Networking Technologies

Chapter 2: Network Services

 

 

Objectives:

Chapter 2 discusses five areas of network services and two common approaches to providing them. The objectives important to this chapter are on page 2-1:

  1. Understanding the five network services.
  2. Understanding the functions provided by each network service.
  3. Determining which network service is needed for specific problems.
  4. Understanding the effect on network services of choosing a centralized or distributed network implementation.
Concepts:

This chapter divides the services that a network can provide into five types on page 2-2:

  • File services
  • Print services
  • Message services
  • Application services
  • Database services

Each type of service is discussed in detail.

File Services

File services are divided into four functions:

  • File Transfer - This is the ability to copy and share files across a network. Before networks, it was necessary to copy files to disk or tape to move a file from one computer to another. Having to physically carry the disks from machine to machine gave us the phrase "sneaker net".
  • File Storage and Data Migration - Files may be stored on media immediately accessible by a computer, like a hard drive. This is called online storage.
    Files may also be stored in larger media than hard drives, like tape carousels and jukeboxes, that take a few moments for the automated functions of a system to load on request. This is called nearline storage. No user intervention is required to load the tape in nearline storage, since it is automated.
    Offline storage is when files are stored on disks or tapes that have to be physically loaded into a machine by an operator before they are available to the system.
  • File Update Synchronization - This is a function that is needed when multiple users access copies of shared data files separately, and the changes made by each user need to be incorporated into all copies of the file. The system may determine which file is the most recently changed and copy it over all other copies. This does not necessarily incorporate all changes, so user intervention is often necessary, unless care is taken to make sure that two copies of a file are not changed differently by two users before synchronization takes place.
  • File Archiving - This is another term for file backup. It is the practice of making a safety copy of files. On a network, an operator may be able to backup several servers at once, instead of having to travel to each one.

Print Services

The functions of print services are listed on page 2-9:

  • Providing Multiple Access - A printer has, at best, two interfaces for user input. Most companies will not want to buy a printer for each employee. Sharing printers on a network makes much more sense, because you will use one interface (where the network connects to the printer) and any network user may send print jobs to it.
  • Eliminating Device Constraints - Printer cables are required to be rather short. This limits the location of printers. Network cables eliminate this problem, since they can be longer. Networked printers may be located in central areas.
  • Handling and Queuing Simultaneous Requests - Most users will not send print jobs at the same time, but in any large organization simultaneous requests will happen, due to size or workflow. The network stores requests in queues or buffers, and processes them as the printers become ready for new jobs.
  • Sharing Specialized Equipment - Special printers, like color lasers, high speed and special format printers are expensive. Sharing them on a network can grant or restrict access to them as necessary for the organization.
  • Network Fax Service - Sending a print job to a networked fax machine is more efficient than printing an image, scanning it into the fax and sending it to the recipient. It saves steps and is easily available to more users. It also sends a clearer, cleaner image than the older method.
Message Services

Instead of functions, the chapter discusses how messages (text, graphics, audio and video) are used by several types of message-enabled applications:

  • Electronic Mail - The purpose of e-mail is to send messages. Current applications also include viewers, schedulers, and such. Critical features are the ability to present different file formats, and to transport the message based on where the sender and recipient are in the network.
  • Integrated E-mail and Voice Mail - Voice mail systems run on specialized computers. Integrating them with the network may eventually lead to hearing a text message over a phone or reading a text version of a phone message on a screen.
  • Object Oriented Applications - Message services are used by objects to pass information back and forth. Objects are like program components that interact with each other in a system to keep from having to repeat the same computer code over and over in many applications.
  • Workgroup Applications - two types of these applications are discussed:
    • Workflow management - This kind of application can eliminate paper flow by sending electronic forms to people on the network. They fill out the form and it is sent to whoever needs it.
    • Linked-object documents - This type of document shares information with other sources, like the Object Linking and Embedding process used in Microsoft Office documents. Messaging services are used to update charts, figures and other parts of the document from actual sources.
  • Directory Services - Also called Directory Synchronization, this service tracks information about resources on the network, and constantly updates directories that retain information about location, address routing and other network related data.
Application Services

Application services perform some processing tasks for clients. Two ways they do this are on page 2-17:

  • Coordinate hardware and software, determining where best to run a utility or application.
  • Increase network capabilities by making more power available to more users.

Two other concepts are discussed here:

  • Specialization of Servers - Servers may be dedicated to specific functions so they can take on common tasks for clients. This frees the client and makes it less necessary for every client to perform every kind of task.
  • Scalability and Growth - Replacing components with faster, newer or more powerful ones is the idea here. For it to work, the application that needs a new processor must be able to run on what may be a new operating system, or the old operating system must be able to run on the new equipment.
Database Services

The last service type discussed in the chapter is Database Services. This is different from file services, in that information is stored and retrieved, not files. Using specialized servers, client-server databases optimize tasks by having the client formulate the data requests, and processing the information returned (called manipulation and presentation). The server receives the requests, searches the information storage and passes it back to the client.

Five Database Services are listed on page 2-20:

  • Dedicated computers are used to store, search and retrieve records.
  • Data storage is controlled geographically. (This is explained in the next two sections.)
  • Data is organized logically between divisions of the organization.
  • Data security is provided.
  • Client access time to data records is reduced (improved).

Two versions of database architectures are discussed:

  • Coordinating Distributed Data - To speed up access to data, it is common to divide the database into logical units. These units of data are stored close to users who need to access them. The Database Services make it look like one database to the system.
  • Replication - This is the creation of copies of units of the database that are located geographically close to users, giving them quicker access, and giving the system redundant fault tolerance. When changes are made in these copies, however, the changes must be synchronized into all copies. Two methods of synchronizing database replicas are illustrated on page 2-31.
    • The Master Database method sends all change requests from users to a central copy of the database. The change is made and then all copies (replicas) are synchronized with the Master. NetWare uses this method for NDS Partition changes.
    • In the Local Database method, the user requests a change in a local database, the change is made and then synchronized to all other local databases. NetWare uses this method for NDS object changes.

The next topics in the chapter are the effects on Network Services of choosing one of two networking strategies:

  • Centralized Network Services - used primarily on Client-Server networks
  • Distributed Network Services - used primarily on Peer-to-Peer networks

Three aspects are discussed first:

  • Control of Resources - A centralized strategy gives more control over resources and makes it easier to track problems.
  • Server Specialization - Using specialized servers implies a centralized strategy. It gives you better performance if you can optimize the server for the tasks assigned to it. Not all servers will need all abilities in this strategy.
  • Choice of Network Operating Systems - The graphics on page 2-35 show the basic strategies inherent in several operating systems. On the Server-Centric side, we can add UNIX and Windows NT. On the Peer-to-Peer side, we can add Windows 95 and 98.

The effects of choosing a Centralized or Distributed strategy are discussed at the end of the chapter. Note the charts on page 2-37 and page 2-38, specifically regarding File Services.
Centralized Benefits

  • Specialized Servers - optimized for tasks
  • Costs of few servers can be distributed over all users
  • Central control is faster to manage and maintain.

Centralized Concerns

  • Single point of failure, not much fault tolerance.
  • File retrieval can be slower than in a distributed system.

Distributed Benefits

  • Fault tolerant, since many servers have replicas.
  • File retrieval times can be faster than centralized systems.
  • Since devices are not specialized, less powerful computers can be used.

Distributed Concerns

  • Complex file management, involving security, synchronization and replication.
  • Access to non-specialized servers is often slower.
  • Expensive equipment will need to be duplicated, since we are not depending on one specialized server to service it, but using several servers for each purpose.