What is meant by multiplexing?

- Multiplexing or muxing is a very important process in computer networks and the telecommunications. 

Using this process, a number of digital data streams or analog message signals are combined as one signal and then transported over the common medium. 

- Multiplexing is used wherever it is required to share a resource that is very expensive. 

- The most common example of multiplexing is of using one wire for several telephone calls. 

- The origin of the multiplexing dates back to 1870s when telegraphy was started.

- Now it is used to a great extent in the field of communications. 

- The telephone carrier multiplexing was developed by George Owen Squire in the field of telephony. 

- The communication channel over which the multiplexed signal might be transmitted might be a physical transmission medium. 

- The high level communication channel’s capacity is divided by multiplexing process in to a number of low level logical channels where for each message or data stream one channel is used. 

- Demultiplexing is the reverse process of multiplexing. 

- This is used for the extraction of the original signals on the reception side.

- A multiplexer or MUX is a device that is used for carrying out the multiplexing process and the demultiplexer or DEMUX is the device that performs demultiplexing. 

- IMUX or inverse multiplexing is another process whose aim is just the opposite of the multiplexing.

- It breaks down a single data stream in to various streams while transferring them at the same time over various communication channels.

- Later, the original stream is recreated.

Types of Multiplexing

Many different types of multiplexing technologies are available today. Each has its own significance:

Ø SDM or space-division multiplexing: 

This technique implies on using different point – to – point wires for individual communication channels. For example, an audio cable of analogue stereo, multi – pair telephone cable, switched star network, mesh network. However typically the wired SDM is not usually considered as multiplexing. In SDM a phased array antenna is formed by multiple antennas. For example MIMO (multiple – input and multiple – output), SIMO (simple – input and multiple – output), MISO (multiple – input and single – output) etc.

Ø  FDM or frequency-division multiplexing: 

This is considered to be an analog process, here the signals are sent in to different frequency ranges over a shared medium. For example, TV and radio broadcasting from satellite stations through the earth’s atmosphere. One cable is given in each house but over this cable many signals can be sent to other subscribers also. For accessing the desired signal, the users require to tune to that particular frequency. WDM or wavelength division multiplexing is a variant of FDM.

Ø TDM or time-division multiplexing: 

Unlike FDM, TDM is a digital technology but very rarely it might be used as an analog technology also. The process involves putting bytes in a sequence for each input stream one by one. this sequencing is done in such a way that the receiver can appropriately  receive them. If this is done quickly, the fact that another logical communication path was served in that circuit time won’t be detected by the receiver.

Ø  CDM or code-division multiplexing: 

In this multiplexing technique, the same frequency spectrum is shared by the several channels at the same time. Also the bandwidth of the spectrum is quite high when compared to the symbol rate or the bit rate. It is implemented in either of the two forms namely direct sequence spread spectrum and frequency hopping.

Some other types of multiplexing techniques which are less prominent are:

Polarization-division multiplexing: Used in optical and radio communications.

Orbital angular momentum multiplexing

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 meant by Conditional Access System?

- Conditional access system or CAS is a system that has been developed as a means for providing the protection to content by setting certain criteria that should be met before the access to it is granted. 

- The concept of conditional access is however related closely to the digital television systems such as the satellite television. 

- The standards for the conditional access system have been defined under the following specification documents of the digital video broadcasting standard (DVB):

1. DVB – CA (conditional access)
2. DVB – CSA (common scrambling algorithm)
3. DVB – CI (common interface)

- All of these standards together define a method for obfuscating a digital television stream, thus providing access to only those possess the ‘valid description smart cards’. 

- All of these specifications are available on the standards page of the DVB web site.

- Conditional access is the result of combination of encryption and scrambling technologies. 

- Firstly, the data stream is subjected to scrambling with a secret key of 48 – bits usually called the control word. 

- The control word is generated in such a pattern in which the successive word cannot be predicted. 

- It is recommended by the DVB that a physical process should be used for that. 

- For a person to unscramble the data stream it is required that he/ she should know about the current control word. 

- In order to provide the protection to control word during the course of transmission, encryption methodology is used. 

- The control word will be de-crypted by the conditional access system only when there is authorization. 

- This authorization is given in the form of EMM or entitlement management message. 

- They are specific for each subscriber. 

- Different ECMs can also transmit the control word at one allowing the usage of many conditional access systems at once and simultaneously. 

- The simulscrypt of the DVB allows all the multiplex operators to cooperate while at the same time saves bandwidth. 

- Channels such as the hot bird satellites and the CNN international make use of several CASs in parallel. 

- The data from the decryption is read from the cards and updated if required through either of the following:

1. CAM or conditional access module
2. PC card – format card reader
3. Built – in ISO/ IEC 7816 card reader

- Conditional access systems include two major types namely analog systems and the digital systems both categorized at shown below:

1. Analog systems:

Ø  Nagravision

Ø  Videocrypt

Ø  Videocipher

Ø  Eurocrypt

2. Digital systems:

Ø  Abel Quintic

Ø  ABV (alliance broadcast vision)

Ø  Accessgate

- These systems are the key component for all the digital TV operations.

- Their purpose is to secure the investments of the operators through the encryption of the signals. 

- The payment from the customers in order to watch TV is also ensured by the conditional access systems. 

- But the TV operators usually have limited knowledge about the conditional access systems. 

- Encryption is involved with the whole conditional access system. 

- Operators have the analyzers for finding the cause of the common signal problems. 

- But they do lack tools that can debug the CASs. 

- Most of the conditional access system monitoring tools follow the ETSI TR 101 290 standard (a DVB promoted standard) even though the CAS is not fully covered by this specification. 

- Analyzing tools as such let the operators to know about the problems of conditional access system before the customers notice it. 

- But practically also, it is not possible to take care of all the problems that may affect the subscriber. 

What are the advantages and drawbacks of QTP?

Though the quick test professional is quite useful like every other object in this world, it has got its advantages and drawbacks. The same thing is going to be discussed here in this article. 

Advantages of Quick Test Professional

1. The quick test professional has got such simplicity that it can be understood easily by even a non programmer and can begin with the testing by adding the test cases.

2. The quick test professional provides support for record and playback of the scripts. Plus it comes with the ability to edit the scripts just after the recording is complete. Different recording modes or options are available with the quick test professional:

    a)   Normal or context sensitive recording mode

    b)   Analog recording mode and

    c)   Low level recording mode

3. Quick test professional (QTP) possesses an excellent object identification mechanism.

4. Quick test professional comes with an extended support for the following add ins:

    a)   SAP

    b)   Oracle

    c)   Java

    d)   .NET

    e)   Siebel

    f)    Main frame or terminal emulators

    g)   Web services

    h)   Web forms and so on.

5. With quick test professional the existing tests and test scripts can be enhanced even if the AUT or application under test in not there with the help of an active screen.

6. Almost all of the popular test automation frame works (such as key word driven, data driven, modular testing approach, hybrid and so on) are supported by the quick test professional.

7. An IDE or integrated development environment is inbuilt in to the quick test professional which is quite easy to use and has got a simple interface. 

8.The scripting language “VBScript” which is used by the quick test professional is just like English and can be learned easily. There are plenty of sources via which one can learn VBScript.

9. In many of the test automation suites it is found difficult to implant the MOM i.e., the Microsoft object model but in quick test professional it is an easy task to do.

10.To make quick test professional further better it can be embellished with other test management tools such as the HP QC or quality center, functional testing tools such as winrunner and test management tools such as test director and so on. Once the tests have been mapped to the automation scripts, they can be executed directly from the HP quality center. Also, winrunner test execution can be kicked off by the quick test professional.

11.With quick test professional several types of suites can be maintained namely:

    a)   Sanity

    b)   Smoke

    c)   Regression and so on.

12. Like most of the test automation suites it provides support for xml.

13. Excel or data base files can be used easily in quick test professional with a variety of methods to use them.

14. Data and the test iterations can be easily maintained.

15. Test reports provided by the quick test professional are complete with all details required for analysis.

Drawbacks of Quick Test Professional

1. Proves to be costlier than any other test automation suite in terms of license and maintenance.

2. It does not support multiple threads and instances execution.

3. It will be surprising to know that the quick test professional has got no high speed rather it has got much slower speed than most of the open source tools such as selenium.

4. Support is provided from HP QC only on the renewal of the license.

5. Add- ins (such as SAP, oracle, java, .net, Seibel, peoplesoft and so on) have to be purchased separately.

How many types of recording modes are there in QTP? Which will be used when?

Unlike Winrunner, Quick test professional comes with three types of recording modes. Two of them are same as what the winrunner had and one is new. 

The 3 types of recording modes available in quick test professional are:

1. Context sensitive recording
2. Analog recording and
3. Low level recording

Now let us discuss about these three recording modes in much detail one by one:

Context Sensitive Recording Mode

- The context sensitive recording mode is considered to be the normal recording mode in the quick test professional. 

- In this recording, full advantage of the test object model of the quick test professional can be taken. 

- In this mode the quick test professional can recognize a particular GUI object perfectly even if its location on the application screen is not known. 

- This recording takes place regardless of the location of the GUI objects on the application interface. 

- In this mode, the objects present in the software application and the operations that are performed in them are recorded. 

Analog Recording Mode

- Each and every movement of the mouse is tracked and recorded as it is dragged across the screen or some window. 

- The analog recording data captured by the quick test professional is saved as what are called tracks and these tracks are in turn saved in the directory of the test that you are going to perform. 

- This technique proves to be useful essentially for recording those operations whose recording cannot be carried out at the level of an object. 

- An example for such operation is making a signature by dragging of the mouse.

- With analog recording two types of recording can be done as stated below:

1. Recording relative to the screen and
2. Recording relative to the window.

- When you have to perform the analog operations pertain to just one window then you can very well go for the second one i.e., the recording relative to the window.

- On the other hand when your analog recording is to consist of multiple screen actions like dragging and dropping objects from one window to the other the former option i.e., the recording relative to the screen can be used.

The Low Level Recording Mode

 

- It is used for making recording easy at the level of the objects. 

- With this recording, you can record operations being performed on any of the objects present in the application software. 

- With this mode, even all of the run time objects can be recorded as:

1. Window test objects or
2. winObject test objects.

- This mode of recording is independent of whether the quick test professional is able to recognize the specific operation or the specific object. 

- This recording mode should only be used when your tests demand the exact coordinates of the objects.

- An example of this can be given by a hash map. 

-This mode is also used when the recording of a test in a particular environment is not provided recognition by the quick test professional.

- Using this, x and y coordinates of any click can be recorded.

- It has been noted that more disk space is used up in the analog and low level recording modes as compared to the context sensitive mode. 

- For both of the last two modes, one thing should always be kept in mind is that the position of the screen during the run time as well the record should remain identical otherwise the test script may fail. 

What are different types of functions available in WinRunner?

Functions form a very important aspect of the winrunnner. When you work with the winrunner you are provided with functions from all the categories. 

Some of the functions are there which are suppose to work only when the applications have been developed in specific environments like those mentioned below:

1. Visual basic
2. Power builder and so on.

What is a Function?

- The availability of a specific function can be checked by clicking up on the availability button available at the top of the help screen for that particular function. 

- The loadrunner GUI vusers types make use of the winrunner for the creation of the system load.

- This makes it possible to make use of any of the categories. 

- The standard functions can be used apart from the loadrunner functions in loadrunner scenario scripts on UNIX platform only.

- There are certain TSL functions that have the support for both the Xrunner and winrunner. 

- The winrunner provides 4 major types of TSL functions as stated below and a different and unique requirement is addressed by each of them:

1. Context sensitive functions: These functions hold the responsibility for performing the operations on GUI objects.

2. Analog functions: These functions hold the responsibility for carrying out some user like events like mouse and key board input.

3. Customization functions: These functions hold the responsibility for configuring the settings of the testing tool according to one’s requirement. 

4. Standard functions: These functions hold the responsibility for performing the basic programming language operations.

Context Sensitive Functions

- The actions taking place on the software system or application which is under test can be depicted by these functions in terms of the GUI objects like buttons, lists and windows and so on. 

- These functions are available for the following platforms:

      a)   Winrunner

      b)   Load runner GUI vusers

   - Parameters that make a reference to the logical name of some object are involved in most of the context sensitive functions. 

   - However, it should be checked that the logical name of an object has been given a place with the physical description. 

    - What happens during recording is that the logical name is automatically put to use by the system. 

   - In some cases the function is expected to work with the physical description of the object as well.

    Analog Functions

   - Operations are recorded and executed at the screen coordinates of the screen as specified in the test script. 

   - When the winrunner recording is done in this mode, these functions come in to play for depicting the following:

a)     Mouse clicks

b)     Key board input

c)     Exact coordinates travelled by the mouse

- The purpose of these functions is to run and retrace the mouse tracks and submit the exact input that was recorded by the user. 

- There are some other particular operations that are supported by analog functions:

a)   Synchronization

b)   Verification

c)   Text manipulation etc.

- The analog functions are available for the same platforms as for the context sensitive functions.

Customization Functions

- With these functions, the testing tools you are using can be enhanced further to suit better with your testing style and other specific needs. 

- One particular example for this function is the function using which functions can be added to the function generator and custom GUI check points can be created. 

- These are only available for winrunner.

Standard Functions

- These functions are for general elements of the programming language like:

a)   Basic input and out put

b)   Control flow

c)   Mathematical functions

d)   Array functions and so on.

- Unlike other function the standard functions are available for all the products of mercury interactive.  

When do you go for Context Sensitive and Analog recordings? What's the difference between them?

Winrunner was developed as an automated load testing tool by the firm of HP (Hewett Packard) and it operated at the level of graphical user interface layer. You must be wondering why it was operated only at the GUI level? This is so because it was possible only at this level to record as well as play back the user actions from a large number of software systems and applications just as a real world user would do.

The winrunner provides us with two types of recordings namely context sensitive recording and analog recording. 

Context Sensitive Recording

- With the help of this mode all your actions on the software system or application in terms of the user interactions can  be recorded  while physically ignoring the location of the objects (windows, lists, buttons and so on) on the graphical user interface.

- Whenever some action is performed on the software system or application which is under question, the generation of a statement in TSL takes place. 

- As the recording goes on, a unique description for each and every selected object is written to a map called GUI map by the winrunner tool. 

- This GUI map keeps within itself all the files from the test scripts created by you and maintains all of them separately. 

- Suppose you fall in a condition where you got to change the graphical user interface of your software system or application.

- In such a case you only need to update all the test scripts present in the GUI map and you do not need to perform any 1000 number of tests. 

- With such a facility you not only save time but you are also able to reuse all of your context sensitive test scripts for the future versions of your software system or application that you plan to develop further. 

- In context sensitive recording, playing or running a test is quite easy and you just need to play back the associated test script. 

- A real world user is emulated by the winrunner software by carrying out user like options such as pointing the mouse to some object, selecting or deselecting the objects, entering words from keyboard and so on. 

- The object descriptions that we earlier had fed to the GUI map are read by the winrunner and then those objects are searched in the software application under test to match.  

- The objects can be tracked even if their placements have been changed. 

Analog Recording

- In this type of winrunner recording, mostly the events like those mentioned below are recorded by the winrunner:

a)    Mouse clicks

b)    Input entered via the key board

c)   The exact x and y coordinates travelled by the mouse pointer and so on.

- While the test is executed or the test scripts are played the mouse tracks are re- traced by the winruner.

- However, one thing should always be kept in mind that winrunner is used only when the exact mouse coordinates are of utmost important to your test case and also when you have to test some drawing software application. 

- It is almost impossible to capture the GUI elements like objects and windows with the analog recording. 

- To say it simply, the analog recording holds good for the software systems and applications containing bitmap areas.

If you starting afresh with the winrunner it is recommended that you go for context sensitive recording since here you don’t have to go for exact mouse tracks. 

What are different methods of verification and validation?

Verification and validation together can be defined as a process of reviewing and testing and inspecting the software artifacts to determine that the software system meets the expected standards. There are various methodologies for verification different kinds of data in software applications. The different methods have been discussed below:

- File verification
It is used to check the integrity and the level of correctness of file. It is used to detect errors in the file.
- CAPTCHA
It is a kind of device that is used to verify that the user of the website is a human being and not some false program intended to hamper the security of the system.
- Speech verification
This kind of verification is used to check the correctness of the spoken statements and sentences.
- Verify command in DOS.

Apart from verification techniques for software applications there are several other techniques for verification during the development of software. They have been discussed below:

- Intelligence verification
This type of verification is used to adapt the test bench changes to the changes in RTL automatically.
- Formal verification
It is used to verify the algorithms of the program for their correctness by some mathematical techniques.
- Run time verification
Run time verification is carried out during execution. It is done to determine if the program is able to execute properly and within the specified time or not.
- Software verification
This verification type uses several methodologies for the verification of the software.

There are several other techniques used for verification in circuit development. - Functional verification
- Physical verification
- Analog verification

Verification, Validation, and Testing of Engineered Systems

Fundamentals of Verification and Validation

Verification and Validation in Computational Science and Engineering