1
NET 221D:COMPUTER
NETWORKS FUNDAMENTALS
Networks and
Communication
Department
Lecture 1: Introduction
Introduction
•
The merging of computers and communications
has had a profound influence on the way computer
systems are organized.
•
Computer Network: a collection of autonomous
computers interconnected by a single technology.
•
Networks are usually connected together to make
larger networks.
Uses of Computer Networks
•
Business Applications.
•
Home Applications.
•
Mobile Users.
1. Business Applications of Networks
•
Goals: sharing physical resources such as
printers or sharing information (product
information, inventories, financial statements,
…).
•
Model: one or more databases with company
information and some number of employees
who need to access them remotely.
1. Business Applications cont.
•
A network with two clients and one server.
1. Business Applications cont.
•
•
•
A computer network can provide a powerful
communication medium among employees : Email.
Telephone calls between employees may be carried
by the computer network instead of by the phone
company: IP telephony or Voice over IP (VoIP).
Doing Business electronically: E-commerce.
2. Home Network Applications
•
Home users can access information, communicate
with other people, and buy products and services
with e-commerce.
•
Much of this information is accessed using the clientserver model.
•
A different model for accessing information is peerto-peer communication.
2. Home Applications cont.
•
Peer-to-peer communication is often used to share
music and videos.
•
Social network applications: the flow of information
is driven by the relationships that people declare
between each other.
•
Groups of people can work together to create a
content: a wiki.
•
Many people already pay their bills, manage their
bank accounts, …
2. Home Applications cont.
Peer to peer model:
Some forms of E-commerce:
3. Mobile Network Users
•
•
Combinations of wireless networks and mobile
computing.
Example:
Physical Structures: A. Types of connections
11
1. Point –to-point
Dedicated link between two devices. Most of them uses an
actual length of wire or cable to connect the two ends but
other options ,such as microwave satellite are possible.
Behrouz A. Forouzan” Data communications and Networking
Physical Structures: Types of connections
2. 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 as:
Spatially: If several devices can use the link simultaneously
 Temporally: If users must take turns, it is a timeshared
connection.

Physical Structures: B. physical topology
13


The way in which a network is laid out physically.
It is the geometric representation of the relationship of
all the links and linking devices (nodes) to one another.
Behrouz A. Forouzan” Data communications and Networking
1. Fully connected mesh topology
14
• Every device has a dedicated point-to-point link to
every other devices.
• Fully connected mesh network has n(n-1)/2 physical
connection to link n devices.
• Every device on the network must have n-1
input/output (I/O) ports.
Behrouz A. Forouzan” Data communications and Networking
1. Mesh topology
15
Advantages:
 Privacy or security: every message travels along a dedicated
line, and only the intended recipient sees it.
 Eliminating the traffic problems: the use of dedicated links
guarantees that each connection can carry its own data load;
that can occur when links must be shared by multiple
devices.
 A mesh is robust. If one link becomes unusable, it does not
incapacitate the entire system.
 Fault identification and fault isolation easy. This enables the
network manager to discover the precise location of fault and
aids in finding its cause and solution.
Behrouz A. Forouzan” Data communications and Networking
1. Mesh topology
16
Disadvantages

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 can accommodate.

The H.W required to connect each link (I/O ports and cable)
expensive.
Behrouz A. Forouzan” Data communications and Networking
2. Start topology
17
Each device has a dedicated point-to-point link only to a
central controller (hub).
Unlike a mesh , a star topology does not allow direct traffic
between devices.
If one device want to send data to another , it send it to the
hub, which send it to other device.
Behrouz A. Forouzan” Data communications and Networking
2. Start topology
18
Advantages



Easy to install and to reconfigure and it is less
expensive.
Robustness: if one link fails, only that link affected
and other links remain active.
Identification and fault isolation.
Disadvantages
 The dependency of the whole topology on one single
point, the hub. If the hub goes down, the whole
system is dead.
Behrouz A. Forouzan” Data communications and Networking
3. Tree topology
19
 Not every device plugs directly into the central hub. The
majority of devices connect to secondary hub that in turn is
connected to the central hub.
 The advantages and disadvantages of tree topology are
generally the same as those of star .
Behrouz A. Forouzan” Data communications and Networking
4. Bus topology
20
 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.
Behrouz A. Forouzan” Data communications and Networking
4. Bus topology
21
Advantages
Easy of insulation: use less cabling than mesh or star.
Disadvantages
 A fault in bus cable (break) stops all transmissions
even between devices on the same side of the
problem.
 Reconnection
It can be difficult to add new devices (adding more
require modification or replacement of the backbone).
Behrouz A. Forouzan” Data communications and Networking
5. Ring topology
22
 Each device has a dedicated point-to-point connection
only with the two devices on either side of it.
 A signal is passed along the ring in one direction from
device until it reaches its destination.
 Each device in the ring incorporate as repeater.
 Repeater :regenerates the signal. It receives a weakened
signal, creates a copy, bit for bit, at the original strength.
Behrouz A. Forouzan” Data communications and Networking
5. Ring topology
23
Advantages:
o Easy to install and reconfigure: Each device is linked
only to its immediate neighbors.
o Fault isolation is simplified : A signal is circulating at all
times (token) if one device does not receive a signal
within specified period, it can issue an alarm.
Disadvantages

Unidirectional traffic: A break in the ring (such as
disabled station) can disable the entire network. This can
be solved by use dual ring.
Hybrid topology
24
Behrouz A. Forouzan” Data communications and Networking
Network categories
25
Today when we speak about networks, we are
generally referring to two primary categories : local area networks and wide – area networks.
The categories are :
 PAN
 LAN
 MAN
 WAN
23-Mar-16
Networks and Communication Department
1. Personal Area Network
•
Bluetooth is used to connect wirelessly a computer
with its peripherals.
•
PANs can also be built with other technologies that
communicate over short ranges, such as RFID.
2. Local Area Networks
•
A LAN is a privately owned network that operates within
and nearby a single building like a home, office or factory.
•
Every computer has a radio modem and an antenna that it
uses to communicate with other computers.
•
Each computer talks to a device in the ceiling called an AP
(Access Point), wireless router, or base station.
•
IEEE 802.11, popularly known as WiFi is a communication
standard for LANs. It runs at speeds anywhere from 11to
hundreds of Mbps.
Local Area Networks
Local Area Networks
•
•
•
•
Most of wired LANs use copper wires, but some
use optical fiber.
Typically, wired LANs run at speeds of 100 Mbps
to 1 Gbps, have low delay (microseconds or
nanoseconds), and make very few errors.
• It is just easier to send signals over a wire or
through a fiber than through the air.
The topology of many wired LANs is built from
point-to-point links.
IEEE 802.3, popularly called Ethernet, is, by far, the
most common type of wired LAN.
3. Metropolitan Area Networks
•
A metropolitan area network based on cable TV.
4. Wide Area Networks
•
WAN (Wide Area Network) spans a
geographical area, often a country or continent.
•
Consist of hosts and communication subnets.
•
•
large
In most WANs, the subnet consists of two distinct
components: transmission lines and switching
elements.
When data arrive on an incoming line, the switching
element must choose an outgoing line on which to
forward them.
Wide Area Networks
•
The hosts and subnet are owned and operated by
different people.
•
The routers (one of the switching elements) will
usually connect different kinds of networking
technology.
•
What is connected to the subnet? This could be
individual computers, as was the case for connecting
to LANs, or it could be entire LANs.
Wide Area Networks
•
Relation between hosts on LANs and the subnet.
Wide Area Networks
•
A stream of packets from sender to receiver.
Internetworks
(interconnection of networks)
•
People connected to one network often want to
communicate with people attached to a different one.
•
A collection of interconnected networks is called an
internetwork or internet.
•
ISP : The Internet ( as an example of internet) uses ISP
networks to connect enterprise networks, home networks,
and many other networks.
•
Gateway: connects two or more networks and provides
the necessary translation, both in terms of hardware and
software.
Network Software,
protocols and standards
•
Most networks are organized as a stack of layers or levels,
each one built upon the one below it.
•
The purpose of each layer is to offer certain services to the
higher layers.
•
A protocol is an agreement between the communicating
parties on how communication is to proceed.
•
The interface defines which primitive operations and
services the lower layer makes available to the upper one.
Network Software
Protocol Hierarchies
Layers, protocols, and interfaces.
Protocol Hierarchies – an example
•
The philosopher-translator-secretary architecture.
Protocols
39
Set of rules that governs data communications.
Protocol defines :
 What is communicated?
 How it communicated?
 When it is communicated?
 Key elements of a protocol:
 Syntax,
 Semantics
 and timing.

Behrouz A. Forouzan” Data communications and Networking
Protocols
40

Syntax:
 Structure or format of the data, meaning the order in
which they are presented.
Example: A simple protocol might expect the first byte
of data to be the address of the sender, the second byte to
be the address of the receiver and the reset of the stream
to be the message itself.
Behrouz A. Forouzan” Data communications and Networking
Protocols
41


Semantics:
 Refers to the meaning of each section of bits.
Example: does an address identify the route to be taken
or the final destination of the message
Timing:
 When data to should be sent?
 How fast they can be sent?
Example: If a sender produces data at 100Mpbs but
the receiver can process data at only 1Mpbs,
transmission will overload the receiver and data will
be largely lost.
Behrouz A. Forouzan” Data communications and Networking
Standards
42

Standards provide guidelines to manufactures,
venders, government agencies, and other service
providers to ensure the kind of interconnectivity
necessary in today’s marketplace and in
international communications .
Behrouz A. Forouzan” Data communications and Networking
The END
43
Behrouz A. Forouzan” Data communications and Networking