Differentiate between transparent and nontransparent fragmentation?

A number of problems are encountered because of the size of the data packets. There is no ability in the data link layer by means of which it could handle these problems and so the bridges also don’t work here. 

The Ethernet also experiences a number of problems because of the following:

Ø  Different way in which the maximum packet size is defined.

Ø  Maximum packet size that can be handled by a router.

Ø  The maximum length slot that are used for transmission

Ø  Errors due to the packet length

Ø  Standards

The data packets can be fragmented in two ways namely:

1. Transparent and
2. Non – transparent

Both these ways can be followed based on a network by network basis. We can also say that no such end – to – end agreement exists based up on which it can be decided which process is to be used.

Transparent Fragmentation: 

- This type of fragmentation is followed when a packet is split in to smaller fragments by a router.

- These fragments are sent to the next router which does just the opposite i.e., it reassembles the fragments and combine them to form original packet. 

- Here, the next network does not come to know whether any fragmentation has taken place. 

- Transparency is maintained between the small packet networks when compared to the other subsequent networks.

- For example, transparent fragmentation is used by the ATM networks by means of some special hardware. 

- There are some issues with this type of fragmentation. 

- It puts some burden on the performance of the network since all the fragments have to be transmitted through the same gateway. 

- Also, sometimes the repeated fragmentation and reassembling has to be done for small packet network in series. 

- Whenever an over-sized packet reaches a router, it is broken up in to small fragments. 

- These fragments are transported to the next exit router. 

- The fragments are assembled by this exit router which then forwards them to the next router.

- Awareness regarding this fragmentation is not maintained for the subsequent networks. 

- For a single packet fragmentation is done many times before the destination is finally reached. 

- This of course consumes a lot of time because the repeated fragmentation and assembling has to be carried out. 

- Sometimes, it also presents the reason of corrupting the packet’s integrity.

Non-Transparent Fragmentation: 

- In this type, the packet is split in to fragments by one router. 

- But the difference is that these fragments are not reassembled until the fragments reach their destination. 

- They remain split till then. 

- Since in this type of fragmentation the fragments are assembled only at the destination host, the fragments can be routed independent of each other. 

- This type of fragmentation also experiences some problems such as header has to be carried by each of the fragments till they reach their destination. 

- Numbering has to be done for all the fragments so that no problem is experienced in reconstructing the data stream.

Whichever type of fragmentation we use, one thing has to be made sure which is that later we should be able to form the original packets using the fragments. This insists on having some type of labeling for the fragments. 

Segmentation is another name for the fragmentation. A packet is injected in to the data link layer by the IP layer but it is not responsible for reliable transmission of the packets. Some maximum value on the size of the packets is imposed by each layer for their reasons. For a large packet that travels through the network for which the MTU is small, fragmentation is very much needed. 

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 congestion control and flow control?

Flow control and congestion control are similar sounding concepts and often confuse us sometimes. In this article we shall discuss about the differences between these two. 

- Computer networks use the flow control mechanism for keeping control over the data flow between two nodes in such a way that the receiver if it is slower when compared to the sender is not outrun by it. 

- The mechanism of flow control also provides ways to the receiver to maintain control over the speed with which it transmits the information.

- On the other side, the congestion control provides mechanism for the controlling the data flow under the condition of actual congestive collapse. 

- The mechanism keeps a control over the entry of data in to the network so that this traffic can be handled by the network effectively.  

- The mechanism of flow control does not let the receiving node get overwhelmed by the traffic that is being sent by another node. 

There are several reasons why this flow of data gets out of control and affects the network negatively. 

- First reason being that the receiving node might not be capable of processing the incoming data as fast as it is being sent by the sender node. 

- Based on these reasons there are various types of flow control mechanisms available. 

- However, the most common categorization is based on the fact whether the feedback is being sent to the sender or not. 

- There is another flow control mechanism called the open loop flow control mechanism. 

- In this mechanism no feedback is sent to the sender by the receiver and this perhaps the most widely used flow control mechanism. 

- Opposite of open loop flow control mechanism is the closed loop flow control. 

- In this mechanism, the receiver sends back congestion information to the sender. 

- Other commonly used flow control mechanisms are:

Ø  Network congestion

Ø  Windowing flow control

Ø  Data buffer etc.

- Congestion control offers such methods that can be used for regulating the incoming traffic in the network to such an extent where the network itself can manage all that.
- In congestion control, the network is prevented from falling in to a state of congestive collapse. 
- In such a state either little or no communication happens.
- This little communication is of no help. 
- Switching networks usually require congestion control measures than any other type of networks. 
- The congestion control is driven by the goal of keeping the number of data packets at such a level that the performance of the network would be reduced dramatically.
- Congestion control mechanism can be seen even in protocols such as UDP (user datagram protocol), TCP (transport control protocol) and other transport layer protocols. 
- TCP makes use of the exponential back off and slow start algorithms. 
- We classify the congestion control algorithms based up on the feedback that is given by the network, the performance aspect that has to be improved, and modifications that have to be made for the present network, fairness criterion that is being used and so on. 

- Congestion and flow control are two very important mechanisms used for keeping the traffic flow in order. 
- Flow control is a mechanism that stretches from one end to another i.e., between the sender and the receiver where the speed of sender is much higher than that of the receiving node. 
- Congestion control is implemented for preventing packet loss as well as delay that is caused as a side effect of the network congestion. 
- Congestion is meant for controlling the traffic of the entire whereas flow control is limited to transmission between two nodes.

Comparison between Virtual Circuit and Datagram subnets

Difference #1:

- In virtual circuits the packets are allowed to contain in them the circuit number rather than storing the full address of the destination. 

- This reduces the requirement for a much larger memory and bandwidth. 

- This also makes it cheaper in cost. 

- On the other hand, the data-grams have to contain the full destination address rather than a single circuit number.

- This causes a significant overhead in the data-gram sub nets. 

- Also, this leads to wastage of the bandwidth. 

- All this implies that the data-gram sub nets are more costly when compared to the virtual circuits. 

Difference #2:

- A set up phase is required for the virtual circuits. 

- For establishing this phase a lot of resources are required along with a lot of time. 

- Data-gram sub net in contrast does not require establishment of set up phase. 

- Hence, there is no requirement of resources.

Difference #3:

- In virtual circuits, for indexing purpose the circuit numbers are used by the router. 

- These numbers are stored in a table and are used for finding out the destination of the packet. 

- This procedure is quite simple when compared with the one used by the data-gram sub nets. 

- The procedure used in data-gram sub nets for determining the destination of the packet is quite complex. 

Difference #4:

- Virtual circuits allow for reserving the resources in advance on the establishment of the resources.

- This has a great advantage which is that the congestion is avoided in the sub net. 

- However, in the data-gram sub nets, it is quite difficult to avoid congestion. 

Difference #5:

- If a crash occurs in a router, then it will lose its memory. 

- Even if it backs up after sometime, all the virtual circuits that pass via it must be aborted. 

- This is not a major problem in the case of the data-gram sub nets. 

- Here, if the router crashes, the only packets that will have to suffer will be the ones that were queued for that router at that instant of time. 

Difference #6:

- The virtual circuits can vanish as a result of the loss or fault on the current communication line.

- In data-gram sub nets, it is comparatively easy to compensate for the fault or loss on the communication line. 

Difference #7:

- In virtual circuits there is one more cause for the traffic congestion. 

- This cause in the use of the fixed routes for the transmission of the data packets throughout the network. 

- This also leads to the problem of unbalanced traffic. 

- In data gram sub nets the routers are given the responsibility of balancing the traffic over the entire traffic.

- This has been made possible because it is allowed to change the routers halfway between the connections. 

Difference #8:

- Virtual circuits are one way of implementing the connection-oriented services. 

- For various types of data gram sub nets, a number of protocols are defined by the internet protocol. 

- Internet protocol provides the data-gram service at the internet layer. 

- In contrast with the virtual circuits, data gram sub nets are connection-less service. 

- It is the best effort message delivery service but at the same time is very unreliable. 

- There are a number of high level protocols such as TCP that are dependent up on the data gram service of the internet protocol.

- This calls for additional functionality. 

- The data gram service of IP is even used by the UDP. 

- The fragments of a data gram might be referred to as the data packets. 

- The IP and UDP both provide unreliable services and this is why both of them are termed as data grams. 

- The fragments of TCP are referred to as TCP fragments to distinguish it from data-grams. 

Differentiate between persistent and non-persistent CSMA?

- CSMA or Carrier Sense Multiple Access makes use of LBT or listen before technique before making any transmission. 

- It senses the channel for its status and if found free or idle, the data frames are transmitted otherwise the transmission is deferred till the channel becomes idle again. 

- In simple words, we can say that CSMA is an analogy to human behavior of not interrupting others when busy. 

- There are number of protocols out which the persistent and the non – persistent are the major ones. 

- CSMA is based on the idea that if the state of the channel can be listened or sensed prior to transmitting a packet, better throughput can be achieved.

- Also, using this methodology a number of collisions can be avoided. 

- However, it is necessary to make the following assumptions in CSMA technology:

1. The length of the packets is constant.
2. The errors can only be caused by collisions except which there are no errors.
3. Capture effect is absent.
4. The transmissions made by all the other hosts can be sensed by each of the hosts.
5. The transmission time is always greater than the propagation delay.

About Persistent CSMA

- This protocol first senses the transmission channel and acts accordingly. 

- If the channel is found to be occupied by some other transmission, it keeps listening or sensing the channel and as soon as the channel becomes free or idle, starts its transmission. 

- On the other hand, if the channel is found empty, then it does not wait and starts transmitting immediately. 

- There are possibilities of collisions. 

- If one occurs, the transmitter must wait for random time duration and start again with the transmission. 

- It has a type called 1 – persistent protocol which makes transmission of probability 1 whenever the channel is idle. 

- In persistent CSMA there are possibilities of occurrence of collisions even if the propagation delay is 0. 

- However, collisions can only be avoided if the stations do not act so greedy. 

- We can say that this CSMA protocol is aggressive and selfish. 

- There is another type of this protocol called the P – persistent CSMA. 

- This is the most optimal strategy. 

- Here the channels are assumed to be slotted where one slot equals the period of contention i.e., 1 RTT delay. 

- The protocol has been named so because it transmits the packet with probability p if the channel is idle otherwise it waits for one slot and then transmits.

About Non–Persistent CSMA

- It is deferential and less aggressive when compared to its persistent counterpart. 

- It senses the channel and if it is busy it just waits and then again after sometime senses the channel unlike persistent CSMA which keeps on sensing the channel continuously. 

- As and when the channel is found free, the data packet is transmitted immediately. 

- If there occurs a collision it waits and starts again.

- In this protocol, even if the two stations become greedy in midst of transmission of some other station they do not collide probably whereas, in persistent CSMA they collide.

- Also, if only one of the stations become greedy in midst of some other transmission in progress, it has no choice but to wait. 

- In persistent CSMA this greedy stations takes over the channel up on completion of the current transmission.

- Using non – persistent CSMA can reduce the number of collisions whereas persistent CSMA only increases the risk. 

- But the non – persistent CSMA is less efficient when compared to the persistent CSMA.

- Efficiency lies in the ability of the protocols of detecting the collisions before starting the transmission. 

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 the difference between functions and actions in QTP?

Functions and actions are two different aspects of quick test professional. In this article we will talk regarding the two and see what the differences between them are. 

What are functions in QTP?

- In most of the cases you may need to use some segments of code again and again in your tests. 

- In such a case a user defined function can be created which will not only reduce the length of your tests but will also make it easier to be designed, read and maintained. 

- The created functions can consist of the following:

1. Sub routines,
2. Modules
3. VBScript functions and so on.

- Three simple tests need to be followed if a function is to be used in the test from a library:

1. For creating a new function you can select new function library from the file menu. This cause a new tab to open up in the quick test professional window. You can even opt for multiple tests in the same file.
2. Next you need to save the function with extension “.qfl” and associate the appropriate library with that particular function. “Associate function library” option can be accessed from file menu.
3. Now the third step is to invoke the function using the usual simple function call. Quite advanced or sophisticated functions can be generated using DCOM and COM.

What are actions in QTP?

- Whenever you have a piece of code at your disposal you get two options in quick test professional i.e., to either turn it in to a function or a reusable action.

- When the quick test 9 was introduced by the mercury, an argument was made in the favor of the functions for making them more compelling by the means of multi document interface. 

- Many function libraries can be used at once but still mostly one test is used. 

- If you want a reusable action to be edited, the current test has to be shut down. 

- Then you need to open the test in which you want to make the changes, make the changes and then save and close. 

- Later, the original test can be re-opened.

Differences between Functions and Actions in QTP

Now we are stating below some very basic differences between the functions and actions in quick test professional:

1. Passing of Data: 

Functions can access a variety of objects advance as well as primitive like arrays, test objects (such as frames, pages and so on.) and dictionary objects. But this is not the case with the actions rather they can accept only primitive kind of data types such as parameters like integers, strings and so on.

2. Use of Resources: 

Each action is provided with the following basic things:

a)   A folder

b)   3 separate VBS files

c)   A subfolder for keeping snapshots

d)   A certain amount of load time

e)   An excel spreadsheet and

f)    A local object repository.

On the other hand a function consists of only the code and nothing else.

3. API: 

For functions, the function libraries can be attached to the test scripts through the API of the quick test professional. As for the actions the insertion of the calls in the existing actions is not possible through the API of the quick test professional rather it can be done only through the interface.

4. Return Values: 

Actions return the values which are quite difficult to be read in syntax as well as understand. On the other hand the return values of the functions work well and similarly in all the languages.

With actions your test can be divided in to business processes or logical units along with the scripts that are more efficient as well as modular.

What is keyword view and Expert view in QTP?

In quick test professional you get two types of views namely the key word view and the expert view which we are going to discuss in this article. 

Keyword View in Quick Test Professional

- The key word view is more like a table like view which consists of a finite number of rows and columns. 

- Each row in the key word view represents a separate step. 

-On the other side, different parts of the steps are marked by the corresponding columns. 

- There is a separate column called the item column which holds the item on which the user wants the step to be carried out. 

- Also, in this column the hierarchy of the GUI object on which the operations are to be carried out. 

- There is another column by the name operation column consisting of the operations to be performed on the item as required. 

- One more column called value column holds the values of the arguments for the operation selected by the user. 

- Each step is documented by the quick test professional under the column titled documentation by the quick test professional for the ease of understanding. 

- The above mentioned 4 columns are the default columns provided by the quick test professional however comment and assignment columns in key word view can be used. 

- With the aid of key word view you can create as well as view the steps of the operation in a tabular, modular and key word driven format. 

- Full access is granted to the test automation experts via some debugging environment and integrated scripting that has round trip synchronizing with the key word view. 

Expert View and Differences between expert and keyword view in Quick Test Professional

- The expert view also displays the same object hierarchy all mapped to the same arguments value and same operation. 

- The basic difference lies between the formats in which the same data is arranged in the two. 

- Both have a distinct way of representing the data though the data displayed is same. 

- All the operations like creation of a step, modifying it and so on can be carried out in the key word view. 

- Expert view as the name suggests is a more advanced way of dealing with the data and requires mastery over the tool. 

- After all of the operations are recorded, they are displayed as steps in the quick test professional in key word view whereas these steps are generated as scripts in the expert view. 

- Expert view is for the advanced testers, using which they can view as well as edit their tests as and when required. 

- This also reveals the underlying standard of the VBscript that is used by the quick test professional. 

- Any changes that are made to the scripts in the expert view are automatically reflected in the key word view also.  

- However, in key word view the exact recording of the applications can be viewed. 

- In key word view, the steps can be created as well as viewed and can be modified by making a selection of the operations as well as items in the key word view and putting in the information as and where required. 

- In the expert view each of the operations that are performed in the form of VB script statements are displayed. 

- There exists a row in the key word view for every method and object. 

- To put it simply, it can be said that the VBscript format is used by the expert view whereas the scripts are represented using graphical user interface which is divided in to 4 default columns as mentioned earlier in the article in key word view. 

- Also, in the expert view all the test statements are displayed.