Data Communications and the Internet
Chapter 6
1
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.
2
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]
3
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.
4
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]
5
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.
6
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
7
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.
8
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).
9
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.
10
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.
11
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.
12
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.
13
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.
14
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.
15
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.
16
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).
17
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.
18
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.
19
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.
20
Transmission Line Types, Uses, and Speeds
21
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]
22
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.
23
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.
24
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.
25
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.
26
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.
27
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).
28
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
29
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.
30
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.
31
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.
32
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.
33
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.
34
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).
35
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.
36
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 userfriendly 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.
37
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-leveldomain (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.
38
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.
39
Top-Level Domains, 2005
40
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.
41
Wide Area Network Using VPN
42
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.
43
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://.
44
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.
45