This chapter discusses topics from the third and fourth layers of the ISO-OSI model, having to do with connecting circuits together to make networks. The objectives important to this chapter are:
Concepts:The chapter begins with the statement on page 397 that a network is a collection of circuits connecting nodes. It is helpful to classify networks as being of various types, because each classification method tells us something about the network.
The first classification method is topology, the way the network
is shaped and constructed. Six topologies are described:
Networks can be classified by ownership. A private network is owned by one entity (such as a company) for its own use: outsiders are not meant to intrude. A public network is one that the public is meant to use, like the public telephone network or the Internet. A Value Added Network is one that adds features, for a price, to what is available to the public. For example, any Internet user can access the public features of America Online, but only subscribers to that service can access email through it.
We briefly discussed the three geographic classifications for
networks last week:
Networks can also be classified by transmission technology, how
they make connections and send messages.
Let's continue on page 417 with the discussion of routing messages.
Since many paths are possible through many networks, there are several
methods of determining the actual path or route to be used. Two
major types of routing are discussed on page 417, and it could have been
clearer. Try it this way:
The next section of the chapter introduces more vocabulary. It is important to know that when you are talking about connecting any two (or more) networks together, you are talking about internetworking. I carefully avoided putting that word at the beginning of a sentence. If I had capitalized it, you could confuse it with Internetworking, which is making connections through the Internet, the network that was originally set up by ARPA, and is now available to the world. Any network that is connected to another network can be referred to as a subnetwork.
In the discussion of TCP/IP on page 420, you should learn that the Internet model is different from the ISO-OSI model, but does the same kind of things, as we have discussed in other chapters. The Internet Protocol (IP) works on the layer 3 of the ISO-OSI model, and does routing and addressing of packets. The Transmission Control Protocol (TCP) works on layer 4 of the ISO-OSI model, and is concerned with error-free, loss-free, connection-oriented delivery of packets. The combination is more reliable than either of the two would be separately.
Mr. Rowe's discussion of the Internet on pages 423-424 is a bit dated, but that is expected in a phenomenon that reinvents itself daily. He mentions four categories of user activity. You should be able to list several others.
It is important for people who do not use the Internet (yet) to become familiar with it, so the vocabulary on the next couple of pages is assigned: telnet, domain name, usenet group, HTML, URL, browser and home page.
Before discussing the next several pages, you should know that the word host has two very different meanings, depending on what kind of network you are talking about. If you are talking about a network of clients and servers, in which the clients request services (like storage space, print service, etc.) the server is called a host. However, if you are talking about an IP network, every device on that has an IP address is called a host. Confusing, isn't it?
Two basic ways of attaching a computer to network are described on pages 426 and 427. The direct connection method resembles the description of a star network from early in this chapter. (Remember that a star can mean a network that uses hubs or concentrators, too.) The other method is to use a Front End Processor, which is a computer that takes on some of the burdens that the host computer would normally have to carry.
The next several pages are not necessary for this course, so we will skip ahead to page 439 and the discussion of two proprietary architectures: SNA and DNA.
Systems Network Architecture (SNA) is an IBM invention. The SNA model is based on a similar but different series of concepts from the ISO-OSI model.
Several layers in it will be familiar and some have new concepts:
Digital Network Architecture (DNA) from Digital Equipment Corporation is introduced on page 442. Note the term DECnet, which is used to denote any product that uses DNA.
DECnet supports connecting to other types of networks well. Its model only has five layers in it, but it actually supports all the features of the ISO-OSI model in its five layers.