PART 1 Overview 1
Chapter 1 Introduction
1.1 DATA COMMUNICATIONS
ComponentsData Representation
DataFlow
1.2 NETWORKS
Distributed ProcessingNetwork Criteria
Physical Structures
Network Models
Categories of Networks
Interconnection of Networks: Internetwork IS
1.3 THE INTERNET
A Brief HistoryThe Internet Today
1.4 PROTOCOLS AND STANDARDS
ProtocolsStandards
Standards Organizations
Internet Standards
1.5 RECOMMENDED READING
BooksSites
RFCs
1.6 KEY TERMS
1.7 SUMMARY
1.8 PRACTICE SET
Review QuestionsExercises
Research Activities
PART 1
Objectives
Part 1 provides a general idea of what we will see in the rest of the book. Four major
concepts are discussed: data communications, networking, protocols and standards,
and networking models.
Networks exist so that data may be sent from one place to another-the basic concept of data communications. To fully grasp this subject, we must understand the data communication components, how different types of data can be represented, and how to create a data flow.
Data communications between remote parties can be achieved through a process called networking, involving the connection of computers, media, and networking devices. Networks are divided into two main categories: local area networks (LANs) and wide area networks (WANs). These two types of networks have different characteristics and different functionalities. The Internet, the main focus of the book, is a collection of LANs and WANs held together by internetworking devices.
Protocols and standards are vital to the implementation of data communications and networking. Protocols refer to the rules; a standard is a protocol that has been adopted by vendors and manufacturers.
Network models serve to organize, unify, and control the hardware and software components of data communications and networking. Although the term "network model" suggests a relationship to networking, the model also encompasses data communications.
Chapters
This part consists of two chapters: Chapter 1 and Chapter 2.Chapter 1
In Chapter 1, we introduce the concepts of data communications and networking. We discuss data communications components, data representation, and data flow. We then move to the structure of networks that carry data. We discuss network topologies, categories of networks, and the general idea behind the Internet. The section on protocols and standards gives a quick overview of the organizations that set standards in data communications and networking.
Chapter 2
The two dominant networking models are the Open Systems Interconnection (OSI) and the Internet model (TCP/IP).The first is a theoretical framework; the second is the actual model used in today's data communications. In Chapter 2, we first discuss the OSI model to give a general background. We then concentrate on the Internet model, which is the foundation for the rest of the book.
CHAPTER 1
Introduction
Data communications and networking are changing the way we do business and the waywe live. Business decisions have to be made ever more quickly, and the decision makers
require immediate access to accurate information. Why wait a week for that report
from Germany to arrive by mail when it could appear almost instantaneously through
computer networks? Businesses today rely on computer networks and internetworks.
But before we ask how quickly we can get hooked up, we need to know how networks
operate, what types of technologies are available, and which design best fills which set
of needs.
The development of the personal computer brought about tremendous changes for
business, industry, science, and education. A similar revolution is occurring in data
communications and networking. Technological advances are making it possible for
communications links to carry more and faster signals. As a result, services are evolving
to allow use of this expanded capacity. For example, established telephone services
such as conference calling, call waiting, voice mail, and caller ID have been extended.
Research in data communications and networking has resulted in new technologies.
One goal is to be able to exchange data such as text, audio, and video from all
points in the world. We want to access the Internet to download and upload information
quickly and accurately and at any time.
This chapter addresses four issues: data communications, networks, the Internet,
and protocols and standards. First we give a broad definition of data communications.
Then we define networks as a highway on which data can travel. The Internet is discussed
as a good example of an internetwork (i.e., a network of networks). Finally, we
discuss different types of protocols, the difference between protocols and standards,
and the organizations that set those standards.
1.1 DATA COMMUNICATIONS
When we communicate, we are sharing information. This sharing can be local orremote. Between individuals, local communication usually occurs face to face, while
remote communication takes place over distance. The term telecommunication, which
includes telephony, telegraphy, and television, means communication at a distance (tele
is Greek for "far").
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. 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 (programs). The effectiveness
of a data communications 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. For example, let us assume that video packets
are sent every 3D ms. If some of the packets arrive with 3D-ms delay and others with
4D-ms delay, an uneven quality in the video is the result.
Components
A data communications system has five components (see Figure 1.1).
forms of information include text, numbers, pictures, audio, and video.
2.Sender The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera, and so on.
3.Receiver The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television, and so on.
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. Without a protocol, two
devices may be connected but not communicating, just as a person speaking French
cannot be understood by a person who speaks only Japanese.
Data Representation
Information today comes in different forms such as text, numbers, images, audio, andvideo.
Text
In data communications, text is represented as a bit pattern, a sequence of bits (Os orIs). 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. Today, the
prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or
character used in any language in the world. The American Standard Code for Information
Interchange (ASCII), developed some decades ago in the United States, now
constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.
Appendix A includes part of the Unicode.
Numbers
Numbers are also represented by bit patterns. However, a code such as ASCII is not usedto represent numbers; the number is directly converted to a binary number to simplify
mathematical operations. Appendix B discusses several different numbering systems.
Images
Images are also represented by bit patterns. In its simplest form, an image is composedof a matrix of pixels (picture elements), where each pixel is a small dot. The size of the
pixel depends on the resolution. For example, an image can be divided into 1000 pixels
or 10,000 pixels. In the second case, there is a better representation of the image (better
resolution), but more memory is needed to store the image.
After an image is divided into pixels, each pixel is assigned a bit pattern. The size
and the value of the pattern depend on the image. For an image made of only black and-
white dots (e.g., a chessboard), a I-bit pattern is enough to represent a pixel.
If an image is not made of pure white and pure black pixels, you can increase the
size of the bit pattern to include gray scale. For example, to show four levels of gray
scale, you can use 2-bit patterns. A black pixel can be represented by 00, a dark gray
pixel by 01, a light gray pixel by 10, and a white pixel by 11.
There are several methods to represent color images. One method is called RGB,
so called because each color is made of a combination of three primary colors: red,
green, and blue. The intensity of each color is measured, and a bit pattern is assigned to
it. Another method is called YCM, in which a color is made of a combination of three
other primary colors: yellow, cyan, and magenta.
Audio
Audio refers to the recording or broadcasting of sound or music. Audio is by naturedifferent from text, numbers, or images. It is continuous, not discrete. Even when we
use a microphone to change voice or music to an electric signal, we create a continuous
signal. In Chapters 4 and 5, we learn how to change sound or music to a digital or an
analog signal.
Video
Video refers to the recording or broadcasting of a picture or movie. Video can either beproduced as a continuous entity (e.g., by a TV camera), or it can be a combination of
images, each a discrete entity, arranged to convey the idea of motion. Again we can
change video to a digital or an analog signal, as we will see in Chapters 4 and 5.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex asshown in Figure 1.2.
Simplex
In simplex mode, the communication is unidirectional, as on a one-way street. Only oneof the two devices on a link can transmit; the other can only receive (see Figure 1.2a).
Keyboards and traditional monitors are examples of simplex devices. The keyboard
can only introduce input; the monitor can only accept output. The simplex mode
can use the entire capacity of the channel to send data in one direction.
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 (see Figure 1.2b).
The half-duplex mode is like a one-lane road with traffic allowed in both directions.
When cars are traveling in one direction, cars going the other way must wait. In a
half-duplex transmission, the entire capacity of a channel is taken over by whichever of
the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios
are both half-duplex systems.
The half-duplex mode is used in cases where there is no need for communication
in both directions at the same time; the entire capacity of the channel can be utilized for
each direction.
Full-Duplex
In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously
(see Figure 1.2c).
The full-duplex mode is like a two-way street with traffic flowing in both directions
at the same time. In full-duplex mode, signals going in one direction share the
capacity of the link: with signals going in the other directon. This sharing can occur in
two ways: Either the link must contain two physically separate transmission paths, one
for sending and the other for receiving; or the capacity of the channel is divided
between signals traveling in both directions.
One common example of full-duplex communication is the telephone network.
When two people are communicating by a telephone line, both can talk and listen at the
same time.
The full-duplex mode is used when communication in both directions is required
all the time. The capacity of the channel, however, must be divided between the two
directions.
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.
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.
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.
Transit time is the amount of time required for a message to travel from one device to
another. Response time is the elapsed time between an inquiry and a response. The performance
of a network depends on a number of factors, including the number of users,
the type of transmission medium, the capabilities of the connected hardware, and the
efficiency of the software.
Performance is often evaluated by two networking metrics: throughput and delay.
We often need more throughput and less delay. However, these two criteria are often
contradictory. If we try to send more data to the network, we may increase throughput
but we increase the delay because of traffic congestion in the network.
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.
Physical Structures
Before discussing networks, we need to define some network attributes.
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. For visualization purposes, it is
simplest to imagine any link as a line drawn between two points. For communication to
occur, two devices must be connected in some way to the same link at the same time.
There are two possible types of connections: point-to-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
(see Figure 1.3a). When you change television channels by infrared remote control,
you are establishing a point-to-point connection between the remote control and the
television's control system.
Multipoint A multipoint (also called multidrop) connection is one in which more
than two specific devices share a single link (see Figure 1.3b).
In a multipoint environment, the capacity of the channel is shared, either spatially
or temporally. If several devices can use the link simultaneously, it is a spatially shared
connection. If users must take turns, it is a timeshared connection.
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 (see Figure 1.4).
Mesh In a mesh topology, every device has a dedicated point-to-point link to every
other device. The term dedicated means that the link carries traffic only between the
two devices it connects. To find the number of physical links in a fully connected mesh
network with n nodes, we first consider that each node must be connected to every
other node. Node 1 must be connected to n - I nodes, node 2 must be connected to n - 1
nodes, and finally node n must be connected to n - 1 nodes. We need n(n - 1) physical
links. However, if each physical link allows communication in both directions (duplex
mode), we can divide the number of links by 2. In other words, we can say that in a
mesh topology, we need
n(n -1) /2
duplex-mode links.
To accommodate that many links, every device on the network must have n - 1
input/output (I/O) ports (see Figure 1.5) to be connected to the other n - 1 stations.
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.
Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate
the entire system. Third, there is the advantage of privacy or security. When every
message travels along a dedicated line, only the intended recipient sees it. Physical
boundaries prevent other users from gaining access to messages. Finally, point-to-point
links make fault identification and fault isolation easy. Traffic can be routed to avoid
links with suspected problems. This facility enables the network manager to discover the
precise location of the fault and aids in finding its cause and solution.
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. For these reasons a mesh topology is usually implemented in a
limited fashion, for example, as a backbone connecting the main computers of a hybrid
network that can include several other topologies.
One practical example of a mesh topology is the connection of telephone regional
offices in which each regional office needs to be connected to every other regional office.
Star Topology In a star topology, each device has a dedicated point-to-point 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. The controller acts as an exchange: 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 (see Figure 1.6) .
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. This factor also
makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions,
moves, and deletions involve only one connection: between that device and the hub.
Other advantages include robustness. If one link fails, only that link is affected. All
other links remain active. This factor also lends itself to easy fault identification and
fault isolation. As long as the hub is working, it can be used to monitor link problems
and bypass defective links.
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).
The star topology is used in local-area networks (LANs), as we will see in Chapter 13.
High-speed LANs often use a star topology with a central hub.
Bus Topology The preceding examples all describe point-to-point connections. A bus
topology, on the other hand, is multipoint. One long cable acts as a backbone to link all
the devices in a network (see Figure 1.7).
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. As a signal travels along the backbone, some of its energy is transformed
into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For
this reason there is a limit on the number of taps a bus can support and on the distance
between those taps.
Advantages of a bus topology include ease of installation. Backbone cable can be
laid along the most efficient path, then connected to the nodes by drop lines of various
lengths. In this way, a bus uses less cabling than mesh or star topologies. In a star, for
example, four network devices in the same room require four lengths of cable reaching
all the way to the hub. In a bus, this redundancy is eliminated. Only the backbone cable
stretches through the entire facility. Each drop line has to reach only as far as the nearest
point on the backbone.
Disadvantages include difficult reconnection and fault isolation. A bus is usually
designed to be optimally efficient at installation. It can therefore be difficult to add new
devices. Signal reflection at the taps can cause degradation in quality. This degradation
can be controlled by limiting the number and spacing of devices connected to a given
length of cable. Adding new devices may therefore require modification or replacement
of the backbone.
In addition, 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.
Bus topology was the one of the first topologies used in the design of early localarea
networks. Ethernet LANs can use a bus topology, but they are less popular now for
reasons we will discuss in Chapter 13.
Ring Topology In a ring topology, each device has a dedicated point-to-point 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 (see Figure 1.8).
A ring is relatively easy to install and reconfigure. Each device is linked to only its
immediate neighbors (either physically or logically). To add or delete a device requires
changing only two connections. The only constraints are media and traffic considerations
(maximum ring length and number of devices). In addition, fault isolation is simplified.
Generally in a ring, a signal is circulating at all times. 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.
However, 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. This weakness can
be solved by using a dual ring or a switch capable of closing off the break.
Ring topology was prevalent when IBM introduced its local-area network Token
Ring. Today, the need for higher-speed LANs has made this topology less popular.
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 as shown in Figure 1.9.
Network Models
Computer networks are created by different entities. 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. This book is based on the Internet
model with occasional references to the OSI model.
Categories of Networks
Today when we speak of networks, we are generally referring to two primary categories:
local-area networks and wide-area networks. The category into which a network
falls is determined by its size. A LAN normally covers an area less than 2 mi; aWAN can
be worldwide. Networks of a size in between are normally referred to as metropolitanarea
networks and span tens of miles.
Local Area Network
A local area network (LAN) is usually privately owned and links the devices in a single
office, building, or campus (see Figure 1.10). Depending on the needs of an organization
and the type of technology used, a LAN can be as simple as two PCs and a printer in
someone's home office; or it can extend throughout a company and include audio and
video peripherals. Currently, LAN size is limited to a few kilometers.
LANs are designed to allow resources to be shared between personal computers or
workstations. The resources to be shared can include hardware (e.g., a printer), software
(e.g., an application program), or data. A common example of a LAN, found in many
business environments, links a workgroup of task-related computers, for example, engineering
workstations or accounting PCs. One of the computers may be given a largecapacity
disk drive and may become a server to clients. Software can be stored on this
central server and used as needed by the whole group. In this example, the size of the
LAN may be determined by licensing restrictions on the number of users per copy of software,
or by restrictions on the number of users licensed to access the operating system.
In addition to size, LANs are distinguished from other types of networks by their
transmission media and topology. In general, a given LAN will use only one type of
transmission medium. The most common LAN topologies are bus, ring, and star.
Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range. Today,
however, speeds are normally 100 or 1000 Mbps. LANs are discussed at length in
Chapters 13, 14, and 15.
Wireless LANs are the newest evolution in LAN technology. We discuss wireless
LANs in detail in Chapter 14.
Wide Area Network
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. In Chapters 17 and 18 we discuss wide-area networks in
greater detail. A WAN can be as complex as the backbones that connect the Internet or as
simple as a dial-up line that connects a home computer to the Internet. We normally refer
to the first as a switched WAN and to the second as a point-to-point WAN (Figure 1.11).
The switched WAN connects the end systems, which usually comprise a router (internetworking
connecting device) that connects to another LAN or WAN. The point-to-point
WAN is normally a line leased from a telephone or cable TV provider that connects a
home computer or a small LAN to an Internet service provider (lSP). This type of WAN
is often used to provide Internet access.
An early example of a switched WAN is X.25, a network designed to provide connectivity
between end users. As we will see in Chapter 18, X.25 is being gradually
replaced by a high-speed, more efficient network called Frame Relay. 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. We will discuss ATM in Chapter 18.
Another example ofWANs is the wireless WAN that is becoming more and more popular.
We discuss wireless WANs and their evolution in Chapter 16.
Metropolitan Area Networks
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. Another
example is the cable TV network that originally was designed for cable TV, but today
can also be used for high-speed data connection to the Internet. We discuss DSL lines
and cable TV networks in Chapter 9.
Interconnection of Networks: Internetwork
Today, it is very rare to see a LAN, a MAN, or a LAN in isolation; they are connected
to one another. When two or more networks are connected, they become an
internetwork, or internet.
As an example, assume that an organization has two offices, one on the east coast
and the other on the west coast. The established office on the west coast has a bus topology
LAN; the newly opened office on the east coast has a star topology LAN. The president of
the company lives somewhere in the middle and needs to have control over the company
from her horne. To create a backbone WAN for connecting these three entities (two
LANs and the president's computer), a switched WAN (operated by a service provider
such as a telecom company) has been leased. To connect the LANs to this switched
WAN, however, three point-to-point WANs are required. These point-to-point WANs
can be a high-speed DSL line offered by a telephone company or a cable modern line
offered by a cable TV provider as shown in Figure 1.12.
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. Count the ways you've
used the Internet recently. Perhaps you've sent electronic mail (e-mail) to a business
associate, paid a utility bill, read a newspaper from a distant city, or looked up a local
movie schedule-all by using the Internet. Or maybe you researched a medical topic,
booked a hotel reservation, chatted with a fellow Trekkie, or comparison-shopped for a
car. The Internet is a communication system that has brought a wealth of information to
our fingertips and organized it for our use.
The Internet is a structured, organized system. We begin with a brief history of the
Internet. We follow with a description of the Internet today.
A Brief History
A network is a group of connected communicating devices such as computers and
printers. 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 of thousands of interconnected networks.
Private individuals as well as various organizations such as government agencies,
schools, research facilities, corporations, and libraries in more than 100 countries
use the Internet. Millions of people are users. Yet this extraordinary communication system
only came into being in 1969.
In the mid-1960s, mainframe computers in research organizations were standalone
devices. Computers from different manufacturers were unable to communicate
with one another. 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, thereby reducing costs and eliminating
duplication of effort.
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 a specialized computer, called an inteiface message processor (IMP). The
IMPs, in tum, 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.
By 1969, ARPANET was a reality. Four nodes, at the University of California at
Los Angeles (UCLA), the University of California at Santa Barbara (UCSB), Stanford
Research Institute (SRI), and the University 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, both of whom were part of the core ARPANET
group, collaborated on what they called the Internetting Projec1. Cerf and Kahn's landmark
1973 paper outlined the protocols to achieve end-to-end delivery of packets. This
paper on Transmission Control Protocol (TCP) included concepts such as encapsulation,
the datagram, and the functions of a gateway.
Shortly thereafter, authorities made a decision to split TCP into two protocols:
Transmission Control Protocol (TCP) and Internetworking Protocol (lP). IP would
handle datagram routing while TCP would be responsible for higher-level functions
such as segmentation, reassembly, and error detection. The internetworking protocol
became known as TCPIIP.
The Internet Today
The Internet has come a long way since the 1960s. The Internet today is not a simple
hierarchical structure. It is made up of many wide- and local-area networks joined by
connecting devices and switching stations. It is difficult to give an accurate representation
of the Internet because it is continually changing-new networks are being
added, existing networks are adding addresses, and networks of defunct companies are
being removed. Today most end users who want Internet connection use the services of
Internet service providers (lSPs). There are international service providers, national
service providers, regional service providers, and local service providers. The Internet
today is run by private companies, not the government. Figure 1.13 shows a conceptual
(not geographic) view of the Internet.
International Internet Service Providers
At the top of the hierarchy are the international service providers that connect nations
together.
National Internet Service Providers
The national Internet service providers 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). Some national ISP networks are also
connected to one another by private switching stations called peering points. These
normally operate at a high data rate (up to 600 Mbps).
Regional Internet Service Providers
Regional internet service providers or regional ISPs are smaller ISPs that are connected
to one or more national ISPs. They are at the third level of the hierarchy with a smaller
data rate.
Local Internet Service Providers
Local Internet service providers provide direct service to the end users. The local
ISPs can be connected to regional ISPs or directly to national ISPs. Most end users are
connected to the local ISPs. Note that in this sense, a local ISP can be a company that
just provides Internet services, a corporation with a network that supplies services to its
own employees, or a nonprofit organization, such as a college or a university, that runs
its own network. Each of these local ISPs can be connected to a regional or national
service provider.
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.
Protocols
In computer networks, communication occurs between entities in different systems. An
entity is anything capable of sending or receiving information. However, two entities cannot
simply send bit streams to each other and expect to be understood. 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.
1 Syntax. The term syntax refers to the structure or format of the data, meaning the
order in which they are presented. For example, a simple protocol might expect the
first 8 bits of data to be the address of the sender, the second 8 bits to be the address
of the receiver, and the rest of the stream to be the message itself.
2 Semantics. The word semantics refers to the meaning of each section of bits.
How is a particular pattern to be interpreted, and what action is to be taken based
on that interpretation? For example, does an address identify the route to be taken
or the final destination of the message?
3 Timing. The term timing refers to two characteristics: when data should be sent
and how fast they can be sent. For example, if a sender produces data at 100 Mbps
but the receiver can process data at only 1 Mbps, the transmission will overload the
receiver and some data will be lost.
Standards
Standards are essential in creating and maintaining an open and competitive market for
equipment manufacturers and in guaranteeing national and international interoperability
of data and telecommunications technology and processes. Standards provide guidelines
to manufacturers, vendors, government agencies, and other service providers to ensure
the kind of interconnectivity necessary in today's marketplace and in international communications.
Data communication standards fall into two categories: de facto (meaning
"by fact" or "by convention") and de jure (meaning "by law" or "by regulation").
1 De facto. Standards that have not been approved by an organized body but have
been adopted as standards through widespread use are de facto standards. De facto
standards are often established originally by manufacturers who seek to define the
functionality of a new product or technology.
2 De jure. Those standards that have been legislated by an officially recognized body
are de jure standards.
Standards Organizations
Standards are developed through the cooperation of standards creation committees,
forums, and government regulatory agencies.
Standards Creation Committees
While many organizations are dedicated to the establishment of standards, data telecommunications
in North America rely primarily on those published by the following:
1 International Organization for Standardization (ISO). The ISO is a multinational
body whose membership is drawn mainly from the standards creation committees
of various governments throughout the world. The ISO is active in developing
cooperation in the realms of scientific, technological, and economic activity.
2 International Telecommunication Union-Telecommunication Standards
Sector (ITU-T). By the early 1970s, a number of countries were defining national
standards for telecommunications, but there was still little international compatibility.
The United Nations responded by forming, as part of its International
Telecommunication Union (ITU), a committee, the Consultative Committee
for International Telegraphy and Telephony (CCITT). This committee was
devoted to the research and establishment of standards for telecommunications in
general and for phone and data systems in particular. On March 1, 1993, the name
of this committee was changed to the International Telecommunication UnionTelecommunication
Standards Sector (ITU-T).
3 American National Standards Institute (ANSI). Despite its name, the American
National Standards Institute is a completely private, nonprofit corporation not affiliated
with the U.S. federal government. However, all ANSI activities are undertaken
with the welfare of the United States and its citizens occupying primary importance.
4 Institute of Electrical and Electronics Engineers (IEEE). The Institute of
Electrical and Electronics Engineers is the largest professional engineering society in
the world. International in scope, it aims to advance theory, creativity, and product
quality in the fields of electrical engineering, electronics, and radio as well as in all
related branches of engineering. As one of its goals, the IEEE oversees the development
and adoption of international standards for computing and communications.
5 Electronic Industries Association (EIA). Aligned with ANSI, the Electronic
Industries Association is a nonprofit organization devoted to the promotion of
electronics manufacturing concerns. Its activities include public awareness education
and lobbying efforts in addition to standards development. In the field of information
technology, the EIA has made significant contributions by defining physical connection
interfaces and electronic signaling specifications for data communication.
Forums
Telecommunications technology development is moving faster than the ability of standards
committees to ratify standards. Standards committees are procedural bodies and
by nature slow-moving. To accommodate the need for working models and agreements
and to facilitate the standardization process, many special-interest groups have developed
forums made up of representatives from interested corporations. The forums
work with universities and users to test, evaluate, and standardize new technologies. By
concentrating their efforts on a particular technology, the forums are able to speed
acceptance and use of those technologies in the telecommunications community. The
forums present their conclusions to the standards bodies.
Regulatory Agencies
All communications technology is subject to regulation by government agencies such
as the Federal Communications Commission (FCC) in the United States. The purpose
of these agencies is to protect the public interest by regulating radio, television,
and wire/cable communications. The FCC has authority over interstate and international
commerce as it relates to communications.
Internet Standards
An 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.
There is a strict procedure by which a specification attains Internet standard
status. A specification begins as an Internet draft. An Internet draft is a working document
(a work in progress) with no official status and a 6-month lifetime. Upon recommendation
from the Internet authorities, a draft may be published as a Request for
Comment (RFC). Each RFC is edited, assigned a number, and made available to all
interested parties. RFCs go through maturity levels and are categorized according to
their requirement level.
1.5 RECOMMENDED READING
For more details about subjects discussed in this chapter, we recommend the following
books and sites. The items enclosed in brackets [...] refer to the reference list at the end
of the book.
Books
The introductory materials covered in this chapter can be found in [Sta04] and [PD03].
[Tan03] discusses standardization in Section 1.6.
Sites
The following sites are related to topics discussed in this chapter.
1 www.acm.org/sigcomm/sos.html This site gives the status of varililus networking
standards.
2 www.ietf.org/ The Internet Engineering Task Force (IETF) home page.
RFCs
The following site lists all RFCs, including those related to IP and TCP. In future chapters
we cite the RFCs pertinent to the chapter material.
3 www.ietf.org/rfc.html
1.6 KEY TERMS
Advanced Research Projects forum
Agency (ARPA) full-duplex mode, or duplex
American National Standards half-duplex mode Institute (ANSI) hub American Standard Code for image Information Interchange (ASCII) Institute of Electrical and Electronics ARPANET Engineers (IEEE) audio International Organization for
backbone Standardization (ISO)
Basic Latin International Telecommunication
bus topology Union-Telecommunication
code Standards Sector (ITU-T)
Consultative Committee for Internet
International Telegraphy Internet draft
and Telephony (CCITT) Internet service provider (ISP)
data Internet standard
data communications Internetwork or internet
de facto standards local area network (LAN)
de jure standards local Internet service providers
delay mesh topology
distributed processing message
Electronic Industries Association (EIA) metropolitan area network (MAN)
entity multipoint or multidrop connection
Federal Communications Commission national Internet service provider
(FCC) network
network access points (NAPs) sender
node simplex mode
performance star topology
physical topology syntax
point-to-point connection telecommunication
protocol throughput
receiver timing
regional ISP Transmission Control Protocol!
reliability Internetworking Protocol (TCPIIP)
Request for Comment (RFC) transmission medium
ROB Unicode
ring topology video
security wide area network (WAN)
semantics YCM
1.7 SUMMARY
1.Data communications are the transfer of data from one device to another via some
form of transmission medium.
2.A data communications system must transmit data to the correct destination in an
accurate and timely manner.
3.The five components that make up a data communications system are the message,
sender, receiver, medium, and protocol.
4.Text, numbers, images, audio, and video are different forms of information.
5.Data flow between two devices can occur in one of three ways: simplex, half-duplex,
or full-duplex.
6.A network is a set of communication devices connected by media links.
7.In a point-to-point connection, two and only two devices are connected by a
dedicated link. In a multipoint connection, three or more devices share a link.
8.Topology refers to the physical or logical arrangement of a network. Devices may
be arranged in a mesh, star, bus, or ring topology.
9.A network can be categorized as a local area network or a wide area network.
10.A LAN is a data communication system within a building, plant, or campus, or
between nearby buildings.
11.A WAN is a data communication system spanning states, countries, or the whole
world.
12.An internet is a network of networks.
13.The Internet is a collection of many separate networks.
14.There are local, regional, national, and international Internet service providers.
15.A protocol is a set of rules that govern data communication; the key elements of
a protocol are syntax, semantics, and timing.
16.Standards are necessary to ensure that products from different manufacturers can
work together as expected.
17.The ISO, ITD-T, ANSI, IEEE, and EIA are some of the organizations involved
in standards creation.
18.Forums are special-interest groups that quickly evaluate and standardize new
technologies.
19.A Request for Comment is an idea or concept that is a precursor to an Internet
standard.
1.8 PRACTICE SET
Review Questions
1. Identify the five components of a data communications system.
2. What are the advantages of distributed processing?
3. What are the three criteria necessary for an effective and efficient network?
4. What are the advantages of a multipoint connection over a point-to-point
connection?
5. What are the two types of line configuration?
6. Categorize the four basic topologies in terms of line configuration.
7. What is the difference between half-duplex and full-duplex transmission modes?
8. Name the four basic network topologies, and cite an advantage of each type.
9. For n devices in a network, what is the number of cable links required for a mesh,
ring, bus, and star topology?
10. What are some of the factors that determine whether a communication system is a
LAN or WAN?
1I. What is an internet? What is the Internet?
12. Why are protocols needed?
13. Why are standards needed?
Exercises
14. What is the maximum number of characters or symbols that can be represented by
Unicode?
15. A color image uses 16 bits to represent a pixel. What is the maximum number of
different colors that can be represented?
16. Assume six devices are arranged in a mesh topology. How many cables are needed?
How many ports are needed for each device?
17. For each of the following four networks, discuss the consequences if a connection fails.
a. Five devices arranged in a mesh topology
b. Five devices arranged in a star topology (not counting the hub)
c. Five devices arranged in a bus topology
d. Five devices arranged in a ring topology
18. You have two computers connected by an Ethernet hub at home. Is this a LAN, a
MAN, or a WAN? Explain your reason.
19. In the ring topology in Figure 1.8, what happens if one of the stations is unplugged?
20. In the bus topology in Figure 1.7, what happens if one ofthe stations is unplugged?
21. Draw a hybrid topology with a star backbone and three ring networks.
22. Draw a hybrid topology with a ring backbone and two bus networks.
23. Performance is inversely related to delay. When you use the Internet, which of the
following applications are more sensitive to delay?
a. Sending an e-mail
b. Copying a file
c. Surfing the Internet
24. When a party makes a local telephone call to another party, is this a point-to-point
or multipoint connection? Explain your answer.
25. Compare the telephone network and the Internet. What are the similarities? What
are the differences?
Research Activities
26. Using the site \\iww.cne.gmu.edu/modules/network/osi.html, discuss the OSI model.
27. Using the site www.ansi.org, discuss ANSI's activities.
28. Using the site www.ieee.org, discuss IEEE's activities.
29. Using the site www.ietf.org/, discuss the different types of RFCs.
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