To understand how networks function, you need to become familiar with the basic elements of a network. This chapter explains networks by introducing fundamental computer and network concepts and the characteristics, functions, benefits, metrics, and attributes used to describe network features and performance. This chapter also introduces the Open System Interconnection (OSI) reference model, data communications terms and concepts, and the TCP/IP protocol, which serves as the de facto standard for most of today's computer networks. Finally, this chapter provides you with an opportunity to connect two PCs in a point-to-point serial network.
What Is a Network?
The first task in understanding how to build a computer network is defining what a network is and understanding how it is used to help a business meet its objectives. A network is a connected collection of devices and end systems, such as computers and servers, that can communicate with each other.
Networks carry data in many types of environments, including homes, small businesses, and large enterprises. In a large enterprise, a number of locations might need to communicate with each other, and you can describe those locations as follows:
Main office: A main office is a site where everyone is connected via a network and where the bulk of corporate information is located. A main office can have hundreds or even thousands of people who depend on network access to do their jobs. A main office might use several connected networks, which can span many floors in an office building or cover a campus that contains several buildings.
Remote locations: A variety of remote access locations use networks to connect to the main office or to each other.
Branch offices: In branch offices, smaller groups of people work and communicate with each other via a network. Although some corporate information might be stored at a branch office, it is more likely that branch offices have local network resources, such as printers, but must access information directly from the main office.
Home offices: When individuals work from home, the location is called a home office. Home office workers often require on-demand connections to the main or branch offices to access information or to use network resources such as file servers.
Mobile users: Mobile users connect to the main office network while at the main office, at the branch office, or traveling. The network access needs of mobile users are based on where the mobile users are located.
Figure 1-1 shows some of the common locations of networks that can be used to connect users to business applications.
Many different types and locations of networks exist. You might use a network in your home or home office to communicate via the Internet, to locate information, to place orders for merchandise, and to send messages to friends. You might have work in a small office that is set up with a network that connects other computers and printers in the office. You might work in a large enterprise in which many computers, printers, storage devices, and servers communicate and store information from many departments over large geographic areas. All of these networks share many common components.
Common Physical Components of a Network
The physical components are the hardware devices that are interconnected to form a computer network. Depending on the size of the network, the number and size of these components varies, but most computer networks consist of the basic components shown in Figure 1-2.
Common Network Components
These are the four major categories of physical components in a computer network:
Personal computers (PCs): The PCs serve as endpoints in the network, sending and receiving data.
Interconnections: The interconnections consist of components that provide a means for data to travel from one point to another point in the network. This category includes components such as the following:
Network interface cards (NICs) that translate the data produced by the computer into a format that can be transmitted over the local network
Network media, such as cables or wireless media, that provide the means by which the signals are transmitted from one networked device to another
Connectors that provide the connection points for the media
Switches: Switches are devices that provide network attachment to the end systems and intelligent switching of the data within the local network.
Routers: Routers interconnect networks and choose the best paths between networks.
Interpreting a Network Diagram
When designing and describing a computer network, you use a drawing or diagram to describe the physical components and how they are interconnected.
The network diagram uses common symbols to capture information related to the network for planning, reference, and troubleshooting purposes. The amount of information and the details of that information differ from organization to organization. The network topology is commonly represented by a series of lines and icons. Figure 1-3 shows a typical network diagram.
In this diagram:
A cloud represents the Internet or WAN connection.
A cylinder with arrows represents a router.
A rectangular box with arrows represents a workgroup switch.
A tower PC represents a server.
A laptop or computer and monitor represent an end user PC.
A straight line represents an Ethernet link.
A Z-shaped line represents a serial link.
Typical Network Diagram
Other information can be included as space allows. For example, it is sometimes desirable to identify the interface on a device in the format of s0/0/0 for a serial interface or fa0/0 for a Fast Ethernet interface. It is also common to include the network address of the segment in the format such as 10.1.1.0/24, where 10.1.1.0 indicates the network address and /24 indicates the subnet mask.
Resource-Sharing Functions and Benefits
The main functions of computer networks in business today are to simplify and streamline business processes through the use of data and application sharing. Networks enable end users to share both information and hardware resources. By providing this interconnection between the users and common sets of data, businesses can make more efficient use of their resources. The major resources that are shared in a computer network include the following:
Data and applications: When users are connected through a network, they can share files and even software application programs, making data more easily available and promoting more efficient collaboration on work projects.
Physical resources: The resources that can be shared include both input devices, such as cameras, and output devices, such as printers.
Network storage: Today the network makes storage available to users in several ways. Direct attached storage (DAS) directly connects physical storage to a PC or a shared server. Network attached storage (NAS) makes storage available through a special network appliance. Finally, storage area networks (SAN) provide a network of storage devices.
Backup devices: A network can also include backup devices, such as tape drives, that provide a central means to save files from multiple computers. Network storage is also used to provide archive capability, business continuance, and disaster recovery.
Figure 1-4 shows some common shared resources.
The overall benefit to users who are connected by a network is an efficiency of operation through commonly available components used in everyday tasks, sharing files, printing, and storing data. This efficiency results in reduced expenditures and increased productivity.
In recent years, the open access to devices that was once pervasive in networking has been replaced with a need for caution. There have been many well-advertised acts of "cyber vandalism," in which both end systems and network devices have been broken into; therefore, the need for network security has to be balanced with the need for connectivity.
Network User Applications
The key to utilizing multiple resources on a data network is having applications that are aware of these communication mechanisms. Although many applications are available for users in a network environment, some applications are common to nearly all users.
The most common network user applications include the following:
E-mail: E-mail is a valuable application for most network users. Users can communicate information (messages and files) electronically in a timely manner, to not only other users in the same network but also other users outside the network (suppliers, information resources, and customers, for example). Examples of e-mail programs include Microsoft Outlook and Eudora by Qualcomm.
Web browser: A web browser enables access to the Internet through a common interface. The Internet provides a wealth of information and has become vital to the productivity of both home and business users. Communicating with suppliers and customers, handling orders and fulfillment, and locating information are now routinely done electronically over the Internet, which saves time and increases overall productivity. The most commonly used browsers are Microsoft Internet Explorer, Netscape Navigator, Mozilla, and Firefox.
Instant messaging: Instant messaging started in the personal user-to-user space; however, it soon provided considerable benefit in the corporate world. Now many instant messaging applications, such as those provided by AOL and Yahoo!, provide data encryption and logging, features essential for corporate use.
Collaboration: Working together as individuals or groups is greatly facilitated when the collaborators are on a network. Individuals creating separate parts of an annual report or a business plan, for example, can either transmit their data files to a central resource for compilation or use a workgroup software application to create and modify the entire document, without any exchange of paper. One of the best-known traditional collaboration software programs is Lotus Notes. A more modern web-based collaboration application is a wiki.
Database: This type of application enables users on a network to store information in central locations (such as storage devices) so that others on the network can easily retrieve selected information in the formats that are most useful to them. Some of the most common databases used in enterprises today are Oracle and Microsoft SQL Server.
The Impact of User Applications on the Network
The key to user applications is that they enable users to be connected to one another through the various types of software. As a business begins to rely on these applications as part of the day-to-day business process, the network that the applications operate in becomes a critical part of the business. A special relationship exists between these applications and the network. The applications can affect network performance, and network performance can affect applications. Therefore, you need to understand some common interactions between user applications and the network. Figure 1-5 characterizes some of the interactions for different types of applications.
Historically, when the interaction between the network and the applications that ran on the network was considered, bandwidth was the main concern. Batch applications such as FTP, TFTP, and inventory updates, which simply used the network to transfer bulk data between systems, would be initiated by a user and then run to completion by the software with no further direct human interaction. Bandwidth was important but not critical because little human interaction occurred. As long as the time the application took to complete did not become excessive, no one really cared.
Interactive applications, such as Enterprise Resource Planning (ERP) software, perform tasks, such as inventory inquiries and database updates, that require more human interaction. The user requests some type of information from the server and then waits for a reply. With these types of applications, bandwidth becomes more important because users are intolerant of slow responses. However, application response is not solely dependant on the bandwidth of the network; the server and storage devices also play a part. However, in cases where the network becomes a problem, other features such as quality of service (QoS) can alleviate some bandwidth limitations by giving the traffic from interactive applications preference over batch applications.
Another type of application that can be affected heavily by the network is a real-time application. Like interactive applications, real-time applications such as Voice over IP (VoIP) and video applications involve human interaction. Because of the amount of information that is transmitted, bandwidth is critical. In addition, because these applications are time-critical, latency (delay through the network) is critical. Even variations in the amount of latency (jitter) can affect the application. Not only is proper bandwidth mandatory, but QoS is also mandatory. VoIP and video applications must be given the highest priority.