HCW - 5.02 - datacomm - intro
When we start talking about data communications, we have to talk about timing again. Timing seems to show up an awful lot, in terms of computers, doesn't it? Well, yes it does. And here we are again.
If you study into the details of data communication ..., well, I know you won't. Very few people do. You use it absolutely every single day, but almost nobody, even those who do go into studying information technology quite extensively, goes into the details of data communications. If you do, you quickly run into this issue of timing. As a matter of fact, when you start out, with the basics, you will probably run into terms like synchronous communications, bi-synchronous communications, and asynchronous communications. Synchronous, and bi-synchronous, communications were the originals. When people started to do data communications, this issue of timing was so important that there was actually a timing signal over the communications channel. And you could only send a packet of data immediately after the timing signal had pinged down the line. Bi-synchronous just meant that the timing signal was sent in both directions. Asynchronous doesn't mean that timing is not involved: definitely not. It just means that you didn't have to wait for a specific timing signal: you could start communicating anytime you wanted to.
There is another pair of terms that you might hear when you start talking about data communications. This is serial, and parallel.
You might not hear these terms being used, an awful lot, these days. They used to be very important, a while ago. They generally referred to the connections, almost always by cables, that you made with the peripherals that you attached to your computer. Serial means that the information was sent one bit at a time, in a stream of bits. And, when we talked about data communications today, this is, most often, the type of communication that we are talking about.
Parallel communications sends a number of bits, all at once. Back in the days when serial communication was problematic, and noisy, and slow, using parallel communication was a way of speeding things up. Obviously, it is difficult to send a number of hits, together, along a single wire. Signalling happens by changing the signal, from off to on, or high volume to low volume, or high frequency to low frequency. When this change happens, you can only send one bit of information with it. Parallel communication used to have ribbon cables, where a number of wires were packaged together, side by side, in a plastic ribbon. (Each wire carried one signal, in the same way as the serial communications.) You don't see this as much anymore, and it was mostly short range anyway.
However, parallel data communication may start to have a comeback sometime soon. A lot of people are experimenting with various forms of quantum networking and communication. Quantum entities can carry more than one bit of information, and so it may be possible to create parallel communication technologies, once again. (Actually, there is a form of data communications called quadrature amplitude phase shift keying that uses a combination of high and low frequencies, and volumes, and other changes that can carry up to seven bits of data at a time, but that's getting a bit advanced for now.)
Recently, somebody asked me what was the difference between wifi and the Internet. This is a fairly common question. Wifi is one of the various short-range communications technologies that you use to connect to the much wider and larger network known as the Internet. Internetworking, or just internet, with a lowercase i, used to be a technical term to describe what you needed to do to connect two devices that used different communications technologies, or were made by different manufacturers, since pretty much every manufacturer had invented their own communications protocols. However, once people really started to build communications networks between computers which were all made by different manufacturers, we needed internetworking on a very large scale. And there were formal attempts to ensure that all computers could talk to pretty much any other computer. This, and the links between all of those computers, became the Internet, with a capital i. The Internet is this large scale network that connects pretty much every computer to every other computer, all around the world.
That may not explain very much, quite yet. And we're going to go into some further details. But before I do, I want to introduce something that became very important in ensuring that we could connect every computer to every other computer. This is the OSI model.
OSI is the international organization for standards. (Yes, I know, the three letter acronym doesn't seem to match the name. That actually involves politics, and we don't need to get into it right now. As a matter of fact, for my part, I'd be glad *never* to get into it. It's *much* more complicated than computer are or were.) Anyway, these are the people who make sure that technology has standards, so that when one person says that this device is a standard what's it, the other person knows what particular functions this what's it has. And, of course, they were very involved with the protocols and standards to ensure that computers could communicate with each other.
(Of course, this resulted in a completely new set of standards. The nice thing about computer standards is that there are so many of them. And if you don't like any of the ones that we have now, just wait until next year, when there will be a whole bunch of new ones.)
The one really good thing that came out of all of this was the OSI model. When we were talking about programming and software, we talked about layers: layers of hardware, layers of operating systems, layers of utilities, and layers of other types of software. The OSI model is based on layers. It has seven layers, that divide aspects of data communications into specific functional areas. If you stick with these layers, and ensure that the layer you make can talk to the layer below you, and you can talk to the layer above you, you can create your own communications technologies and protocols, and it will work with pretty much everything else.
There are seven layers to the model. Everybody has their own favorite mnemonic to remember the names of the different layers. My personal favorite it is Please Do Not Take Sales Person’s Advice. This allows me to remember that the actual layers of the OSI model are Physical, Data Link, Network, Transport, Session, Presentation, and Application.
We are going to go through at least the first three layers of the model. It turns out that the OSI model is not only good for building new communications technologies and protocols, but it also makes a really great way to separate the functions, and therefore, it gives a nice structure for teaching about how data communications work.
Introduction and ToC: https://fibrecookery.blogspot.com/2025/12/how-computers-work-from-ground-up.html
Next: TBA
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