NET 121b: Essentials of Networking

Chapter 2: The OSI Model


This chapter discusses the OSI network model. The topics of this chapter are:

  1. The ISO networking model
  2. Functions of the layers of the ISO model

The way a network works can be understood in terms of a model of a network that was created by the International Organization for Standardization, called the ISO for short. (No it isn't an acronym, it is from the Greek word isos, meaning equal.) Their model is called the Open Systems Interconnection (OSI) Reference Model, hence the ISO-OSI model.

Once you understand this model, you will have a general, powerful reference for examining and comparing networks. In the Novell curriculum, we spent nine chapters on this model. I will present an overview here, in keeping with your text. For those who would like to understand the subject better, look over the notes for course CNE 250.

The seven layers of the model are usually written in a list, numbering the top as layer seven and the bottom as layer one.

  • 7 - Application
  • 6 - Presentation
  • 5 - Session
  • 4 - Transport
  • 3 - Network
  • 2 - Data Link
  • 1 - Physical

Several mnemonic sentences exist to help us remember the proper order. I recommend "Please Do Not Throw Sausage Pizza Away", because this is in the correct numeric order (bottom to top). If you must have one that goes from top to bottom, try "All People Studying This Need Drastic Psychotherapy".  Warning: you MUST remember the correct order, the correct numbers, and the correct details for each layer.

The processes that happen in each layer communicate with the next layer. Which way is next, up or down? It depends whether data is being passed out of the stack (down) or into it (up). Typically, a computer generates a request starting at the top layer, and working down. The request is passed across the network (probably to a server) and the received request is passed up the layers. When a response is generated, the process reverses.

The Novell texts for this subject provide another metaphor for the model: think of each layer of the model as being like a shelf in a bookcase. There are seven shelves, and each shelf has several books on it. The books represent the topics that we discuss in our study of that layer. In fact, in each book are discussions of the various methods that relate to that topic. A method is a way of implementing a task the topic covers. For instance, the Physical layer includes the Connection Type topic (a task), and we will see that there are two classic methods for making connections (ways to do it).

Using the metaphor of the bookcase, consider that the ISO might made different choices about which shelf some the books belong on. Some books fit the subject matter of more than one shelf. Other network models exist that place some topics on different layers. This explains why some actual networks do not fit the ISO model as cleanly as they might. There is no definitive rule that says either is right, only that each must follow its own standard.

The chapter continues with an overview of the functions of the seven layers:

  • Layer 1, Physical - Protocols define structure, physical specifications for media, rules for transmitting bits. Most network models do not specify what happens on this layer, making it possible to use many kinds of wiring for different kinds of networks. This layer tells us how the network is physically set up.
  • Layer 2, Data Link - Bits are formed into frames, headers give address information. This layer tells us how the network is physically accessed, how some errors are handled, how data flow is handled and how entities on the network are addressed. Sub-layers:
    • Media Access Control (MAC) - Rules to access the media, logical topologies, hardware addressing. The book discusses other addresses here, although they belong on other layers: Network addresses belong in the Network layer headers, Service addresses can be placed in the Data Link or Network layer headers.
    • Logical Link Control (LLC) - Frame synchronization, connection services and error control
  • Layer 3, Network - Datagram packets are routed to other networks. Connection services are introduced. The Network Layer is concerned with moving data to specific locations across networks. This might have been called the Internetwork Layer, but it was not, so we'll live with it.
  • Layer 4, Transport - The Transport Layer is concerned with segment development (building message units) and moving data to specific processes or services in a reliable way. Associate this layer with the words "reliable" and "dependable". End-to-end control and error checking. In some networks, this may be the last or next-to-last layer.
  • Layer 5, Session -The Session Layer is concerned with communications between service requesters and providers. Dialogs are set up, maintained, and terminated at this layer.
  • Layer 6, Presentation - The Presentation Layer is concerned with translation of signals into formats network entities can understand. This includes translating network protocols, characters, and file standards.
  • Layer 7, Application - The Application Layer is concerned with all the network services, as well as service advertisement, and service use
  • Layers 5, 6 and 7: Session, Presentation and Application- Some networks group all these functions into one group of Upper Layer Protocols. The Session layer functions control dialog between nodes, the Presentation layer functions format data and bits, and the Application layer functions share network services.

As a request is passed down the layers, a header is added to the request at each new layer. The header holds information that is added to the request by processes at that layer. So, by the time the request leaves the computer, it has had six more parts added to it. When the request is received by the server, the receiving layers strip off the information added to the request by their counterpart layers. The request increases in size as it leave the requester, and decreases in size as it is processed by the receiver. This is a simplification to get the idea across to you, but then ,"it's only a model". (Patsy, Monty Python and the Holy Grail.)