1.1

Chapter 1

Introduction

Copyright © NDSL, Chang Gung University. Permission required for reproduction or display.

長庚大學資訊工程學系 陳仁暉 副教授Tel: (03) 211-8800 Ext: 5990

Email: jhchen@mail.cgu.edu.tw

URL: http://www.csie.cgu.edu.tw/~jhchen

1.2

1-1 DATA COMMUNICATIONS1-1 DATA COMMUNICATIONS

The term The term telecommunicationtelecommunication means communication at a means communication at a distance. The word distance. The word datadata refers to information presented refers to information presented in whatever form is agreed upon by the parties creating in whatever form is agreed upon by the parties creating and using the data. and using the data. Data communicationsData communications are the are the exchange of data between two devices via some form of exchange of data between two devices via some form of transmission medium such as a wire cable. transmission medium such as a wire cable.

ComponentsData RepresentationData Flow

Topics discussed in this section:Topics discussed in this section:

1.3

Data communications

• For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (program).

• Fundamental characteristics influence the effectiveness of a data communications system– Delivery– Accuracy– Timeliness– Jitter

1.4

Components of data communications• Message:

– The information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

• Sender:– The device that sends the data message.– It can be a computer, workstation, telephone handset, video camera, and so

on.

• Receiver:– The device that receives the data message.

• Transmission medium:– The physical path by which a message travels from sender to receiver.– e.g., twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

• Protocol:– A set of rules that govern data communications.– It represents an agreement between the communication devices.

1.5

Figure 1.1 Five components of data communication

1.6

Data representation

• Text– In data communications, text is represented as a bit

pattern, a sequence of bits (0s or 1s).– Different sets of bit patterns have been designed to

represent text symbols. Each set is called a code, and the process of representing symbols is called coding.

• Numbers• Images

– RGB (red, green, blue)– YCM (yellow, cyan, and magenta)

• Audio• Video

1.7

Data flow

• Communication between two devices can be simplex, half-duplex, or full-duplex (as shown in next slide).

• Simplex– The communication is unidirectional, as on a one-way

street.

• Half-duplex– Each station can both transmit and receive, but not at

the same time.

• Full-duplex– Both stations can transmit and receive simultaneously.

1.8

Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

1.9

1-2 NETWORKS1-2 NETWORKS

A A networknetwork is a set of devices (often referred to as is a set of devices (often referred to as nodesnodes) ) connected by communication connected by communication linkslinks. A node can be a . A node can be a computer, printer, or any other device capable of sending computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the and/or receiving data generated by other nodes on the network.network.

Distributed ProcessingNetwork CriteriaPhysical StructuresNetwork ModelsCategories of NetworksInterconnection of Networks: Internetwork

Topics discussed in this section:Topics discussed in this section:

1.10

Distributed processing

• Most networks use distributed processing, in which a task is divided among multiple computers.

• Network Criteria– Performance

• Throughput• Delay

– Reliability• Measured by the frequency of failure• The time it takes a link to recover from a failure• The network robustness in a catastrophe

– Security

1.11

Figure 1.3 Types of connections: point-to-point and multipoint

multidrop

1.12

Physical Topology

• The term physical topology refers to the way in which a network is laid out physically.

• Two or more devices connect to a link; two or more links form a topology.

• The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another.

• Four basic topologies– Mesh– Star– Bus– Ring

1.13

Figure 1.4 Categories of topology

1.14

Mesh Topology

• Several advantages– The use of dedicated links– A mesh topology is robust– The advantage of privacy or security– Point-to-point links make fault identification and fault isolation easy

• Main disadvantages– The amount of cabling and the number of I/O ports required– Because every device must be connected to every other device,

installation and reconnection are difficult.– The sheer bulk of the wiring can be greater than the available space

(in walls, ceilings, or floors) can accommodate.– The hardware required to connect each link (I/O ports and cable)

can be prohibitively expensive.

1.15

Figure 1.5 A fully connected mesh topology (five devices)

1.16

Star Topology

• Deach device has a dedicated point-to-point link only to a central controller, usually called a hub.

• Not similar with mesh topology, if one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device.

• Advantages– A star topology is less expensiveless expensive than a mesh topology.– Easy to install and reconfigure– Far less cabling needs to be housed– RobustnessRobustness: If one link fails, only that link is affected.

• Disadvantage– The dependency of the whole topology on one single point, the hub.

If the hub goes down, the whole system is dead.

1.17

Figure 1.6 A star topology connecting four stations

1.18

Bus Topology

• A bus topology, on the other hand, is multipoint.• One long cable acts as a backbone to link all the devices in

a network.• Advantages

– Ease of installation– Backbone cable can be laid along the most efficient path, then

connected to the nodes by drop lines of various lengths.

• Disadvantages– Difficult reconnection and fault isolation– Signal reflection at the taps can cause degradation in quality– A fault or break in the bus cable stops all transmission, even

between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions.

1.19

Figure 1.7 A bus topology connecting three stations

1.20

Ring Topology

• Relatively easy to install and reconfigure• Each device is linked to only its immediate neighbors (either

physically or logically).• Advantages

– To add or delete a device requires changing only two connections.– Fault isolation is simplified.– A signal is circulating at all times. If one device does not receive a

signal within a specified period, it can issue an alarm.

• Disadvantages– Unidirectional traffic– In a simple ring, a break in the ring (such as a disable station) can

disable the entire network.– This weakness (above) can be solved by using a dual ring or a

switch capable of closing off the break.

1.21

Figure 1.8 A ring topology connecting six stations

1.22

Figure 1.9 A hybrid topology: a star backbone with three bus networks

1.23

Categories of Networks

• Local area network (LAN)– usually privately owned and links the devices in a single

office, building, or campus.– A LAN can be as simple as two PCs and a printer in

someone’s home office.

• Wide area network (WAN)– Long distance transmission of data, image, audio, and

video information over large geographic areas.

• Metropolitan area network (MAN)– A network with a size between a LAN and a WAN

1.24

Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet

1.25

Figure 1.11 WANs: a switched WAN and a point-to-point WAN

1.26

Figure 1.12 A heterogeneous network made of four WANs and two LANs

1.27

1-3 THE INTERNET1-3 THE INTERNET

The The InternetInternet has revolutionized many aspects of our daily has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a way we spend our leisure time. The Internet is a communication system that has brought a wealth of communication system that has brought a wealth of information to our fingertips and organized it for our use. information to our fingertips and organized it for our use.

A Brief HistoryThe Internet Today (ISPs)

Topics discussed in this section:Topics discussed in this section:

1.28

A Brief History

• A network is a group of connected communicating devices such as computers and printers.

• An internet is two or more networks that can communicate with each other.

• The most notable internet (note the lowercase letter i) is called the Internet (uppercase letter I), a collaboration of more than hundreds of thousands of interconnected networks.

• In the mid-1960s, mainframe computers in research organizations were stand alone devices.

• The Advanced Research Projects Agency (ARPA) in the Department of Defense (DoD)

1.29

A Brief History (continued)

• In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA presented its ideas for ARPANET, a small network of connected computers.

• The idea was that each host computer (not necessarily from the same manufacturer) would be attached to specialized computer, called an interface message processor (IMP).

• The IMPs, in turn, would be connected to one another.

• Each IMP had to be able to communicate with other IMPs as well as with its own attached host.

1.30

A Brief History (continued)

• By 1969, ARPANET was a reality. Four nodes, at the U. California at Los Angeles (UCLA), U. California at Santa Barbara (UCSB), Stanford Research Institute (SRI), and the U. of Utah, were connected via the IMPs to form a network.

• Software called the Network Control Protocol (NCP) provided communication between the hosts.

• In 1972, Vint Cerf and Bob Kahn outlined the protocols (TCP) to achieve end-to-end delivery of packets.

• Shortly thereafter, authorities split

TCP into two protocols: Transmission

Control Protocol (TCP) and

Internetworking Protocol (IP).

V. Cerf B. Kahn W. Bush

1.31

The Internet Today

• Internet Service Providers (ISPs)– National ISPs

• Use network access points (NAPs)

– Regional ISPs• Connect to one or more national ISPs• The third level of the hierarchy

– Local ISPs

• Taiwan– National ISPs and Local ISPs only

1.32

Figure 1.13 Hierarchical organization of the Internet

1.33

1-4 PROTOCOLS AND STANDARDS1-4 PROTOCOLS AND STANDARDS

In this section, we define two widely used terms: In this section, we define two widely used terms: protocolsprotocols and and standardsstandards. First, we define protocol, which is . First, we define protocol, which is synonymous with rule. Then we discuss standards, which synonymous with rule. Then we discuss standards, which are agreed-upon rules.are agreed-upon rules.

ProtocolsStandardsStandards OrganizationsInternet Standards

Topics discussed in this section:Topics discussed in this section:

1.34

Protocols

• An entity is anything capable of sending or receiving information.

• A protocol defines what is communicated, how it is communicated, and when it is communicated.

• Syntax– Refers to the structure or format of the data, meaning the

order in which they are presented.

• Semantics– Refers to the meaning of each section of bits.

• Timing– When data should be sent and how fast they can be sent.

1.35

Standards• De facto standards

– de facto means “by fact” or “by convention”

• De jure standards– De jure means “by law” or “by regulation”

• Standard organizations– International Organization for Standardization (ISO)– International Telecommunication Union-Telecommunication standards

sector (ITU-T)• Consultative Committee for International Telegraphy and Telephony (CCITT)

– American National Standards Institute (ANSI)– Institute of Electrical and Electronics Engineers (IEEE)– Electronic Industries Association (EIA)

• Government agencies– Federal Communications Commission (FCC)

• Internet Standards– Internet draft– Request for Comment (RFC)

1.36

Appendix: Analog & Digital. What the difference?

• Analog phone lines, analog signals, digital security. digital PBX, Analog-to-digital adapters, and so on.

• What does it all mean?• In the telecom world, understanding analog versus

digital isn't as simple as comparing one technology to another.

• It depends on what product—and in some cases, which product feature—you happen to be talking about.

1.37

Analog at a Glance• As a technology, analog is the process of taking an audio or

video signal (in most cases, the human voice) and translating it into electronic pulses.

• Digital on the other hand is breaking the signal into a binary format where the audio or video data is represented by a series of "1"s and "0"s.

• Simple enough when it's the device—analog or digital phone, fax, modem, or likewise—that does all the converting for you.

• Is one technology better than the other?– Analog technology has been around for decades. It's not that

complicated a concept and it's fairly inexpensive to use. That's why we can buy a $10 telephone or watch a few TV stations with the use of a well-placed antenna.

– The trouble is, analog signals have size limitations as to how much data they can carry. So with our $10 phones and inexpensive TVs, we only get so much.

1.38

Digital World

• The newer of the two, digital technology breaks your voice (or television) signal into binary code—a series of 1s and 0s—transfers it to the other end where another device (phone, modem or TV) takes all the numbers and reassembles them into the original signal.

• The beauty of digital is that it knows what it should be when it reaches the end of the transmission.

• That way, it can correct any errors that may have occurred in the data transfer.

• What does all that mean to you?– Clarity. In most cases, you'll get distortion-free

conversations and clearer TV pictures.

1.39

Phone Line

• Analog lines, also referred to as POTS (Plain Old Telephone Service), support standard phones, fax machines, and modems. These are the lines typically found in your home or small office. Digital lines are found in large, corporate phone systems.

• How do you tell if the phone line is analog or digital?– Look at the back of the telephone connected to it. Look

at the phone's dialpad. Are there multiple function keys? Do you need to dial "9" for an outside line? These are indicators that the phone and the line are digital. Otherwise, it is analog.