When data in either digital or analog forms needs to be sent over an
analog media it must first be converted into analog signals. There can
be two cases according to data formatting. Bandpass: In real world scenarios, filters are used to filter
and pass frequencies of interest. A bandpass is a band of frequencies
which can pass the filter. Low-pass: Low-pass is a filter that passes low frequencies signals.
When digital data is converted into a bandpass analog signal, it is
called digital-to-analog conversion. When low-pass analog signal is
converted into bandpass analog signal it is called analog-to-analog
conversion.
Digital-to-Analog Conversion
When data from one computer is sent to another via some analog
carrier, it is first converted into analog signals. Analog signals are
modified to reflect digital data, i.e. binary data.
An analog is characterized by its amplitude, frequency and phase.
There are three kinds of digital-to-analog conversions possible:
Amplitude shift keying
In this conversion technique, the amplitude of analog carrier signal is modified to reflect binary data. [Image: Amplitude Shift Keying]
When binary data represents digit 1, the amplitude is held otherwise
it is set to 0. Both frequency and phase remain same as in the original
carrier signal.
Frequency shift keying
In this conversion technique, the frequency of the analog carrier signal is modified to reflect binary data. [Image: Frequency shift keying]
This technique uses two frequencies, f1 and f2. One of them, for
example f1, is chosen to represent binary digit 1 and the other one is
used to represent binary digit 0. Both amplitude and phase of the
carrier wave are kept intact.
Phase shift keying
In this conversion scheme, the phase of the original carrier signal is altered to reflect the binary data. [Image: Phase shift keying]
When a new binary symbol is encountered, the phase of the signal is
altered. Amplitude and frequency of the original carrier signal is kept
intact.
Quadrature Phase Shift Keying
QPSK alters the phase to reflect 2 binary digits at once. This is
done in two different phases. The main stream of binary data is divided
equally into two sub-streams. The serial data is converted in to
parallel in both sub-streams and then each stream is converted to
digital signal using NRZ technique. Later, both the digital signals are
merged together.
Analog-to-analog conversion
Analog signals are modified to represent analog data. This
conversion is also known as Analog Modulation. Analog modulation is
required when bandpass is used. Analog to analog conversion can be done
in three ways:
[Image: Types of Modulation]
Amplitude Modulation
In this modulation, the amplitude of the carrier signal is modified to reflect the analog data. [Image: Amplitude Modulation]
Amplitude modulation is implemented by means of a multiplier. The
amplitude of modulating signal (analog data) is multiplied by the
amplitude of carrier frequency, which then reflects analog data.
The frequency and phase of carrier signal remain unchanged.
Frequency Modulation
In this modulation technique, the frequency of the carrier signal is
modified to reflect the change in the voltage levels of the modulating
signal (analog data). [Image: Frequency Modulation]
The amplitude and phase of the carrier signal are not altered.
Phase Modulation
In the modulation technique, the phase of carrier signal is modulated
in order to reflect the change in voltage (amplitude) of analog data
signal. [Image: Phase Modulation]
Phase modulation practically is similar to Frequency Modulation, but
in Phase modulation frequency of the carrier signal is not increased.
Frequency is carrier is signal is changed (made dense and sparse) to
reflect voltage change in the amplitude of modulating signal.
Data or information can be stored in two ways, analog and digital.
For a computer to use that data is must be in discrete digital form.
Like data, signals can also be in analog and digital form. To transmit
data digitally it needs to be first converted to digital form.
Digital-to-digital conversion
This section explains how to convert digital data into digital
signals. It can be done in two ways, line coding and block coding. For
all communications, line coding is necessary whereas block coding is
optional.
Line Coding
The process for converting digital data into digital signal is said
to be Line Coding. Digital data is found in digital format, which is
binary bits. It is represented (stored) internally as series of 1s and
0s. [Image: Line Coding]
Digital signals which represents digital data, represented as
discrete signals. There are three types of line coding schemes
available: [Image: Line Coding Schemes]
Uni-Polar Encoding
Unipolar encoding schemes uses single voltage level to represent
data. In this case, to represent binary 1 high voltage is transmitted
and to represent 0 no voltage is transmitted. It is also called
Unipolar-Non-return-to-zero, because there’s no rest condition i.e. it
either represents 1 or 0. [Image: UniPolar NRZ Encoding]
Polar Encoding
Polar encoding schemes multiple voltage levels are used to represent
binary values. Polar encodings are available in four types:
Polar-NRZ (Non-return to zero)
It uses two different voltage levels to represent binary values,
generally positive voltage represents 1 and negative value represents 0.
It is also NRZ because there’s no rest condition.
NRZ scheme has two variants: NRZ-L and NRZ-I. [Image: NRZ-L and NRZ-I]
NRZ-L changes voltage level at when a different bit is encountered whereas NRZ-I changes voltage when a 1 is encountered.
RZ (Return to zero)
Problem with NRZ was the receiver cannot conclude when a bit ended
and when the next bit is started, in case when sender and receiver’s
clock are not synchronized. [Image: Return-to-Zero Encoding]
RZ uses three voltage levels, positive voltage to represent 1,
negative voltage to represent 0 and zero voltage for none. Signals
change during bits not between bits.
Manchester
This encoding scheme is a combination of RZ and NRZ-L. Bit time is
divided into two halves. It transitions at the middle of the bit and
changes phase when a different bit is encountered.
Differential Manchester
This encoding scheme is a combination of RZ and NRZ-I. It also
transitions at the middle of the bit but changes phase only when 1 is
encountered.
Bipolar Encoding
Bipolar encoding uses three voltage levels, positive, negative and
zero. Zero voltage represents binary 0 and bit 1 is represented by
altering positive and negative voltages. [Image: Bipolar Encoding]
Block Coding
To ensure accuracy of data frame received, redundant bits are used.
For example, in even parity one parity bit is added to make the count of
1s in the frame even. This way the original number of bits are
increased. It is called Block Coding.
Block coding is represented by slash notation, mB/nB, that is m-bit
block is substituted with n-bit block where n > m. Block coding
involves three steps: division, substitution and combination.
After block coding is done it is line coded for transmission.
Analog-to-digital conversion
Microphones creates analog voice and camera creates analog videos,
which here in our case is treated is analog data. To transmit this
analog data over digital signals we need an analog to digital
conversion.
Analog data is wave form continuous stream of data whereas digital
data is discrete. To convert analog wave into digital data we use Pulse
Code Modulation.
Pulse Code Modulation is one of the most commonly used method to
convert analog data into digital form. It involves three steps:
Sampling, Quantization and Encoding.
Sampling
[Image: Sampling of Analog Signal]
The analog signal is sampled every T interval. Most important factor
in sampling is the rate on which analog signal is sampled. According
to Nyquist Theorem, the sampling rate must be at least two times of the
highest frequency of the signal.
Quantization
[Image: Quantization of sampled analog signal]
Sampling yields discrete form of continuous analog signal. Every
discrete pattern shows the amplitude of the analog signal at that
instance. The quantization is done between the maximum amplitude value
and the minimum amplitude value. Quantization is approximation of the
instantaneous analog value.
Encoding
[Image: Encoding from quantization]
In encoding, each approximated value is then converted into binary format.
Transmission Modes
How data is to be transferred between to computer is decided by the
transmission mode they are using. Binary data i.e. 1s and 0s can be
sent in two different modes: Parallel and Serial.
Parallel Transmission
[Image: Parallel Transmission]
The binary bits are organized in to groups of fixed length. Both
sender and receiver are connected in parallel with the equal number of
data lines. Both computer distinguish between high order and low order
data lines. The sender sends all the bits at once on all lines.
Because data lines are equal to the number of bits in a group or data
frame, a complete group of bits (data frame) is sent in one go.
Advantage of Parallel transmission is speed and disadvantage is the cost
of wires, as it is equal to the number of bits needs to send
parallelly.
Serial Transmission
In serial transmission, bits are sent one after another in a queue
manner. Serial transmission requires only one communication channel as
oppose parallel transmission where communication lines depends upon bit
word length. [Image: Serial Transmission]
Serial transmission can be either asynchronous or synchronous.
Asynchronous Serial Transmission
It is named so because there’s no importance of timing. Data-bits
have specific pattern and helps receiver recognize when the actual data
bits start and where it ends. For example, a 0 is prefixed on every
data byte and one or more 1s added at the end.
Two continuous data-frames (bytes) may have gap between them.
Synchronous Serial Transmission
It is up to the receiver to recognize and separate bits
into bytes. The advantage of synchronous transmission is speed and it
has no overhead of extra header and footer bits as in asynchronous
transmission.
Physical layer in the OSI model plays the role of interacting with
actual hardware and signaling mechanism. Physical layer is the only
layer of OSI which actually deals with the physical connectivity two
different stations. This layer defines the hardware equipments,
cabling, wiring, frequencies, pulses used to represent binary signals
etc.
Physical layer provides its services to Data-link layer. Data-link
layer hands over frames to physical layer and physical layer converts it
to electrical pulses which represents binary data and sends over to the
wired or wireless media.
Signals
When data is sent over physical medium it needs to be first converted
into electromagnetic signals. Data itself can be analog such as human
voice, or digital such as file on the disk. Data (both analog and
digital) can be represented in digital or analog signals.
Digital Signals
Digital signals are discrete in nature and represents sequence of
voltage pulses. Digital signals are used within the circuitry of a
computer system.
Analog Signals
Analog signals are in continuous wave form in nature and represented by continuous electromagnetic waves.
Transmission impairment
When signals travel through the medium they tend to deteriorate. This may have many reasons:
Attenuation:
When signal passes through the medium it tends to get weaker as it
covers distance. It loses is strength. For the receiver to interpret
the data signal must be sufficiently strong.
Dispersion:
As signal travels through the media it tends to spread and overlaps. The amount of dispersion depends upon the frequency used.
Delay distortion:
Signals are sent over media with pre-defined speed and frequency. If
the signal speed (velocity) and frequency does not match, there are
possibilities that signal reach destination in arbitrary fashion. In
digital media, this is very critical that some bits reach earlier than
the previously sent.
Noise:
Random disturbance or fluctuation in analog or digital signals is
said to be Noise in signal, which may distort the actual information
being carried. Noise can be characterized in one of the following
class:
Thermal Noise:
Heat agitates the electronic conductors of a medium which may
introduce noise in the media. Up to a certain level thermal noise is
unavoidable.
Intermodulation:
When more than frequency shares a medium their interference can
cause noise in the media. Intermodulation noise occurs say, if two
different frequencies sharing a media and one of them has excessive
strength or the component itself is not functioning properly, then the
resultant frequency may not be delivered as expected.
Crosstalk:
This sort of noise happens when a foreign signal enters into the
media. This is because signal in one media is affecting the signal of
second media.
Impulse:
This noise is introduced because of irregular disturbances like
lightening, electricity short-circuit or faulty components. Digital
data is mostly affected by this sort of noise.
Transmission Media
The medium over which the information between two computer systems is
sent, called Transmission Media. Transmission media comes in two
forms.
Guided Media
All communication wires/cables comes into this type of media, such as
UTP, Coaxial and Fiber Optics. In this media the sender and receiver
are directly connected and the information is send (guided) through it.
Unguided Media
Wireless or open air space is said to be unguided media, because
there is no connectivity between the sender and receiver. Information
is spread over the air, and anyone including the actual recipient may
collect the information.
Channel Capacity
The speed of transmission of information is said to be the channel
capacity. We count it as data rate in digital world. It depends on
numerous factors:
Bandwidth: The physical limitation of underlying media.
Error-rate: Incorrect reception of information because of noise.
Encoding: number of levels used for signaling.
Multiplexing
Multiplexing is a technique to mix and send multiple data stream over
a single media. This technique requires system hardware called
Multiplexer for multiplexing streams and sending them on a media and
De-Multiplexer which takes information from the media and distributes to
different destinations.
Switching
Switching is a mechanism by which data/information sent from source
towards destination which are not directly connected. Networks have
interconnecting devices, which receives data from directly connected
sources, stores data, analyze it and then forwards to the next
interconnecting device closest to the destination.
Switching can be categorized as: [Image: Switching]
When first networking was used, it was limited to Military and
Universities for Research and development purposes. Later when all
networks merge together and formed Internet, user’s data use to travel
through public transit network, where users are not scientists or
computer science scholars. Their data can be highly sensitive as bank’s
credentials, username and passwords, personal documents, online
shopping or secret official documents.
All security threats are intentional i.e. they occur only if
intentionally triggered. Security threats can be divided into the below
mentioned categories:
Interruption:
Interruption is a security threat in which availability of
resources is attacked. For example, a user is unable to access its
web-server or the web-server is hijacked.
Privacy-breach:
In this threat, the privacy of a user is compromised. Someone,
who is not the authorized person is accessing or intercepting data sent
or received by the original authenticated user.
Integrity:
This type of threat includes any alteration or modification in
the original context of communication. The attacker intercepts and
receives the data sent by the Sender and the attacker then either
modifies or generate false data and sends to the receiver. The receiver
receive data assuming that it is being sent by the original Sender.
Authenticity:
When an attacker or security breacher, represents himself as if
he is the authentic person and access resources or communicate with
other authentic users.
No technique in the present world can provide 100% security. But
steps can be taken to secure data while it travels in unsecured network
or internet. The most widely used technique is Cryptography. [Image: Cryptography]
Cryptography is a technique to encrypt the plain-text data which
makes it difficult to understand and interpret. There are several
cryptographic algorithm available present day as described below:
Secret Key
Public Key
Message Digest
Secret Key Encryption
Both sender and receiver have one secret key. This secret key is
used to encrypt the data at sender’s end. After encrypting the data, it
is then sent on the public domain to the receiver. Because the
receiver knows and has the Secret Key, the encrypted data packets can
easily be decrypted.
Example of secret key encryption is DES. In Secret Key encryption it
is required to have a separate key for each host on the network making
it difficult to manage.
Public Key Encryption
In this encryption system, every user has its own Secret Key and it
is not in the shared domain. The secret key is never revealed on public
domain. Along with secret key, every user has its own but public key.
Public key is always made public and is used by Senders to encrypt the
data. When the user receives the encrypted data, he can easily decrypt
it by using its own Secret Key.
Example of public key encryption is RSA.
Message Digest
In this method, the actual data is not sent instead a hash value is
calculated and sent. The other end user, computes its own hash value
and compares with the one just received. The both hash values matches,
it is accepted otherwise rejected.
Example of Message Digest is MD5 hashing. It is mostly used in
authentication where user’s password is cross checked with the one saved
at Server.
Networking at engineering level is a complicated task. It involves
software, firmware, chip level engineering, hardware and even electric
pulses. To ease network engineering, the whole networking concept is
divided into multiple layers. Each layer is involved in some particular
task and is independent of all other layers. But as a whole the almost
all networking task depends on all of these layers. Layers share data
between them and they depend on each other only to take input and give
output.
Layered tasks
In layered architecture of Network Models, one whole network process
is divided into small tasks. Each small task is then assigned to a
particular layer which works dedicatedly to process the task only.
Every layer does only specific work.
In layered communication system, one layer of a host deals with the
task done by or to be done by its peer layer at the same level on the
remote host. The task is either initiated by layer at the lowest level
or at the top most level. If the task is initiated by top most layer it
is then passed on to the layer below it for further processing. The
lower layer does the same thing, it processes the task and pass on to
lower layer. If the task is initiated by lowest most layer the reverse
path is taken. [Image: Layered Tasks]
Every layer clubs together all procedures, protocols, methods which
it requires to execute its piece of task. All layers identify their
counterparts by means of encapsulation header and tail.
OSI Model
Open System Interconnect is an open standard for all communication
systems. OSI model is established by International Standard
Organization. This model has seven layers: [Image: OSI Model]
Application Layer: This layer is responsible for providing
interface to the application user. This layer encompasses protocols
which directly interacts with the user.
Presentation Layer: This layer defines how data in the native format of remote host should be presented in the native format of host.
Session Layer: This layer maintains sessions between
remote hosts. For example, once user/password authentication is done,
the remote host maintains this session for a while and does not ask for
authentication again in that time span.
Transport Layer: This layer is responsible for end-to-end delivery between hosts.
Network Layer: This layer is responsible for address assignment and uniquely addressing hosts in a network.
Data Link Layer: This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer.
Physical Layer: This layer defines the hardware, cabling and wiring, power output, pulse rate etc.
Internet Model
Internet uses TCP/IP protocol suite, also known as Internet suite.
This defines Internet Model which contains four layered architecture.
OSI Model is general communication model but Internet Model is what
Internet uses for all its communication. Internet is independent of its
underlying network architecture so is its Model. This model has the
following layers: [Image: Internet Model]
Application Layer: This layer defines the protocol which enables user to internet with the network such as FTP, HTTP etc.
Transport Layer: This layer defines how data should flow
between hosts. Major protocol at this layer is Transmission Control
Protocol. This layer ensures data delivered between hosts is in-order
and is responsible for end to end delivery.
Internet Layer: IP works on this layer. This layer facilitates host addressing and recognition. This layer defines routing.
Link Layer: This layer provides mechanism of sending and
receiving actual data. But unlike its OSI Model’s counterpart, this
layer is independent of underlying network architecture and hardware.
A Network Topology is the way computer systems or network equipment
connected to each other. Topologies may define both physical and
logical aspect of the network. Both logical and physical topologies
could be same or different in a same network.
Point-to-point
Point-to-point networks contains exactly two hosts (computer or
switches or routers or servers) connected back to back using a single
piece of cable. Often, the receiving end of one host is connected to
sending end of the other end and vice-versa. [Image: Point-to-point Topology]
If the hosts are connected point-to-point logically, then may have
multiple intermediate devices. But the end hosts are unaware of
underlying network and see each other as if they are connected directly.
Bus Topology
In contrast to point-to-point, in bus topology all device share
single communication line or cable. All devices are connected to this
shared line. Bus topology may have problem while more than one hosts
sending data at the same time. Therefore, the bus topology either uses
CSMA/CD technology or recognizes one host has Bus Master to solve the
issue. It is one of the simple forms of networking where a failure of a
device does not affect the others. But failure of the shared
communication line make all other devices fail. [Image: Bus Topology]
Both ends of the shared channel have line terminator. The data is
sent in only one direction and as soon as it reaches the extreme end,
the terminator removes the data from the line.
Star Topology
All hosts in star topology are connected to a central device, known
as Hub device, using a point-to-point connection. That is, there exists
a point to point connection between hosts and Hub. The hub device can
be Layer-1 device (Hub / repeater) or Layer-2 device (Switch / Bridge)
or Layer-3 device (Router / Gateway). [Image: Star Topology]
As in bus topology, hub acts as single point of failure. If hub
fails, connectivity of all hosts to all other hosts fails. Every
communication happens between hosts, goes through Hub only. Star
topology is not expensive as to connect one more host, only one cable is
required and configuration is simple.
Ring Topology
In ring topology, each host machine connects to exactly two other
machines, creating a circular network structure. When one host tries to
communicate or send message to a host which is not adjacent to it, the
data travels through all intermediate hosts. To connect one more host
in the existing structure administrator may need only one more extra
cable. [Image: Ring Topology]
Failure of any host results in failure of the whole ring. Thus every
connection in the ring is point of failure. There exists methods which
employs one more backup ring.
Mesh Topology
In this type of topology, a host is connected to one or two or more
than two hosts. This topology may have hosts having point-to-point
connection to every other hosts or may also have hosts which are having
point to point connection to few hosts only. [Image: Full Mesh Topology]
Hosts in Mesh topology also work as relay for other hosts which do
not have direct point-to-point links. Mesh technology comes into two
flavors:
Full Mesh: All hosts have a point-to-point connection to
every other host in the network. Thus for every new host n(n-1)/2
cables (connection) are required. It provides the most reliable network
structure among all network topologies.
Partially Mesh: Not all hosts have point-to-point connection
to every other host. Hosts connect to each other in some arbitrarily
fashion. This topology exists where we need to provide reliability to
some host whereas others are not as such necessary.
Tree Topology
Also known as Hierarchical Topology is the most common form of
network topology in use present day. This topology imitates as extended
Star Topology and inherits properties of Bus topology.
This topology divides the network in to multiple levels/layers of
network. Mainly in LANs, a network is bifurcated into three types of
network devices. The lowest most is access-layer where user’s computer
are attached. The middle layer is known as distribution layer, which
works as mediator between upper layer and lower layer. The highest most
layer is known as Core layer, and is central point of the network, i.e.
root of the tree from which all nodes fork. [Image: Tree Topology]
All neighboring hosts have point-to-point connection between them.
Like bus topology, if the root goes down, the entire network suffers.
Though it is not the single point of failure. Every connection serves
as point of failure, failing of which divides the network into
unreachable segment and so on.
Daisy Chain
This topology connects all its hosts in a linear fashion. Similar to
Ring topology, all hosts in this topology are connected to two hosts
only, except the end hosts. That is if the end hosts in Daisy Chain are
connected then it represents Ring topology. [Image: Daisy Chain Topology]
Each link in Daisy chain topology represents single point of failure.
Every link failure splits the network into two segment. Every
intermediate host works as relay for its immediate hosts.
Hybrid Topology
A network structure whose design contains more than one topology is
said to be Hybrid Topology. Hybrid topology inherits merits and
demerits of all the incorporating topologies. [Image: Hybrid Topology]
The above picture represents an arbitrarily Hybrid topology. The
combining topologies may contain attributes of Star, Ring, Bus and
Daisy-chain topologies. Most WANs are connected by means of dual Ring
topology and networks connected to them are mostly Star topology
networks. Internet is the best example of largest Hybrid topology
Ethernet is a Local Area Network implemenation technology which is
widely deployed. This technology was invented by Bob Metcalfe and D.R.
Boggs in early 70s. It was standardized in IEEE 802.3 in 1980.
Ethernet is network technology which shares media. Network which uses
shared media has high probability of data collision. Ethernet uses
CSMA/CD technology to detect collisions. CSMA/CD stands for Carrier
Sense Multi Access/Collision Detection. When a collision happens in
Ethernet, all its host rolls back and waits for some random amount of
time and then re-transmit data.
Ethernet connector, i.e. Network Interface cards are equipped with
48-bits MAC address. This help other Ethernet devices to identify and
communicate with remote devices in Ethernet.
Traditional Ethernet uses 10BASE-T specifications. 10 is for 10mpbs
speed, BASE stands for using baseband and T stands for Thick net or
Thick Ethernet. 10BASE-T Ethernet provides transmission speed up to
10mbps and uses Coaxial cable or Cat-5 Twisted Pair cable with RJ-5
connector. Ethernet follows Star Topology with segment length up to 100
meters. All devices are connected to a Hub/Switch in a Star Fashion.
Fast-Ethernet
To encompass need of fast emerging software and hardware
technologies, Ethernet extends itself as Fast-Ethernet. It can run on
UTP, Optical Fiber and can be wireless too. It can provide speed up to
100 mbps. This standard is named as 100BASE-T in IEEE 803.2 using Cat-5
Twisted pair cable. It uses CSMA/CD technique for wired media sharing
among Ethernet hosts and CSMA/CA (Collision Avoidance) technique for
wireless Ethernet LAN.
Fast Ethernet on fiber is defined under 100BASE-FX standard which
provides speed up to 100mbps on fiber. Ethernet over Fiber can be
extended up to 100 meters in half-duplex mode and can reach maximum of
2000 meters in full-duplex over multimode fibers.
Giga-Ethernet
After being introduced in 1995, Fast-Ethernet could enjoy its high
speed status only for 3 years till Giga-Ethernet introduced.
Giga-Ethernet provides speed up to 1000 mbits/seconds. IEEE802.3ab
standardize Giga-Ethernet over UTP using Cat-5, Cat-5e and Cat-6 cables.
IEEE802.3ah defines Giga-Ethernet over Fiber.
Virtual LAN
LAN uses Ethernet which in turn works on shared media. Shared media
in Ethernet create one single Broadcast domain and one single Collision
domain. Introduction of switches to Ethernet has removed single
collision domain issue and each device connected to switch works in its
separate collision domain. But even Switches cannot divide a network
into separate Broadcast domain.
Virtual LAN is a method to divide a single Broadcast domain into more
than one Broadcast domains. Host in one VLAN cannot speak to a host in
another. By default, all hosts are placed into same VLAN. [Image: Virtual LAN]
In above pictures, different VLANs are depicted in different color
codes. Hosts in one VLAN, even if connected on the same Switch cannot
see or speak to other hosts in different VLANs. VLAN is Layer-2
technology which works closely on Ethernet. To route packets between
two different VLANs a Layer-3 device (such as Router) is required.
