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.

What is meant by Ovonic Unified Memory?

There is much requirement in the IT industry for a high – speed memory plus that is non–volatile too. A solution to this is provided by the ovonic unified memory. 

- Ovonic Unified Memory is an approach to such a memory. Further, it offers reduced bit rate and cost. 

- There are some other characteristic features of this memory:

1. High endurance
2. Low power consumption
3. Non – volatile RAM
4. Readily scaled.
5. Merged memory/ logic simplified

- Since the ovonic unified memory is readily scaled it is not required to scale the barriers of flash and DRAM memories.

- This represents a new semiconductor technology – a creation of the Energy Conversion Devices, inc. however later it was licensed to the ovonyx inc.

- This technology makes use of a structural phase change that is reversible i.e., from a crystalline phase to the amorphous phase. 

- The material used here is the thin–film chalcogenide alloy. 

- This all constitutes the data storage mechanism of ovonic unified memory (OUM). 

- Each memory cell consists of an active medium in a small volume that acts as the programmable resistor. 

- This resistor switches between the low and high resistance in the dynamic range of greater than 40x. 

- The phase change technology is currently being used in the PD, CD RW, DVD RW and DVD RAM. 

- The basic advantage offered by OUM is in the terms of performance and cost when compared to its conventional counterparts namely the flash and the DRAM memories. 

- OUM has got compatibility with the merged memory/log. 

- A conventional CMOS process is used in the OUM technology along with some additional layers in order to form the memory elements. 

- The OUM products have been commercialized under various licensing agreements. 

- The alloy used in OUM consists of Se and Te elements.

- They exhibit the property of electronic threshold switching phenomenon because of which the OUM memory cells can be programmed at quite low voltages irrespective of which state they are in i.e., whether conductive or resistive. 

- The measurement of the resistance of cell is used to read the information stored. 

- The programming of the OUM devices is done electrically by the alteration of its structure of the alloy. 

- These OUM devices show metallic behavior are independent of the temperature.

- The OUM devices are known for their excellent data retention property in the case of high density array applications. 

- Also, the OUM cells have more than normal life cycle i.e., they can tolerate up to 10¹³ write and erase cycles without any failure. 

- These devices possess quite a large dynamic range.

- This allows them to be programmed for enabling the multi–state data storage at intermediate resistance values.

- For multi–stage data storage, every cell needs to support multiple–bit storage. 

- The technology behind the ovonic unified memory is the device modeling. 

- Here, simple analytical methods show the trends in the properties and size of the material for structures that are spherically equivalent. 

- Other numerical models are inclusive of the mesh evaluation plus the device geometry.

- The behavior of the OUM devices can be predicted using the numerical simulation. 

- The behavior of the OUM material depends up on its bulk properties which have a characteristic that they can be quantified. 

- There are 3 considerations of this model:

1. Phase–change: It includes heat of fusion, crystal growth and nucleation.
2. Electrical: It includes current density, electric field and percolation conduction.
3. Thermal: It includes percolation conduction and the heat equations.

Apart from the cost, another advantage of OUM is its near – idle memory qualities such as:

1. Static
2. Random accessible
3. Non – destructive read

What is Object Proxy Mechanism in IBM Rational Functional Tester?

The proxy pattern is recognized as a pattern for software designing in the field of computer science. In the most general term, the proxy can be defined as a class which functions as an interface to some other program or application. 

Where is Proxy used?

- A proxy can be used as an interface to anything such as the following:

1. A network connection
2. A large object present in the memory
3. A file

- Or in other words, we can say that it can be used for any resource that is quite expensive and it is not affordable to duplicate it.

- An example of the proxy can be given by a reference counting pointer object. 

- There are situations where it is required to have multiple copies of a complex object. In such situations proxy pattern comes to play a big role. 

- It is adapted for incorporating the fly weight pattern for reducing the memory foot print of the application. 

- In typical situations, the instance created for a complex object is only one in count and multiple proxy objects are created. 

- All these proxy objects consist of a reference to an original object that is both single and complex. 

- The operations that are performed in a proxy object are carried forward to the original object. 

- If all the instances created for the proxy lose their scope, the memory of the complex object is de–allocated for that particular instance. 

- A virtual proxy pattern is used in java. 

- A proxy class can be used to access the remote methods. 

About Object Proxy Mechanism

- Firstly, an interface is created against which the rational functional tester creates the classes using the proxy patterns. 

- An interface may consist of many methods. 

- These methods need to be coded by all the classes which are to implement those particular methods. 

- If a proxy class is running on some other system rather than the original class, then the proxy class can represent its real image itself over there. 

- All the information regarding the image is obtained from the disk. 

- The multiple loading of the image can be avoided by using the proxy pattern with the code of the image. 

- This happens so because the image is accessed in a memory saving manner from some other system. 

- The primary purpose of the proxy object is to act as an intermediate object between the accessible object and the client. 

- The proxy object then serves the purpose of monitoring the life span of the object that is accessible. 

- Also, if the accessible object is not destroyed, all the calls are forwarded to it. - The proxy object checks for the availability of the accessible objects whenever a client makes calls to its accessible properties for getting some info. - If the object is found to be available, the client’s call is passed on to that accessible object. 

- If the case is opposite i.e., if the object is unavailable, an error is returned by the proxy object. 

- Returning of an error message by the proxy object is an indication that the destruction of the object has taken place and this indication is recommended.

- The error returned is “CO E OBJNOTCONNECTED”.

- The COM or the component object model returns this error after CoDisconnectObject is called by the server. 

- A transparency is maintained between the client and the proxy object. 

- Whenever client calls an accessible object, a pointer to the interface of that object is returned to the client. 

- In turn when this pointer is used by the client to call any of the object’s properties or methods, the execution of the code takes place within proxy object. 

- It is important for the rational functional tester to understand the interface of a given object in order to interact with it. 

- Usually, it is easy to interact with objects such as buttons, list boxes etc, however there are some custom objects in an application for which a proxy needs to be created.