What is the difference between Multimode and single mode step index fiber ?

Multimode fiber is normally driven by an LED. The light is somewhat focused but bounces all around off the walls of the fiber. Different parts of the beam will travel different distances, such that what started out as a nice square wave pulse will get all smeared out after some distance and harder to detect. Both the fiber and the transmitters/detectors are cheap.

 Single mode fibers are driven by a laser. A laser beam is “coherent”, meaning all parts of the beam are pointing in exactly the same direction. Thus a nice pulse can travel huge distances and still look good, even if the fiber bends. In fact, a single mode fiber has been used to transmit a signal clear across the US without repeaters. This stuff is lots more expensive and is only therefore only used for long hauls.

 In addition, single mode fiber can carry multiple beams of different colors at the same time. The technique for doing this is a very inexpensive prism at each end. This is called Wave Length Division Multiplexing (WDM) and a single fiber can be used to carry multiple transmissions at one time. Last time I checked they were up to 128 channels at one time - explaining why long distance calls are basically free now.

What is modem ? Types of modem ? How does modem works?

What is modem ?

       The name modem means modulator demodulator. A modem connects our computer to a standard phone line or to our cable, which allows us to send data or receive data.
       A modem is a conversion device that converts signals from one device into signals another device can read.
 For example,
  a modem may covert the digital data of a computer into an analog signal that can be read and carried by a telephone line. The modulator part of the modem converts digital signals to analog signals, and the demodulator part converts analog signals to digital signals. ?

Your home internet connection medium will be either wireless or wired cables. When a wireless media or wired media transmits data it should be in analog form.

So the data you are transmitting or being transmitted to you is of analog form ie, timebased voltage variations, till it reaches your modem. So a modem converts those transmitted incoming analog signals to machine readable digital signals(digital signals are the signals that computers and electronic devices will accept or will generate) and outgoing digital signals to transmitable analog signals.

For transmission of a signal via a particular medium the signal needs to be modulated so that the frequency or amplitude or phase of the orginal wave can be changed to another amplitude or frequency or phase. The changing of the orginal frequency or amplitude or phase of the analog wave helps in better transmission and less noise while transmitting via the medium.

So a modem does another job too, to modulate orginal wave after converting it to analog form before transmission and to demodulate incoming transmitted wave before converting to digital form.

In short we define modem as follows:-

The name modem means modulator demodulator. A modem connects our computer to a standard phone line or to our cable, which allows us to send data or receive data. A modem is a conversion device that converts signals from one device into signals another device can read. For example, a modem may covert the digital data of a computer into an analog signal that can be read and carried by a telephone line. The modulator part of the modem converts digital signals to analog signals, and the demodulator part converts analog signals to digital signals.

What is wireless modem ?

   A wireless modem is a special type of modem that have special functionality to skip the telephone system and connects directly to a wireless network in order to access Internet by multiple devices provided by Internet Service Provider.

Types of modem ?

1.Internal modem:-
A modem that plugs into an expansion slot within the computer. Unlike an external modem, an internal modem does not provide a series of display lights that inform the user of the changing modem states. The user must rely entirely on the communications program.

2.External modem:-
An external modem is a network device that is in a self-contained enclosure external to a computer. This is in contrast to an internal modem that is contained on a printed circuit board with a computer.

What is a firewall ? How firewall works ? Why a firewall is needed?

Firewalls :-

 A firewall is a networking device, either hardware or software based, that

controls access to your organization’s network. This controlled access is
designed to protect data and resources from an outside threat. To do this,
firewalls are typically placed at entry/exit points of a network.

- for example, placing a firewall between an internal network and the Internet. Once there,
it can control access in and out of that point.

- Although firewalls typically protect internal networks from public networks,
they are also used to control access between specific network segments with-
in a network.
- for example, placing a firewall between the Accounts and the
Sales departments.
- As mentioned, firewalls can be implemented through software or through a
dedicated hardware device. Organizations implement software firewalls
through network operating systems (NOS) such as Linux/UNIX, Windows
servers, and Mac OS servers.

https://youtu.be/x1YLj06c3hM

working of firewalls  :-

1. Network traffic is comprised of packets — there are typically many of them for any given request      like loading a web page. The packets contain information about where they are coming from and        where they are going and of course carry information such as the request for a web page or the             response. The exact format of the packets depends on the type of software that is trying to                  communicate, but there are prescribed formats in various protocols.

2. A typical firewall is software that can look at those packets read the protocols and decide if the           packet will be allowed to continue to the next step on the path between the “where they are coming     from” and “where they are going”.

3. This software could be on your computer system, it could be in your cable modem, it could be in       your internet service provider’s systems, it could be on the web services’ network, or the web            services’ computer system. More than likely it will be on more than one of those.

4. Different firewall software varies in how it looks at the packets and how you can tell it about            packets that you don’t want to traverse. The firewall on your computer for example should look at      packets coming to your computer from anywhere and decide if they are following all the rules.
  If any rules are being broken then the firewall software can take action including simply not           forwarding the packet.

https://youtu.be/KZc1KaE1OKU

Why a Firewalls is needed?

   There is no need for a firewall if each and every host of a private network is properly
secured. Unfortunately, in practice the situation is different. A private network may
consist of different platforms with diverse OS and applications running on them. Many of
the applications were designed and developed for an ideal environment, without
considering the possibility of the existence of bad guys. Moreover, most of the corporate
networks are not designed for security. Therefore, it is essential to deploy a firewall to
protect the vulnerable infrastructure of an enterprise.

Access Control Policies :-

 Access control policies play an important role in the operation of a firewall. The policies

can be broadly categorized in to the following four types:

Service Control:-
- Determines the types of internet services to be accessed
- Filters traffic based on IP addresses and TCP port numbers
- Provides Proxy servers that receives and interprets service requests before it is
passed on.

Direction Control:-
- Determines the direction in which a particular service request may be initiated and
allowed to flow through the firewall.

User Control:-
- Controls access to a service according to which user is attempting to access it
-  Typically applied to the users inside the firewall perimeter
-  Can be applied to the external users too by using secure authentication technique

Behavioral Control:-
- Controls how a particular service is used
- For example, a firewall may filter email to eliminate spam
- Firewall may allow only a portion of the information on a local web server to an
external user

firewall capability :-

  Important capabilities of a firewall system are listed below:  

- It defines a single choke point to keep unauthorized users out of protected

  network
- It prohibits potentially vulnerable services from entering or leaving the network
- It provides protection from various kinds of IP spoofing
- It provides a location for monitoring security-related events
- Audits and alarms can be implemented on the firewall systems
- A firewall is a convenient platform for several internet functions that are not
  security related
- A firewall can serve as the platform for IPSec using the tunnel mode capability
  and can be used to implement VPNs.

Types of firewalls :-

The firewalls can be broadly categorized into the following three types:

• Packet Filters
• Application-level Gateways
• Circuit-level Gateways

1. Application-level Gateway:

   Application level gateway, also called a Proxy Server act as a relay of application level traffic.       Users contact gateways using an application and the request is successful after authentication. The      application gateway is service specific such as FTP, TELNET, SMTP or HTTP.

2.Circuit Level Gateway: 

  Circuit-level gateway can be a standalone or a specialized system. It does not allow end-to-end TCP    connection; the gateway sets up two TCP connections. Once the TCP connections are established,      the gateway relays TCP segments from one connection to the other without examining the contents.    The security function determines which connections will be allowed and which are to be disallowed.

3. Packet Filters:

  Packet filtering router applies a set of rules to each incoming IP packet and then forwards or       discards it. Pac ket filter is typically set up as a list of rules based on matches of fields in the IP or       TCP header

What is optical fibre ? Classification of optical fibre ? Explain step index and graded Index fibre?

OPTICAL FIBRE CABLE :-

  A thin flexible and transparent wire prepared for light propagation is called optical fibre.

The optical fibre has been constructed for the following reasons:
- The light wave cannot traverse long distance in air without any losses.
-  To make loss less light wave communication, the optical waves can be guided through optical fibre.
-The optical fibre can be used for the many of industrial application and medical
applications as well.
-The optical fibre consists of two media kept one inside the other.
- The centre transparent medium of optical fibre is called “core” and the outer is cladding.

Classification :-
 In the optical fibres the materials used, refractive index and mode of propagation of light are used for classification as follows:
Material Based optical fibres-
The material based classification results to the following types:
1. Plastic made fibres.
2. Glass made fibres.
The plastic made fibres are obtained from polymers of transparent to light, flexibility and
interaction less to light etc,.
For example poly methyl metha acrylate (PMMA), polyethylene (PE), polystyrene (PS) are used as core materials.
Glass made fibre is also fabricated from flexible glass as core with suitable drawing
technique in presence of impurities. Therefore, the above types of optical fibres are limited to some application.
Classification of optical fibres based on refractive index

The types of optical fibre can be classified based on the refractive index are

1. Step index fibres.
2. Graded index fibres.

Step index
 If the refractive index of core remains the same from the centre of the core to the core-cladding interface; those optical fibres are known as step index fibres .
 The distance room centre of core to interface of fibre vs. refractive index is shown

Graded Index:-

These types are made of varying refractiveindex of core material. Therefore,the refractive index 

changes with distance form the centre of fibre to core-cladding interface. The profile of above is as shown in fig.

*Modes of propagation based optical fibre:- 

Based on modes of propagation of light through core, the following are the types of optical
fibre identified.
- Single mode fibres
- Multimode fibres

* Single mode fibres 

Considering the light as electromagnetic radiation, the possible modes of propagation are
transverse electric (TE) modes and transverse magnetic (TM) modes.
  If the core size is adjusted to allow only one mode of light wave
propagation is single mode fibre, whose profile  is shown below:

*Multi mode optical fibre
If the core size has been adjusted to allow more than one mode of propagation of light
wave is multimode fibre. The profile of multimode fibres is as shown below:

    

Combination of optical fibres

The refractive index of core and mode of propagation of light in optical fibre usedto 

form combination types of optical fibre.

-Step index-single mode fibres

-Step index-multi mode fibres

-Graded index-Single mode fibres

-Graded index-Multi mode fibres

Application :-
- as fibres are very flexible ,they are used in flexible digital cameras.
- fibres are used in mechanical imaging i.e. for inspection of mechanical welds in pipes and engines of rockets,space shuttle,airp airps.
- it is used in medical imaging such as endoscopes and laparoscopes.
- it is used under sea communication
- used in military appreciation such as aircraft,shi sh, thank etc
- nuclear testing application use optical fibre phase sensor and transducer.
-it is used in LAN system of office, industrial plants and Collegs etc.

https://youtu.be/aqazAcE19vw

* Why is cladding necessary in an optical fibre cable?

   Since for a low loss transmission, we need to guide ligh by total reflection not by reflection from a mirror surface. Indeed although we can guide light using a mirror for the outer layer of the core, but it has a huge loss (even with perfect mirrors used in professional telescopes).

  On the other hand, light can be reflected by the total reflection property of transparent media without any loss. SO till now we are convinced that we must guide light through total intenal reflection of light. But how can it be done? For total intenal reflection we need two transparent media with different refraction indices. So despite the core which is used for the transmitting the light, we need another transparent medium which has a lower index than the core (to have total internal reflection) and it is the reason of existance of cladding in the fiber.