Chapter 1
Introduction
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
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Copyright © NDSL, Chang Gung University. Permission required for reproduction or display.
1-1 DATA COMMUNICATIONS
The term telecommunication means communication at a
distance. The word data refers to information presented
in whatever form is agreed upon by the parties creating
and using the data. Data communications are the
exchange of data between two devices via some form of
transmission medium such as a wire cable.
Topics discussed in this section:
Components
Data Representation
Data Flow
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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
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Delivery
Accuracy
Timeliness
Jitter
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.
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Figure 1.1 Five components of data communication
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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
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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.
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Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)
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1-2 NETWORKS
A network is a set of devices (often referred to as nodes)
connected by communication links. A node can be a
computer, printer, or any other device capable of sending
and/or receiving data generated by other nodes on the
network.
Topics discussed in this section:
Distributed Processing
Network Criteria
Physical Structures
Network Models
Categories of Networks
Interconnection of Networks: Internetwork
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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
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Figure 1.3 Types of connections: point-to-point and multipoint
multidrop
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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
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Mesh
Star
Bus
Ring
Figure 1.4 Categories of topology
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Mesh Topology
• Several advantages
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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.
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Figure 1.5 A fully connected mesh topology (five devices)
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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
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A star topology is less expensive than a mesh topology.
Easy to install and reconfigure
Far less cabling needs to be housed
Robustness: 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.
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Figure 1.6 A star topology connecting four stations
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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.
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Figure 1.7 A bus topology connecting three stations
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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.
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Figure 1.8 A ring topology connecting six stations
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Figure 1.9 A hybrid topology: a star backbone with three bus networks
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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
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Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet
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Figure 1.11 WANs: a switched WAN and a point-to-point WAN
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Figure 1.12 A heterogeneous network made of four WANs and two LANs
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1-3 THE INTERNET
The Internet has revolutionized many aspects of our daily
lives. It has affected the way we do business as well as the
way we spend our leisure time. The Internet is a
communication system that has brought a wealth of
information to our fingertips and organized it for our use.
Topics discussed in this section:
A Brief History
The Internet Today (ISPs)
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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)
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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.
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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
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B. Kahn
W. Bush
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
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Figure 1.13 Hierarchical organization of the Internet
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1-4 PROTOCOLS AND STANDARDS
In this section, we define two widely used terms: protocols
and standards. First, we define protocol, which is
synonymous with rule. Then we discuss standards, which
are agreed-upon rules.
Topics discussed in this section:
Protocols
Standards
Standards Organizations
Internet Standards
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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.
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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)
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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.
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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.
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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.
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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.
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