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Data Communications and the Internet

Chapter 6

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Learning Objectives

Know basic telecommunications terminology. Know the definition and characteristics of LAN, WANs, and

internets. Understand the nature of processing in a layered communications

protocol. Know the purpose of the five layers of the TCP/IP-OSI protocol. Understand Ethernet and wireless LANs. Understand the characteristics of WANs using personal computers

with modems to the Internet, networks of leased lines, PSDNs and virtual private networks.

Know basic concepts involved in the operation of the Internet.

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What Is Telecommunications?

Telecommunications is any "...process that permits the passage from a sender to one or more receivers of information of any nature delivered in any usable form (printed copy, fixed or moving pictures, visible or audible signals, etc.) by means of any electromagnetic system (electrical transmission by wire, radio, optical transmission, guided waves, etc.)." [Martin, James. Introduction to Teleprocessing. Englewood cliffs, NJ:Prentice-hall, 1972]

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What Is Data Communications? Data communications "... can be defined as that part of

telecommunications that relates to computer systems, or the electronic transmission of computer data. This definition excludes the transmission of data to local peripherals such as disk, tape, and printers."[Stamper, David A. Business Data Communications. Redwood city, CA: Benjamin/Cummings, 1989]

Data communications is sometimes called networking because it involves the transmission of data over a network.

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What are the Electromagnetic Waves? The motion of electrically charged particles produces electromagnetic

waves. These waves are also called "electromagnetic radiation" because they radiate from the electrically charged particles.

Radio waves, microwaves, visible light, and x rays are all examples of electromagnetic waves that differ from each other in wave length.

Electromagnetic waves need no material medium for transmission. Light and radio waves can travel through interplanetary and interstellar space from the sun and stars to the earth. Regardless of their frequency and wavelength, electromagnetic waves travel at a speed of 299,792 km (186,282 mi) per second in a vacuum. [Berkeley National Lab]

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What is the Electromagnetic Spectrum? The "electromagnetic spectrum" is a term used to describe

the entire range of frequencies of electromagnetic radiation from zero to infinity.

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What is Electromagnetic Signal Frequency?

Frequency refers to the number of times a current (electromagnetic wave) passes through a complete cycle. The measure of frequency is a Hertz (Hz), which represents one cycle per second.

Frequencies are represented from a number from 0 Hertz to 300 GHz (called the electromagnetic spectrum).

K=Kilo=1,000, M=Mega=1,000,000, G=Giga=1,000,000,000

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What is a Communication Line Bandwidth? Communication line bandwidth is the difference between

the minimum and maximum range of frequencies allowed by the communication line.

Bandwidth = higher frequency - lower frequency. For example: A wire transmits in a frequency range from 100 Hz to 2,500 Hz. Its bandwidth is 2,400 Hz.

Bandwidth is important because it indicates how fast data can be transmitted over a specific channel. A wider bandwidth allows faster transmission speeds.

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Digital Versus Analog Data The world we live is an analog world. Light, voice, video are

examples of analog data because the signal comes in a continuous form. A light bulb, for example, emits a steady stream of light.

Analog data refers to physical quantities, which in data communications take the form of voltage and variations in the properties of waves [Stamper]

Digital data is composed of items that are distinct from one another. That means that the items are discrete. Digital computers manipulate this type of data by converting it to binary data (1 or 0).

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Components of Communication Networks In order for communication to occur, there must be a

source, a medium, a receiver and a message. The source (sender) is the device that sends the message

(transmitter) and is any device that can be connected to the network like a PC or a telephone.

The medium connects the source with the receiver and can be a copper cable, a fiber-optic cable, airwaves or another physical path.

The receiver is the device that accepts the message. The message can be a file, a request, a response, a status

message, a control message or correspondence. The message must be understandable.

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Communications Protocols A protocol is a standard means for coordinating an activity

between two or more entities. A communications protocol is a means for coordinating

activity between two or more communicating computers. Two machines must agree on the protocol to use, and they

must follow that protocol as they send messages back and forth.

Communications protocols are broken into levels of layers.

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Fundamental Networking Concepts

A computer network is a collection of computers that communicate with one another over transmission lines.

Three basic types of networks are:– Local area networks (LANs)–connects computers that reside in a single

geographic location on the premises of the company that operates the LAN.

– Wide area networks (WANs)–connects computers at different geographic sites.

– Internets–a network of networks

The networks that comprise an Internet use a large variety of communication methods and conventions, and data must flow seamlessly across them.

To provide seamless flow, an elaborate scheme called a layered protocol is used.

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Local Area Networks A local area network (LAN) is a group of computers connected together

on a single company site. Usually the computers are located within a half mile or so of each other,

although longer distances are possible.– The key distinction, however, is that all of the computers are located on

property controlled by the company that operates the LAN. Computers and printers are connected via a switch, which is a special-

purpose computer that receives and transmits messages on the LAN.

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NIC Interface Card Each device on a LAN (computer, printer, etc.) has a

hardware component called a network interface card (NIC) that connects the device’s circuitry to the cable.– The NIC works with programs in each device to implement Layer

1 and Layer 2 protocols.

– Each NIC has a unique identifier, which is called the (MAC) media access control address.

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Contacted and Radiated Media A Communication network cannot exist without a medium

to connect the source and receiver. If this medium can be seen physically, it is considered a contacted medium.

Radiated media, or wireless media, do not use physical wires to transmit data. With radiated media, the signal is radiated through the air, water and vacuum of space.

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Communications Media The computers, printers, switches, and other devices on a LAN are

connected using one of two media.– Most connections are made using unshielded, twisted pair (UTP) cable.

– A device called an RJ-45 connector is used to connect the UTP cable into NIC devices on the LAN.

The connection between switches can use UTP cable, but if they carry a lot of traffic or are far apart UTP cable may be replaced by optical fiber cables.

– The signals on such cables are light rays, and they are reflected inside the glass core of the optical fiber cable.

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LANs with Wireless Connections

Wireless connections have become popular with LANs. The NIC for wireless devices have been replaced by wireless NIC (WNIC)

– For laptop computers, such devices can be cards that slide into the PCMA slot or they can be built-in, onboard devices.

Several different wireless standard exist– As of 2005, the most popular is IEEE 802.11g– The current standard, 802.11g allows speeds of up to 54 Mbps.– The WNICs operate according to the 802.11 protocol and connect to an

access point (AP).

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IEEE 802.3 or Ethernet Protocol The committee that addresses LAN standards is called the IEEE 802

Committee.– Thus, IEEE LAN protocols always start with the number 802.

Today, the world’s most popular protocol for LAN is the IEEE 802.3 protocol.– This protocol standard, also called Ethernet, specifies hardware

characteristics such as which wire carries which signals. – It also describes how messages are to be packaged and processed for

transmission over the LAN.

Most personal computers today are equipped with an onboard NIC that supports what is called 10/100/1000 Ethernet.– These products conform to the 802.3 specification and allow for

transmission at a rate of 10, 100, or 1,000 Mbps.

– Communications speeds are expressed in bits, whereas memory sizes are expressed in bytes.

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Wide Area Networks

A wide area network (WAN) connects computers located at physically separated sites.– A company with offices in Detroit and Atlanta must use a WAN to connect

the computers together.– Because the sites are physically separated, the company cannot string wire

from one site to another.

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Networks of Leased Lines

A WAN connects computers located at geographically distributed company sites.

The lines that connect these sites are leased from telecommunication companies that are licensed to provide them.

A variety of access devices connect each site to the transmission.– These devices are typically special-purposed computers.– The particular devices required depend on the line used and other factors– Sometimes switches and routers are employed.

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Transmission Line Types, Uses, and Speeds

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What is the Internet The Internet is a collection of interconnected networks, all

freely exchanging information. “The Internet is the most famous computer network that’s

ever build. It’s actually a network of networks: tens of thousands of computers connected in a web, talking to one another through a common communications protocol."[PC Magazine]

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Connecting the Personal Computer to an ISP An Internet service provider (ISP) has three important functions:

– It provides you with a legitimate Internet address.– It serves as your gateway to the Internet.– It receives the communication from your computer and passes them on to the Internet,

and it receives communication from the Internet and passes them on to you. Home computers and those of small businesses are commonly connected to an

ISP in one of three ways:– Using a regular telephone line– Using a special telephone line called a DSL line– Using cable TV line

All three ways require that the digital data in the computer be converted to an analog, or wavy, signal.

A device called a modem, or modulator/demodulator performs this conversion.

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Dial-Up Modems

A dial-up modem performs the conversion between analog and digital in such a way that the signal can be carried on a regular telephone line.– You dial the number for your ISP and connect.

– The maximum transmission speed for a switch is 56 kbps. The way messages are packaged and handled between your modem and

the ISP is governed by a protocol known as Point-to-Point Protocol (PPP).– This protocol is used for networks that involve just two computers.

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DSL Modems

A DSL modem is the second modem type. DSL stands for digital subscriber line. DSL modems operate on the same lines as voice telephones and dial-up

modems.– They operate so that their signals do not interfere with voice telephone

service.

– They provide much faster data transmission speeds than dial up modems.

– They always maintain a connection. DSL data transmission and telephone conversations can occur

simultaneously. DSL lines that have different upload and download speeds are called

asymmetric digital subscriber lines (ADSL). Symmetrical digital subscriber lines (SDSL) offers the same speed in

both directions.

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Cable Modems

A cable modem is the third modem type. Cable modems provide high-speed data transmission using cable

television lines. At the maximum, users can download data up to 10 Mbps and can

upload data at 256 kbps. Narrowband lines typically have transmission speeds less than 56

kbps. Broadband lines have speeds in excess of 256 kbps.

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IP Addressing Schemes Two IP addressing schemes exist: IPv4 and IPv6.IPv4

– IPv4 constructs addresses having 32 bits.– These bits are divided into four groups of 8 bits, and a decimal number

represents each group.– IPv4 addresses appear as 63.224.57.59.– The largest decimal number that can appear between the period are 255 (0 –

255).IPv6

– Due to growth of the Internet IPv6 was developed.– IPv6 construct addresses having 128 bits.– Currently both IPv4 and IPv6 are used on the Internet.

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Using TCP/IP-OSI Protocols over the Internet Network Address Translation

For Internet traffic, only public IP addresses can be used.– These addresses are assigned in blocks to large companies and organizations

like ISPs.

All Internet traffic aimed at any computer within an organization's LAN will be sent over the Internet using the router’s IP public address for the given computer.– The router will receive all packets for all computers for the organization’s

computers.– When the router receives a packet, it determines the internal IP address

within the LAN for that computer.– It then changes the address in the packet from the router’s IP public address

to the internal IP address of a computer in the organization's LAN, the packet’s true destination.

The process of changing public IP addresses into private IP addresses, and the reverse, is called Network Address Translation (NAT).

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Network Addresses: MAC and IP On most networks, and on every internet, two address schemes identify

computers and other devices.– Programs that implement Layer 2 protocols use physical addresses, or MAC addresses.– Programs that implement Layer 3, 4, and 5 protocols use logical addresses, or IP

addresses.

Physical Addresses (MAC Addresses)– Each NIC is given an address at the factory that is the device’s physical address or

MAC address.– By agreement among computer manufacturers, such addresses are assigned in such a

way that no two NIC devices will ever have the same MAC address.– Physical addresses are only known, shared, and used within a particular network or

network segment.

Logical Address (IP Addresses)– Internets, including the Internet, and many private networks use logical addresses,

which are also called IP addresses, for example, 192.168.2.28

Public Versus Private Addresses– Public IP addresses are used on the Internet and assigned to major institutions in

blocks by the Internet Corporation for Assigned Names and Numbers (ICANN).– Each IP address is unique across all computers on the Internet.– Private IP addresses are used within private networks and internets. They are

controlled only by the company that operates the private network or internet

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Dynamic Host Configuration Protocol The DHCP server is a computer or router that hosts a program called

Dynamic Host Configuration Protocol (DHCP). When the program finds such a device, your computer will request a

temporary IP address from the DNCP server which is loaned to you while you are connected to the LAN.

When you disconnect, that IP address becomes available, and the DHCP server will reuse it when needed.

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The TCP/IP-OSI Architecture

The International Organization for Standardization (ISO) developed the Reference Model for Open Systems Interconnection (OSI), an architecture that has seven layers.

The Internet Engineering Task Force (IETP), developed a four-layer scheme called the TCP/IP (Transmission Control Program Internet Protocol) architecture.

The most commonly used architecture today is a five-layer blend of these two architectures called the TCP/IP-OSI architecture.

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Layer 5 Layer 5 generates and receives email (and attachments like photos) according to one of the

standard email protocols generated for layer 5– Most likely its Simple Mail Transfer (SMTP)– Hypertext Transfer Protocol (HTTP) is used for the processing of Web pages.

The Web and the Internet are not the same thing.– The Web, which is a subset of the Internet, consists of sites and users that process the HTTP protocol.– The Internet is the communications structure that supports all application-layer protocols, including

HTTP, SMTP, and other protocols. FTP, or the File Transfer Protocol is another application layer protocol.

– You can use FTP to copy files from one computer to another. An architecture is an arrangement of protocol layers in which each layer is given specific tasks to

accomplish. At each level of the architecture, there are one or more protocols.

– Each protocol is a set of rules that accomplish the tasks assigned to its layer.

A program is a specific computer product that implements a protocol. Programs that implement the HTTP protocol of the TCP/IP-OSI architecture are

called browsers. Two common browsers are Netscape and Microsoft Internet Explorer.

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Layer 4

An email program (which uses SMTP) interacts with another protocol called TCP, or Transmission Control Program (TCP).

TCP operates at layer 4 of the TCP/IP-OSI architecture. The TCP program examines your data (files, email, pictures, etc.) and

breaks these lengthy messages into pieces called segments. TCP places identifying data in front of each segment that are akin to the

To and From addresses that you would put on a letter for the postal mail.

TCP programs also provide reliability. If data is sent from a Dell computer to a Macintosh computer, TCP

program translates the segments from Windows (Dell) to Macintosh format, reassembles the segments into a coherent whole, and makes that assembly available to Macintosh computer.

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Layer 3

TCP/IP interacts with protocols that operate at Level 3, the next layer down.

For the TCP/IP architecture, the layer-3 protocol is the Internet Protocol (IP).

The chief purpose of IP is to route messages across an internet. Routers are special-purpose computers that implements the IP protocol.

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Layers 1 and 2 Basic computer connectivity is accomplished using Layers 1 and 2 of

the TCP/IP-OSI architecture. Computing devices called switches facilitate data communication. A program implementing a Layer-2 protocol will package each of your

packets into frames, which are the containers used at Layers 1 and 2 (Segments go into packets and packets go into frames).

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Switches and Routers Switches work with frames at Layer 2.

– They send frames from switch to switch until they arrive at their destination.

– They use MAC addresses.

– All switches have a table of data called a switch table. Routers work with packets at Layer 3.

– They send packets from router to router until they arrive at their destination.

– They use IP addresses.

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Domain Name System IP addresses are useful for computer-to-computer communication, but

they are not well suited for human use. The purpose of the domain name system (DNS) is to convert user-

friendly names into their IP addresses. Any registered, valid name is called a domain name. The process of changing a name into its IP address is called resolving

the domain name. Every domain name must be unique, worldwide. To ensure duplicate domain names do not occur, an agency registers

names and records the corresponding IP addresses in a global directory.

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Domain Name Registration

ICANN is a nonprofit organization that is responsible for administering the registration of domain names.

ICANN does not register domain names itself; instead it licenses other organizations to register names.

ICANN is also responsible for managing the domain name resolution system.

The last letter in any domain name is referred to as the top-level-domain (TLD).– In the domain www.icann.org the top level domain is .org

A uniform resource locator (URL) is a document’s address on the Web. URLs begin with a domain and then are followed by optional data that

locates a document with that domain.– Thus, in the URL www.prenhall.com/kroenke , the domain name is

www.prenhall.com , and /kroenke is a directory within that domain.

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Domain Name Resolution Domain name resolution is the process of converting a domain name

into a public IP address. The process starts from the TLD and works to the left across the URL. As of 2005, ICANN manages 13 special computers called root servers

that are distributed around the world. Each root server maintains a list of IP addresses of servers that each

resolve each type of TLD. Domain name resolution proceeds quickly because there are thousands

of computers called domain name resolvers that store the correspondence of domain names and IP addresses– These resolvers reside at ISPs, academic institutions, large companies,

government organizations, etc.– For example, if a domain name solver is on your campus and whenever

anyone on your campus needs to resolve a domain name, that resolver will store, or cache, the domain name and IP address on a local file.

• When someone else on the campus needs to resolve the same domain name, the resolver can supply the IP address from the local file.

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Top-Level Domains, 2005

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Virtual Private Network Virtual private network (VPN) is the fourth WAN alternative. A VPN uses the Internet or a private internet to create the appearance of

private point-to-point connections. A VPN uses the public Internet to create the appearance of a private

connection. A connection called a tunnel, is a virtual pathway over a public or

shared network from the VPN client to the VPN server. VPN communications are secure.

– The VPN client software encrypts, or codes, the original messages so that its contents are hidden.

Virtual private networks offer the benefit of point-to-point leased lines, and they enable remote access, both by employees and by any others who have been registered with the VPN server.

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Wide Area Network Using VPN

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Criteria for Comparing Network Alternatives Many different computer networking alternatives are available, each with

different characteristics. There are three types of costs that need to be considered.

– Setup costs include the costs of acquiring transmission lines and necessary equipment, such as switches, routers, and access devices.

– Operational costs include lease fees for lines and equipment, charges of the ISP, the cost of ongoing training, etc.

– Maintenance costs include those for periodic maintenance, problem diagnosis and repair, and mandatory upgrades.

There are six considerations with regard to performance:– Speed– Latency– Availability– Loss rate– Transparency– Performance guarantees

Other criteria to consider when comparing network alternatives include the growth potential (greater capacity) and the length of contract commitment.

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Security Guide–Encryption Encryption is the process of transforming clear text into coded, unintelligible

text for secure storage or communication. Considerable research has gone into developing encryption algorithms that are

difficult to break. A key is a number used to encrypt data.

– The encryption algorithm applies the key to the original message to produce the coded message.

– Decoding (decrypting) a message is similar; a key is applied to code the message to recover the original text.

In symmetric encryption, the same key is used to encode and decode. In asymmetric encryption, different keys are used;

– One key encodes the message, and the other key decodes the message. A special version of asymmetric encryption, public key/private key, is popular

on the Internet. With this method, each site has a public key for encoding messages and a private key for decoding them.

Most secure communication over the Internet uses a protocol called HTTPS. With HTTPS, data are encrypted using a protocol called the Secure Socket

Layer/Transport Layer Security (SSL/TLS). SSL uses a combination of public key/private key and symmetric encryption.

The use of SSL/TLS makes it safe to send sensitive data like credit card numbers and bank balances. Just be certain that you see https//: in your browser and not just http://.

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Ethics Guide–Personal Email at Work

You send a personal email to your friend at his job that contains both

text and a picture (6.2 megabytes in size). This email during its transmission from you to your friend at his job,

consumes his company’s entire computing infrastructure. Finally, if your friend reads his email during his working hours, he will

be consuming company resources, his time and attention, which the company has paid while he is at work.