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Introduction to Networks: Exploring the Network

Chapter Description

This chapter explores the purpose, function, technology, and implications of modern networking technology in home and enterprise environments.

LANs, WANs, and the Internet (1.2)

Many different components are required to allow a network to provide services and resources. These various components work together to ensure that resources are delivered in an efficient manner to those requiring the services.

Components of a Network (1.2.1,

The network infrastructure contains three categories of network components—devices, media, and services—as shown in Figure 1-6.

Figure 1-6

Figure 1-6 Components of the Network Infrastructure

The path that a message takes from source to destination can be as simple as a single cable connecting one computer to another or as complex as a network that literally spans the globe. This network infrastructure is the platform that supports the network. It provides the stable and reliable channel over which our communications can occur.

Devices (Figure 1-6a) and media (Figure 1-6b) are the physical elements, or hardware, of the network. Hardware is often the visible components of the network platform such as a laptop, PC, switch, router, wireless access point, or the cabling used to connect the devices. Occasionally, some components might not be so visible. In the case of wireless media, messages are transmitted using invisible radio frequency or infrared waves without requiring any physical connecting media.

Network components are used to provide services and processes (Figure 1-6c). These are the communication programs, called software, that run on the networked devices. A network service provides information in response to a request. Services include many of the common network applications people use every day, like email-hosting services and web-hosting services. Processes provide the functionality that directs and moves the messages through the network. Processes are less obvious to us but are critical to the operation of networks.

End Devices (

The network devices that people are most familiar with are called end devices, or hosts. These devices form the interface between users and the underlying communication network.

Some examples of end devices are

  • Computers (work stations, laptops, file servers, web servers)
  • Network printers
  • VoIP phones
  • TelePresence endpoints
  • Security cameras
  • Mobile handheld devices (such as smartphones, tablets, PDAs, and wireless debit/credit card readers and bar-code scanners)

A host device is either the source or destination of a message transmitted over the network. To distinguish one host from another, each host on a network is identified by an address. When a host initiates communication, it uses the address of the destination host to specify where the message should be sent. Data originates with an end device, flows through the network, and arrives at an end device. Messages can take alternate routes through the network between end devices.

Intermediary Network Devices (

Intermediary devices interconnect end devices. These devices provide connectivity and work behind the scenes to ensure that data flows across the network. Intermediary devices connect the individual hosts to the network and can connect multiple individual networks to form an internetwork.

Examples of intermediary network devices are

  • Network access (switches and wireless access points)
  • Internetworking (routers)
  • Security (firewalls)

The management of data as it flows through the network is also a role of the intermediary devices. Intermediary devices direct the path of the data but do not generate or change the data content. These devices use the destination host address, in conjunction with information about the network interconnections, to determine the path that messages should take through the network.

Processes running on the intermediary network devices perform these functions:

  • Regenerate and retransmit data signals
  • Maintain information about what pathways exist through the network and internetwork
  • Notify other devices of errors and communication failures
  • Direct data along alternate pathways when there is a link failure
  • Classify and direct messages according to quality of service (QoS) priorities
  • Permit or deny the flow of data, based on security settings

Network Media (

Communication across a network is carried on a medium. The medium provides the channel over which the message travels from source to destination.

Modern networks primarily use three types of media to interconnect devices and to provide the pathway over which data can be transmitted. As shown in Figure 1-7, these media are

  • Metallic wires within cables
  • Glass or plastic fibers (fiber-optic cable)
  • Wireless transmission
Figure 1-7

Figure 1-7 Network Media

The signal encoding that must occur for the message to be transmitted is different for each medium type. On metallic wires, the data is encoded into electrical impulses that match specific patterns. Fiber-optic transmissions rely on pulses of light, within either infrared or visible light ranges. In wireless transmission, patterns of electromagnetic waves depict the various bit values.

Different types of network media have different features and benefits. Not all network media have the same characteristics and are appropriate for the same purpose. The criteria for choosing network media are

  • The distance the medium can successfully carry a signal
  • The environment in which the medium is to be installed
  • The amount of data and the speed at which it must be transmitted
  • The cost of the medium and installation

Network Representations (

When conveying complex information such as displaying all the devices and media in a large internetwork, it is helpful to use visual representations. A diagram provides an easy way to understand the way the devices in a large network are connected. Such a diagram uses symbols to represent the different devices and connections that make up a network. This type of “picture” of a network is known as a topology diagram.

Like any other language, the language of networking uses a common set of symbols to represent the different end devices, network devices, and media, as shown in Figure 1-8. The ability to recognize the logical representations of the physical networking components is critical to being able to visualize the organization and operation of a network. Throughout this course and labs, you will learn both how these devices operate and how to perform basic configuration tasks on these devices.

Figure 1-8

Figure 1-8 Network Symbols

In addition to these representations, specialized terminology is used when discussing how each of these devices and media connect to each other. Important terms to remember are

  • Network interface card (NIC): A NIC, or LAN adapter, provides the physical connection to the network at the PC or other host device. The medium connecting the PC to the networking device plugs directly into the NIC.
  • Physical port: A connector or outlet on a networking device where the medium is connected to a host or other networking device.
  • Interface: Specialized ports on an internetworking device that connect to individual networks. Because routers are used to interconnect networks, the ports on a router are referred to as network interfaces.

Topology Diagrams (

Topology diagrams, as shown in Figure 1-9, are mandatory for anyone working with a network. They provide a visual map of how the network is connected.

Figure 1-9

Figure 1-9 Network Topologies

There are two types of topology diagrams:

  • Physical topology diagrams (Figure 1-9a): Identify the physical location of intermediary devices, configured ports, and cable installation.
  • Logical topology diagrams (Figure 1-9b): Identify devices, ports, and IP addressing scheme.

LANs and WANs (1.2.2)

Network infrastructures can vary greatly in terms of

  • Size of the area covered
  • Number of users connected
  • Number and types of services available

For this reason, networks are often classified into various types based on a number of characteristics.

Types of Networks (

Figure 1-10 illustrates the two most common types of network infrastructures:

  • Local-area network (LAN): A network infrastructure that provides access to users and end devices in a small geographical area.
  • Wide-area network (WAN): A network infrastructure that provides access to other networks over a wide geographical area.
Figure 1-10

Figure 1-10 LANs and WANs

Other types of networks include

  • Metropolitan-area network (MAN): A network infrastructure that spans a physical area larger than a LAN but smaller than a WAN (for example, a city). MANs are typically operated by a single entity such as a large organization.
  • Wireless LAN (WLAN): Similar to a LAN but wirelessly interconnects users and endpoints in a small geographical area.
  • Storage-area network (SAN): A network infrastructure designed to support file servers and provide data storage, retrieval, and replication. It involves high-end servers, multiple disk arrays, and Fibre Channel interconnection technology.

Local-Area Networks (

Local-area networks (LAN) are a network infrastructure that spans a small geographical area. Specific features of LANs include

  • LANs interconnect end devices in a limited area such as a home, school, office building, or campus.
  • A LAN is usually administered by a single organization or individual. The administrative control that governs the security and access control policies are enforced on the network level.
  • LANs provide high-speed bandwidth to internal end devices and intermediary devices.

Wide-Area Networks (

Wide-area networks (WAN) are a network infrastructure that spans a wide geographical area. WANs are typically managed by service providers (SP) or Internet service providers (ISP).

Specific features of WANs include

  • WANs interconnect LANs over wide geographical areas such as between cities, states, provinces, countries, or continents.
  • WANs are usually administered by multiple service providers.
  • WANs typically provide slower-speed links between LANs.

The Internet (1.2.3,

Although there are benefits to using a LAN or WAN, most individuals need to communicate with a resource on another network, outside of the local network within the home, campus, or organization. This is done using the Internet.

As shown in Figure 1-11, the Internet is a worldwide collection of interconnected networks (internetworks or internet for short), cooperating with each other to exchange information using common standards. Through telephone wires, fiber-optic cables, wireless transmissions, and satellite links, Internet users can exchange information in a variety of forms.

Figure 1-11

Figure 1-11 Internet

The Internet is a conglomerate of networks and is not owned by any individual or group. Ensuring effective communication across this diverse infrastructure requires the application of consistent and commonly recognized technologies and standards as well as the cooperation of many network administration agencies. There are organizations that have been developed for the purpose of helping to maintain the structure and standardization of Internet protocols and processes. These organizations include the Internet Engineering Task Force (IETF), the Internet Corporation for Assigned Names and Numbers (ICANN), and the Internet Architecture Board (IAB), plus many others.

Intranet and Extranet (

There are two other terms that are similar to the term Internet:

  • Intranet
  • Extranet

Intranet is a term often used to refer to a private connection of LANs and WANs that belongs to an organization, and is designed to be accessible only by the organization’s members, employees, or others with authorization. Intranets are basically an internet that is usually only accessible from within the organization.

Organizations can publish web pages on an intranet about internal events, health and safety policies, staff newsletters, and staff phone directories. For example, schools can have intranets that include information on class schedules, online curricula, and discussion forums. Intranets usually help eliminate paperwork and speed workflows. The intranet can be accessible to staff working outside of the organization by using secure connections to the internal network.

An organization can use an extranet to provide secure and safe access to individuals who work for a different organization, but require company data. Examples of extranets include

  • A company providing access to outside suppliers/contractors
  • A hospital providing a booking system to doctors so that they can make appointments for their patients
  • A local office of education providing budget and personnel information to the schools in its district. Figure 1-12 shows how intranets, extranets, and the Internet relate.
Figure 1-12

Figure 1-12 Intranets, Extranets, and the Internet

Internet Access Technologies (

There are many different ways to connect users and organizations to the Internet.

Home users, teleworkers (remote workers), and small offices typically require a connection to an Internet service provider (ISP) to access the Internet. Connection options vary greatly between ISP and geographical location. However, popular choices include broadband cable, broadband digital subscriber line (DSL), wireless WANs, and mobile services.

Organizations typically require access to other corporate sites and the Internet. Fast connections are required to support business services, including IP phones, video conferencing, and data center storage.

Business-class interconnections are usually provided by service providers (SP). Popular business-class services include business DSL, leased lines, and Metro Ethernet.

Connecting Remote Users to the Internet (

Figure 1-13 illustrates some common connection options for small office and home office users, which include

  • Cable: Typically offered by cable television service providers, the Internet data signal is carried on the same coaxial cable that delivers cable television. It provides a high-bandwidth, always-on connection to the Internet. A special cable modem separates the Internet data signal from the other signals carried on the cable and provides an Ethernet connection to a host computer or LAN.
  • DSL: Provides a high-bandwidth, always-on connection to the Internet. It requires a special high-speed modem that separates the DSL signal from the telephone signal and provides an Ethernet connection to a host computer or LAN. DSL runs over a telephone line, with the line split into three channels. One channel is used for voice telephone calls. This channel allows an individual to receive phone calls without disconnecting from the Internet. A second channel is a faster download channel, used to receive information from the Internet. The third channel is used for sending or uploading information. This channel might be slower than the download channel. The quality and speed of the DSL connection depends mainly on the quality of the phone line and the distance from your phone company’s central office. The farther you are from the central office, the slower the connection.
  • Cellular: Cellular Internet access uses a cell phone network to connect. Wherever you can get a cellular signal, you can get cellular Internet access. Performance will be limited by the capabilities of the phone and the cell tower to which it is connected. The availability of cellular Internet access is a real benefit in those areas that would otherwise have no Internet connectivity, or for those constantly on the move.
  • Satellite: Satellite service is a good option for homes or offices that do not have access to DSL or cable. Satellite dishes require a clear line of sight to the satellite, so service might be difficult in heavily wooded areas or places with other overhead obstructions. Speeds will vary depending on the contract, though they are generally good. Equipment and installation costs can be high (although check the provider for special deals), with a moderate monthly fee thereafter. The availability of satellite Internet access is a real benefit in those areas that would otherwise have no Internet connectivity.
  • Dialup telephone: An inexpensive option that uses any phone line and a modem. To connect to the ISP, a user calls the ISP access phone number. The low bandwidth provided by a dialup modem connection is usually not sufficient for large data transfer, although it is useful for mobile access while traveling. A modem dialup connection should only be considered when higher-speed connection options are not available.
Figure 1-13

Figure 1-13 Common Internet Connection Options

Many homes and small offices are now being connected directly with fiber-optic cables. This enables an Internet service provider to provide higher bandwidth speeds and support more services such as Internet, phone, and TV.

The choice of connection varies depending on geographical location and service provider availability.

What are your options for connecting to the Internet?

Connecting Businesses to the Internet (

Corporate connection options differ from home user options. Businesses often require higher bandwidth, dedicated bandwidth, and managed services. Connection options available differ depending on the number of service providers located nearby.

Figure 1-14 illustrates common connection options for organizations, which include

  • Dedicated leased line: This is a dedicated connection from the service provider to the customer premises. Leased lines are actually reserved circuits that connect geographically separated offices for private voice and/or data networking. The circuits are typically rented at a monthly or yearly rate, which tends to make them expensive. In North America, common leased line circuits include T1 (1.54 Mbps) and T3 (44.7 Mbps), while in other parts of the world, they are available in E1 (2 Mbps) and E3 (34 Mbps).
  • Metro Ethernet: Metro Ethernet is typically available from a provider to the customer premises over a dedicated copper or fiber connection providing bandwidth speeds of 10 Mbps to 10 Gbps. Ethernet over Copper (EoC) is more economical than fiber-optic Ethernet service in many cases, is widely available, and reaches speeds of up to 40 Mbps. However, Ethernet over Copper is limited by distance. Fiber-optic Ethernet service delivers the fastest connections available at an economical megabit-per-second price. Unfortunately, there are still many areas where this service is unavailable.
  • DSL: Business DSL is available in various formats. A popular choice is symmetric digital subscriber lines (SDSL), which are similar to asymmetric digital subscriber lines (ADSL), but provide the same upload and download speeds. ADSL is designed to deliver bandwidth at different rates downstream than upstream. For example, a customer getting Internet access might have downstream rates that range from 1.5 to 9 Mbps, whereas upstream bandwidth ranges are from 16 to 640 kbps. ADSL transmissions work at distances up to 18,000 feet (5,488 meters) over a single copper twisted pair.
  • Satellite: Satellite service can provide a connection when a wired solution is not available. Satellite dishes require a clear line of sight to the satellite. Equipment and installation costs can be high, with a moderate monthly fee thereafter. Connections tend to be slower and less reliable than its terrestrial competition, which makes it less attractive than other alternatives.

The choice of connection varies depending on geographical location and service provider availability.

Figure 1-14

Figure 1-14 Internet Connectivity Options for Businesses

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