Definition
of Network:
A computer
network is a set of computers or devices that are connected with each other to
carry on data and share information. In computing, it is called a network as a
way to interconnect two or more devices to each other using cables, signals,
waves or other methods with the ultimate goal of transmitting data, share
information, resources and services.
Purpose
of networking:
The purpose
of a network is, generally, to facilitate and expedite communications between
two or more instances on the same physical space or connected remotely. Such
systems also allow cost savings and time.
The most
known type of network is the Intranet, which is a private network that uses
Internet as a basic architecture in order to connect various devices. Internet,
however, is a technology that connects devices throughout the world, and that
is why it is called “network of networks.”
Classifications
of Networks:
The networks
are classified by range (personal, local, campus, metropolitan or wide area),
as well as by method of connection (cable, fiber optics, radio, infrared,
wireless, etc..) or by functional relationship (client – server or
peer-to-peer). Also in the topology field there is a classification to be aware
of (bus, star, ring, mesh, tree etc.) and directional (simplex, half duplex or
full duplex).
Use of a
network:
The use of a
network in an office, for example, in which all employees have the same access
to resources such as programs and applications or devices like a printer or
scanner. Moreover, configuring a large-scale network facilitates communication
among different geographic locations, so a company with multiple branches in
the world can keep in communication with its members in a simple and quick.
Finally, a network can be used as a home to share files or maximize the
available space.
Analog
Network Signaling:
An analog
signal is best compared to a wave. It
has similar properties to an ocean wave, and can be described using three
specific characteristics: amplitude, frequency, and wavelength.
To use the
ocean wave analogy an analog signal's amplitude is like the height of a wave
rolling in onto the beach. The frequency of an analog signal can be compared to
how fast the waves roll in. Wavelength
can be compared to the distance between one wave and the next wave. Wavelength is measured as the distance
between the peak of one wave and the next.
Advantages
and Disadvantages of Analog Signals
Analog
signals are variable and can convey more subtly than a digital signal. For example the human voice is analog, and
has more tone than a digital representation of the same voice. However, analog signals are very vulnerable
to interference from outside forces and other waves which can cancel them out.
Digital
Network Signaling:
A digital
signal is made up of on/off states.
Unlike the smooth curve of an analog wave, the digital signal cuts on
and off. This happens to perfectly fit
the type of communication inside a computer, which is made up of on/off states
as well.
Digital
signals are much more reliable than analog signals because they are less
vulnerable to interference and errors. However, digital equipment costs more
and is much more complex.
Modulation
(AM, FM, PM)
In
telecommunications, modulation is the process of conveying a message signal,
for example a digital bit stream or an analog audio signal, inside another
signal that can be physically transmitted. Modulation of a sine waveform is
used to transform a baseband message signal into a pass band signal, for
example low-frequency audio signal into a radio-frequency signal (RF signal).
In radio communications, cable TV systems or the public switched telephone
network for instance, electrical signals can only be transferred over a limited
pass band frequency spectrum, with specific (non-zero) lower and upper cutoff
frequencies. Modulating a sine-wave carrier makes it possible to keep the
frequency content of the transferred signal as close as possible to the centre
frequency (typically the carrier frequency) of the pass band.
A device
that performs modulation is known as a modulator and a device that performs the
inverse operation of modulation is known as a demodulator (sometimes detector
or demod). A device that can do both operations is a modem
(modulator–demodulator).
Amplitude
Modulation (AM)
Amplitude
modulation (AM) is a method of impressing data onto an alternating-current (AC)
carrier waveform. The highest frequency of the modulating data is normally less
than 10 percent of the carrier frequency. The instantaneous amplitude(overall
signal power) varies depending on the instantaneous amplitude of the modulating
data. In AM, the carrier itself does not fluctuate in amplitude. Instead, the
modulating data appears in the form of signal components at frequencies
slightly higher and lower than that of the carrier. These components are called
sidebands. The lower sideband (LSB) appears at frequencies below the carrier
frequency; the upper sideband (USB) appears at frequencies above the carrier frequency.
The LSB and USB are essentially "mirror images" of each other in a
graph of signal amplitude versus frequency, as shown in the illustration. The
sideband power accounts for the
variations in the overall amplitude of the signal.
When a
carrier is amplitude-modulated with a pure sine wave, up to 1/3 (33percent) of
the overall signal power is contained in the sidebands. The other 2/3 of the
signal power is contained in the carrier, which does not contribute to the
transfer of data. With a complex modulating signal such as voice, video, or
music, the sidebands generally contain 20 to 25 percent of the overall signal
power; thus the carrier consumes75 to 80 percent of the power. This makes AM an
inefficient mode. If an attempt is made to increase the modulating data input
amplitude beyond these limits, the signal will become distorted, and will
occupy a much greater bandwidth than it should. This is called over modulation,
and can result in interference to signals on nearby frequencies.
Frequency
Modulation (FM)
Frequency
modulation (FM) is a method of impressing data onto an alternating-current (AC)
wave by varying the instantaneous (immediate) frequency of the wave. This
scheme can be used with analog or digital data.
Analog
FM
In analog
FM, the frequency of the AC signal wave, also called the carrier, varies in a
continuous manner. Thus, there are infinitely many possible carrier
frequencies. In narrowband FM, commonly used in two-way wireless
communications, the instantaneous carrier frequency varies by up to 5 kilohertz
(kHz, where 1 kHz = 1000 hertz or alternating cycles per second) above and
below the frequency of the carrier with no modulation. In wideband FM, used in
wireless broadcasting, the instantaneous frequency varies by up to several megahertz
(MHz, where 1 MHz = 1,000,000 Hz). When the instantaneous input wave has
positive polarity, the carrier frequency shifts in one direction; when the
instantaneous input wave has negative polarity, the carrier frequency shifts in
the opposite direction. At every instant in time, the extent of
carrier-frequency shift (the deviation) is directly proportional to the extent
to which the signal amplitude is positive or negative.
Digital
FM
In digital
FM, the carrier frequency shifts abruptly, rather than varying continuously.
The number of possible carrier frequency states is usually a power of 2. If
there are only two possible frequency states, the mode is called
frequency-shift keying (FSK). In more complex modes, there can be four, eight,
or more different frequency states. Each specific carrier frequency represents
a specific digital input data state.
Phase
Modulation (PM):
Phase
modulation (PM) is a method of impressing data onto an alternating-current (AC)
waveform by varying the instantaneous phase of the wave. This scheme can be
used with analog or digital data.
Analog PM,
The phase of the AC signal wave, also called the carrier, varies in a
continuous manner. Thus, there are infinitely many possible carrier phase
states. When the instantaneous data input waveform has positive polarity, the
carrier phase shifts in one direction; when the instantaneous data input
waveform has negative polarity, the carrier phase shifts in the opposite
direction. At every instant in time, the extent of carrier-phase shift(the
phase angle) is directly proportional to the extent to which the signal
amplitude is positive or negative.
Digital
PM
In digital
PM, the carrier phase shifts abruptly, rather than continuously back and forth.
The number of possible carrier phase states is usually a power of2. If there
are only two possible phase states, the mode is called biphase modulation. In
more complex modes, there can be four, eight, or more different phase states.
Each phase angle (that is, each shift from one phase state to
another)represents a specific digital input data state.
Phase modulation
is similar in practice to frequency modulation (FM). When the instantaneous
phase of a carrier is varied, the instantaneous frequency changes as well. The
converse also holds: When the instantaneous frequency is varied, the
instantaneous phase changes. But PM and FM are not exactly equivalent,
especially in analog applications. When an FM receiver is used to demodulate a
PM signal, or when FM signal is intercepted by a receiver designed for PM, the
audio is distorted. This is because the relationship between phase and
frequency variations is not linear; that is, phase and frequency do not vary in
direct proportion.
Direction
of communication flow(Simplex, Halfduplex, FullDuplex)
In data
communications, flow control is the process of managing the pacing of data
transmission between two nodes to prevent a fast sender from outrunning a slow
receiver. It provides a mechanism for the receiver to control the transmission
speed, so that the receiving node is not overwhelmed with data from
transmitting node. Flow control should be distinguished from congestion
control, which is used for controlling the flow of data when congestion has
actually occurred. Flow control mechanisms can be classified by whether or not
the receiving node sends feedback to the sending node.
Flow control
is important because it is possible for a sending computer to transmit
information at a faster rate than the destination computer can receive and
process them. This can happen if the receiving computers have a heavy traffic
load in comparison to the sending computer, or if the receiving computer has
less processing power than the sending computer.
Data flow is
the flow of data between two points. The direction of the data flow can be
described as:
Simplex:
Data flows
in only one direction on the data communication line (medium). Examples are
radio and television broadcasts. They go from the TV station to your home
television.
HALF-DUPLEX
Half-Duplex:
data flows in both directions but only one direction at a time on the data
communication line. For example, a conversation on walkie-talkies is a
half-duplex data flow. Each person takes turns talking. If both talk at once -
nothing occurs!
Bi-directional
but only 1 direction at a time!
Full-Duplex:
Data flows
in both directions simultaneously. Modems are configured to flow data in both
directions.
Bi-directional
both directions simultaneously!
Simplex vs. Duplex
SIMPLEX
Simplex
communication is permanent unidirectional communication. Some of the very first
serial connections between computers were simplex connections. For example,
mainframes sent data to a printer and never checked to see if the printer was
available or if the document printed properly since that was a human job.
Simplex links are built so that the transmitter (the one talking) sends a
signal and it's up to the receiving device (the listener) to figure out what
was sent and to correctly do what it was told. No traffic is possible in the
other direction across the same connection.
You must use
connectionless protocols with simplex circuits as no acknowledgement or return
traffic is possible over a simplex circuit. Satellite communication is also
simplex communication. A radio signal is transmitted and it is up to the
receiver to correctly determine what message has been sent and whether it
arrived intact. Since televisions don't talk back to the satellites (yet),
simplex communication works great in broadcast media such as radio, television
and public announcement systems.
HALF
DUPLEX
A half-duplex
link can communicate in only one direction, at a time. Two way communication is
possible, but not simultaneously. Walkie-talkies and CB radios sort of mimic
this behavior in that you cannot hear the other person if you are talking.
Half-duplex connections are more common over electrical links. Since
electricity won't flow unless you have a complete loop of wire, you need two
pieces of wire between the two systems to form the loop. The first wire is used
to transmit, the second wire is referred to as a common ground. Thus, the flow
of electricity can be reversed over the transmitting wire, thereby reversing
the path of communication. Electricity cannot flow in both directions
simultaneously, so the link is half-duplex.
FULL
DUPLEX
Full duplex
communication is two-way communication achieved over a physical link that has
the ability to communicate in both directions simultaneously. With most
electrical, fiber optic, two-way radio and satellite links, this is usually
achieved with more than one physical connection. Your telephone line contains
two wires, one for transmit, the other for receive. This means you and your
friend can both talk and listen at the same time.
Half or
Full-Duplex is required for connection-oriented protocols such as TCP. A duplex
circuit can be created by using two separate physical connections running in
half duplex mode or simplex mode. Two way satellite communications is achieved
using two simplex connections.
Types of Network
The types of
network are categorized on the basis of the number of systems or devices that
are under the networked area. Computer Networking is one of the most important
wings of computing. Networking is the process by which two or more computers
are linked together for a flawless communication. By creating a network,
devices like printers and scanners, software, and files and data that are
stored in the system can be shared. It helps the communication among multiple
computers easy. By computer networking the user access may be restricted when
necessary.
There
three types of networks:
Local
Area Network (LAN):
A local area
network (LAN) is a computer network that connects computers and devices in a
limited geographical area such as home, school, computer laboratory or office
building. The defining characteristics of LANs, in contrast to wide area
networks (WANs), include their usually higher data-transfer rates, smaller
geographic area, and lack of a need for leased telecommunication lines. The
Local Area Network is also referred as LAN. This system spans on a small area
like a small office or home. The computer systems are linked with cables. In
LAN system computers on the same site could be linked.
Wide
Area Network (WAN):
A wide area
network (WAN) is a computer network that covers a broad area (i.e., any network
whose communications links cross metropolitan, regional, or national
boundaries). This is in contrast with personal area networks (PANs), local area
networks (LANs), campus area networks (CANs), or metropolitan area networks
(MANs) which are usually limited to a room, building, campus or specific
metropolitan area (e.g., a city) respectively. A Wide Area Network or WAN is a
type of networking where a number of resources are installed across a large
area such as multinational business. Through WAN offices in different countries
can be interconnected. The best example of a WAN could be the Internet that is
the largest network in the world. In WAN computer systems on different sites
can be linked.
Metropolitan
area network (MAN):
A
metropolitan area network (MAN) is a computer network that usually spans a city
or a large campus. A MAN usually interconnects a number of local area networks
(LANs) using a high-capacity backbone technology, such as fiber-optical links,
and provides up-link services to wide area networks (or WAN) and the Internet.
The IEEE
802-2002 standard describes a MAN as being:
A MAN is optimized for a larger
geographical area than a LAN, ranging from several blocks of buildings to
entire cities. MANs can also depend on communications channels of
moderate-to-high data rates. A MAN might be owned and operated by a single
organization, but it usually will be used by many individuals and organizations. MANs might also be owned and operated as
public utilities. They will often provide means for internetworking of local
networks.
The types of
networks can be further classified into two more divisions:
Peer to peer
Network:
Peer to peer
is an approach to computer networking where all computers share equivalent
responsibility for processing data. Peer-to-peer networking (also known simply
as peer networking) differs from client-server networking, where certain
devices have responsibility for providing or "serving" data and other
devices
Consume or
otherwise act as "clients" of those servers.
Characteristics
of a Peer Network:
Peer to peer
networking is common on small local area networks (LANs), particularly home
networks. Both wired and wireless home networks can be configured as peer to
peer environments.
Computers in
a peer to peer network run the same networking protocols and software. Peer
networks are also often situated physically near to each other, typically in
homes, small businesses or schools. Some peer networks, however, utilize the
Internet and are geographically dispersed worldwide.
Home
networks that utilize broadband routers are hybrid peer to peer and
client-server environments. The router provides centralized Internet connection
sharing, but file, printer and other resource sharing is managed directly
between the local computers involved.
Peer
to Peer and P2P Networks:
Internet-based
peer to peer networks emerged in the 1990s due to the development of P2P file
sharing networks like Napster. Technically, many P2P networks (including the
original Napster) are not pure peer networks but rather hybrid designs as they
utilize central servers for some functions such as search.
Peer
to Peer and Ad Hoc Wi-Fi Networks
Wi-Fi
wireless networks support so-called ad hoc connections between devices. Ad hoc
Wi-Fi networks are pure peer to peer compared to those utilizing wireless
routers as an intermediate device.
Benefits of
a Peer to Peer Network:
You can
configure computers in peer to peer workgroups to allow sharing of files,
printers and other resources across all of the devices. Peer networks allow
data to be shared easily in both directions, whether for downloads to your
computer or uploads from your computer.
On the
Internet, peer to peer networks handle a very high volume of file sharing
traffic by distributing the load across many computers. Because they do not
rely exclusively on central servers, P2P networks both scale better and are
more resilient than client-server networks in case of failures or traffic
bottlenecks.
Client
Server Networks
The term
client-server refers to a popular model for computer networking that utilizes
client and server devices each designed for specific purposes. The
client-server model can be used on the Internet as well as local area networks
(LANs). Examples of client-server systems on the Internet include Web browsers
and Web servers, FTP clients and servers, and DNS.
Client
and Server Devices:
Client/server
networking grew in popularity many years ago as personal computers (PCs) became
the common alternative to older mainframe computers. Client devices are
typically PCs with network software applications installed that request and
receive information over the network. Mobile devices as well as desktop
computers can both function as clients. A server device typically stores files
and databases including more complex applications like Web sites. Server
devices often feature higher-powered central processors, more memory, and larger
disk drives than clients.
Client-Server
Applications:
The
client-server model distinguishes between applications as well as devices.
Network clients make requests to a server by sending messages, and servers
respond to their clients by acting on each request and returning results. One
server generally supports numerous clients, and multiple servers can be
networked together in a pool to handle the increased processing load as the
number of clients grows.
A client
computer and a server computer are usually two separate devices, each
customized for their designed purpose. For example, a Web client works best
with a large screen display, while a Web server does not need any display at
all and can be located anywhere in the world. However, in some cases a given
device can function both as a client and a server for the same application.
Likewise, a device that is a server for one application can simultaneously act
as a client to other servers, for different applications.
[Some of the
most popular applications on the Internet follow the client-server model
including email, FTP and Web services. Each of these clients features a user
interface (either graphic- or text-based) and a client application that allows
the user to connect to servers. In the case of email and FTP, users enter a
computer name (or sometimes an IP address) into the interface to set up
connections to the server.
Local
Client-Server Networks:
Many home
networks utilize client-server systems without even realizing it. Broadband
routers, for example, contain DHCP servers that provide IP addresses to the
home computers (DHCP clients). Other types of network servers found in home
include print servers and backup servers.
Client-Server
vs Peer-to-Peer and Other Models:
The
client-server model was originally developed to allow more users to share
access to database applications. Compared to the mainframe approach,
client-server offers improved scalability because connections can be made as
needed rather than being fixed. The client-server model also supports modular
applications that can make the job of creating software easier. In so-called
"two-tier" and "three-tier" types of client-server systems,
software applications are separated into modular pieces, and each piece is
installed on clients or servers specialized for that subsystem.
Client-server
is just one approach to managing network applications The primary alternative,
peer-to-peer networking, models all devices as having equivalent capability
rather than specialized client or server roles. Compared to client-server, peer
to peer networks offer some advantages such as more flexibility in growing the
system to handle large number of clients. Client-server networks generally
offer advantages in keeping data secure.
Network topology
Network
topology is the layout pattern of interconnections of the various elements
(links, nodes, etc.) of a computer or biological network. Network topologies
may be physical or logical. Physical topology refers to the physical design of
a network including the devices, location and cable installation. Logical
topology refers to how data is actually transferred in a network as opposed to
its physical design. In general physical topology relates to a core network
whereas logical topology relates to basic network.
Topology can
be understood as the shape or structure of a network. This shape does not
necessarily correspond to the actual physical design of the devices on the
computer network. The computers on a home network can be arranged in a circle
but it does not necessarily mean that it represents a ring topology.
Any
particular network topology is determined only by the graphical mapping of the
configuration of physical and/or logical connections between nodes. The study
of network topology uses graph theory. Distances between nodes, physical
interconnections, transmission rates, and/or signal types may differ in two
networks and yet their topologies may be identical.
A local area
network (LAN) is one example of a network that exhibits both a physical
topology and a logical topology. Any given node in the LAN has one or more
links to one or more nodes in the network and the mapping of these links and
nodes in a graph results in a geometric shape that may be used to describe the
physical topology of the network. Likewise, the mapping of the data flow
between the nodes in the network determines the logical topology of the
network. The physical and logical topologies may or may not be identical in any
particular network.
Bus
Topology:
In local
area networks where bus topology is used, each node is connected to a single
cable. Each computer or server is connected to the single bus cable. A signal
from the source travels in both directions to all machines connected on the bus
cable until it finds the intended recipient. If the machine address does not
match the intended address for the data, the machine ignores the data.
Alternatively, if the data does match the machine address, the data is
accepted. Since the bus topology consists of only one wire, it is rather
inexpensive to implement when compared to other topologies. However, the low
cost of implementing the technology is offset by the high cost of managing the
network. Additionally, since only one cable is utilized, it can be the single
point of failure. If the network cable breaks, the entire network will be down.
Advantages
and disadvantages of a bus network:
Advantages
·
Easy to implement and extend.
·
Easy to install.
·
Well-suited for temporary or small networks not
requiring high speeds (quick setup), resulting in faster networks.
·
less expensive than other topologies (But in
recent years has become less important due to devices like a switch)
·
Cost effective; only a single cable is used.
·
Easy identification of cable faults.
Disadvantages
·
Limited cable length and number of stations.· If there is a problem with the cable, the entire network breaks down.
· Maintenance costs may be higher in the long run.
· Performance degrades as additional computers are added or on heavy traffic (shared bandwidth).
· Proper termination is required (loop must be in closed path).
· Significant Capacitive Load (each bus transaction must be able to stretch to most distant link).
· It works best with limited number of nodes.
· Commonly has a slower data transfer rate than other topologies.
· Only one packet can remain on the bus during one clock pulse
Star
Topology:
Star
networks are one of the most common computer network topologies. In its
simplest form, a star network consists of one central switch, hub or computer,
which acts as a conduit to transmit messages. This consists of a central node,
to which all other nodes are connected; this central node provides a common
connection point for all nodes through a hub. Thus, the hub and leaf nodes, and the
transmission lines between them, form a graph with the topology of a star. If
the central node is passive, the originating node must be able to tolerate the
reception of an echo of its own transmission, delayed by the two-way
transmission time (i.e. to and from the central node) plus any delay generated
in the central node. An active star network has an active central node that
usually has the means to prevent echo-related problems.
The star
topology reduces the chance of network failure by connecting all of the systems
to a central node. When applied to a bus-based network, this central hub
rebroadcasts all transmissions received from any peripheral node to all
peripheral nodes on the network, sometimes including the originating node. All
peripheral nodes may thus communicate with all others by transmitting to, and
receiving from, the central node only. The failure of a transmission line
linking any peripheral node to the central node will result in the isolation of
that peripheral node from all others, but the rest of the systems will be
unaffected.
It is also
designed with each node (file servers, workstations, and peripherals) connected
directly to a central network hub, switch, or concentrator.
Data on a
star network passes through the hub, switch, or concentrator before continuing
to its destination. The hub, switch, or concentrator manages and controls all
functions of the network. It is also acts as a repeater for the data flow. This
configuration is common with twisted pair cable. However, it can also be used
with coaxial cable or optical fiber cable.
Advantages
·
Better performance: star topology prevents the
passing of data packets through an excessive number of nodes. At most, 3
devices and 2 links are involved in any communication between any two devices.
Although this topology places a huge overhead on the central hub, with adequate
capacity, the hub can handle very high utilization by one device without
affecting others.
·
Isolation of devices: Each device is inherently
isolated by the link that connects it to the hub. This makes the isolation of
individual devices straightforward and amounts to disconnecting each device
from the others. This isolation also prevents any non-centralized failure from
affecting the network.
·
Benefits from centralization: As the central hub
is the bottleneck, increasing its capacity, or connecting additional devices to
it, increases the size of the network very easily. Centralization also allows
the inspection of traffic through the network. This facilitates analysis of the
traffic and detection of suspicious behavior.
·
Easy to detect faults and to remove parts.
·
No disruptions to the network when connecting or
removing devices.
Disadvantages
·
High dependence of the system on the functioning
of the central hub
·
Failure of the central hub renders the network
inoperable
Ring
Topology:
A ring
network is a network topology in which each node connects to exactly two other
nodes, forming a single continuous pathway for signals through each node - a
ring. Data travels from node to node, with each node along the way handling
every packet. Because a ring topology provides only one pathway between any two
nodes, ring networks may be disrupted by the failure of a single link.[1] A
node failure or cable break might isolate every node attached to the ring.
Fiber
Distributed Data Interface(FDDI) networks overcome this vulnerability by
sending data on a clockwise and a counterclockwise ring: in the event of a
break data is wrapped back onto the complementary ring before it reaches the
end of the cable, maintaining a path to every node along the resulting
"C-Ring". Many ring networks add a "counter-rotating ring"
to form a redundant topology. Such "dual ring" networks include
Spatial Reuse Protocol, Fiber Distributed Data Interface (FDDI), and Resilient
Packet Ring.
Advantages
·
Very orderly network where every device has
access to the token and the opportunity to transmit
·
Performs better than a bus topology under heavy
network load
·
Does not require network server to manage the
connectivity between the computers
Disadvantages
·
One malfunctioning workstation can create
problems for the entire network
·
Moves, adds and changes of devices can affect
the network
·
Network adapter cards much more expensive than
Ethernet cards and hubs
·
Much slower than an Ethernet network under
normal load
Tree
Topology:
The type of
network topology in which a central 'root' node (the top level of the
hierarchy) is connected to one or more other nodes that are one level lower in
the hierarchy (i.e., the second level) with a point-to-point link between each
of the second level nodes and the top level central 'root' node, while each of
the second level nodes that are connected to the top level central 'root' node
will also have one or more other nodes that are one level lower in the
hierarchy (i.e., the third level) connected to it, also with a point-to-point
link, the top level central 'root' node being the only node that has no other
node above it in the hierarchy (The hierarchy of the tree is symmetrical.) Each
node in the network having a specific fixed number, of nodes connected to it at
the next lower level in the hierarchy, the number, being referred to as the
'branching factor' of the hierarchical tree. This tree has individual
peripheral nodes.
A network
that is based upon the physical hierarchical topology must have at least three
levels in the hierarchy of the tree, since a network with a central 'root' node
and only one hierarchical level below it would exhibit the physical topology of
a star.
A network
that is based upon the physical hierarchical topology and with a branching
factor of 1 would be classified as a physical linear topology.
The
branching factor, f, is independent of the total number of nodes in the network
and, therefore, if the nodes in the network require ports for connection to
other nodes the total number of ports per node may be kept low even though the
total number of nodes is large – this makes the effect of the cost of adding
ports to each node totally dependent upon the branching factor and may
therefore be kept as low as required without any effect upon the total number
of nodes that are possible.
The total
number of point-to-point links in a network that is based upon the physical
hierarchical topology will be one less than the total number of nodes in the
network.
If the nodes
in a network that is based upon the physical hierarchical topology are required
to perform any processing upon the data that is transmitted between nodes in
the network, the nodes that are at higher levels in the hierarchy will be
required to perform more processing operations on behalf of other nodes than
the nodes that are lower in the hierarchy. Such a type of network topology is
very useful and highly recommended.
Tree
topology advantages:
·
It is the best topology for a large computer
network for which a star topology or ring topology are unsuitable due to the
sheer scale of the entire network. Tree topology divides the whole network into
parts that are more easily manageable.
·
Tree topology makes it possible to have a point
to point network.
·
All computers have access to their immediate
neighbors in the network and and also the central hub. This kind of network
makes it possible for multiple network devices to be connected with the central
hub.
·
It overcomes the limitation of star network
topology, which has a limitation of hub connection points and the broadcast
traffic induced limitation of a bus network topology.
·
A tree network provides enough room for future
expansion of a network.
Tree
topology disadvantages:
·
Dependence of the entire network on one central
hub is a point of vulnerability for this topology. A failure of the central hub
or failure of the main data trunk cable, can cripple the whole network.
·
With increase in size beyond a point, the
management becomes difficult.
Mesh
Topology:
Mesh networking
(topology) is a type of networking where each node must not only capture and
disseminate its own data, but also serve as a relay for other sensor nodes,
that is, it must collaborate to propagate the data in the network.
The
self-healing capability enables a routing based network to operate when one
node breaks down or a connection goes bad. As a result, the network is
typically quite reliable, as there is often more than one path between a source
and a destination in the network. Although mostly used in wireless scenarios,
this concept is also applicable to wired networks and software interaction.
Advantages
of Mesh Topology
·
There are dedicated links used in the topology,
which guarantees, that each connection is able to carry its data load, thereby
eliminating traffic problems, which are common, when links are shared by
multiple devices.
·
It is a robust topology. When one link in the
topology becomes unstable, it does not cause the entire system to halt.
·
If the network is to be expanded, it can be done
without causing any disruption to current users of the network.
·
It is possible to transmit data, from one node
to a number of other nodes simultaneously
·
Troubleshooting, in case of a problem, is easy
as compared to other network topologies.
·
This topology ensures data privacy and security,
as every message travels along a dedicated link.
Disadvantages
of Mesh Topology
·
The first disadvantage of this topology is that,
it requires a lot more hardware (cables, etc.) as compared to other Local Area
Network (LAN) topologies.
·
The implementation (installation and
configuration) of this topology is very complicated and can get very messy. A large
number of Input / Outout (I/O) ports are required.
·
It is an impractical solution, when large number
of devices are to be connected to each other in a network.
·
The cost of installation and maintenance is
high, which is a major deterrent.
Transmission Media
Various
physical media can be used to transport a stream of bits from one device to
another. Each has its own characteristics in terms of bandwidth, propagation
delay, cost, and ease of installation and maintenance. Media can be generally classified
as guided (e.g. copper and fiber cable) and unguided (wireless) media. The main
categories of transmission media used in data communications networks. Some
Bound Media are Coaxial Cable, Twisted Pair cable and Optical Fiber Cable.
Coaxial cable:
Coaxial cable:
A coaxial
cable has a central copper wire core, surrounded by an insulating (dielectric)
material. Braided metal shielding surrounds the dielectric and helps to absorb
unwanted external signals (noise), preventing it from interfering with the data
signal travelling along the core. A plastic sheath protects the cable from
damage. A terminating resistor is used at each end of the cable to prevent
transmitted signals from being reflected back down the cable. The following
diagram illustrates the basic construction of a coaxial cable.
Construction
of coaxial cable
Coaxial
cable has a fairly high degree of immunity to noise, and can be used over
longer distances (up to 500 meters) than twisted pair cable. Coaxial cable has,
in the past, been used to provide network backbone cable segments. Coaxial
cable has largely been replaced in computer networks by optical fiber and
twisted pair cable, with fiber used in the network backbone, and twisted pair
used to connect workstations to network hubs and switches.
Thick net
cable (also known as 10Base5) is a fairly thick cable (0.5 inches in diameter).
The 10Base5 designation refers to the 10 Mbps maximum data rate , baseband
signaling and 500 meter maximum segment length . Thick net was the original
transmission medium used in Ethernet networks, and supported up to 100 nodes per
network segment. An Ethernet transceiver was connected to the cable using a
vampire tap , so called because it clamps onto the cable, forcing a spike
through the outer shielding to make contact with the inner conductor, while two
smaller sets of teeth bite into the outer conductor. Transceivers could be
connected to the network cable while the network was live. A separate drop
cable with an attachment unit interface (AUI) connector at each end connected
the transceiver to the network interface card in the workstation (or other
network device). The drop cable was typically a shielded twisted pair cable,
and could be up to 50 meters in length. The minimum cable length between
connections ( taps ) on a cable segment was 2.5 meters.
Thick net
(10Base5) coaxial cable
Thin net
cable (also known as 10Base2) is thinner than Thick net (approximately 0.25
inches in diameter) and as a consequence is cheaper and far more flexible. The
10Base2 designation refers to the 10 Mbps maximum data rate , baseband
signaling and 185 (nearly 200) meter maximum segment length . A T-connector is
used with two BNC connectors to connect the network segment directly to the
network adapter card. The length of cable between stations must be at least 50
centimeters, and Thin net can support up to 30 nodes per network segment.
Thin net
(10Base2) coaxial cable
Coaxial
cable has the following advantages and disadvantages:
Advantages
·
Highly resistant to EMI (electromagnetic
interference)
·
Highly resistant to physical damage
Disadvantages
·
Expensive
·
Inflexible construction (difficult to install)
·
Unsupported by newer networking standards
Twisted
pair cable:
Twisted pair
copper cable is still widely used, due to its low cost and ease of
installation. A twisted pair consists of two insulated copper cables, twisted
together to reduce electrical interference between adjacent pairs of wires.
This type of cable is still used in the subscriber loop of the public telephone
system (the connection between a customer and the local telephone exchange),
which can extend for several kilometers without amplification. The subscriber
loop is essentially an analogue transmission line, although twisted pair cables
are also be used in computer networks to carry digital signals over short
distances.
The
bandwidth of twisted pair cable depends on the diameter of wire used, and the
length of the transmission line. The type of cable currently used in local area
networks has four pairs of wires. Until recently, category 5 or category 5E
cable has been used, but category 6 is now used for most new installations. The
main difference between the various categories is in the data rate supported -
category 6 cable will support gigabit Ethernet. The main disadvantage of UTP
cables in networks is that, due to the relatively high degree of attenuation
and a susceptibility to electromagnetic interference, high speed digital
signals can only be reliably transmitted over cable runs of 100 metres or less.
Unshielded
twisted pair cable
Shielded
Twisted Pair (STP) cable was introduced in the 1980s by IBM as the recommended
cable for their Token Ring network technology. It is similar to unshielded
twisted pair cable except that each pair is individually foil shielded, and the
cable has a braided drain wire that is earthed at one end during installation.
The popularity of STP has declined for the following reasons:
High cost of
cable and connectors
More
difficult to install than UTP
Ground loops
can occur if incorrectly installed
There is
still a cable length limitation of 100 meters
Shielded
twisted pair cable
Advantages
·
It is a thin, flexible cable that is easy to
string between walls.
·
More lines can be run through the same wiring
ducts.
·
UTP costs less per meter/foot than any other
type of LAN cable.
·
Electrical noise going into or coming from the
cable can be prevented.
·
Cross-talk is minimized.
Disadvantages
·
Twisted pair’s susceptibility to electromagnetic
interference greatly depends on the pair twisting schemes (usually patented by
the manufacturers) staying intact during the installation. As a result, twisted
pair cables usually have stringent requirements for maximum pulling tension as
well as minimum bend radius. This relative fragility of twisted pair cables
makes the installation practices an important part of ensuring the cable’s
performance.
·
In video applications that send information
across multiple parallel signal wires, twisted pair cabling can introduce
signaling delays known as skew which results in subtle color defects and
ghosting due to the image components not aligning correctly when recombined in
the display device. The skew occurs because twisted pairs within the same cable
often use a different number of twists per meter so as to prevent common-mode
crosstalk between pairs with identical numbers of twists. The skew can be
compensated by varying the length of pairs in the termination box, so as to
introduce delay lines that take up the slack between shorter and longer pairs,
though the precise lengths required are difficult to calculate and vary
depending on the overall cable length.
·
Optical
fiber:
Optical
fibers are thin, solid strands of glass that transmit information as pulses of
light. The fiberd has a core of high-purity glass, between 6μm and 50μm in
diameter, down which the light pulses travel. The core is encased in a covering
layer made of a different type of glass, usually about 125 μm in diameter,
known as the cladding. An outer plastic covering, the primary buffer, provides
some protection, and takes the overall diameter to about 250 μm. The structure
of an optical fiber is shown below.
The basic
construction of an optical fiber
The cladding
has a slightly lower refractive index than the core (typical values are 1.47
and 1.5 respectively), so that as the pulses of light travel along the fiber
they are reflected back into the core each time they meet the boundary between
the core and the cladding. Optical fibers lose far less of its signal energy
than copper cables, and can be used to transmit signals of a much higher
frequency. More information can be carried over longer distances with fewer
repeaters. The bandwidth achievable using optical fiber is almost unlimited,
but current signaling technology limits the data rate to 1 Gbps due to time required
to convert electronic digital signals to light pulses and vice versa. Digital
data is converted to light pulses by either a light emitting diode (LED) or a
laser diode. Although some light is lost at each end of the fiber, most is
passed along the fiber to the receiver, where the light pulses are converted
back into electronic signals by a photo-detector.
As the ray
passes along the fiber it meets the boundary between the core and the cladding
at some point. Because the refractive index of the cladding is lower than that
of the core, the ray is reflected back into the core material, as long as the
angle of incidence ( θ i ) is greater than the critical angle ( θ c ). The
critical angle depends on the refractive indices of the two materials. In the case
of an optical fiber, the values are chosen so that almost all of the light is
reflected back into the fiber, and there is virtually no loss through the walls
of the fiber. This is called total internal reflection . The critical angle for
a particular fiber can be calculated using Snell's Law. This states that:
n 1 sin θ 1
= n 2 sin θ 2
where θ 1 is
the angle of incidence, θ 2 is the angle of refraction, and n 1 and n 2 are the
refractive indices of the core and cladding respectively. The effect on a ray
of light passing along the fiber is shown below.
Light
transmission in an optical fiber
In
step-index fibers, the refractive index of both the core and the cladding has a
constant value, so that the refractive index of the fiber steps from one value
to the next.
A step-index
fiber
A multi-mode
step-index fiber
Multi-mode
and mono-mode fibers
One way to
improve the performance of multi-mode fiber is to use a graded index fiber
instead of a step index fiber. The refractive index of this type of fiber varies
across the diameter of the core in such a way that light is made to follow a
curved path along the fiber (see below). Light near the edges of the core
travels faster than light at the centre of the core, so although some rays
follow a longer path than others, they all tend to arrive at the same time,
resulting in far less modal dispersion than would occur in a step-index
multi-mode fiber.
A graded
index fiber
Light paths
in a graded index fiber
Advantages
·
Better security - very hard to tap a fibre
without being noticed.
·
Longer cable runs
·
Greater bandwidth.
·
Not affected by electromagnetic interference.
·
Can connect between buildings with different
earth potentials. These would could cause problems with a copper wired system.
·
Not effected by near-miss lightning strikes
·
Lower cost for 2 to 3 km fibre runs. CAT5
twisted-pair is limited to 100 metres.
·
Carrier signals on different frequencies
(colours) can be used to increase the capacity (frequency division multiplexing).
·
Single mode or monomode fibre has a very thin
inner glass layer.
·
This is so thin that it behaves like a wave
guide and the light can't follow different paths.
·
This reduces the dispersion and makes longer
fibres possible. 30km.
Disadvantages
·
Attenuation is still a problem and this limits
the maximum cable length
·
Dispersion is still a problem and this also
limits the maximum cable length
·
Scattering occurs when there are imperfections
in the fibre. This causes attenuation or energy loss.
·
Higher installation cost for small networks. For
major backbones fibre works out cheaper per megabit of bandwidth.
·
Optical fibres can be fragile although they are
reinforced with kevlar fibres and an outer protective plastic layer
·
Optical fibres are difficult to connect to the
transmitting light source and the receiving light detector. A complex cutting
and polishing operation is needed to make the fibre ends flat and free from
dirt or imperfections.
Unbound
Media or Unguided Media:
Unguided
Media: It is one that does not guide the data signals instead it uses the
multiple paths for transmitting data signals. In this type the data cable are
not bounds to a cable media. So it is called “Unbound media” basically there
are 2 types.
a)
Microwave
b)
Satellite Technology.
a)
Microwave:
Microwaves
are radio waves that are used to provide high-speed transmission. Both voice
and data can be transmitted through microwave. Data is transmitted through the
air form one microwave station to other similar to radio signals.
Microwave
uses line-of-sight transmission. It means that the signals travel in straight
path and cannot bend. Microwave stations or antennas are usually installed on
high towers or buildings. Microwave stations are placed within 20 to 30 miles
to each other. Each station receives signal from previous station and transfer
to next station. In this way, data transferred from one place to another. There
should be no buildings on mountains between microwave stations.
Advantages:
1. It has
the medium capacity slightly higher than “Bound Media”
2. Medium
cost.
3. It can
cover longer distance that cannot be possible by bound media.
Disadvantages:
1. Noise
interference is more.
2. Since it
uses less susceptible signal so it has got greater influence from rain &
fog.
3. It is not
secure & reliable.
b)
Satellite Communication:
Communication
satellite is a space station. It receives microwave signals from earth station.
It amplifies the signal and retransmits them back to earth. Communication
satellite is established in space about 22,300 miles above the earth. The data
transfer speed of communication satellite is very high.
The
transmission from earth station to satellite is called uplink. The transmission
from satellite to earth station is called downlink. An important advantage at
satellite is that a large volume of data can be communicated at once. The
disadvantage is that bad weather can severely affect the quality of satellite
transmission.
Advantages:
1. Low cost
per user (for pay TV)
2. High
Capacity
3. Very
large coverage area.
Disadvantages:
1. High
Installing & managing cost.
2. Receive
dishes & decoders required.
3. Delays
involved in the reception of the signal.
Wireless
Media:
Wireless
telecommunications, is the transfer of information between two or more points
that are physically not connected. Distances can be short, as a few meters as
in television remote control; or long ranging from thousands to millions of
kilometers for deep-space radio communications. It encompasses various types of
fixed, mobile, and portable two-way radios, cellular telephones, personal
digital assistants (PDAs), and wireless networking. Other examples of wireless
technology include GPS units, garage door openers and or garage doors, wireless
computer mice, keyboards and headsets, satellite television and cordless
telephones.
Device
Network Connecting Device: (Modem, NIC, Switch / Hub, Router, Gateway, Repeater,
Bluetooth, IR, WiFi):
Computer
networking devices are units that mediate data in a computer network. Computer
networking devices are also called network equipment, Intermediate Systems (IS)
or InterWorking Unit (IWU). Units which are the last receiver or generate data
are called hosts or data terminal equipment.
Modem
A modem
(modulator-demodulator) is a device that modulates an analog carrier signal to
encode digital information, and also demodulates such a carrier signal to
decode the transmitted information. The goal is to produce a signal that can be
transmitted easily and decoded to reproduce the original digital data. Modems
can be used over any means of transmitting analog signals, from light emitting
diodes to radio. The most familiar example is a voice band modem that turns the
digital data of a personal computer into modulated electrical signals in the
voice frequency range of a telephone channel. These signals can be transmitted
over telephone lines and demodulated by another modem at the receiver side to
recover the digital data.
Modems are
generally classified by the amount of data they can send in a given unit of
time, usually expressed in bits per second (bit/s, or bps). Modems can
alternatively be classified by their symbol rate, measured in baud. The baud
unit denotes symbols per second, or the number of times per second the modem
sends a new signal. For example, the ITU V.21 standard used audio
frequency-shift keying, that is to say, tones of different frequencies, with
two possible frequencies corresponding to two distinct symbols (or one bit per
symbol), to carry 300 bits per second using 300 baud. By contrast, the original
ITU V.22 standard, which was able to transmit and receive four distinct symbols
(two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second
(600 baud) using phase shift keying.
Network
interface controller(NIC)
A network
interface controller (also known as a network interface card, network adapter,
LAN adapter and by similar terms) is a computer hardware component that
connects a computer to a computer network.
Whereas
network interface controllers were commonly implemented on expansion cards that
plug into a computer bus, the low cost and ubiquity of the Ethernet standard
means that most newer computers have a network interface built into the
motherboard.
The network
controller implements the electronic circuitry required to communicate using a
specific physical layer and data link layer standard such as Ethernet, Wi-Fi,
or Token Ring. This provides a base for a full network protocol stack, allowing
communication among small groups of computers on the same LAN and large-scale
network communications through routable protocols, such as IP.
Switch /
Hub
A network
switch or switching hub is a computer networking device that connects network
segments.
The term
commonly refers to a multi-port network bridge that processes and routes data
at the data link layer (layer 2) of the OSI model. Switches that additionally
process data at the network layer (Layer 3) and above are often referred to as
Layer 3 switches or multilayer switches.
The network
switch plays an integral part in most modern Ethernet local area networks
(LANs). Mid-to-large sized LANs contain a number of linked managed switches.
Small office/home office (SOHO) applications typically use a single switch, or
an all-purpose converged device such as a gateway to access small office/home
broadband services such as DSL or cable internet. In most of these cases, the
end-user device contains a router and components that interface to the
particular physical broadband technology. User devices may also include a
telephone interface for VoIP.
An Ethernet
switch operates at the data link layer of the OSI model to create a separate
collision domain for each switch port. With 4 computers (e.g., A, B, C, and D)
on 4 switch ports, A and B can transfer data back and forth, while C and D also
do so simultaneously, and the two conversations will not interfere with one
another. In the case of a hub, they would all share the bandwidth and run in
half duplex, resulting in collisions, which would then necessitate
retransmissions. Using a switch is called microsegmentation. This allows
computers to have dedicated bandwidth on a point-to-point connections to the
network and to therefore run in full duplex without collisions.
Router
A router is
a device that forwards data packets between telecommunications networks,
creating an overlay internetwork. A router is connected to two or more data
lines from different networks. When data comes in on one of the lines, the
router reads the address information in the packet to determine its ultimate
destination. Then, using information in its routing table or routing policy, it
directs the packet to the next network on its journey or drops the packet. A
data packet is typically forwarded from one router to another through networks
that constitute the internetwork until it gets to its destination node.
The most familiar type of routers are home and
small office routers that simply pass data, such as web pages and email,
between the home computers and the owner's cable or DSL modem, which connects
to the Internet (ISP). However more sophisticated routers range from enterprise
routers, which connect large business or ISP networks up to the powerful core
routers that forward data at high speed along the optical fiber lines of the
Internet backbone.
Gateway
A network
gateway is an internetworking system capable of joining together two networks
that use different base protocols. A network gateway can be implemented
completely in software, completely in hardware, or as a combination of both.
Depending on the types of protocols they support, network gateways can operate
at any level of the OSI model.
Because a
network gateway, by definition, appears at the edge of a network, related
capabilities like firewalls tend to be integrated with it. On home networks, a
broadband router typically serves as the network gateway although ordinary
computers can also be configured to perform equivalent functions.
(1) A node
on a network that serves as an entrance to another network. In enterprises, the
gateway is the computer that routes the traffic from a workstation to the
outside network that is serving the Web pages. In homes, the gateway is the ISP
that connects the user to the internet.
In
enterprises, the gateway node often acts as a proxy server and a firewall. The
gateway is also associated with both a router, which use headers and forwarding
tables to determine where packets are sent, and a switch, which provides the
actual path for the packet in and out of the gateway.
(2) A
computer system located on earth that switches data signals and voice signals
between satellites and terrestrial networks.
(3) An
earlier term for router, though now obsolete in this sense as router is
commonly used.
Repeater
A repeater
is an electronic device that receives a signal and retransmits it at a higher
level and/or higher power, or onto the other side of an obstruction, so that
the signal can cover longer distances. The term "repeater" originated
with telegraphy and referred to an electromechanical device used by the army to
regenerate telegraph signals. Use of the term has continued in telephony and
data communications.
In
telecommunication, the term repeater has the following standardized meanings:
An analog
device that amplifies an input signal regardless of its nature (analog or
digital).
A digital
device that amplifies, reshapes, retimes, or performs a combination of any of
these functions on a digital input signal for retransmission. Because repeaters
work with the actual physical signal, and do not attempt to interpret the data
being transmitted, they operate on the Physical layer, the first layer of the
OSI model.
Bluetooth
Bluetooth is
a proprietary open wireless technology standard for exchanging data over short
distances (using short wavelength radio transmissions in the ISM band from
2400-2480 MHz) from fixed and mobile devices, creating personal area networks
(PANs) with high levels of security. Created by telecoms vendor Ericsson in
1994,[1] it was originally conceived as a wireless alternative to RS-232 data
cables. It can connect several devices, overcoming problems of synchronization.
Bluetooth is
managed by the Bluetooth Special Interest Group, which has more than 14,000
member companies in the areas of telecommunication, computing, networking, and
consumer electronics.[2] The SIG oversees the development of the specification,
manages the qualification program, and protects the trademarks.[3] To be
marketed as a Bluetooth device, it must be qualified to standards defined by
the SIG. A network of patents are required to implement the technology and are
only licensed to those qualifying devices; thus the protocol, whilst open, may
be regarded as proprietary.
Infrared
Infrared
(IR) light is electromagnetic radiation with a wavelength longer than that of
visible light, measured from the nominal edge of visible red light at 0.7
micrometers, and extending conventionally to 300 micrometres. These wavelengths
correspond to a frequency range of approximately 1 to 430 THz, and include most
of the thermal radiation emitted by objects near room temperature.
Microscopically, IR light is typically emitted or absorbed by molecules when
they change their rotational-vibrational movements.
Sunlight at
zenith provides an irradiance of just over 1 kilowatt per square meter at sea
level. Of this energy, 527 watts is infrared radiation, 445 watts is visible
light, and 32 watts is ultraviolet radiation.
WiFi
Wi-Fi is a
wireless standard for connecting electronic devices. A Wi-Fi enabled device
such as a personal computer, video game console, smartphone, and digital audio
player can connect to the Internet when within range of a wireless network
connected to the Internet. A single access point (or hotspot) has a range of
about 20 meters indoors. Wi-Fi has a greater range outdoors and multiple
overlapping access points can cover large areas.
"Wi-Fi"
is a trademark of the Wi-Fi Alliance and the term was originally created as a
simpler name for the "IEEE 802.11" standard. Wi-Fi is used by over
700 million people, there are over 4 million hotspots (places with Wi-Fi
Internet connectivity) around the world, and about 800 million new Wi-Fi
devices every year.[citation needed] Wi-Fi products that complete the Wi-Fi
Alliance interoperability certification testing successfully can use the Wi-Fi
CERTIFIED designation and trademark.
OSI Reference Model
The Open
Systems Interconnection model (OSI model) was a product of the Open Systems
Interconnection effort at the International Organization for Standardization.
It is a way of sub-dividing a communications system into smaller parts called
layers. Similar communication functions are grouped into logical layers. A
layer provides services to its upper layer while receiving services from the
layer below. On each layer, an instance provides service to the instances at
the layer above and requests service from the layer below.
For example,
a layer that provides error-free communications across a network provides the
path needed by applications above it, while it calls the next lower layer to
send and receive packets that make up the contents of that path. Two instances
at one layer are connected by a horizontal connection on that layer.
Layer 1:
Physical Layer
The Physical
Layer defines electrical and physical specifications for devices. In
particular, it defines the relationship between a device and a transmission
medium, such as a copper or optical cable. This includes the layout of pins,
voltages, cable specifications, hubs, repeaters, network adapters, host bus
adapters (HBA used in storage area networks) and more.
To
understand the function of the Physical Layer, contrast it with the functions
of the Data Link Layer. Think of the Physical Layer as concerned primarily with
the interaction of a single device with a medium, whereas the Data Link Layer
is concerned more with the interactions of multiple devices (i.e., at least
two) with a shared medium. Standards such as RS-232 do use physical wires to
control access to the medium.
The major
functions and services performed by the Physical Layer are:
Establishment
and termination of a connection to a communications medium.
Participation
in the process whereby the communication resources are effectively shared among
multiple users. For example, contention resolution and flow control.
Modulation,
or conversion between the representation of digital data in user equipment and
the corresponding signals transmitted over a communications channel. These are
signals operating over the physical cabling (such as copper and optical fiber)
or over a radio link.
Layer 2:
Data Link Layer
The Data
Link Layer provides the functional and procedural means to transfer data
between network entities and to detect and possibly correct errors that may
occur in the Physical Layer. Originally, this layer was intended for
point-to-point and point-to-multipoint media, characteristic of wide area media
in the telephone system. Local area network architecture, which included
broadcast-capable multiaccess media, was developed independently of the ISO
work in IEEE Project 802. IEEE work assumed sublayering and management
functions not required for WAN use. In modern practice, only error detection,
not flow control using sliding window, is present in data link protocols such
as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2
LLC layer is not used for most protocols on the Ethernet, and on other local
area networks, its flow control and acknowledgment mechanisms are rarely used.
Sliding window flow control and acknowledgment is used at the Transport Layer
by protocols such as TCP, but is still used in niches where X.25 offers
performance advantages.
Layer 3:
Network Layer
The Network
Layer provides the functional and procedural means of transferring variable
length data sequences from a source host on one network to a destination host
on a different network, while maintaining the quality of service requested by
the Transport Layer (in contrast to the data link layer which connects hosts
within the same network). The Network Layer performs network routing functions,
and might also perform fragmentation and reassembly, and report delivery
errors. Routers operate at this layer—sending data throughout the extended
network and making the Internet possible. This is a logical addressing scheme –
values are chosen by the network engineer. The addressing scheme is not
hierarchical.
Careful
analysis of the Network Layer indicated that the Network Layer could have at
least three sublayers:
Subnetwork
Access – that considers protocols that deal with the interface to networks,
such as X.25;
Subnetwork
Dependent Convergence – when it is necessary to bring the level of a
transit network up to the level of networks on either side;
Subnetwork
Independent Convergence – which handles transfer across multiple networks.
The best
example of this latter case is CLNP, or IPv7 ISO 8473. It manages the
connectionless transfer of data one hop at a time, from end system to ingress
router, router to router, and from egress router to destination end system. It
is not responsible for reliable delivery to a next hop, but only for the
detection of erroneous packets so they may be discarded. In this scheme, IPv4
and IPv6 would have to be classed with X.25 as subnet access protocols because
they carry interface addresses rather than node addresses.
A number of
layer management protocols, a function defined in the Management Annex, ISO
7498/4, belong to the Network Layer. These include routing protocols, multicast
group management, Network Layer information and error, and Network Layer
address assignment. It is the function of the payload that makes these belong to
the Network Layer, not the protocol that carries them.
Layer 4:
Transport Layer
The
Transport Layer provides transparent transfer of data between end users,
providing reliable data transfer services to the upper layers. The Transport
Layer controls the reliability of a given link through flow control,
segmentation/desegmentation, and error control. Some protocols are state- and
connection-oriented. This means that the Transport Layer can keep track of the
segments and retransmit those that fail. The Transport layer also provides the
acknowledgement of the successful data transmission and sends the next data if
no errors occurred.
Although not
developed under the OSI Reference Model and not strictly conforming to the OSI
definition of the Transport Layer, typical examples of Layer 4 are the
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Of the
actual OSI protocols, there are five classes of connection-mode transport
protocols ranging from class 0 (which is also known as TP0 and provides the
least features) to class 4 (TP4, designed for less reliable networks, similar
to the Internet). Class 0 contains no error recovery, and was designed for use
on network layers that provide error-free connections. Class 4 is closest to
TCP, although TCP contains functions, such as the graceful close, which OSI
assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes
provide expedited data and preservation of record boundaries, both of which TCP
is incapable.
Layer 5:
Session Layer
The Session
Layer controls the dialogues (connections) between computers. It establishes,
manages and terminates the connections between the local and remote
application. It provides for full-duplex, half-duplex, or simplex operation,
and establishes checkpointing, adjournment, termination, and restart
procedures. The OSI model made this layer responsible for graceful close of
sessions, which is a property of the Transmission Control Protocol, and also
for session checkpointing and recovery, which is not usually used in the
Internet Protocol Suite. The Session Layer is commonly implemented explicitly
in application environments that use remote procedure calls.
Layer 6:
Presentation Layer
The
Presentation Layer establishes context between Application Layer entities, in
which the higher-layer entities may use different syntax and semantics if the
presentation service provides a mapping between them. If a mapping is
available, presentation service data units are encapsulated into session
protocol data units, and passed down the stack.
This layer
provides independence from data representation (e.g., encryption) by
translating between application and network formats. The presentation layer
transforms data into the form that the application accepts. This layer formats
and encrypts data to be sent across a network. It is sometimes called the
syntax layer.[5]
The original
presentation structure used the basic encoding rules of Abstract Syntax
Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text
file to an ASCII-coded file, or serialization of objects and other data
structures from and to XML.
Layer 7:
Application Layer
The
Application Layer is the OSI layer closest to the end user, which means that
both the OSI application layer and the user interact directly with the software
application. This layer interacts with software applications that implement a
communicating component. Such application programs fall outside the scope of
the OSI model. Application layer functions typically include identifying
communication partners, determining resource availability, and synchronizing
communication. When identifying communication partners, the application layer
determines the identity and availability of communication partners for an
application with data to transmit. When determining resource availability, the
application layer must decide whether sufficient network or the requested
communication exist. In synchronizing communication, all communication between
applications requires cooperation that is managed by the application layer.
Communication Protocol: TCP/IP, SMTP, POP3, FTP,
HTTPs, Telnet protocol
A
communications protocol is a formal description of digital message formats and
the rules for exchanging those messages in or between computing systems and in
telecommunications.
Protocols
may include signaling, authentication and error detection and correction
capabilities.
A protocol
defines the syntax, semantics, and synchronization of communication, and the
specified behaviour is typically independent of how it is to be implemented. A
protocol can therefore be implemented as hardware or software or both.
Communicating
systems use well-defined formats for exchanging messages. Each message has an
exact meaning intended to provoke a defined response of the receiver. A
protocol therefore describes the syntax, semantics, and synchronization of
communication. A programming language describes the same for computations, so
there is a close analogy between protocols and programming languages: protocols
are to communications what programming languages are to computations. (A less
technical reader might appreciate this similar analogy: protocols are to
communications what grammar is to writing.)
The
communications protocols in use on the Internet are designed to function in
very complex and diverse settings. To ease design, communications protocols are
structured using a layering scheme as a basis. Instead of using a single
universal protocol to handle all transmission tasks, a set of cooperating
protocols fitting the layering scheme is used.
The layering
scheme in use on the Internet is called the TCP/IP model. The actual protocols
are collectively called the Internet protocol suite. The group responsible for
this design is called the Internet Engineering Task Force (IETF).
TCP/IP
TCP/IP
(Transmission Control Protocol/Internet Protocol) is the basic communication language
or protocol of the Internet. It can also be used as a communications protocol
in a private network (either an intranet or an extranet).
TCP/IP is a
two-layer program. The higher layer, Transmission Control Protocol, manages the
assembling of a message or file into smaller packets that are transmitted over
the Internet and received by a TCP layer that reassembles the packets into the
original message. The lower layer, Internet Protocol, handles the address part
of each packet so that it gets to the right destination. Each gateway computer
on the network checks this address to see where to forward the message. Even
though some packets from the same message are routed differently than others,
they'll be reassembled at the destination.
As with all
other communications protocol, TCP/IP is composed of layers:
IP - is
responsible for moving packet of data from node to node. IP forwards each
packet based on a four byte destination address (the IP number). The Internet
authorities assign ranges of numbers to different organizations. The
organizations assign groups of their numbers to departments. IP operates on
gateway machines that move data from department to organization to region and
then around the world.
TCP - is
responsible for verifying the correct delivery of data from client to server.
Data can be lost in the intermediate network. TCP adds support to detect errors
or lost data and to trigger retransmission until the data is correctly and
completely received.
Sockets - is
a name given to the package of subroutines that provide access to TCP/IP on
most systems.
TCP/IP uses
the client/server model of communication in which a computer user (a client)
requests and is provided a service (such as sending a Web page) by another
computer (a server) in the network. TCP/IP communication is primarily
point-to-point, meaning each communication is from one point (or host computer)
in the network to another point or host computer. TCP/IP and the higher-level
applications that use it are collectively said to be "stateless"
because each client request is considered a new request unrelated to any
previous one (unlike ordinary phone conversations that require a dedicated
connection for the call duration). Being stateless frees network paths so that
everyone can use them continuously. (Note that the TCP layer itself is not
stateless as far as any one message is concerned. Its connection remains in
place until all packets in a message have been received.)
Many
Internet users are familiar with the even higher layer application protocols
that use TCP/IP to get to the Internet. These include the World Wide Web's
Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet
(Telnet) which lets you logon to remote computers, and the Simple Mail Transfer
Protocol (SMTP). These and other protocols are often packaged together with
TCP/IP as a "suite."
Personal
computer users with an analog phone modem connection to the Internet usually
get to the Internet through the Serial Line Internet Protocol (SLIP) or the
Point-to-Point Protocol (PPP). These protocols encapsulate the IP packets so
that they can be sent over the dial-up phone connection to an access provider's
modem.
Protocols
related to TCP/IP include the User Datagram Protocol (UDP), which is used
instead of TCP for special purposes. Other protocols are used by network host
computers for exchanging router information. These include the Internet Control
Message Protocol (ICMP), the Interior Gateway Protocol (IGP), the Exterior
Gateway Protocol (EGP), and the Border Gateway Protocol (BGP).
Simple
Mail Transfer Protocol (SMTP)
SMTP (Simple
Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving
e-mail. However, since it is limited in its ability to queue messages at the
receiving end, it is usually used with one of two other protocols, POP3 or IMAP
, that let the user save messages in a server mailbox and download them
periodically from the server. In other words, users typically use a program
that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail.
On Unix-based systems, sendmail is the most widely-used SMTP server for e-mail.
A commercial package, Sendmail, includes a POP3 server. Microsoft Exchange
includes an SMTP server and can also be set up to include POP3 support.
SMTP usually
is implemented to operate over Internet port 25. An alternative to SMTP that is
widely used in Europe is X.400. Many mail servers now support Extended Simple
Mail Transfer Protocol (ESMTP), which allows multimedia files to be delivered
as e-mail.
POP3
(Post Office Protocol 3)
In
computing, the Post Office Protocol (POP) is an application-layer Internet
standard protocol used by local e-mail clients to retrieve e-mail from a remote
server over a TCP/IP connection. POP and IMAP (Internet Message Access
Protocol) are the two most prevalent Internet standard protocols for e-mail
retrieval. Virtually all modern e-mail clients and servers support both. The
POP protocol has been developed through several versions, with version 3 (POP3)
being the current standard. Like IMAP, POP3 is supported by most webmail
services such as Hotmail, Gmail and Yahoo! Mail.
POP3 is
designed to delete mail on the server as soon as the user has downloaded it.
However, some implementations allow users or an administrator to specify that
mail be saved for some period of time. POP can be thought of as a
"store-and-forward" service.
An
alternative protocol is Internet Message Access Protocol (IMAP). IMAP provides
the user more capabilities for retaining e-mail on the server and for
organizing it in folders on the server. IMAP can be thought of as a remote file
server.
POP and IMAP
deal with the receiving of e-mail and are not to be confused with the Simple
Mail Transfer Protocol (SMTP), a protocol for transferring e-mail across the
Internet. You send e-mail with SMTP and a mail handler receives it on your
recipient's behalf. Then the mail is read using POP or IMAP.
File
Transfer Protocol(FTP)
File
Transfer Protocol (FTP) is a standard network protocol used to transfer files
from one host to another over a TCP-based network, such as the Internet. FTP is
built on a client-server architecture and utilizes separate control and data
connections between the client and server. FTP users may authenticate
themselves using a clear-text sign-in protocol but can connect anonymously if
the server is configured to allow it.
The first FTP
client applications were interactive command-line tools, implementing standard
commands and syntax. Graphical user interface clients have since been developed
for many of the popular desktop operating systems in use today.
HTTPS
(HTTP over SSL or HTTP Secure)
HTTPS (HTTP
over SSL or HTTP Secure) is the use of Secure Socket Layer (SSL) or Transport
Layer Security (TLS) as a sublayer under regular HTTP application layering.
HTTPS encrypts and decrypts user page requests as well as the pages that are
returned by the Web server. The use of HTTPS protects against eavesdropping and
man-in-the-middle attacks. HTTPS was developed by Netscape.
HTTPS and
SSL support the use of X.509 digital certificates from the server so that, if
necessary, a user can authenticate the sender. Unless a different port is
specified, HTTPS uses port 443 instead of HTTP port 80 in its interactions with
the lower layer, TCP/IP.
Suppose you
visit a Web site to view their online catalog. When you're ready to order, you
will be given a Web page order form with a Uniform Resource Locator (URL) that
starts with https://. When you click "Send," to send he page back to
the catalog retailer, your browser's HTTPS layer will encrypt it. The
acknowledgement you receive from the server will also travel in encrypted form,
arrive with an https:// URL, and be decrypted for you by your browser's HTTPS
sublayer.
The
effectiveness of HTTPS can be limited by poor implementation of browser or
server software or a lack of support for some algorithms. Furthermore, although
HTTPS secures data as it travels between the server and the client, once the
data is decrypted at its destination, it is only as secure as the host
computer. According to security expert Gene Spafford, that level of security is
analogous to "using an armored truck to transport rolls of pennies between
someone on a park bench and someone doing business from a cardboard box."
HTTPS is not
to be confused with S-HTTP, a security-enhanced version of HTTP developed and
proposed as a standard by EIT.
Getting
started with HTTPS
To explore
how HTTPS is used in the enterprise, here are some additional resources for
learning about HTTPS and Web page security:
Enabling
HTTPS in J2EE Web components: The HTTPS protocol is a valuable security feature
for J2EE Web components. Expert Ramesh Nagappan explains how to implement HTTPS
in JSPs and servlets.
Authentication
and authorization for Web applications: Web applications need robust
authentication and authorization mechanisms, such as HTTPS. Expert Ramesh Nagappan
explains what measures are needed before you deploy Web apps.
How to
create a secure login page using ASP.NET: A secure ASP.NET login page is easier
to create than one might assume. Expert Dan Cornell explains how to use
authentication, authorization and HTTPS to ensure your login page is safe.
TELNET
Protocol
Telnet is a
user command and an underlying TCP/IP protocol for accessing remote computers.
Through Telnet, an administrator or another user can access someone else's
computer remotely. On the Web, HTTP and FTP protocols allow you to request
specific files from remote computers, but not to actually be logged on as a
user of that computer. With Telnet, you log on as a regular user with whatever
privileges you may have been granted to the specific application and data on
that computer.
A Telnet
command request looks like this (the computer name is made-up):
>telnet
the.libraryat.whatis.edu
The result
of this request would be an invitation to log on with a userid and a prompt for
a password. If accepted, you would be logged on like any user who used this
computer every day.
Telnet is
most likely to be used by program developers and anyone who has a need to use
specific applications or data located at a particular host computer.
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