|The “Seven-Layer” Vocabulary of
In the movies constituting our Ethernet series, and especially in the Internet movie entitled: “Ethernet Delivers the Internet,” we’ve shown how a message can originate anywhere in the world, can be sliced up and formatted inside IP packets, which can then be encapsulated and removed from various types of framing to traverse the entire world and end up at the doorstep of our local network. In a typical home-office or small-office environment, our local DSL modem or cable modem delivers that information to our local router inside an Ethernet frame, on a tiny little subnet managed and owned by our Internet service provider that only knows about one IP address for all of our computers. Other movies published at AskMisterWizard.com explain how our small local router asserts the additional intelligence necessary so that our local network can have more IP addresses and send that packet to one computer within our premises.
Experience has shown, however, that just getting the message to the right computer on our network is not sufficiently precise to allow real and useful work. Most computers today are controlled by “multitasking” or “multiprocessing” operating systems that are running several programs, or tasks, at any given moment. Even a typical small computer in a home is often running twenty or thirty different programs all at once, and somehow, our message from the Internet has to find the right program to receive and process it. Furthermore, most modern operating systems are oriented around a multi-user philosophy, where any of several people could be using the computer at any given time (sometimes, two or three people at once; sometimes just one). But in any event, work has to be associated with and attributed to a particular person among a group of people, and because different people have grown up in different cultures and speak different languages, reading and writing with different alphabets, it’s also important that our Internet message is somehow able to express itself in an appropriate character set and language.
So, stepping back and looking at this situation from the big picture demands that we allow a specific message, using a specific character set in a specific language, to be delivered or attributed to a specific person, using a specific program or process, on a specific computer inside our network, which is a subset of the world-wide Internet. This is a lot of selection and focus, and layer upon layer of intelligence.
At one end of this spectrum, the data represents real information in a useful format that will make real sense in a specific context to a specific person or a specific application; whereas, at the other end of the spectrum, the data just represents bits in transit from somewhere to somewhere else. In between these two extremes, various levels of intelligence can be individually identified, and perhaps interchanged with counterparts at the other end of the network at the same level of intelligence.
In the physical world, using the analogy of packages being transported by a postal service, we are already accustomed to thinking about counterparts at each end of the delivery, each performing similar functions. For example, the delivery van at the source end of a shipment has a counterpart delivery van at the destination. And the shipping company’s office at the source end of a shipment has a counterpart with a shipping company’s office at the destination end of a shipment.
Back in the early days of computer networking (in the 1970’s) computer networks were created in a proprietary fashion by a lot of different companies that used competing specifications, and they could not interoperate with each other at any level. Back in those days, if you bought your computers from IBM, you also bought your network from IBM. And your IBM computers really could not speak very effectively with computers from DEC, or Data General, or Hewlett Packard, etc. Attempts to make these competing network types interoperate sensibly resulted in a lot of frustration. Eventually, it became clear that these proprietary network systems would have to be abandoned, and would eventually be replaced by new network architectures built along an interoperable design.
This is the text from the movie entitled "7-LayerVocabulary of Modern Internetworking", published at www.AskMisterWizard.com. (The text also includes a few small, static illustrations from the movie). You can learn more about the video version and purchase an inexpensive personal license to view it by clicking right here. You can learn more about our "Internet Series" of licensed videos by clicking here. Our "Internet Series" is part of a larger set of "Networking" videos, and you can learn about all of them by clicking here.