Networking Technologies

Chapter 12: The OSI Application Layer

 

 

Objectives:

Chapter 12 is the last chapter about OSI model layers. The objectives important to this chapter are on page 12-1:

  1. Understanding the overall purpose of this layer.
  2. Understanding two service advertisement methods.
  3. Understanding the service use methods.
Concepts:

The overview of the chapter on page 12-2 shows that the Application Layer is concerned with all the network services in Chapter 2, as well as service advertisement and service use.

The topics and methods for this layer are on page 12-3:

  • Application - This layer has 3 topics and is associated with Messages
    • Network Services
      • File services
      • Print services
      • Message services
      • Application services
      • Database services
    • Service Advertisement - how services become known
    • Service Use - how services are obtained

The first topic, Network Services, is covered in Chapter 2. It has been several weeks since we spoke of it, so I suggest you review Chapter 2 before proceeding. Go ahead... I can wait... I am only a document....

The new material in this chapter begins on page 12-4, concerning Service Advertisement. This is the process by which services discussed in Chapter 2 become known to the network. (You did review it, right?) Two methods are discussed:

  • Active service advertisement - in this method, the server can advertise, or the client can poll the network. Both techniques lead to the client keeping a list of services and service addresses that are available on the network. Clients are aware of the schedule for servers to advertise their services. Services that have not been advertised at the last scheduled time are assumed to be unavailable.
  • Passive service advertisement - This is the third method we discussed in the notes for Chapter 9, that of a directory of servers and service addresses that is maintained for clients.

The last topic of the model (... the bar opens in five minutes...) is Service Use. There are three methods discussed:

  • OS Call Interception - In this method, special client software must intercept requests made to the local operating system and route them to the network. In a Novell network this is done with Netx, VLMs or Client32, or Novell Client software. More on each of them in later classes. The main idea is that it intercepts requests, for instance a signal sent to a printer port, and routes it to a network card and a server, in this case becoming a job for a print queue. The workstation does not have to be aware that it is on a network.
  • Remote Operation - In this method, the network is not aware that it has clients. Clients are aware they are on the net and run programs on the server through network requests. This is like a UNIX system, in which a client logs in and runs a "virtual machine" on the main UNIX computer.
  • Collaborative Computing - typically used on a peer-to-peer system, all entities on the network need to be aware of each other, sharing resources and capabilities. The local operating system must be "network aware", and entities must be set up to cooperate with each other.