Transcript Topics discussed in this section

Chapter 1
Introduction
1.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter Outline
This chapter addresses four issues:
1.
2.
3.
4.
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Data Communications
Networks
Internet
Protocols and Standards
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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1-1 DATA COMMUNICATIONS
The effectiveness of data communication system depends on four
fundamental characteristics: delivery, accuracy, timeliness, and jitter.
1. Delivery: The system must deliver data to the correct destination. Data must be
received by the intended device or user and only by that device or user.
2. Accuracy: The system must deliver the data accurately. Data that have been
altered in transmission and left uncorrected are unusable.
3. Timeliness: The system must deliver data in a timely manner. Data delivered
late are useless. In the case of video and audio, timely delivery means delivering
data as they are produced, in the same order that they are produced, and without
significant delay. This kind of delivery is called real-time transmission.
4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven
delay in the delivery of audio or video packets.
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Figure 1.1 Five components of data communication
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Data Communication Components
1. Message: The message is the information (data) to be communicated.
2. Sender: The sender is the device that sends the data message.
3. Receiver: The receiver is the device that receives the message.
4. Transmission medium: The transmission medium is the physical path by
which a message travels from sender to receiver. Some examples of
transmission media include twisted-pair wire, coaxial cable, fiber-optic
cable, and radio waves.
5. Protocol: A protocol is a set of rules that govern data communications. It
represents an agreement between the communicating devices.
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Data Representation
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Text
Numbers
Images
Audio
Video
Data Representation - Text
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Data Communications use bit patters to
represent text.
Bit Pattern – sequence of bits (1s and 0s)
Different sets of bit patterns represent the text
symbols.
Each set is called a “code”.
Unicode
ASCII
Data Representation - Numbers
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Also represented by bit patterns.
Numbers are converted to binary bit streams.
Data Representation - Images
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Represented by bit patterns.
Image is composed of a matrix of pixels (pixel
being 1 dot)
Size of the pixel depends on the resolution
The more pixels the better the resolution
Data Representation - Audio
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Audio is different than text, numbers or images.
It is continuous, not discrete
In chapters 4 and 5 we will discuss how we can
represent sound as a data communications
signal
Data Representation - Video
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Video can be produced as a continuous entity or
can be a combination of images arranged to
convey the idea of motion
Chapter 4 and 5.
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)
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Data Flow
Communication between two devices can be simplex, half-duplex, fullduplex:
1. Simplex: In simplex mode, the communication is unidirectional, as on a
one-way street. Only one of the two devices on a link can transmit; the
other can only receive.
2. Half-duplex: In half-duplex mode, each station can both transmit and
receive, but not at the same time. When one device is sending, the other
can only receive, and vice versa. Walkie-talkies and CB (citizens band)
radios are both half-duplex systems.
3. Full-duplex: In full-duplex mode (also called duplex), both stations can
transmit and receive simultaneously.
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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
Distributed Processing: Most networks use distributed processing,
in which a task is divided among multiple computers. Instead of one single
large machine being responsible for all aspects of a process, separate
computers (usually a personal computer or workstation) handle a subset.
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Network
 Set of devices (Nodes) connected by media link
 Use distributed processing(task divided between
multiple computers)
 Advantages:
 Security/encapsulation
 Distributed database
 Faster problem solving
 Security through redundancy
 Collaborative processing
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Network Criteria
Network Criteria: A network must be able to meet a certain number
of criteria. The most important of these are performance, reliability, and
security.
 Performance: Performance can be measured in many ways, including
transit time and response time. Performance is often evaluated by two
networking metrics: throughput and delay. We often need more
throughput and less delay.
 Reliability: In addition to accuracy of delivery, network reliability is
measured by the frequency of failure, the time it takes a link to recover
from a failure, and the network's robustness in a catastrophe.
 Security: Network security issues include protecting data from
unauthorized access, protecting data from damage and development,
and implementing policies and procedures for recovery from breaches
and data losses.
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Physical Structure
Type of Connection: A network is two or more devices connected
through links. A link is a communications pathway that transfers data from
one device to another. There are two possible types of connections: pointto-point and multipoint.
Point-to-Point: A point-to-point connection provides a dedicated link
between two devices. The entire capacity of the link is reserved for
transmission between those two devices. Most point-to-point connections
use an actual length of wire or cable to connect the two ends, but other
options, such as microwave or satellite links, are also possible).
Multipoint: A multipoint (also called multidrop) connection is one in which
more than two specific devices share a single link. In a multipoint
environment, the capacity of the channel is shared, either spatially
or temporally.
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Figure 1.3 Types of connections: point-to-point and multipoint
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Physical Structure
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. There are four basic
topologies possible: mesh, star, bus, and ring.
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Physical Structure
Mesh Topology : In a mesh topology, every device has a dedicated pointto-point link to every other device. The term dedicated means that the link
carries traffic only between the two devices it connects. We need n(n - 1)
physical links. However, if each physical link allows communication in both
directions (duplex mode), then we need n(n-1)/2.
 Advantages:
 A mesh offers several advantages over other network topologies. First, the
use of dedicated links guarantees that each connection can carry its own
data load, thus eliminating the traffic problems that can occur when links
must be shared by multiple devices.
 A mesh topology is robust (strong).
 Third, there is the advantage of privacy or security. When every message
travels along a dedicated line, only the intended recipient sees it.
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Physical Structure
 Disadvantages:
 The main disadvantages of a mesh are related to the amount of cabling and
the number of I/O ports required.
 First, because every device must be connected to every other device,
installation and reconnection are difficult.
 Second, the sheer bulk of the wiring can be greater than the available
space (in walls, ceilings, or floors) can accommodate.
 Finally, 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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Physical Structure
Star Topology: In a star topology, each device has a dedicated point-topoint link only to a central controller, usually called a hub. The devices are
not directly linked to one another. Unlike a mesh topology, a star topology
does not allow direct traffic between devices. Used in LANs.
 Advantages:
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A star topology is less expensive than a mesh topology.
In a star, each device needs only one link and one I/O port to connect it to
any number of others.
Other advantages include robustness. If one link fails, only that link is
affected.
 Disadvantages:
 One big disadvantage of a star topology is the dependency of the whole
topology on one single point, the hub. If the hub goes down, the whole
system is dead.
 Although a star requires far less cable than a mesh, each node must be
linked to a central hub. For this reason, often more cabling is required in a
star than in some other topologies (such as ring or bus).
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Figure 1.6 A star topology connecting four stations
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Physical Structure
BusTopology: A bus topology, on the other hand, is multipoint. One
long cable acts as a backbone to link all the devices in a network. Nodes
are connected to the bus cable by drop lines and taps. A drop line is a
connection running between the device and the main cable. A tap is a
connector that either splices into the main cable or punctures the
sheathing of a cable to create a contact with the metallic core. Ethernet
LANs can use a bus topology.
 Advantages:
 Advantages of a bus topology include ease of installation.
 A bus uses less cabling than mesh or star topologies.
 Disadvantages:
 Disadvantages include difficult reconnection and fault isolation.
 a fault or break in the bus cable stops all transmission, even between
devices on the same side of the problem.
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Figure 1.7 A bus topology connecting three stations
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Physical Structure
Ring Topology: In a ring topology, each device has a dedicated point-topoint connection with only the two devices on either side of it. A signal is
passed along the ring in one direction, from device to device, until it
reaches its destination. Each device in the ring incorporates a repeater.
When a device receives a signal intended for another device, its repeater
regenerates the bits and passes them along.
 Advantages:
 Easy to install and reconfigure.
 If one device does not receive a signal within a specified period, it can
issue an alarm. The alarm alerts the network operator to the problem and
its location.
 Disadvantages:
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Unidirectional traffic can be a disadvantage. In a simple ring, a break in
the ring (such as a disabled station) can disable the entire network.
Figure 1.8 A ring topology connecting six stations
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Physical Structure
Hybrid Topology: A network can be hybrid. For example, we can have a
main star topology with each branch connecting several stations in a bus
topology.
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Network Models
 Standards are needed so that these heterogeneous networks
can communicate with one another.
 The two best-known standards are the OSI model and the
Internet model. In Chapter 2 we discuss these two models.
The OSI (Open Systems Interconnection) model defines a
seven-layer network; the Internet model defines a five-layer
network.
 Categories of Networks: The category into which a network falls is
determined by its size. A LAN normally covers an area less than 2 mi; a
WAN can be worldwide.
 A Local Area Network (LAN): is usually privately owned and links the
devices in a single office, building, or campus. The most common LAN
topologies are bus, ring, and star. Today, speeds are normally 100 or 1000
Mbps.
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Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet
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Network Models
 A Wide Area Network (WAN): provides long-distance transmission of data,
image, audio, and video information over large geographic areas that may
comprise a country, a continent, or even the whole world. The switched
WAN connects the end systems, which usually comprise a router
(internetworking connecting device) that connects to another LAN or WAN.
 A good example of a switched WAN is the asynchronous transfer mode
(ATM) network, which is a network with fixed-size data unit packets called
cells.
 A Metropolitan Area Network (MAN): is a network with a size between a
LAN and a WAN. It normally covers the area inside a town or a city.
 It is designed for customers who need a high-speed connectivity, normally to
the Internet, and have endpoints spread over a city or part of city. A good
example of a MAN is the part of the telephone company network that can
provide a high-speed DSL line to the customer.
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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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Metropolitan Area Networks- MAN
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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
 An internet (note the lowercase letter i) is two or more networks that can
communicate with each other. The most notable internet is called the
Internet (uppercase letter I), a collaboration of more than hundreds or
thousands of interconnected networks.
 The Advanced Research Projects Agency (ARPA) in the Department of
Defense (DoD) was interested in finding a way to connect computers so
that the researchers they funded could share their findings.
 In 1967, at an Association for Computing Machinery (ACM) meeting,
ARPA presented its ideas for ARPANET, a small network of connected
computers.
 By 1969, ARPANET was a reality. Four nodes, at the University of
California at Los Angeles (UCLA)
 In 1973 authorities made decision to split TCP in two protocols
Transmission Control Protocol (TCP) and Internetworking Protocol (IP).
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The Internet Today
 Today most end users who want Internet connection use the services of
Internet Service Providers (lSPs). The Internet today is run by private
companies, not the government.
 The national Internet Service Provider are backbone networks created
and maintained by specialized companies. There are many national
ISPs operating in North America; some of the most well known are
SprintLink, PSINet, UUNet Technology, AGIS, and internet Mel.
 To provide connectivity between the end users, these backbone
networks are connected by complex switching stations (normally run by
a third party) called Network Access Points (NAPs).
 Regional Internet Service Provider or regional ISPs are smaller ISPs
that are connected to one or more national ISPs.
 Local Internet Service Provider provide direct service to the end users.
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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 & Standards
 An entity is anything capable of sending or receiving information. For
communication to occur, the entities must agree on a protocol.
 A protocol is a set of rules that govern data communications. A protocol
defines what is communicated, how it is communicated, and when it is
communicated.
 The key elements of a protocol are syntax, semantics, and timing.
 Syntax: The term syntax refers to the structure or format of the data,
meaning the order in which they are presented.
 Semantics: The word semantics refers to the meaning of each section of
bits.
 Timing: The term timing refers to two characteristics: when data should be
sent and how fast they can be sent.
 Standards provide guidelines to manufacturers, vendors, government
agencies, and other service providers
 de facto [by fact] / [by convention]
 de jure [by law].
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Standards
 Standards Organizations: Standards are developed through the
cooperation of standards creation committees, forums, and government
regulatory agencies.
 Standards Creation Committees: International Organization for
Standardization (ISO), International Telecommunication Union –
Telecommunication Standard Sector (ITU-T), America National Standards,
Institute (ANSI), Institute of electrical and Electronics Engineers (IEEE), and
Electronic Industries Association (EIA).
 Forums: The forums work with universities and users to test, evaluate, and
standardize new technologies.
 Regulatory Agencies: All communications technology is subject to
regulation by government agencies such as the Federal Communications
Commission (FCC) in the United States.
 Internet standard is a thoroughly tested specification that is useful to and
adhered to by those who work with the Internet. It is a formalized
regulation that must be followed.
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