Introduction to computer network

Introduction To Computer
K. A. M Lutfullah,PRINCE2, ITIL

Introduction to Networks
 A network consists of two or more entities or
objects sharing resources and information.
 A computer network consists of two or more
computing devices connected to each other to
share resources and information.
 The network becomes a powerful tool when
computers communicate and share resources with
other computers on the same network or entirely
distinct networks.
 Computers on a network can act as a client or a
server.

Introduction to Networks
 A client is a computer that requests for resources.
 A server is a computer that controls and provides
access to resources.
 Data is a piece of information.
 The computing concept ‘hierarchy of data’ is used
when planning a network.
 It is essential to maintain a hierarchy of data to
manage and control resources among computers.
 Network access to data must be evaluated
carefully to avoid security issues.

Communication Model
 Source
– generates data to be transmitted
 Transmitter
– Converts data into transmittable signals
 Transmission System
– Carries data
 Receiver
– Converts received signal into data
 Destination
– Takes incoming data

Transmission Modes
 Simplex
– One direction
• e.g. Television
 Half duplex
– Either direction, but only one
way at a time
• e.g. police radio
 Full duplex
– Both directions at the same
time
• e.g. telephone

Need for Networks
 A computer that operates independently from
other computers is called a stand-alone computer.
 The process of printing or transferring data from
one system to another using various storage
devices is called sneakernet.
 Enhance communication.
 Share resources.
 Facilitate centralized management.

Enhance Communication
 Computer networks use electronic mail (e-mail) as
the choice for most of the communication.
 By using networks, information can be sent to a
larger audience in an extremely fast and efficient
manner.

Share Resources
 A copy of data or application stored at a single
central location is shared over a network.
 Computer peripheral devices, referred to as
additional components, can be attached to a
computer and be shared in a network.
 Peripheral devices include faxes, modems,
scanners, plotters, and any other device that
connects to the computers.
 Equipments having common requirements can be
shared in order to reduce maintenance cost.

Share Resources
 Important data can also be stored centrally to
make it accessible to users, thereby saving storage
space on individual computers.
 Computer applications, which take up a
considerable amount of storage space, can be
installed centrally on the network, saving storage
space.

Facilitate Centralized Management
 Networks are used to assist in management tasks
associated with their own operation and
maintenance.
 Using networks results in increased efficiency and
a resultant reduction in maintenance costs.
 Software:
– Software is a set of instructions or programs that
control the operation of a computer.
– Software can be installed at a central location using
servers, where the installation files are made accessible
over the network.

Networking devices
 Equipment that connects directly to a network
segment is referred to as a device. These devices
are broken up into two classifications. The first
classification is end-user devices. End-user
devices include computers, printers, scanners, and
other devices that provide services directly to the
user. The second classification is network devices.
Network devices include all the devices that
connect the end-user devices together to allow
them to communicate.

Networking devices (Contd…)
 End-user devices that provide users
with a connection to the network are
also referred to as hosts. These
devices allow users to share, create,
and obtain information. The host
devices can exist without a network,
but without the network the host
capabilities are greatly reduced. NICs
are used to physically connect host
devices to the network media. They
use this connection to send e-mails,
print reports, scan pictures, or access
databases.

 A NIC is a printed circuit board that
fits into the expansion slot of a bus on
a computer motherboard. It can also
be a peripheral device. NICs are
sometimes called network adapters.
Laptop or notebook computer NICs
are usually the size of a PCMCIA
card. Each NIC is identified by a
unique code called a Media Access
Control (MAC) address. This address
is used to control data
communication for the host on the
network. More about the MAC
address will be covered later. As the
name implies, the NIC controls host
access to the network.

 There are no standardized symbols for
end-user devices in the networking
industry. They appear similar to the real
devices to allow for quick recognition.
 Network devices are used to extend cable
connections, concentrate connections,
convert data formats, and manage data
transfers. Examples of devices that
perform these functions are repeaters,
hubs, bridges, switches, and routers. All
of the network devices mentioned here
are covered in depth later in the course.
For now, a brief overview of networking
devices will be provided.

 A repeater is a network device used to
regenerate a signal. Repeaters regenerate
analog or digital signals that are distorted
by transmission loss due to attenuation.
A repeater does not make intelligent
decision concerning forwarding packets
like a router.
 Hubs concentrate connections. In other
words, they take a group of hosts and
allow the network to see them as a single
unit. This is done passively, without any
other effect on the data transmission.
Active hubs concentrate hosts and also
regenerate signals.

 Bridges convert network data formats
and perform basic data transmission
management. Bridges provide
connections between LANs. They also
check data to determine if it should cross
the bridge. This makes each part of the
network more efficient.
 Workgroup switches add more
intelligence to data transfer management.
They can determine if data should
remain on a LAN and transfer data only
to the connection that needs it. Another
difference between a bridge and switch is
that a switch does not convert data
transmission formats.

 Routers have all the capabilities listed above. Routers can
regenerate signals, concentrate multiple connections, convert
data transmission formats, and manage data transfers. They can
also connect to a WAN, which allows them to connect LANs
that are separated by great distances. None of the other devices
can provide this type of connection.

Classification of Networks
 Classification by network geography.
– Networks are frequently classified according to the
geographical boundaries spanned by the network itself.
• LAN, MAN, WAN, SAN are the basic types of classification,
of which LAN and WAN are frequently used.
 Classification by component roles.
– Networks can also be classified according to the roles
that the networked computers play in the network’s
operation.
• Peer-to-peer and client/server based are the types of roles into
which networks are classified.

Local area network (LAN)
 A LAN covers a relatively small area such as a
classroom, school, or a single building.
 LANs are inexpensive to install and also provide
higher speeds.

Local area network (LAN)
 LANs consist of the following components:
– Computers
– Network interface cards
– Peripheral devices
– Networking media
– Network devices
 LANs allow businesses to locally share computer files and
printers efficiently and make internal communications
possible. A good example of this technology is e-mail.
LANs manage data, local communications, and computing
equipment.
 Some common LAN technologies include the following:
– Ethernet
– Token Ring
– FDDI

Metropolitan area network (MAN)
 A MAN spans the distance of a typical
metropolitan city.
 The cost of installation and operation is higher.
 MANs use high-speed connections such as fiber
optics to achieve higher speeds.

Wide area network (WAN)
 WANs interconnect LANs, which then provide access to
computers or file servers in other locations. Because
WANs connect user networks over a large geographical
area, they make it possible for businesses to communicate
across great distances. WANs allow computers, printers,
and other devices on a LAN to be shared with distant
locations. WANs provide instant communications across
large geographic areas.

Wide area network (WAN)
 WANs are designed to do the following:
– Operate over a large and geographically separated area
– Allow users to have real-time communication capabilities with
other users
– Provide full-time remote resources connected to local services
– Provide e-mail, Internet, file transfer, and e-commerce services
 Some common WAN technologies include the following:
– Modems
– Integrated Services Digital Network (ISDN)
– Digital subscriber line (DSL)
– Frame Relay
– T1, E1, T3, and E3
– Synchronous Optical Network (SONET)

Storage area network (SAN)
 A storage-area network (SAN) is a dedicated,
high-performance network used to move data
between servers and storage resources. Because it
is a separate, dedicated network, it avoids any
traffic conflict between clients and servers.
 SAN technology allows high-speed server-to-
storage, storage-to-storage, or server-to-server
connectivity. This method uses a separate network
infrastructure that relieves any problems
associated with existing network connectivity.

Storage area network (SAN)
 SANs offer the following features:
– Performance – SANs allow concurrent access of disk or
tape arrays by two or more servers at high speeds. This
provides enhanced system performance.
– Availability – SANs have built-in disaster tolerance.
Data can be duplicated on a SAN up to 10 km (6.2
miles) away.
– Scalability – A SAN can use a variety of technologies.
This allows easy relocation of backup data, operations,
file migration, and data replication between systems.

Virtual private network (VPN)
 A vitual private network (VPN) is a private
network that is constructed within a public
network infrastructure such as the global Internet.
Using VPN, a telecommuter can remotely access
the network of the company headquarters.
Through the Internet, a secure tunnel can be built
between the PC of the telecommuter and a VPN
router at the company headquarters.

Intranets and extranets
 One common configuration of a LAN is an
intranet. Intranet Web servers differ from public
Web servers in that the public must have the
proper permissions and passwords to access the
intranet of an organization. Intranets are designed
to permit users who have access privileges to the
internal LAN of the organization. Within an
intranet, Web servers are installed in the network.
Browser technology is used as the common front
end to access information on servers such as
financial, graphical, or text-based data.

Intranets and extranets
 Extranets refer to applications and services that
are Intranet based, and use extended, secure access
to external users or enterprises. This access is
usually accomplished through passwords, user
IDs, and other application-level security. An
extranet is the extension of two or more intranet
strategies with a secure interaction between
participant enterprises and their respective
intranets.

Peer-to-peer
 In a peer-to-peer network, all computers are
considered equal.
 Each computer controls its own information and is
capable of functioning as either a client or a server
depending upon the requirement.
 Peer-to-peer networks are inexpensive and easy to
install.
 They are popular as home networks and for use in
small companies.
 Most operating systems come with built-in peer-
to-peer networking capability.
 The maximum number of peers that can operate
on a peer-to-peer network is ten.

Peer-to-peer
 Each peer shares resources and allows others open
access to them.
 Peer-to-peer networks become difficult to manage
when more security is added to resources, since
the users control their security by password-
protecting shares.
 Shares can be document folders, printers,
peripherals, and any other resource that they
control on their computers.

Peer-to-peer
 Advantages:
– Low cost
– Simple to configure
– User has full accessibility of the computer
 Disadvantages:
– Does not scale well to large networks and administration becomes
unmanageable.
– Each user must be trained to perform administrative task
– Less Secure
– All machines sharing resources negatively impact the performance.
 Where peer-to-peer network is appropriate:
– 10 or less users
– No specialized services required
– Security is not an issue
– Only limited growth in the foreseeable future

Client/Server
 In a client/server arrangement, network services
are located on a dedicated computer called a
server. The server responds to the requests of
clients. The server is a central computer that is
continuously available to respond to requests from
clients for file, print, application, and other
services. Most network operating systems adopt
the form of a client/server relationship. Typically,
desktop computers function as clients and one or
more computers with additional processing power,
memory, and specialized software function as
servers.

Client/Server
 Servers are designed to handle requests from
many clients simultaneously. Before a client can
access the server resources, the client must be
identified and be authorized to use the resource.
Each client is assigned an account name and
password that is verified by an authentication
service. The authentication service guards access
to the network. With the centralization of user
accounts, security, and access control, server-
based networks simplify the administration of
large networks.

Client/Server
 The centralized functions in a client/server network has
substantial advantages and some disadvantages
 Advantages
– Provides better security
– Easier to administer when network is large because administration
is centralized
– All data cab ne backed up on one central location
 Disadvantages
– Requires expensive specialized network administrative and
operational software
– Requires expensive, more powerful hardware and server machine
– Requires a professional administrator
– Has a single point off failure. User data is unavailable if the server
is down.

Network topology
 Network topology defines the structure of the
network. One part of the topology definition is the
physical topology, which is the actual layout of the
wire or media. The other part is the logical
topology, which defines how the hosts access the
media to send data. The physical topologies that
are commonly used are as follows
– BUS
– Ring
– Star
– Extended Star
– Hierarchical
– Mesh

BUS
 A bus topology uses a single backbone cable that
is terminated at both ends. All the hosts connect
directly to this backbone.
 Advantages
– Easy to setup
– Small amount of wire
 Disadvantages
– Slow
– Easy to crash

Ring
 A ring topology connects one host to the next and the last
host to the first. This creates a physical ring of cable.
 Tokens used to transmit data
– Nodes must wait for token to send
 Advantages
– Time to send data is known
– No data collisions
 Disadvantages
– Slow
– Lots of cable

Star & Extended Star
 A star topology connects all cables to a central
point.
 An extended star topology links individual stars
together by connecting the hubs or switches.
 Advantages
– Easy to setup
– One cable can not crash network
 Disadvantages
– One hub crashing downs entire network
– Uses lots of cable

Hierarchical
 A hierarchical topology is similar to an extended
star. However, instead of linking the hubs or
switches together, the system is linked to a
computer that controls the traffic on the topology.
 Advantages
– Scaleable
– Easy Implementation
– Easy Troubleshooting

Mesh
 A mesh topology is implemented to provide as much
protection as possible from interruption of service. For
example, a nuclear power plant might use a mesh topology
in the networked control systems. As seen in the graphic,
each host has its own connections to all other hosts.
Although the Internet has multiple paths to any one
location, it does not adopt the full mesh topology.
 Advantage
– Data will always be delivered
 Disadvantages
– Lots of cable
– Hard to setup

Network protocols
 Protocol suites are collections of protocols that
enable network communication between hosts. A
protocol is a formal description of a set of rules
and conventions that govern a particular aspect of
how devices on a network communicate. Protocols
determine the format, timing, sequencing, and
error control in data communication. Without
protocols, the computer cannot make or rebuild
the stream of incoming bits from another
computer into the original format.

Network protocols
 Protocols control all aspects of data
communication, which include the following:
– How the physical network is built
– How computers connect to the network
– How the data is formatted for transmission
– How that data is sent
– How to deal with errors

Network protocols
 These network rules are created and maintained by
many different organizations and committees.
Included in these groups are the Institute of
Electrical and Electronic Engineers (IEEE),
American National Standards Institute (ANSI),
Telecommunications Industry Association (TIA),
Electronic Industries Alliance (EIA) and the
International Telecommunications Union (ITU),
formerly known as the Comité Consultatif
International Téléphonique et Télégraphique
(CCITT).

Networking Models
 In order for data packets to travel from a source to
a destination on a network, it is important that all
the devices on the network speak the same
language or protocol. A protocol is a set of rules
that make communication on a network more
efficient. For example, while flying an airplane,
pilots obey very specific rules for communication
with other airplanes and with air traffic control.
 A data communications protocol is a set of rules or
an agreement that determines the format and
transmission of data.

Networking Models
 Layer 4 on the source computer communicates
with Layer 4 on the destination computer. The
rules and conventions used for this layer are
known as Layer 4 protocols. It is important to
remember that protocols prepare data in a linear
fashion. A protocol in one layer performs a certain
set of operations on data as it prepares the data to
be sent over the network. The data is then passed
to the next layer where another protocol performs
a different set of operations.

Networking Models
 Once the packet has been sent to the destination,
the protocols undo the construction of the packet
that was done on the source side. This is done in
reverse order. The protocols for each layer on the
destination return the information to its original
form, so the application can properly read the data.
 Two models
– OSI Model
– TCP/IP model

OSI Model
 To address the problem of network
incompatibility, the International Organization for
Standardization (ISO) researched networking
models like Digital Equipment Corporation net
(DECnet), Systems Network Architecture (SNA),
and TCP/IP in order to find a generally applicable
set of rules for all networks. Using this research,
the ISO created a network model that helps
vendors create networks that are compatible with
other networks.

OSI Model
 The Open System Interconnection (OSI) reference model
released in 1984 was the descriptive network model that
the ISO created. It provided vendors with a set of standards
that ensured greater compatibility and interoperability
among various network technologies produced by
companies around the world.
 The OSI reference model has become the primary model
for network communications. Although there are other
models in existence, most network vendors relate their
products to the OSI reference model. This is especially true
when they want to educate users on the use of their
products. It is considered the best tool available for
teaching people about sending and receiving data on a
network.

OSI Model Peer-to-Peer Communications

 In order for data to travel from the source to the destination, each layer
of the OSI model at the source must communicate with its peer layer at
the destination. This form of communication is referred to as peer-to-
peer. During this process, the protocols of each layer exchange
information, called protocol data units (PDUs). Each layer of
communication on the source computer communicates with a layer-
specific PDU, and with its peer layer on the destination computer as
illustrated in Figure .
 Data packets on a network originate at a source and then travel to a
destination. Each layer depends on the service function of the OSI
layer below it. To provide this service, the lower layer uses
encapsulation to put the PDU from the upper layer into its data field.
Then it adds whatever headers and trailers the layer needs to perform
its function. Next, as the data moves down through the layers of the
OSI model, additional headers and trailers are added. After Layers 7, 6,
and 5 have added their information, Layer 4 adds more information.
This grouping of data, the Layer 4 PDU, is called a segment.

 The network layer provides a service to the transport layer, and the
transport layer presents data to the internetwork subsystem. The
network layer has the task of moving the data through the
internetwork. It accomplishes this task by encapsulating the data and
attaching a header creating a packet (the Layer 3 PDU). The header
contains information required to complete the transfer, such as source
and destination logical addresses.
 The data link layer provides a service to the network layer. It
encapsulates the network layer information in a frame (the Layer 2
PDU). The frame header contains information (for example, physical
addresses) required to complete the data link functions. The data link
layer provides a service to the network layer by encapsulating the
network layer information in a frame.
 The physical layer also provides a service to the data link layer. The
physical layer encodes the data link frame into a pattern of 1s and 0s
(bits) for transmission on the medium (usually a wire) at Layer 1.

TCP/IP Model
 The U.S. Department of Defense (DoD) created
the TCP/IP reference model, because it wanted to
design a network that could survive any
conditions, including a nuclear war. In a world
connected by different types of communication
media such as copper wires, microwaves, optical
fibers and satellite links, the DoD wanted
transmission of packets every time and under any
conditions. This very difficult design problem
brought about the creation of the TCP/IP model.

TCP/IP Model
 The TCP/IP model has the following four layers:
– Application layer
– Transport layer
– Internet layer
– Network access layer

Detailed Encapsulation Process

 All communications on a network originate at a
source, and are sent to a destination. The
information sent on a network is referred to as
data or data packets. If one computer (host A)
wants to send data to another computer (host B),
the data must first be packaged through a process
called encapsulation.
 Encapsulation wraps data with the necessary
protocol information before network transit.
Therefore, as the data packet moves down through
the layers of the OSI model, it receives headers,
trailers, and other information.

 To see how encapsulation occurs, examine the
manner in which data travels through the layers as
illustrated in Figure . Once the data is sent from
the source, it travels through the application layer
down through the other layers. The packaging and
flow of the data that is exchanged goes through
changes as the layers perform their services for
end users. As illustrated in Figure , networks must
perform the following five conversion steps in
order to encapsulate data:

 Build the data – As a user sends an e-mail message, its
alphanumeric characters are converted to data that can
travel across the internetwork.
 Package the data for end-to-end transport – The data is
packaged for internetwork transport. By using segments,
the transport function ensures that the message hosts at
both ends of the e-mail system can reliably communicate.
 Add the network IP address to the header – The data is
put into a packet or datagram that contains a packet header
with source and destination logical addresses. These
addresses help network devices send the packets across the
network along a chosen path.

 Add the data link layer header and trailer – Each
network device must put the packet into a frame. The
frame allows connection to the next directly-connected
network device on the link. Each device in the chosen
network path requires framing in order for it to connect to
the next device.
 Convert to bits for transmission – The frame must be
converted into a pattern of 1s and 0s (bits) for transmission
on the medium. A clocking function enables the devices to
distinguish these bits as they travel across the medium. The
medium on the physical internetwork can vary along the
path used. For example, the e-mail message can originate
on a LAN, cross a campus backbone, and go out a WAN
link until it reaches its destination on another remote LAN.

Transmission Media
 Two main categories:
– Guided
• Cables (Coper, Fiber)
– Unguided
• wireless transmission, e.g. radio
• microwave, infrared
• sound, sonar
 We will concentrate on guided media here:
– Coaxial cables
– Twisted-Pair cables:
• Shielded Twisted Pair (STP) Cables
• Screened Twisted Pair (ScTP) Cables
• Unshielded Twisted Pair (UTP) Cables
– Fiber-optic cables

Cable specifications
 The following Ethernet specifications relate to
cable type:
 10BASE-T
 10BASE5
 10BASE2

Coaxial cable
 Coaxial cable consists of a copper conductor
surrounded by a layer of flexible insulation. The
center conductor can also be made of tin plated
aluminium cable allowing for the cable to be
manufactured inexpensively. Over this insulating
material is a woven copper braid or metallic foil
that acts as the second wire in the circuit and as a
shield for the inner conductor. This second layer,
or shield also reduces the amount of outside
electromagnetic interference. Covering this shield
is the cable jacket.

Coaxial cable
 For LANs, coaxial cable offers several
advantages. It can be run longer distances than
shielded twisted pair, STP, unshielded twisted pair,
UTP, and screened twisted pair, ScTP, cable
without the need for repeaters. Repeaters
regenerate the signals in a network so that they
can cover greater distances. Coaxial cable is less
expensive than fiber-optic cable and the
technology is well known. It has been used for
many years for many types of data communication
such as cable television.

Twisted Pair Cables
 If the pair of wires are not twisted,
electromagnetic noises from, e.g., motors, will
affect the closer wire more than the further one,
thereby causing errors

Shielded Twisted Pair (STP) Cables
 STP cable combines the techniques of cancellation,
shielded, and twisted wires. Each pair of wires is wrapped
in metallic foil. The two pairs of wires are wrapped in an
overall metallic braid or foil. It is usually 150-ohm cable.
As specified for use in Token Ring network installations,
STP reduces electrical noise within the cable such as pair
to pair coupling and crosstalk. STP also reduces electronic
noise from outside the cable such as electromagnetic
interference (EMI) and radio frequency interference (RFI).
STP cable shares many of the advantages and
disadvantages of UTP cable. STP provides more protection
from all types of external interference. However, STP is
more expensive and difficult to install than UTP.

Shielded Twisted Pair (STP) Cables

Screened Twisted Pair (ScTP) Cables
 A new hybrid of UTP is Screened UTP (ScTP), which is
also known as foil screened twisted pair (FTP). ScTP is
essentially UTP wrapped in a metallic foil shield, or
screen. ScTP, like UTP, is also 100-ohm cable. Many cable
installers and manufacturers may use the term STP to
describe ScTP cabling. It is important to understand that
most references made to STP today actually refer to four-
pair shielded cabling. It is highly unlikely that true STP
cable will be used during a cable installation job.

Screened Twisted Pair (ScTP) Cables

Unshielded Twisted-Pair (UTP)
 UTP is a four-pair wire medium used in a variety
of networks. Each of the eight copper wires in the
UTP cable is covered by insulating material. In
addition, each pair of wires is twisted around each
other. This type of cable relies on the cancellation
effect produced by the twisted wire pairs to limit
signal degradation caused by EMI and RFI. To
further reduce crosstalk between the pairs in UTP
cable, the number of twists in the wire pairs
varies. Like STP cable, UTP cable must follow
precise specifications as to how many twists or
braids are permitted per foot of cable.

UTP Cabling
 UTP cable has many advantages. It is easy to
install and is less expensive than other types of
networking media. In fact, UTP costs less per
meter than any other type of LAN cabling.
However, the real advantage is the size. Since it
has such a small external diameter, UTP does not
fill up wiring ducts as rapidly as other types of
cable. This can be an extremely important factor to
consider, particularly when a network is installed
in an older building. When UTP cable is installed
with an RJ-45 connector, potential sources of
network noise are greatly reduced and a good solid
connection is almost guaranteed.

Categories of UTP Cables
EIA classifies UTP cables according to the quality:
 Category 1 ― the lowest quality, only good for voice,
mainly found in very old buildings, not recommended now
 Category 2 ― good for voice and low data rates (up to
4Mbps for low-speed token ring networks)
 Category 3 ― at least 3 twists per foot, for up to 10 Mbps
(common in phone networks in residential buildings)
 Category 4 ― up to 16 Mbps (mainly for token rings)
 Category 5 (or 5e) ― up to 100 Mbps (common for
networks targeted for high-speed data communications)
 Category 6 ― more twists than Cat 5, up to 1 Gbps

Fiber Optic Cable
 The part of an optical fiber through which light
rays travel is called the core of the fiber. Light
rays can only enter the core if their angle is inside
the numerical aperture of the fiber. Likewise, once
the rays have entered the core of the fiber, there
are a limited number of optical paths that a light
ray can follow through the fiber. These optical
paths are called modes. If the diameter of the core
of the fiber is large enough so that there are many
paths that light can take through the fiber, the fiber
is called "multimode" fiber. Single-mode fiber has
a much smaller core that only allows light rays to
travel along one mode inside the fiber.

Advantages and Disadvantages
 Noise resistance ― external light is blocked by outer
jacket
 Less signal attenuation ― a signal can run for miles
without regeneration (currently, the lowest measured loss
is about ~4% or 0.16dB per km)
 Higher bandwidth ― currently, limits on data rates come
from the signal generation/reception technology, not the
fiber itself
 Cost ― Optical fibers are expensive
 Installation/maintenance ― any crack in the core will
degrade the signal, and all connections must be perfectly
aligned

Wireless devices and topologies
 This page describes the devices and related topologies
for a wireless network.
 A wireless network may consist of as few as two
devices. - The nodes could simply be desktop
workstations or notebook computers. Equipped with
wireless NICs, an ‘ad hoc’ network could be
established which compares to a peer-to-peer wired
network. Both devices act as servers and clients in this
environment. Although it does provide connectivity,
security is at a minimum along with throughput.
Another problem with this type of network is
compatibility. Many times NICs from different
manufacturers are not compatible.

PCMCIA NIC
Wireless NIC
USB Wireless NIC

 To solve the problem of compatibility, an access
point (AP) is commonly installed to act as a
central hub for the WLAN infrastructure mode.
The AP is hard wired to the cabled LAN to
provide Internet access and connectivity to the
wired network. APs are equipped with antennae
and provide wireless connectivity over a specified
area referred to as a cell. Depending on the
structural composition of the location in which the
AP is installed and the size and gain of the
antennae, the size of the cell could greatly vary.
Most commonly, the range will be from 91.44 to
152.4 meters (300 to 500 feet).

 To service larger areas, multiple access points may
be installed with a degree of overlap. The overlap
permits "roaming" between cells. This is very
similar to the services provided by cellular phone
companies. Overlap, on multiple AP networks, is
critical to allow for movement of devices within
the WLAN. Although not addressed in the IEEE
standards, a 20-30% overlap is desirable. This rate
of overlap will permit roaming between cells,
allowing for the disconnect and reconnect activity
to occur seamlessly without service interruption.

Access Point
Roaming
Wireless LAN

How wireless LANs communicate
 After establishing connectivity to the WLAN, a
node will pass frames in the same manner as on
any other 802.x network. WLANs do not use a
standard 802.3 frame. Therefore, using the term
wireless Ethernet is misleading. There are three
types of frames: control, management, and data.
Only the data frame type is similar to 802.3
frames. The payload of wireless and 802.3 frames
is 1500 bytes; however, an Ethernet frame may not
exceed 1518 bytes whereas a wireless frame could
be as large as 2346 bytes. Usually the WLAN
frame size will be limited to 1518 bytes as it is
most commonly connected to a wired Ethernet
network.

 Since radio frequency (RF) is a shared medium, collisions
can occur just as they do on wired shared medium. The
major difference is that there is no method by which the
source node is able to detect that a collision occurred. For
that reason WLANs use Carrier Sense Multiple
Access/Collision Avoidance (CSMA/CA). This is
somewhat like Ethernet CSMA/CD.

 When a source node sends a frame, the receiving node
returns a positive acknowledgment (ACK). This can cause
consumption of 50% of the available bandwidth. This
overhead when combined with the collision avoidance
protocol overhead reduces the actual data throughput to a
maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN
rated at 11 Mbps.
 Performance of the network will also be affected by signal
strength and degradation in signal quality due to distance
or interference. As the signal becomes weaker, Adaptive
Rate Selection (ARS) may be invoked. The transmitting
unit will drop the data rate from 11 Mbps to 5.5 Mbps,
from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.

Wireless security
 EAP-MD5 Challenge – Extensible Authentication
Protocol is the earliest authentication type, which is very
similar to CHAP password protection on a wired network.
 LEAP (Cisco) – Lightweight Extensible Authentication
Protocol is the type primarily used on Cisco WLAN access
points. LEAP provides security during credential
exchange, encrypts using dynamic WEP keys, and supports
mutual authentication.
 User authentication – Allows only authorized users to
connect, send and receive data over the wireless network.
 Encryption – Provides encryption services further
protecting the data from intruders.
 Data authentication – Ensures the integrity of the data,
authenticating source and destination devices.

Ethernet media and connector requirements

Connection media
 The RJ-45 connector and jack are the most common. RJ-45
connectors are discussed in more detail in the next section.
 The connector on a NIC may not match the media to which
it needs to connect. As shown in Figure , an interface may
exist for the 15-pin attachment unit interface (AUI)
connector. The AUI connector allows different media to
connect when used with the appropriate transceiver. A
transceiver is an adapter that converts one type of
connection to another. A transceiver will usually convert an
AUI to an RJ-45, a coax, or a fiber optic connector. On
10BASE5 Ethernet, or Thicknet, a short cable is used to
connect the AUI with a transceiver on the main cable.

UTP Implementation
RJ 45 Connector
RJ 45 Jack
RJ 45 Jack

UTP Implementation (Straight Through)
 Use straight-through cables for the following
connections:
– Switch to router
– Switch to PC or server
– Hub to PC or server

UTP Implementation (Crossover)
 Use crossover cables for the following
connections:
– Switch to switch
– Switch to hub
– Hub to hub
– Router to router
– PC to PC
– Router to PC

UTP Implementation (Crossover)

Interconnecting Devices Using Crossover

Cabling LANs (Repeaters)
 A repeater receives a signal, regenerates it, and
passes it on. It can regenerate and retime network
signals at the bit level to allow them to travel a
longer distance on the media.

Cabling LANs (Hub)
 Hubs are actually multiport repeaters. The difference
between hubs and repeaters is usually the number of ports
that each device provides.
 Hubs come in three basic types:
– Passive – A passive hub serves as a physical connection point only.
It does not manipulate or view the traffic that crosses it. It does not
boost or clean the signal. A passive hub is used only to share the
physical media. A passive hub does not need electrical power.
– Active – An active hub must be plugged into an electrical outlet
because it needs power to amplify a signal before it is sent to the
other ports.
– Intelligent – Intelligent hubs are sometimes called smart hubs.
They function like active hubs with microprocessor chips and
diagnostic capabilities. Intelligent hubs are more expensive than
active hubs. They are also more useful in troubleshooting
situations.

Cabling LANs (Bridge)
 There are times when it is necessary to break up a
large LAN into smaller and more easily managed
segments.

Cabling LANs (Bridge)
 When a bridge receives a frame on the network,
the destination MAC address is looked up in the
bridge table to determine whether to filter, flood,
or copy the frame onto another segment. This
decision process occurs as follows:
– If the destination device is on the same segment as the
frame, the bridge will not send the frame onto other
segments. This process is known as filtering.
– If the destination device is on a different segment, the
bridge forwards the frame to the appropriate segment.
– If the destination address is unknown to the bridge, the
bridge forwards the frame to all segments except the
one on which it was received. This process is known as
flooding.

Cabling LANs (Switch)
 A switch is sometimes described as a multiport
bridge. A typical bridge may have only two ports
that link two network segments. A switch can have
multiple ports based on the number of network
segments that need to be linked. Like bridges,
switches learn information about the data frames
that are received from computers on the network.
Switches use this information to build tables to
determine the destination of data that is sent
between computers on the network.

Cabling LANs (Host Connectivity)
 The function of a NIC is to connect a host device to
the network medium. A NIC is a printed circuit board
that fits into the expansion slot on the motherboard or
peripheral device of a computer. The NIC is also
referred to as a network adapter. On laptop or
notebook computers a NIC is the size of a credit card.
 NICs are considered Layer 2 devices because each
NIC carries a unique code called a MAC address. This
address is used to control data communication for the
host on the network. More will be learned about the
MAC address later. NICs control host access to the
medium.

Cabling LANs (Host Connectivity)
 The function of a NIC is to connect a
host device to the network medium. A
NIC is a printed circuit board that fits
into the expansion slot on the
motherboard or peripheral device of a
computer. The NIC is also referred to
as a network adapter. On laptop or
notebook computers a NIC is the size of
a credit card.
 NICs are considered Layer 2 devices because each NIC
carries a unique code called a MAC address. This address
is used to control data communication for the host on the
network. More will be learned about the MAC address
later. NICs control host access to the medium.

IP Addressing
 The identifier used in the IP layer of the
TCP/IP protocol suite to identify each device
connected to the Internet is called the Internet
address or IP address. An IPv4 address is a
32-bit address that uniquely and universally
defines the connection of a host or a router to
the Internet; an IP address is the address of
the interface.
 The address space of IPv4 is 232 or
4,294,967,296.

IP Addressing
Application dataTCP HeaderEthernet Header Ethernet Trailer
Ethernet frame
IP Header
0x4 0x5 0x00 4410
9d08 0102 00000000000002
128.143.137.144
128.143.71.21
12810 0x06 8bff
32 bits
Ethernet frame
IP Header
version
(4 bits)
header
length
Type of Service/TOS
(8 bits)
Total Length (in bytes)
(16 bits)
Identification (16 bits)
flags
(3 bits)
Fragment Offset (13 bits)
Source IP address (32 bits)
Destination IP address (32 bits)
TTL Time-to-Live
(8 bits)
Protocol
(8 bits)
Header Checksum (16 bits)
32 bits

Dotted Decimal Notation
 IP addresses are written in a so-called dotted
decimal notation
 Each byte is identified by a decimal number in the
range [0..255]:
 Example:
1000111110000000 10001001 10010000
1st Byte
= 128
2nd
Byte
= 143
3rd Byte
= 137
4th Byte
= 144
128.143.137.144

Network prefix and Host number
 The network prefix identifies a network and the
host number identifies a specific host (actually,
interface on the network).
 How do we know how long the network prefix
is?
– The network prefix is implicitly defined.
– The network prefix is indicated by a netmask.
network prefix host number

Example
 Example: example.com
 Network id is: 192.168.1.0
 Host number is: 6
 Network mask is: 255.255.255.0 or
ffffff00
 Prefix notation: 192.168.1.0/24
– Network prefix is 24 bits long
192.168 1.6

The old way: Classful IP Addresses
 To accommodate different size networks and aid
in classifying these networks, IP addresses are
divided into groups called classes. This is known
as classful addressing. Each complete 32-bit IP
address is broken down into a network part and a
host part. A bit or bit sequence at the start of each
address determines the class of the address. There
are five IP address classes as shown in Figure .

 When Internet addresses were standardized (early
1980s), the Internet address space was divided up
into classes:
– Class A: Network prefix is 8 bits long
– Class B: Network prefix is 16 bits long
– Class C: Network prefix is 24 bits long
 Each IP address contained a key which identifies
the class:
– Class A: IP address starts with “0”
– Class B: IP address starts with “10”
– Class C: IP address starts with “110”

Class C network id host11 0
Network Prefix
24 bits
Host Number
8 bits
bit # 0 1 23 242 313
Class B 1 network id host
bit # 0 1 15 162
Network Prefix
16 bits
Host Number
16 bits
0
31
Class A 0
Network Prefix
8 bits
bit # 0 1 7 8
Host Number
24 bits
31
Class D multicast group id11 1
bit # 0 1 2 313
0
4
Class E (reserved for future use)11 1
bit # 0 1 2 313
1
4
0
5

Problems with Classful IP Addresses
 The original classful address scheme had a number of problems
 Problem 1. Too few network addresses for large networks
– Class A and Class B addresses are gone
 Problem 2. Two-layer hierarchy is not appropriate for large networks
with Class A and Class B addresses.
– Fix #1: Subnetting
 Problem 3. Inflexible. Assume a company requires 2,000 addresses
– Class A and B addresses are overkill
– Class C address is insufficient (requires 8 Class C addresses)
– Fix #2: Classless Interdomain Routing (CIDR)
 Problem 4: Exploding Routing Tables: Routing on the backbone
Internet needs to have an entry for each network address. In 1993, the
size of the routing tables started to outgrow the capacity of routers.
– Fix #2: Classless Interdomain Routing (CIDR)
 Problem 5. The Internet is going to outgrow the 32-bit addresses
– Fix #3: IP Version 6

Subnetting
 Problem: Organizations have multiple networks which are
independently managed
– Solution 1: Allocate one or more addresses for each network
• Difficult to manage
• From the outside of the organization, each network must be
addressable.
– Solution 2: Add another level of hierarchy to the IP addressing
structure
University Network
Medical
School
Library
Engineering
School

Basic Idea of Subnetting
 Split the host number portion of an IP address into
a subnet number and a (smaller) host
number.
 Result is a 3-layer hierarchy
 Then:
• Subnets can be freely assigned within the organization
• Internally, subnets are treated as separate networks
• Subnet structure is not visible outside the organization
network prefix host number
subnet numbernetwork prefix host number
extended network prefix

Subnet Masks
 Routers and hosts use an extended network prefix
(subnet mask) to identify the start of the host
numbers
* There are different ways of subnetting. Commonly
used netmasks for university networks with /16 prefix
(Class B) are 255.255.255.0 and 255.255.0.0
Class B network host
16 bits
with
subnetting
host
Subnet
mask
(255.255.255.0)
network subnet
Network Prefix (16 bits)
1
1111111111111111111111100000000
0
10
Extended Network Prefix (24 bits)

Subnetting Example
 Address: 192.168.0.1 11000000.10101000.00000000.00000001
 Netmask: 255.255.255.0=24 11111111 .11111111 .11111111 .00000000
 Network: 192.168.0.0/24 11000000.10101000.00000000.00000000
 Broadcast: 192.168.0.255 11000000.10101000.00000000.11111111
 HostMin: 192.168.0.1 11000000.10101000.00000000.00000001
 HostMax: 192.168.0.254 11000000.10101000.00000000.11111110
 Class: C
 Address: 123.200.11.1 01111011 .11001000.00001011.00000001
 Netmask: 255.0.0.0 = 8 11111111 .00000000.00000000.00000000
 Network: 123.0.0.0/8 01111011 .00000000.00000000.00000000
 Broadcast: 123.255.255.255 01111011 .11111111.11111111.11111111
 HostMin: 123.0.0.1 01111011 .00000000.00000000.00000001
 HostMax: 123.255.255.254 01111011 .11111111.11111111.11111110
 Hosts/Net: 16777214
 Class: A

Typical Subnetting Plan for an Organization
 Each layer-2 network (Ethernet segment, FDDI
segment) is allocated a subnet address.
128.143.17.0 / 24
128.143.71.0 / 24
128.143.7.0 / 24
128.143.16.0 / 24
128.143.8.0 / 24
128.143.22.0 / 24
128.143.136.0 / 24
128.143.0.0/16

Advantages of Subnetting
 With subnetting, IP addresses use a 3-layer hierarchy:
– Network
– Subnet
– Host
 Improves efficiency of IP addresses by not consuming an
entire address space for each physical network.
 Reduces router complexity. Since external routers do not
know about subnetting, the complexity of routing tables at
external routers is reduced.
 Note: Length of the subnet mask need not be identical at
all subnetworks.

Reserved IP addresses
 Certain host addresses are reserved and cannot be assigned to devices
on a network. These reserved host addresses include the following:
– Network address – Used to identify the network itself
 In Figure , the section that is identified by the upper box represents the
198.150.11.0 network. Data that is sent to any host on that network
(198.150.11.1- 198.150.11.254) will be seen outside of the local area
network as 198.159.11.0. The only time that the host numbers matter is
when the data is on the local area network. The LAN that is contained
in the lower box is treated the same as the upper LAN, except that its
network number is 198.150.12.0.
– Broadcast address – Used for broadcasting packets to all the devices on a
network

 In Figure , the section that is identified by the
upper box represents the 198.150.11.255 broadcast
address. Data that is sent to the broadcast address
will be read by all hosts on that network
(198.150.11.1- 198.150.11.254). The LAN that is
contained in the lower box is treated the same as
the upper LAN, except that its broadcast address is
198.150.12.255.

 An IP address that has binary 0s in all host bit positions is
reserved for the network address. In a Class A network
example, 113.0.0.0 is the IP address of the network, known
as the network ID, containing the host 113.1.2.3. A router
uses the network IP address when it forwards data on the
Internet. In a Class B network example, the address
176.10.0.0 is a network address, as shown in Figure.

 To send data to all the devices
on a network, a broadcast
address is needed. A broadcast
occurs when a source sends
data to all devices on a
network. To ensure that all the
other devices on the network
process the broadcast, the
sender must use a destination
IP address that they can
recognize and process.
Broadcast IP addresses end
with binary 1s in the entire
host part of the address.

 In the network example, 176.10.0.0, the last 16 bits make
up the host field or host part of the address. The broadcast
that would be sent out to all devices on that network would
include a destination address of 176.10.255.255. This is
because 255 is the decimal value of an octet containing
11111111.

Public Address
 Public IP addresses are unique. No two machines
that connect to a public network can have the same
IP address because public IP addresses are global
and standardized. All machines connected to the
Internet agree to conform to the system. Public IP
addresses must be obtained from an Internet
service provider (ISP) or a registry at some
expense.

Private Address
 Computers on private LANs do not need a public IP
addresses, since they do not need to be accessed by the
public.
 Therefore, certain addresses that will never be registered
publicly are reserved. These are known as private IP
addresses, and are found in the following ranges:
– From 10.0.0.0 to 10.255.255.255
– From 172.16.0.0 to 172.31.255.255
– From 192.168.0.0 to 192.168.255.255
 Devices with private IP addresses cannot connect directly
to the Internet
 Computers outside the network cannot access devices with
a private IP address.
 Access must be obtained through a router.

CIDR - Classless Interdomain Routing
 IP backbone routers have one routing table entry
for each network address:
– With subnetting, a backbone router only needs to know one entry
for each network
– This is acceptable for Class A and Class B networks
• 27 = 128 Class A networks
• 214 = 16,384 Class B networks
– But this is not acceptable for Class C networks
• 221 = 2,097,152 Class C networks
 In 1993, the size of the routing tables started to
outgrow the capacity of routers
 Consequence: The Class-based assignment of IP
addresses had to be abandoned

CIDR - Classless Interdomain Routing
 Goals:
– Restructure IP address assignments to increase
efficiency
– Hierarchical routing aggregation to minimize route
table entries
Key Concept: The length of the network id (prefix)
in the IP addresses is kept arbitrary
 Consequence: Routers advertise the IP address and
the length of the prefix

CIDR Example
 CIDR notation of a network address:
192.0.2.0/18
• "18" says that the first 18 bits are the network part of the
address (and 14 bits are available for specific host addresses)
 The network part is called the prefix
 Assume that a site requires a network address with 1000 addresses
 With CIDR, the network is assigned a continuous block of 1024
addresses with a 22-bit long prefix.

CIDR: Prefix Size vs. Network Size
CIDR Block Prefix # of Host Addresses
/27 32 hosts
/26 64 hosts
/25 128 hosts
/24 256 hosts
/23 512 hosts
/22 1,024 hosts
/21 2,048 hosts
/20 4,096 hosts
/19 8,192 hosts
/18 16,384 hosts
/17 32,768 hosts
/16 65,536 hosts
/15 131,072 hosts
/14 262,144 hosts
/13 524,288 hosts

CIDR and Address assignments
 Backbone ISPs obtain large block of IP addresses space
and then reallocate portions of their address blocks to their
customers.
Example:
 Assume that an ISP owns the address block 206.0.64.0/18,
which represents 16,384 (214) IP addresses
 Suppose a client requires 800 host addresses
 With classful addresses: need to assign a class B address
(and waste ~64,700 addresses) or four individual Class Cs
(and introducing 4 new routes into the global Internet
routing tables)
 With CIDR: Assign a /22 block, e.g., 206.0.68.0/22, and
allocated a block of 1,024 (210) IP addresses.

CIDR and Routing Information
206.0.64.0/18
204.188.0.0/15
209.88.232.0/21
Internet
Backbone
ISP X owns:
Company X :
206.0.68.0/22
ISP y :
209.88.237.0/24
Organization z1 :
209.88.237.192/26
Organization z2 :
209.88.237.0/26

CIDR and Routing Information
206.0.64.0/18
204.188.0.0/15
209.88.232.0/21
Internet
Backbone
ISP X owns:
Company X :
206.0.68.0/22
ISP y :
209.88.237.0/24
Organization z1 :
209.88.237.192/26
Organization z2 :
209.88.237.0/26
Backbone sends everything
which matches the prefixes
206.0.64.0/18, 204.188.0.0/15,
209.88.232.0/21 to ISP X.
ISP X sends everything which
matches the prefix: 206.0.68.0/22
to Company X,
209.88.237.0/24 to ISP y
Backbone routers do not know
anything about Company X, ISP
Y, or Organizations z1, z2.
ISP X does not know about
Organizations z1, z2.
ISP y sends everything which matches
the prefix:
209.88.237.192/26 to Organizations z1
209.88.237.0/26 to Organizations z2

CIDR and Routing Example
 The IP Address: 207.2.88.170
Belongs to:
Cable & Wireless USA 207.0.0.0 - 207.3.255.255
11001111 00000010
207 2
01011000
88
10101010
170
11001111 00000010 01011000 00000000
Belongs to:
City of Charlottesville, VA: 207.2.88.0 - 207.2.92.255
11001111 00000000 00000000 00000000
You can find about ownership of IP addresses in
North America via http://www.arin.net/whois/

CIDR and Routing
 Aggregation of routing table entries:
– 128.143.0.0/16 and 128.142.0.0/16 are represented as
128.142.0.0/15
 Longest prefix match: Routing table lookup finds
the routing entry that matches the longest prefix
What is the outgoing interface for
128.143.137.0 ?
Prefix Interface
128.0.0.0/4 interface #5
128.128.0.0/9 interface #2
128.143.128.0/17 interface #1
Routing table

IPv6 - IP Version 6
 IP Version 6
– Is the successor to the currently used IPv4
– Specification completed in 1994
– Makes improvements to IPv4 (no revolutionary
changes)
 One (not the only !) feature of IPv6 is a significant
increase in size of the IP address to 128 bits (16
bytes)
• IPv6 will solve – for the foreseeable future – the problems with
IP addressing

IPv6 Header
Ethernet frame
IPv6 Header
version
(4 bits)
Traffic Class
(8 bits)
Flow Label
(24 bits)
Payload Length (16 bits)
Next Header
(8 bits)
Hop Limits (8 bits)
Source IP address (128 bits)
32 bits
Destination IP address (128 bits)

IPv6 vs. IPv4: Address Comparison
 IPv4 has a maximum of
232  4 billion addresses
 IPv6 has a maximum of
2128 = (232)4  4 billion x 4 billion x 4 billion x 4 billion
addresses

Notation of IPv6 addresses
 Convention: The 128-bit IPv6 address is written as eight
16-bit integers (using hexadecimal digits for each integer)
CEDF:BP76:3245:4464:FACE:2E50:3025:DF12
 Short notation:
 Abbreviations of leading zeroes:
CEDF:BP76:0000:0000:009E:0000:3025:DF12
 CEDF:BP76:0:0:9E :0:3025:DF12
 “:0000:0000” can be written as “::”
CEDF:BP76:0:0:FACE:0:3025:DF12  CEDF:BP76::FACE:0:3025:DF12
 IPv6 addresses derived from IPv4 addresses have 96 leading zero bits.
Convention allows to use IPv4 notation for the last 32 bits.
::80:8F:89:90  ::128.143.137.144

IPv6 Provider-Based Addresses
 The first IPv6 addresses will be allocated to a provider-
based plan
 Type: Set to “010” for provider-based addresses
 Registry: identifies the agency that registered the address
The following fields have a variable length (recommeded length in “()”)
 Provider: Id of Internet access provider (16 bits)
 Subscriber: Id of the organization at provider (24 bits)
 Subnetwork: Id of subnet within organization (32 bits)
 Interface: identifies an interface at a node (48 bits)
Registry
ID
Provider
ID
010
Subscriber
ID
Interface
ID
Subnetwork
ID

More on IPv6 Addresses
 The provider-based addresses have a similar flavor
as CIDR addresses
 IPv6 provides address formats for:
– Unicast – identifies a single interface
– Multicast – identifies a group. Datagrams sent to a
multicast address are sent to all members of the group
– Anycast – identifies a group. Datagrams sent to an
anycast address are sent to one of the members in the
group.

Introduction to computer network

More Related Content

What's hot

Similar to Introduction to computer network

More from K. A. M Lutfullah

Recently uploaded

Introduction to computer network