This document introduces key concepts in data communications and computer networks. It discusses the basic elements of communication, different types of networks including LANs, MANs and WANs, the OSI reference model, and general implementations such as intranets. The OSI model is presented as a standard for network interconnection with 7 layers governing how data is packaged and communicated between applications and devices.

1. 1-1© 2001 by Prentice Hall
Local Area Networks, 3rd Edition
David A. Stamper
Part 1: Introduction to Data
Communications and Local Area
Networks
Chapter 1
Introduction to Data
communications

2. © 2001 by Prentice Hall
Chapter Preview
In this chapter you will study:
• The requirements for communication
• What constitutes a network
• The various types of networks in
common use today
• The OSI Reference Model
• Some of the basic terminology of
data communications and networks

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Essential Elements of
Communication
• A message
• A sender
• A receiver
• A medium

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Understanding the Message
• In computer systems, data can be
represented by any of several
different codes, the two most
common being:
– the American Standard Code for Information
Interchange (ASCII)
– the Extended Binary Coded Decimal Interchange
Code (EBCDIC)

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Security
• Sensitive data like your credit card
number or other secret data should be
safeguarded during transmission. The
most common mechanism for protecting
data during transmission is encryption.
– Encryption transforms plain text into an (presumably)
undecipherable form called cipher-text.

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Two Types of Networks
• Terminal Network
– consists of a single host computer with attached
terminals
– the host computer does all or most of the processing,
and the terminals imply act as input/output (I/O) devices
through which a person gains access to the host’s
applications.
• Network of Computers
– two or more nodes connected by a data communications
medium.
– individual nodes may have terminals attached to them
– a single node on this network can look just like the
terminal network

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Three Network Subtypes
• Local Area Network (LAN)
• Metropolitan Area Network (MAN)
• Wide Area Network (WAN)

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LAN/MAN/WAN Comparison
Limited—typically up
to 2,500 meters or 2
miles
High—typically in
excess of 10 Mbps—
10,100 and 1,000 are
standard
Locally owned—
twisted-pair wires,
fiber optic cable,
wireless (not satellite)
Can be any, but most
are desktop computers
Limited—typically up
to 200 kilometers or
100 miles
High—typically
100 Mbps
Locally owned and
common carrier—
twisted-pair wires,
fiber optic cable
Can be any, but most
are desktop computers
and minicomputers
Unlimited
Slower—usually 1.5
Mbps
Locally owned and
common carrier—
twisted-pair wires,
coaxial cable, fiber
optic cable, wireless to
include satellite
Can be any, but most
are desktop computers
Distance
Speed
Media
Nodes
LAN MAN WAN

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The OSI Reference Model
• The problem of network interconnection is
so important that the ISO created the OSI
Reference Model that describes the
functions a generic network needs to
provide.
• The OSI Reference Model has become the
basis for many data communications
standards.
– Because these standards are placed in the public domain, they are
called open standards and lead to open systems.

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OSI Peer Layer Communication
ApplicationApplication
PresentationPresentation
SessionSession
TransportTransport
NetworkNetwork
Data LinkData Link
PhysicalPhysical
ApplicationApplication
PresentationPresentation
SessionSession
TransportTransport
NetworkNetwork
Data LinkData Link
PhysicalPhysical
Processor 1 Processor 2
Logical Path Physical Path

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OSI Reference Model Formatting
Trans-Id Data Date=dd/mm/yy
Trans-Id Data Date=mm-dd-yyyy
ID Length Trans-Id Data Date=mm-dd-yyyy
TSAP ChkSum ID Length Trans-Id Data Date=mm-dd-yyyy
Address Seq Nbr TSAP ChkSum ID Length Trans-Id Data Date=mm-dd-yyyy
Header Address Seq Nbr TSAP ChkSum ID Length Trans-Id Data Date=mm-dd-yyyy Chksum
(a) Application Layer
(b) Presentation Layer
(c) Session Layer
(d) Transport Layer
(e) Network Layer
(f) Data Link Layer

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The OSI Model at Work
• Application Layer
– The application on Node A builds a record with a transaction
identifier, the number of the account to be updated, the date and
time of the transaction, and the amount to be deducted.
• Presentation Layer
– The presentation layer is responsible for translating from one format
to another.
• Session Layer
– The session layer’s major functions are to set up and perhaps
monitor a set of dialogue rules by which the two applications can
communicate and to bring a session to an orderly conclusion.

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The OSI Model at Work (cont.)
• Transport Layer
– The transport layer is the first of the OSI layers responsible for
actually transmitting the data.
• Network Layer
– The network layer provides accounting and routing functions.
• Data Link Layer
– The data link layer is responsible for data delineation, error
detection,and logical control of the link.
• Physical Layer
– The physical layer does not append anything to the message. It
simply accepts the message from the data link layer and translates
the bits into signals on the medium.

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Receiving the Message
1. The message is passed over the link connecting Nodes A
and X.
2. The message is passed to the data link layer in Node X. The
message is checked for transmission errors, the PDU
information applied by A’s data link layer is removed, and
the message is sent to X’s network layer.
3. X’s network layer records the accounting information for
the message and then strips off the network layer protocol
data and examines the destination address. The destination
is not Node X in this case, so the network layer consults its
network routing table and determines the next link on the
path to Node M. X’s network layer affixes the proper
network layer protocol data and sends the message to
Node X’s data link layer.

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Receiving the Message (cont.)
4. Node X’s data link layer creates its PDU and sends the
message to Node M.
5. Node M’s data link layer receives the message, strips off
Node X’s data link layer protocol data, checks for
transmission errors, and passes the data up to Node M’s
network layer.
6. Node M’s network layer gathers accounting data, strips off
the network layer protocol data, and fins that the message
is destined for an application in this node.
7. The message is passed up to M’s transport layer, where the
sequence number is checked to ensure that no messages
have been lost. The transport layer protocol data is
removed.

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Receiving the Message (cont.)
8. The message arrives at the session layer, where
relevant protocol data is examined and remove.
9. The message arrives at Node M’s presentation
layer, where appropriate action is taken.
10.The message arrives at the application, where
it is acted on.

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General Network Implementations:
LANs
• A LAN services a limited geographic area at high speeds—
usually 10 million bits per second or higher. All
components of the LAN are commonly owned by the
organization that uses it.
• The nodes in many of today’s LANs are desktop systems
like personal computers. Henceforth, we will also use the
terms workstations, clients, and servers in referring to LAN
nodes.
• A workstation is used here to represent a LAN user’s
computer; other terms used in referring to a workstation
are client and node.
• A server is a network node that is dedicated to providing
services to client nodes.

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General Network Implementations:
MANs
• A MAN (metropolitan area network) is a high-
speed network covering wider distances than
LAN.
• A MAN spans distances of approximately 100
miles; therefore, it is suitable for connecting
devices and LANs in a metropolitan area.
• MAN speeds are typically 100 Mbps or higher.
• The most commonly implemented MAN is the
fiber distributed data interface (FDDI). It operates
at 100 Mbps over fiber optic cable for distances
up to 200 kilometers.

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General Network Implementations:
WANs
• A WAN is the oldest type of network.
• WANs generally span a wide geographic area like
a state, province, country, or multiple countries.
However, some WANs are confined to a limited
geographic area, like a LAN.
• A WAN in a limited geographic area could be
easily extended over a wide area using the same
technologies. The same is not true of a LAN.

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General Network Implementations:
VANs
• A VAN is a network owned by a communications
utility that sells the services of the network to
other companies.
• A communications utility that owns a VAN
provides connectivity to multiple locations.
• The value added by the communications utility is
the maintenance and management of the
communications circuits.

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General Network Implementations:
Enterprise Networks
• An enterprise network is an organization’s
complete network.
• With the advent of LANs, many companies
installed departmental LANs to improve the
productivity of work groups.
• Soon, these companies realized that there was a
benefit to having users on one LAN communicate
with users or applications on other LANs or on
the WAN, and the various networks were
connected together to form one corporate-wide
network, the enterprise network.

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General Network Implementations:
The Internet
• An internet (with a lowercase ‘i’) is the
interconnection of two or more networks. An
enterprise networks just described is an example
of an internet.
• The Internet (with an uppercase ‘I’) is a specific
instance of an internet.
• The Internet is a global network of networks. The
Internet is made up of hundreds of networks,
thousands of nodes, and millions of users
throughout most countries of the world.

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General Network Implementations:
Intranets
• An intranet is an organization’s private Web.
• Companies have found that WWW capabilities
can improve the information flow and availability
in a company.
• Companies may use an intranet rather than
publishing on the Internet because the
information being provided is intended for
corporate use only and not for the public at large.