or it turns the ripple generator off. Everybody that is in communication with the AP is aware of the schedule and all the 10 million time slots. Think of these time slots as dates on your Calendar, and if you have a sunny day, call that a one, while if you have a cloudy day call that a 0. Cloudy days are a binary 1 and clear day a binary 0. After we string together 8 days we have a sequence of 1’s and 0’s and a full byte. Now 8 days is a long time to transmit a byte, that is why the AP does not use 24 hours for a time slot, but it could , if we were some laid back hippie society where time did not matter.
So let’s go back over what we have learned and plug in some realistic parameters.
Let’s start with a frequency of 2.4 gigahertz. The fastest an AP can realistically turn this ripple generator off and on is about 1/4 the frequency or about 600 time slots/bits per second. This assumes a perfect world and all the bits get out without any interference from other things generating ripples (like your microwave) or something. So in reality the effective rate might be more on the order of 100 million bits a second.
Now let’s say there are 20 users in the room, sharing the available bits equally. They would all be able to run 5 megabits each. But again, there is over head switching between these users (sometimes they talk at the same time and have to constantly back off and re-synch) Realistically with 20 users all competing for talk time, 1 to 2 megabits per user is more likely.
Other factors that can affect the number of users.
As you can imagine the radio AP manufacturers do all sorts of things to get better numbers. The latest AP’s have multiple antennas and run in two frequencies (two ripple generators) for more bits.
There are also often interference problems with multiple AP’s in the area , all making ripples . The transmission of ripples for one AP do not stop at a fixed boundary, and this complexity will cause the data rates to slow down while the AP’s sort themselves out.
For related readings on Users and Access Points: