What are the differences between bridges and repeaters?

Bridges and repeaters are both important devices in the field of telecommunications and computer networking. In this article we discuss about these two and differences between them. 

- The repeaters are deployed at the physical layer whereas one can find bridges at the MAC layer. 

- Thus, we called repeaters as the physical layer device. 

- Similarly, bridge is known as the MAC layer device. 

- Bridge is responsible for storing as well forwarding the data packets in an Ethernet.

- Firstly, it examines the header of the data frame, selects few of them and then forwards them to the destination address mentioned in the frame. 

- Bridge uses the CSMA/CD for accessing a segment whenever the data frame has to be forwarded to it.

- Another characteristic of a bridge is that its operation is transparent. 

- This means that the hosts in the network do not know that the bridge is also present in the network. 

- Bridges learn themselves; they do not have to be configured again and again. 

- They can be simply plugged in to the network. 

- Installing a bridge causes formation of LAN segments by breaking a LAN. 

- Packets are filtered with the help of bridges. 

- The frames that belong to one LAN segment are not sent to the other segments. 

- This implies separate collision domains are formed. 

The bridge maintains a bridge table consisting of the following entries:

1. LAN address of the node
2. Bridge interface
3. Time stamp
4. Stale table entries

- Bridges themselves learn that which interface can be used for reaching which host. 

- After receiving a frame, it looks for the location of the sending node and records it.

- It keeps the collision domains isolated from one another thus, giving the maximum throughput. 

- It is capable of connecting a number of nodes and offer limitless geographical coverage. 

- Even different types of Ethernet can be connected through it. 

- Even the repeaters are plug and play devices but they do not provide any traffic isolation. 

- Repeaters are used for the purpose of regenerating the incoming signals as they get attenuated with time and distance. 

- If physical media such as the wifi, Ethernet etc. is being used, the signals can travel only for a limited distance and after that their quality starts degrading. 

- The work of the repeaters is to increase the extent of the distance over which the signals can travel till they reach their destination. 

- Repeaters also provide strength to the signals so that their integrity can be maintained. 

- Active hubs are an example of the repeaters and they are often known as the multi-port repeaters. 

- Passive hubs do not serve as repeaters. 

- Another example of the repeaters are the access points in a wifi network. 

- But it is only in repeater mode that they function as repeaters. 

- Regenerating signals using repeaters is a way of overcoming the attenuation which occurs because of the cable loss or the electromagnetic field divergence. 

- For long distances, a series of repeaters is often used. 

- Also, the unwanted noise that gets added up with the signal is removed by the repeaters. 

- The repeaters can only perceive and restore the digital signals.

- This is not possible with the analog signals. 

- Signal can be amplified with the help of amplifiers but they have a disadvantage which is that on using the amplifiers, the noise is amplified as well. 

- Digital signals are more prone to dissipation when compared to analog signals since they are completely dependent up on the presence of the voltages. 

- This is why they have to be repeated again and again using repeaters. 

What is the difference between a passive star and an active repeater in fiber optic network?

There are two important components of a fiber optic network namely passive star coupler and active repeaters. 

Passive Star in Fiber Optic Network

- Passive star couplers are single mode fiber optic couplers with reflective properties.  

- These couplers are used for optical local area networking at very high speeds. 

- These couplers are made from very simple components such as mirrors and 3 db couplers. 

- Besides this, these star couplers save a lot of optical fiber when compared to its trans-missive counterpart. 

- They are free of any multi-paths so as to avoid any interference. 

- A fiber optic network may consist of any number of passive star couplers and each of them is capable of connecting a number of users. 

- The input and output from every passive star coupler is given to the output and input of an active coupler. 

- The round trip transmission tile is stored by the active star coupler. 

- When it receives a signal from a passive star coupler, it stops the output to that coupler for the duration of the signal.

- It also inhibits the incoming data from all the other passive star couplers for the round trip transmission delay plus signal duration. 

- The purpose of a star coupler is to take one input signal and then splitting it in to a number of output signals. 

- In telecommunications industry and fiber optics communication, this coupler is used in network applications being a passive optical device. 

- If an input signal is introduced to one of the input ports, it is distributed to all of the output ports of the coupler. 

- As per the construction of the passive star coupler, the number of ports it will have is given by the power of 2. 

- For example, in a two port coupler or in a directional coupler or splitter, there are 2 input ports and 2 output ports.

- In a four port coupler, there are 4 i/p ports and 4 o/p ports and so on. 

- The digital equipment corporation also sold a device by the name of star coupler which was used for interconnecting the links and computers through coaxial cable instead of using optical fibers. 

Active Repeater in Fiber Optic Network 

- Active repeater is an important telecommunications device used for re transmitting the signal it receives to a higher level and with higher basically to the other side of an obstacle so that long distances can be covered. 

- Repeater is an electro-mechanical device that helps in regenerating the telegraphy signals. 

- It may be defined as an analog device for amplifying the input signal, reshaping it, re-timing it for re-transmission. 

- A re-generator is a repeater that can perform the re-timing operation. 

- Repeaters just tend to amplify the physical signal without interpreting the data transmitted by the signal. 

- The 1^st layer i.e., the physical layer is where the repeaters operate. 

- Repeaters are employed for boosting the signals in optical fiber lines as well as in twisted pair and coaxial cables. 

- When a signal travels through a channel, it gets attenuated with the distance and time because of the energy loss (dielectric losses, conductor resistance etc.). 

- When light travels in optical fibers, it scattered and absorbed and hence is attenuated. 

- Therefore, in long fiber lines, repeaters are installed at proper intervals for regenerating and strengthening the signal. 

Repeater in optical communication performs the following functions:

Ø  Takes the input signal

Ø  Converts it in to electrical signal

Ø  Regenerates it.

Ø  Converts it in to optical signal

Ø  Re-transmits it

- These repeaters are usually employed in submarine as well as transcontinental communication cables as the loss is unacceptable in these cases.  

What is DRAM? In which form does it store data?

The random access memory is of two types out of which one is dynamic random access memory or DRAM and the other one is SRAM or static random access memory. 

Here we shall focus up on the first type i.e., the Dynamic RAM. 

What is Dynamic Random Access Memory (DRAM)?

- In dynamic RAM, each bit of the data is stored in a separate capacitor. 

- All these capacitors are housed within an IC (integrated circuit).

- These capacitors can be in either of the two states:

1. Charged and
2. Discharged

- The two values of a bit are represented by means of these two states only. 

- The two values of bit are 0 and 1. 

- However, there is a disadvantage of the dynamic RAM. 

- These capacitors tend to leak charge and therefore may lose all the stored information. 

- Therefore, it is very important to keep the capacitors flushing with fresh charge. 

- They are refreshed at regular intervals of time. 

- It is because of this refreshing requirement this type of RAM has been named so. 

- The main memory or the physical memory of the CPU is constituted of this dynamic RAM only.

- Apart from desktops, DRAM is also used in workstation systems, laptops, video game consoles etc. 

- The structural simplicity is one of the biggest advantages of the DRAM. 

- For each bit it only requires one capacitor and one transistor, whereas SRAM requires 4 to 6 transistors for the same purpose. 

- This enables the dynamic RAM to attain very high density. 

- DRAM is a volatile memory unlike the flash memory and so it loses data whenever the power supply is cut.

- The capacitors and the transistors it uses are extremely small and so billions of them can be easily be integrated in to one single memory chip.

- DRAM consists of array of charge storage cells arranged in a sort of rectangular way. 

- Each of the cells consists of one transistor and one capacitor. 

- Word lines are the horizontal lines that connect the rows with each other. 

- Two bit lines compose each of the columns of cells. 

- These lines are called the + and – bit lines.

- It is specified by the manufacturers that at what rate the storage cell capacitors are to be refreshed. 

- Typically, it is less than or equal to 64 ms. 

- The DRAM controller consists of the refresh logic that is responsible for automating the periodic refresh. 

- This job cannot be done by any other software and hardware. 

- Thus, the circuit of the controller is very complicated. 

- The capacity of DRAM per unit surface is greater than that of the SRAM. 

- Some systems may refresh one row at one instant while others may refresh all the rows simultaneously every 64 ms.  

- Some systems use an external timer based up on whose timing they refresh a part of the memory. 

- Many of the DRAM chips come with a counter that keeps track of which row is to be refreshed next.

- However, there are some conditions under which the data can be recovered even if the DRAM has not been refreshed since few minutes. 

- Bits of the DRAM might flip to opposite state spontaneously because of the electromagnetic interference in the system. 

- Background radiation is the major cause for the occurrence of the majority of the soft errors.

- Because of these errors the contents of the memory cells may change and circuitry might be harmed. 

- Redundant memory bits along with the memory controllers are one potential solution to this problem. 

- These bits are within the modules of the RAM. 

- The parity is recorded by these bits which enable the reconstruction of the missing data via ECC or error – correcting code.

What is hard disk and what is its purpose?

- HDD or Hard Disk Drive is for data storage. 

- It is used for storage and retrieval of the digital information or data that is stored on it. 

- The data is stored or retrieved by means of its discs that rotate rapidly. 

Hard Disk and its Purpose

 

- These discs are known as the platters and are coated with some sort of magnetic material. 

- The major characteristic as well as benefit of hard disk drives is that they retain the data even when the power supply is switched off. 

- From hard disk, the data can be read in a manner of random access. 

- This means that the storing and retrieval of the individual blocks of the data can be done either sequentially or in any order that the user may like. 

- A hard disk may consist of one or more than one of those rigid platters. 

- These rotating discs have magnetic heads that are located on an actuator arm that is continuously moving and reads and writes data on their surfaces. 

- IBM was the first to introduce the hard disk in the year of 1956. 

- Hard disk drives are the most dominant and the prominent secondary storage device for the computers since 1960s. 

- Since then, it has been continuously improved. 

- The HDD units are produced by more than 200 companies; among them most prominent developers are Toshiba, Seagate, Western digital etc. 

HDD’s primary characteristics are:

Ø  Capacity and

Ø  Performance

- The former is specified in terms of the unit prefixes. 

- In some systems, the capacity of the hard disk drive might be unavailable to the user since being used by the operating system and the file system and may have a possibility of occurrence of redundancy.

- The latter is specified in terms of the movements of the heads for a file i.e., the average access time in addition to the time taken for moving the file under the head i.e., the average latency and data rate. 

HDDs are available in two most common factors namely:

Ø  3.5 inch for desktop computers

Ø  2.5 inch for laptops

HDDs might be connected to the system by any of the following standard interface cables:

Ø  Serial ATA or SATA cable

Ø  USB cable

Ø  Serial attached SCSI or SAS cable

- In the year of 2012, flash memory emerged as a tough primary competitor for the hard disk drives. 

- These flash memories are some sort of solid state drives or SSDs. 

- However, the HDDs will still continue to dominate the secondary storage for its advantages such as price per unit of storage and recording capacity. 

- But there is a different scenario is the case of portable electronics.

- Here, the flash drives are considered to be more useful then rotating HDDs because here the durability and physical size of the drive has also to be considered more when compared to price and capacity.

HDD uses the magnetic recording technology where the data is recorded by magnetizing a thin film of material that is typically ferromagnetic on a disk. 

- The binary data bits are represented by the sequential change in the direction of the magnetization.

- An encoding scheme is used for encoding the user data. 

- An example of such encoding scheme run – length limited encoding. 

- It is these schemes that determine how the magnetic transitions would represent the data.

The latest HDD technologies are:

Ø  Shingled write

Ø  CPP/ MGR heads

Ø  Heat assisted magnetic recording

Ø  Bit – patterned recording

What is meant by Holographic Data Storage?

Currently the conventional magnetic and optical data storage dominates the field of high – capacity data storage. But another technology called the holographic data storage holds the potential to lead this area. Now what is this technology? We shall discuss about it in this article. 

Difference between Conventional storage methods and Holographic Data Storage

- The conventional methods such as the optical storage and magnetic storage technologies depend up on the recording of the individual data bits on the distinct optical and magnetic on the medium’s surface. 

- In the holographic technology, the information is recorded throughout the medium’s volume.

- Multiple images can be recorded in the same medium area via utilization of the light at varying angles. 

- Further, in the conventional storage methods the recording takes place in a linear fashion whereas, 

- In holographic storage, millions of bits can be recorded and read in parallel thus increasing the rates of the data transfer more than what are offered by the conventional methods.

Features of Holographic Data Storage

Data Recording: 

- The information is stored in a thick photosensitive optical substance with the help of an optical interference pattern. 

- A laser beam is divided into two un-identical optical patterns (formed of light and dark pixels) and is projected towards the medium. 

- A multitude of holograms in one volume is recorded by making adjustments in the wavelength, reference beam angle, media position etc.

Data Reading: 

- For reading the stored data, the same reference beam is reproduced for the creation of the hologram. 

- This light beam is focused up on the photosensitive material that illuminates the interference pattern which leads to the diffraction of the light. 

- The pattern is then projected to the detector. 

- The data is then read in parallel at a rate more than that of millions. 

- This means high data transfer rate. 

- It takes less than 0.2 seconds for accessing the information from a holographic drive. 

- Holographic data storage has offered a solution to many companies for preserving and archiving the information. 

- The WORM (write – once, read many) approach provides assurance about the content security, risk of modification and overwriting of the information. 

- This technology provides hope for storage of data without degradation for 50 plus years which is quite more than the current options.

- However, if the same trend is followed, and it becomes possible to store data for 50 – 100 years in same format, that would still be irrelevant. 

- This is so because the format will be changing in less than 10 years.  

Types of Holographic Media

- Holographic media is of two types namely:

1.  The re-writable media and
2. The write once medium.

- In the former type, the changes can be reversed but in the latter can the changes are irreversible.

There is no point in the competition between the holographic storage and the hard drives since the former can find a market that is based up on the virtues like access speed. 

- The holographic data storage technology does seems to have a future in the video game market. 

In the year of 2009, the GE Global Research did came up with their own demonstration of the holographic storage medium that could stand the discs that had read mechanisms somewhat similar to that of the blu–ray disc players. 

What is a Module? What is a Compiled Module? (In WinRunner)

Modules are the basic elements of any software system or application that make up the foundation of that software system or application. In winrunner usually the term “compiled module” is used. We shall see in this article that what are the compiled modules and how they can be created in winrunner. 

What are Compiled Modules?

- Compiled modules store the frequently used functions and so are considered to be a kind of libraries. 

- In compiled modules, the functions defined by users i.e., the user defined functions can be stored and later these stored user defined functions can be called from the test scripts. 

- Compiled modules are considered to be a library since they are actually a script containing all the user defined functions which are required to be called many times from other tests. 

- Whenever a compiled module is loaded, the functions that it contains are subjected to automatic compilation and reside in the memory itself. 

- After this, they can be called directly and from within any test. 

- With the compiled modules, the performances as well as the organization of the tests that you create are improved significantly.

- This happens so because the compiled modules are debugged before they are used and therefore the tests that we created require very less error- checking. - In addition to all this, the interpretation of a function in a test script is quite slower than the calling of a function that has already been compiled.

There are 2 ways available for compiling a module as mentioned below:

1. Loading the module to be compiled from a test script with the help of the TSL load function.
2. Running the script of the module to be compiled with the help of the winrunner run commands.

- The step commands can be used for performing the incremental compilation if in case you need to debug or modify an already existing module. 

- After this, the part of the compiled module that you made changes to needs to be run so that the entire compiled module will be updated. 

- It is necessary that the user defined functions present in your compiled modules are automatically compiled every time the winrunner starts up. 

- For this you need to add load statements top the start up tests. 

- However, you should know that for loading the recovery compiled module there is no need for adding the load statements to the start up tests or any other tests.

- This is so bacuse the recovery compiled module loads by itself when the winrunner is started. 

- Another point to be taken care of is that the compiled modules cannot load the GUI map files when you are working in the GUI map file per test mode.

- If the compiled module has to make references to any of the GUI objects present in the application software then it is required that the referred GUI objects must also be referred in the test which is responsible for loading that particular compiled module. 

- Like a regular TSL test the compiled module can be opened, edited or saved.  - However, there is a big difference in the contents of a compiled module or ordinary tests. 

- Like a regular test, a compiled module does not contain any analog input or check points. 

- This is so because a compiled module is designed to store user defined functions rather than just performing a test. 

- The purpose why it stores all the frequently user defined functions is to speed up the execution process by providing a quick and convenient access from the other tests. 

- Its contents are:

1. Function definitions and declarations,
2. External functions prototypes
3. Load statements etc. 

Describe some Caching Issues?

We all are familiar with what a cache is? 

"A cache can be defined as a memory component that is held for storing the data transparently in order to speed up the future serving of the data requests."

The data stored in a cache might be the data that has been required earlier for some operations. 

There are two events related to cache as mentioned below:

1. Cache hit: When the requested data is available in cache and

2. Cache miss: When the requested data is not available in cache and has to be looked up in to the RAM.

- The speed of processing is directly proportional to the number of requests that can be served via the cache. 

- Cache memories are quite costly and hence to make it cost efficient and keep the data usage as efficient as possible, smaller caches are used. 

- But since the time of its advent, cache has proven itself in the field of computing. 

"Caching can be thought of as a technique that is aimed at increasing the computing performance by keeping in itself the frequently accessed data."

There are basically three kinds of caching as we have stated below:

1. Caching output caching: In this type of caching the dynamic output that had been generated up on a request.

2. Fragment caching: In this type of caching the portion of the page that is generated by the request is cached since in many situations it is not practical to cache the whole page at once.

3. Data caching: In this type of caching the objects are cached pro grammatically.

What are different caching issues?

In this article we have taken in to discussion some very prominent caching issues. Most of the people experience problem in server caching of certain files. 

There are four major caching issues have been recognized which have been mentioned below:

1. Designing of a custom cache.

2. Securing of a custom cache.

3. Monitoring of a custom cache and lastly.

4. Synchronization of the caches in a server farm.

Besides these four major caching issues, there are many other minor caching issues.

- Some times it happens that the package delivery fails or an object or element appears like it has been corrupted and seems like such a failure has not got anything to do with the connection! 

- In such cases you can go on with a cache clear up. 

- If the situation is worse, you may also require clearing up the proxy cache!

- Web sites and browsers are looked up as a means of optimizing the resources which is done by them so well that they end up breaking down your dynamic web site content. 

- Your web site is not updated as you thought it will be done. 

Let us see an example, suppose you own a music web site which you frequently update with new music. Your clients come to your site every day so what happens is that the cache forces the web site to list the cached version of the play list and so the clients would never be able to listen to new and the latest music.

Such situation though enhances your internet experience, can also cause many other problems! Some times the cache will stash up an old page of the web site instead of showing up the latest one. You should make it a point to empty your browser’s cache from time to time. There are many internet service providers that cache pages to speed up the internet access like AOL. All the web pages that you visit are stashed up in the cache. 

How is password cracking done?

Password as we all know is some secret string of some characters that is typically used for the authentication purpose and as a means of identity proof.

WHAT IS PASSWORD?

- Password is way through which you access your some accounts and resources.
- A password is not meant for the other except the account holder.
- Passwords have been in use since ancient history and of course the password theft or cracking also!
- Nowadays passwords are more known for their use in the log in process to various systems like an operated system, ATMs, cell phones, email accounts and so on.
- It is not necessary that a password should be some meaningful word; rather it can be anything silly that is probably difficult to be guessed by the others.
- There are many types of passwords like passphrase (password formed by more than one word), PIN (personal identification number, numerical password).
- Passwords are again very much vulnerable since they are not as secure as their cryptographic counterparts i.e., protocols.
- These days password theft, password spoofing etc is quite common.

FACTORS AFFECTING THE SECURITY OF PASSWORD

Before we explain to how a password is cracked, you should know what all the factors affect the security of a password.

- Any password protected system is provided protection against the viruses, Trojans etc.
- Physical security measures like shoulder surfing are also implemented.
- Many a times less extreme measures are also used like:
1. Side channel attack
2. Extortion and
3. Rubber hose cryptanalysis

HOW SECURITY OF PASSWORD PROTECTED SYSTEM IS DETERMINED?

- The security of a password protected system is often determined by the rate at which the attacker or hacker can guess the password.

- To overcome this threat a “time out” of a few seconds can be implemented or a fixed number of chances should be given to type in the correct password.

- Many of the computer systems are now implementing these techniques.

- In some systems the cryptographic hash of the password gets stored which makes the password accessible to an attacker.

- The attacker can obtain the actual password from this hash password value.
- Passwords with high guessing rates are commonly used for cryptographic keys generation process.

HOW PASSWORD CRACKING IS DONE?

- Password cracking is recovering of the passwords from the stored or transmitted data in a computer system.
- There are many approaches developed to crack a password:

1. Guessing
This is perhaps the most common approach and does not require any special skills.

2. Changing the password
This method is second on the list of password cracking methodologies. When a user forgets his/ her password, the system allows the changing of the password following an authentication process.

3. Brute force cracking
This type of cracking involves trying every possible password till the right one is achieved.

4. Dictionary attacks
This method is also very much common and involves trying of the candidate passwords using a cracking dictionary.

5. Pattern checking
6. Word list substitution

PURPOSE FOR PASSWORD CRACKING

- The purpose for the cracking of password can be a positive one, for example, the user of a particular account might have forgotten his password and could not access his account.
- The purpose for the cracking of password can be negative one i.e., for gaining unauthorized access to a computer system, mischief purposes etc.

Time taken to crack a password is directly proportional to the strength of its character set or bits.
- The complex a password, the longer it will take to crack it.
- In some password cracking processes, the system is made to generate the similar types of passwords.
- Such passwords are called candidate passwords.
- Password cracking rate depends on the availability of the hash and the limitations of the software authentication.

CPU Caching

The cache on your CPU has become a very important part of today's computing. The cache is a very high speed and very expensive piece of memory, which is used to speed up the memory retrieval process. Without the cache memory every time the CPU requested data it would send a request to the main memory which would then be sent back across the memory bus to the CPU. This is a slow process in computing terms. The idea of the cache is that this extremely fast memory would store and data that is frequently accessed and also if possible the data that is around it.

CPU's however use a 2 level cache system. The level 1 cache is the fastest and smallest memory, level 2 cache is larger and slightly slower but still smaller and faster than the main memory. The main problem with having too much cache memory is that the CPU will always check the cache memory before the main system memory.

Read cache is used to store copies of data and instructions that are retrieved from main memory or mass storage. If the central processing unit (CPU) needs to access the same data or instructions again, it can use the copy in read cache. This is much faster the going back to main memory or mass storage again. Write cache is a temporary store for data that needs to be written to main memory or mass storage. The CPU can move the data into cache very quickly, and then continue executing instructions. The data is subsequently moved to its permanent location by the cache controller, a process that takes more time because main memory and mass storage devices are much slower to access than cache memory.