Exploring the Functions of Networking
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.
- 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
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.
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
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.
Figure 1-3 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.
Figure 1-4 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.
Figure 1-5 Application Interaction
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.
In today’s environment, the end user is bombarded with ads indicating how much money can be saved by converting to VoIP and how installation is as easy as dropping a VoIP router into the network. Although this is often true in the home network, it can result in disaster in a small office network. Applications that used to work start to run so slowly that they are unusable, for example, when someone is on the phone, and voice quality is poor. This type of implementation does not provide enough bandwidth to the Internet, nor does it provide a proper QoS scheme.
Both issues can be overcome with proper network design.
Characteristics of a Network
Many characteristics are commonly used to describe and compare various network designs. When you are determining how to build a network, each of these characteristics must be considered along with the applications that will be running on the network. The key to building the best network is to achieve a balance of these characteristics. Networks can be described and compared according to network performance and structure, as follows:
- Speed: Speed is a measure of how fast data is transmitted over the network. A more precise term would be data rate.
- Cost: Cost indicates the general cost of components, installation, and maintenance of the network.
- Security: Security indicates how secure the network is, including the data that is transmitted over the network. The subject of security is important and constantly evolving. You should consider security whenever you take actions that affect the
- Availability: Availability is a measure of the probability that the network will be available for use when required. For networks that are meant to be used 24 hours a day, 7 days a week, 365 days a year, availability is calculated by dividing the time it is actually available by the total time in a year and then multiplying by 100 to get a
For example, if a network is unavailable for 15 minutes a year because of network outages, its percentage availability can be calculated as follows:
([Number of minutes in a year – downtime] / [Number of minutes in a year]) * 100 = Percentage availability
([525600 – 15] / ) * 100 = 99.9971
- Scalability: Scalability indicates how well the network can accommodate more users and data transmission requirements. If a network is designed and optimized for just the current requirements, it can be very expensive and difficult to meet new needs when the network grows.
- Reliability: Reliability indicates the dependability of the components (routers, switches, PCs, and so on) that make up the network. Reliability is often measured as a probability of failure, or mean time between failures (MTBF).
- Topology: Networks have two types of topologies: the physical topology, which is the arrangement of the cable, network devices, and end systems (PCs and servers), and the logical topology, which is the path that the data signals take through the physical topology.
These characteristics and attributes provide a means to compare different networking solutions. Increasingly, features such as security, availability, scalability, and reliability have become the focus of many network designs because of the importance of the network to the business process.
Physical Versus Logical Topologies
Building a reliable and scalable network depends on the physical and logical topology. Topology defines the interconnection method used between devices including the layout of the cabling and the primary and backup paths used in data transmissions. As previously mentioned, each type of network has both a physical and a logical topology.
The physical topology of a network refers to the physical layout of the devices and cabling. You must match the appropriate physical topology to the type of cabling that will be installed. Therefore, understanding the type of cabling used is important to understanding each type of physical topology. Here are the three primary categories of physical topologies:
- Bus: Computers and other network devices are cabled together in a line.
- Ring: Computers and other network devices are cabled together with the last device connected to the first to form a circle, or ring. This category includes both ring and dual-ring topologies.
- Star: A central cabling device connects the computers and other network devices. This category includes both star and extended-star topologies.
Figure 1-6 shows some common physical topologies used in networking.
Figure 1-6 Common Physical Topologies
The logical topology of a network refers to the logical paths that the signals use to travel from one point on the network to another—that is, the way in which data accesses the network media and transmits packets across it.
The physical and logical topologies of a network can be the same. For example, in anetwork physically shaped as a linear bus, the data travels along the length of the cable.Therefore, the network has both a physical bus topology and a logical bus topology.
On the other hand, a network can have quite different physical and logical topologies. For example, a physical topology in the shape of a star, in which cable segments connect all computers to a central hub, can have a logical ring topology. Remember that in a ring, the data travels from one computer to the next, and inside the hub, the wiring connections are such that the signal actually travels around in a circle from one port to the next, creating a logical ring. Therefore, you cannot always predict how data travels in a network simply by observing its physical layout.
Star topology is by far the most common implementation of LANs today. Ethernet uses a logical bus topology in either a physical bus or a physical star. An Ethernet hub is an example of a physical star topology with a logical bus topology.
Bus Topology Ring Topology Star Topology
Figure 1-7 shows some common logical topologies used in networking.
Figure 1-7 Common Logical Topologies
The bus topology is commonly referred to as a linear bus; all of the devices on a bus topology are effectively connected by one single cable.
As illustrated in Figure 1-8, in a bus topology, a cable proceeds from one computer to the next like a bus line going through a city. The main cable segment must end with a terminator that absorbs the signal when it reaches the end of the line or wire. If no terminator exists, the electrical signal representing the data bounces back at the end of the wire, causing errors
in the network. An example of a physical bus topology is a Thicknet Ethernet cable running through the length of a building with devices taped into it, though this is an antiquated connection method that is no longer used. An example of a logical bus topology is an Ethernet hub.
Figure 1-8 Bus Topology
Star and Extended-Star Topologies
The star topology is the most common physical topology in Ethernet LANs. When a star network is expanded to include an additional network device that is connected to the main network devices, the topology is referred to as an extended-star topology. The following sections describe both the star and extended-star topologies.
When installed, the star topology resembles spokes in a bicycle wheel. It is made up of a central connection point that is a device, such as a hub, switch, or router, where all the cabling segments actually meet. Each device on the network is connected to the central device with its own cable.
Although a physical star topology costs more to implement than the physical bus topology, the advantages of a physical star topology make it worth the additional cost. Each device is connected to the central device with its own wire, so that if that cable has a problem, only that one device is affected, and the rest of the network remains operational. This benefit is
important and is the reason why almost every newly designed Ethernet LAN has a physical star topology. Figure 1-9 depicts a star topology with all transmissions going through a single point.
A common deployment of an extended-star topology is in a hierarchical design such as a WAN or an Enterprise or a Campus LAN. Figure 1-10 shows the topology of an extended star.
16 Chapter 1: Building a Simple Network
Figure 1-9 Star Topology
Figure 1-10 Extended Star TopologyExtended Star Topology
The problem with the pure extended-star topology is that if the central node point fails, large portions of the network can become isolated. For this reason, most extended-star topologies employ a redundant connection to a separate set of connection devices to prevent isolation in the event of a device failure.
As the name implies, in a ring topology all the devices on a network are connected in the form of a ring or circle. Unlike the physical bus topology, a ring type of topology has no beginning or end that needs to be terminated. Data is transmitted in a way that is different from the logical bus topology. In one implementation, a “token” travels around the ring, stopping at each device. If a device wants to transmit data, it adds that data and the destination address to the token. The token then continues around the ring until it finds the destination device, which takes the data out of the token. The advantage of using this type of method is that no collisions of data packets occur. Two types of ring topology exist:
single-ring and dual-ring.
In a single-ring topology, all the devices on the network share a single cable, and the data travels in one direction only. Each device waits its turn to send data over the network. The single ring, however, is susceptible to a single failure, stopping the entire ring from functioning. Figure 1-11 shows the traffic flow in a single-ring topology.
Figure 1-11 Traffic Flow in a Single-Ring Topology
In a dual-ring topology, two rings allow data to be sent in both directions. This setup creates redundancy (fault tolerance), meaning that if one ring fails, data can be transmitted on the other ring. Figure 1-12 shows the traffic flow in a typical dual-ring topology.
Figure 1-12 Traffic Flow in a Dual-Ring Topology
Mesh and Partial-Mesh Topologies
Another type of topology that is similar to the star topology is mesh topology. Mesh topology provides redundancy between devices in a star topology. A network can be fully meshed or partially meshed depending on the level of redundancy needed. This type of topology helps improve network availability and reliability. However, it increases cost and can limit scalability, so you need to exercise care when meshing.
The full-mesh topology connects all devices (or nodes) to one another for redundancy and fault tolerance. Implementing a full mesh topology is expensive and difficult. This method is the most resistant to failures because the failure of any single link does not affect reachability in the network.
Figure 1-13 shows the connections in a full-mesh topology.
Figure 1-13 Full-Mesh Topology
In a partial-mesh topology, at least one device maintains multiple connections to all other devices, without having all other devices fully meshed. This method trades off the cost of meshing all devices by allowing the network designer to choose which nodes are the most critical and appropriately interconnect them.
Figure 1-14 shows an example of a partial-mesh topology.
Figure 1-14 Partial-Mesh Topology
Connection to the Internet
Another key component for most business users today is a connection to the Internet. An Internet connection is a WAN connection, but small- to medium-sized computer networks can use various methods and topologies to interconnect to the Internet.
You have three common methods of connecting the small office to the Internet. Digital subscriber line (DSL) uses the existing telephone lines as the infrastructure to carry the signal. Cable uses the cable television (CATV) infrastructure. Serial uses the classic digital local loops.
In the case of DSL and cable, the incoming lines are terminated into a modem that converts the incoming digital encoding into a digital format for the router to process. In the case of serial this is done by channel service unit (CSU)/digital service unit (DSU). In all three cases (DSL, cable, and serial), the digital output is sent to a router that is part of the customer premises equipment (CPE). Figure 1-15 shows the equipment placement for these different connection methods.
Figure 1-15 Common Internet Connections Methods