Although we would like to assign every user of a wireless system a wideband channel in practice, that is not possible, simply because that user isn't the only user trying to utilize that wireless system. There are many other users who are looking to utilize the same wireless system at the same time. And that is where the concept of sharing the wireless channel comes into picture. So the fundamental question that we have to ask us is, can different users share this data pipe or the wireless channel? And the answer to that question is absolutely yes. There are defined algorithms that allow sharing of the radio channel and that translates to a concept of multiple access. Meaning that when multiple users are trying to access the same wireless channel at the same time, that concept is called multiple access and that is not very difficult to understand. For example, consider that there is only one user at a given point in time. So, the entire pipe for the entire wireless channel will be available to that user and that will be equivalent of a wideband channel. But now, consider that there are three more users trying to utilize the same wireless channel. So, in the interest of fairness or equity, all the users will get a certain sliver of that wireless channel depending upon their application requirements. And the wireless system will have to divide the available resources, the number of users that are trying to access the system at the same time. And that is what makes shared data channel like a shared a pipe. And there are trade offs to be achieved on this front, just like there are tradeoffs with many other engineering problems. In that, because the users now have to share the wireless channel stands to reason that each of them will get fewer wireless resources than they were if they were the only user trying to access the wireless channel. So that is a slight disadvantage, but a big advantage in favor of multiple access is that you don't have to allocate or buy individual dedicated channels for individual users. Which would be prohibitively expensive, given how scarce and costly the frequency spectrum is getting these days. So, multiple access allows us to achieve that desirable trade off between the cost of the frequency spectrum you need to operate your system. We're says the amount of resources individual users get which ultimately determines the speed or the data rate that they will get on their phones. A simpler way to understand this example would be to understand that in terms of a neighborhood and a water pipeline that comes to that neighborhood. Let's say that in the beginning yours is the only house in an up and coming neighborhood. So, the city provides that up and coming neighborhood with a water mains pipeline. And because yours is the only house right now, all the water that is coming from that water pipeline is yours to use. But consider that in the future there are a few more homes built in the same neighborhood. And because there is only one comin in the water mains pipeline for that neighborhood, all the houses will draw their water supply from the same water pipe. So, your water supply that was entirely yours in the past will now be equitably divided among multiple houses in your neighborhood. And that means that if for example you open your faucet at 2:00 AM, you will still get the full water pressure because no one else is likely trying to use water at that time. But if you try to turn your faucet on during morning rush hours when everybody's trying to get ready for school or office, that is when you might notice some fluctuations in the water supply. And that is because all the houses are trying to draw from the same common water supply. So, that is one real life example that you can use to understand how a system might perform when there is only one user and how it might perform when there are multiple users. And the reason we make multiple users use the same wireless channel or make them multiple access is because the underlying fundamental resource. The frequency spectrum is extremely scarce and costly. So, there are several technical details to be understood in terms of multiple access but they are slightly beyond our reach at this point. I will give you a couple of notes. However, you might have heard of a technology called CDMA that was a technology pioneered by qualcomm in the mid nineties. The MA in CDMA stands for multiple access, and what does multiple access mean? In general, just like modulation entails carefully changing certain attributes of the wireless signal you are trying to send out multiple access involves carefully leveraging some of the attributes of the wireless channel. Wireless channel, as we'll see, has different attributes, such as frequency time, etc. And if different users can carefully leverage some of those attributes that constitutes multiple access. So, these are the couple of quick notes about multiple access, which is a fundamental concept that allows multiple users to share the scarce and limited frequency resources, ie a wireless channel.
