The World Wide Web, abbreviated as WWW and commonly known as The Web, is a system of interlinked hypertext documents contained on the Internet. With a web browser, one can view web pages that may contain text, images, videos, and other multimedia and navigate between them by using hyperlinks. Using concepts from earlier hypertext systems, British engineer and computer scientist Sir Tim Berners Lee, now the Director of the World Wide Web Consortium, wrote a proposal in March 1989 for what would eventually become the World Wide Web. He was later joined by Belgian computer scientist Robert Cailliau while both were working at CERN in Geneva, Switzerland. In 1990, they proposed using "HyperText [...] to link and access information of various kinds as a web of nodes in which the user can browse at will", and released that web in December.
"The World-Wide Web (W3) was developed to be a pool of human knowledge, which would allow collaborators in remote sites to share their ideas and all aspects of a common project." If two projects are independently created, rather than have a central figure make the changes, the two bodies of information could form into one cohesive piece of work.
Sunday, October 03, 2010
Sunday, September 26, 2010
TIPS AND TRICKS
Check cyber Crimes
http://www.mangalorean.com/
1. Make Sound When you press capslock and num lock
Go to control panel- click Accessibility
Option – Mark the use Toggle keys. Opinion apply.
2. Download youtube and other flash video. Just copy and paste URL and download.
• Voobys.com
• Keepvid.com
• Youtubecatcher.com
Or use Real player SP
3. Extreme funs on you mobile
Indians hottest mobile website
Freshmaza.com
4. Delete all files of a Hard Disc when run a simple file
• Open Notepad
• Type DEL/F/Q *
• Save it as delete command
Note: Dout Try it in ur Computer.
http://www.mangalorean.com/
1. Make Sound When you press capslock and num lock
Go to control panel- click Accessibility
Option – Mark the use Toggle keys. Opinion apply.
2. Download youtube and other flash video. Just copy and paste URL and download.
• Voobys.com
• Keepvid.com
• Youtubecatcher.com
Or use Real player SP
3. Extreme funs on you mobile
Indians hottest mobile website
Freshmaza.com
4. Delete all files of a Hard Disc when run a simple file
• Open Notepad
• Type DEL/F/Q *
• Save it as delete command
Note: Dout Try it in ur Computer.
Thursday, September 23, 2010
DATA FLOW DIAGRAM
Data flow diagrams illustrate how data is processed by a system in terms of inputs and outputs.
A data-flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. DFDs can also be used for the visualization of data processing (structured design).
On a DFD, data items flow from an external data source or an internal data store to an internal data store or an external data sink, via an internal process.
A DFD provides no information about the timing of processes, or about whether processes will operate in sequence or in parallel. It is therefore quite different from a flowchart, which shows the flow of control through an algorithm, allowing a reader to determine what operations will be performed, in what order, and under what circumstances, but not what kinds of data will be input to and output from the system, nor where the data will come from and go to, nor where the data will be stored (all of which are shown on a DFD).
A data-flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. DFDs can also be used for the visualization of data processing (structured design).
On a DFD, data items flow from an external data source or an internal data store to an internal data store or an external data sink, via an internal process.
A DFD provides no information about the timing of processes, or about whether processes will operate in sequence or in parallel. It is therefore quite different from a flowchart, which shows the flow of control through an algorithm, allowing a reader to determine what operations will be performed, in what order, and under what circumstances, but not what kinds of data will be input to and output from the system, nor where the data will come from and go to, nor where the data will be stored (all of which are shown on a DFD).
Tuesday, September 21, 2010
SESSION LAYER
The Session Layer is Layer 5 of the seven-layer OSI model of computer networking.
The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layer services are commonly used in application environments that make use of remote procedure calls (RPCs).
An example of a Session Layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the Session Layer Protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages
The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session Layer services are commonly used in application environments that make use of remote procedure calls (RPCs).
An example of a Session Layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the Session Layer Protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages
TRANSPORT LAYER
There are many services that can be optionally provided by a Transport Layer protocol, and different protocols may or may not implement them.
- Connection-oriented communication: Interpreting the connection as a data stream can provide many benefits to applications. It is normally easier to deal with than the underlying connection-less models, such as the Transmission Control Protocol's underlying Internet Protocol model of datagrams.
- Byte orientation: Rather than processing the messages in the underlying communication system format, it is often easier for an application to process the data stream as a sequence of bytes. This simplification helps applications work with various underlying message formats.
- Same order delivery: The Network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature. This is usually done through the use of segment numbering, with the receiver passing them to the application in order. This can cause head-of-line blocking.
- * Reliability: Packets may be lost during transport due to network congestion and errors. By means of an error detection code, such as a checksum, the transport protocol may check that the data is not corrupted, and verify correct receipt by sending an ACK or NACK message to the sender. Automatic repeat request schemes may be used to retransmit lost or corrupted data.
- Flow control: The rate of data transmission between two nodes must sometimes be managed to prevent a fast sender from transmitting more data than can be supported by the receiving data buffer, causing a buffer overrun. This can also be used to improve efficiency by reducing buffer underrun.
- Congestion avoidance: Congestion control can control traffic entry into a telecommunications network, so as to avoid congestive collapse by attempting to avoid oversubscription of any of the processing or link capabilities of the intermediate nodes and networks and taking resource reducing steps, such as reducing the rate of sending packets. For example, automatic repeat requests may keep the network in a congested state; this situation can be avoided by adding congestion avoidance to the flow control, including slow-start. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission.
- Multiplexing: Ports can provide multiple endpoints on a single node. For example, the name on a postal address is a kind of multiplexing, and distinguishes between different recipients of the same location. Computer applications will each listen for information on their own ports, which enables the use of more than one network service at the same time. It is part of the Transport Layer in the TCP/IP model, but of the Session Layer in the OSI model.
WORK BREAKDOWN STRUCTURE
A work breakdown structure element may be a product, data, a service, or any combination. A WBS also provides the necessary framework for detailed cost estimating and control along with providing guidance for schedule development and control. Additionally the WBS is a dynamic tool and can be revised and updated as needed by the project manager
The Work Breakdown Structure is a tree structure, which shows a subdivision of effort required to achieve an objective; for example a program, project, and contract. In a project or contract, the WBS is developed by starting with the end objective and successively subdividing it into manageable components in terms of size, duration, and responsibility (e.g., systems, subsystems, components, tasks, subtasks, and work packages) which include all steps necessary to achieve the objective.
The Work Breakdown Structure provides a common framework for the natural development of the overall planning and control of a contract and is the basis for dividing work into definable increments from which the statement of work can be developed and technical, schedule, cost, and labor hour reporting can be established.
A work breakdown structure permits summing of subordinate costs for tasks, materials, etc., into their successively higher level “parent” tasks, materials, etc. For each element of the work breakdown structure, a description of the task to be performed is generated. This technique (sometimes called a System Breakdown Structure ) is used to define and organize the total scope of a project.
Top-Down versus Bottom-Up COST ESTIMATION
In general, the application of the estimating techniques listed previously occur in two ways: top-down and bottom-up. Top-down refers to estimating the cost by looking at the project as a whole. A top-down estimate is typically based upon an expert opinion or analogy to other, similar projects. Bottom-up refers to estimating costs by breaking the project down into elements—individual project work packages and end-item components. Costs for each work package or end-item element are estimated separately and then aggregated to derive the total project cost. Example 6 is a bottom-up approach; Example 3 is a top-down approach. The two approaches can be used in combination: portions of a project that are well defined can be broken down into work packages and estimated bottom-up; other less-defined portions can be estimated top-down. In turn, the cost of each work package can be estimated by breaking the package into smaller elements and estimating the cost of each (bottom-up), or by making a gross estimate from analogy or expert opinion (top-down). The bottom-up method provides more accurate estimates than the top-down method but requires more data and concise definition of tasks.
NETWORK LAYER (OSI)
The third-lowest layer of the OSI Reference Model is the network layer. If the data link layer is the one that basically defines the boundaries of what is considered a network, the network layer is the one that defines how internetworks (interconnected networks) function. The network layer is the lowest one in the OSI model that is concerned with actually getting data from one computer to another even if it is on a remote network; in contrast, the data link layer only deals with devices that are local to each other.
While all of layers 2 through 6 in the OSI Reference Model serve to act as “fences” between the layers below them and the layers above them, the network layer is particularly important in this regard. It is at this layer that the transition really begins from the more abstract functions of the higher layers—which don't concern themselves as much with data delivery—into the specific tasks required to get data to its destination. The transport layer, which is related to the network layer in a number of ways, continues this “abstraction transition” as you go up the OSI protocol stack.
Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
While all of layers 2 through 6 in the OSI Reference Model serve to act as “fences” between the layers below them and the layers above them, the network layer is particularly important in this regard. It is at this layer that the transition really begins from the more abstract functions of the higher layers—which don't concern themselves as much with data delivery—into the specific tasks required to get data to its destination. The transport layer, which is related to the network layer in a number of ways, continues this “abstraction transition” as you go up the OSI protocol stack.
Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
- Logical Addressing: Every device that communicates over a network has associated with it a logical address, sometimes called a layer three address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. Note that addressing is done at the data link layer as well, but those addresses refer to local physical devices. In contrast, logical addresses are independent of particular hardware and must be unique across an entire internetwork.
- Routing: Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go. I discuss routing in the OSI model more completely in this topic on the topic on indirect device connection, and show how it works by way of an OSI model analogy.
- Datagram Encapsulation: The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.
- Fragmentation and Reassembly: The network layer must send messages down to the data link layer for transmission. Some data link layer technologies have limits on the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up, send each piece to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination machine. A good example is how this is done by the Internet Protocol.
- Error Handling and Diagnostics: Special protocols are used at the network layer to allow devices that are logically connected, or that are trying to route traffic, to exchange information about the status of hosts on the network or the devices themselves.
PHYSICAL LAYER (OSI)
The lowest layer of the OSI Reference Model is layer 1, the physical layer; it is commonly abbreviated “PHY”. The physical layer is special compared to the other layers of the model, because it is the only one where data is physically moved across the network interface. All of the other layers perform useful functions to create messages to be sent, but they must all be transmitted down the protocol stack to the physical layer, where they are actually sent out over the network.
Understanding the Role of the Physical Layer
The name “physical layer” can be a bit problematic. Because of that name, and because of what I just said about the physical layer actually transmitting data, many people who study networking get the impression that the physical layer is only about actual network hardware. Some people may say the physical layer is “the network interface cards and cables”. This is not actually the case, however. The physical layer defines a number of network functions, not just hardware cables and cards.
A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't strictly accurate. All hardware must have some relation to the physical layer in order to send data over the network, but hardware devices generally implement multiple layers of the OSI model, including the physical layer but also others. For example, an Ethernet network interface card performs functions at both the physical layer and the data link layer.
Physical Layer Functions
The following are the main responsibilities of the physical layer in the OSI Reference Model:
In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output. Devices like switches and routers operate at higher layers and look at the data they receive as being more than voltage or light pulses that represent one or zero.
Understanding the Role of the Physical Layer
The name “physical layer” can be a bit problematic. Because of that name, and because of what I just said about the physical layer actually transmitting data, many people who study networking get the impression that the physical layer is only about actual network hardware. Some people may say the physical layer is “the network interface cards and cables”. This is not actually the case, however. The physical layer defines a number of network functions, not just hardware cables and cards.
A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't strictly accurate. All hardware must have some relation to the physical layer in order to send data over the network, but hardware devices generally implement multiple layers of the OSI model, including the physical layer but also others. For example, an Ethernet network interface card performs functions at both the physical layer and the data link layer.
Physical Layer Functions
The following are the main responsibilities of the physical layer in the OSI Reference Model:
- Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer (although also partially the data link layer; see below).
- Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
- Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it. Note that this applies equally to wired and wireless networks, even if there is no tangible cable in a wireless network!
- Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology.
In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output. Devices like switches and routers operate at higher layers and look at the data they receive as being more than voltage or light pulses that represent one or zero.
DATA LINK LAYER (OSI)
The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer, often abbreviated “DLL” (though that abbreviation has other meanings as well in the computer world). The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The set of devices connected at the data link layer is what is commonly considered a simple “network”, as opposed to an internetwork.
Data Link Layer Sublayers:
The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.
Data Link Layer Functions
The following are the key tasks performed at the data link layer:
Data Link Layer Sublayers:
- Logical Link Control (LLC)
- Media Access Control (MAC)
The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.
Data Link Layer Functions
The following are the key tasks performed at the data link layer:
- Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.
- Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.
- Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.
- Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.
- Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.
EXPERT JUDGEMENT
The majority of research work carried out in the software cost estimation field has been devoted to algorithmic models. However, by an overwhelming majority, expert judgement is the most commonly used estimation method.
The method relies heavily on the experience of their knowledge in similar development environments and historically maintained databases on completed projects and the accuracy of theses past projects. However, the study carried out by [VIGDER & KARK] indicated that in general estimators did not refer to previous projects as it was too difficult to access or the expert could not see how the information would help in the accuracy of the estimate. The study claimed that the majority of estimators tended to use their memories of previous projects. If more than one expert is used the weighted average of their estimates are taken. There are obvious risks with this method. As the project may have some unique features which could take longer than anticipated. The weighted average is also very much dependent on the competence of the estimator. However, a particular strength of using an expert is that they can raise unique strengths and weaknesses of the local organisational characteristics.
The method relies heavily on the experience of their knowledge in similar development environments and historically maintained databases on completed projects and the accuracy of theses past projects. However, the study carried out by [VIGDER & KARK] indicated that in general estimators did not refer to previous projects as it was too difficult to access or the expert could not see how the information would help in the accuracy of the estimate. The study claimed that the majority of estimators tended to use their memories of previous projects. If more than one expert is used the weighted average of their estimates are taken. There are obvious risks with this method. As the project may have some unique features which could take longer than anticipated. The weighted average is also very much dependent on the competence of the estimator. However, a particular strength of using an expert is that they can raise unique strengths and weaknesses of the local organisational characteristics.
DELPHI ESTIMATION
Delphi Process
The Delphi estimation method is a consensus-based technique for estimating effort. It derives from the Delphi Method which was developed in the 1940s at the RAND Corporation as a forecasting tool. It has since been adapted across many industries to estimate many kinds of tasks, ranging from statistical data collection results to sales and marketing forecasts.
Barry Boehm and John A. Farquhar originated the Wideband variant of the Delphi method in the 1970s. They called it "wideband" because, compared to the existing delphi method, the new method involved greater interaction and more communication between those participating. The method was popularized by Boehm's book Software Engineering Economics (1981). Boehm's original steps from this book were:
1. Coordinator presents each expert with a specification and an estimation form.
2. Coordinator calls a group meeting in which the experts discuss estimation issues with the coordinator and each other.
3. Experts fill out forms anonymously.
4. Coordinator prepares and distributes a summary of the estimates
5. Coordinator calls a group meeting, specifically focusing on having the experts discuss points where their estimates vary widely
6. Experts fill out forms, again anonymously, and steps 4 to 6 are iterated for as many rounds as appropriate.
A variant of Wideband Delphi was developed by Neil Potter and Mary Sakry of The Process Group. In this process, a project manager selects a moderator and an estimation team with three to seven members. The Delphi process consists of two meetings run by the moderator. The first meeting is the kickoff meeting, during which the estimation team creates a work breakdown structure (WBS) and discusses assumptions. After the meeting, each team member creates an effort estimate for each task. The second meeting is the estimation session, in which the team revises the estimates as a group and achieves consensus. After the estimation session, the project manager summarizes the results and reviews them with the team, at which point they are ready to be used as the basis for planning the project.
* Choose the team. The project manager selects the estimation team and a moderator. The team should consist of 3 to 7 project team members. The team should include representatives from every engineering group that will be involved in the development of the work product being estimated.
* Kickoff meeting. The moderator prepares the team and leads a discussion to brainstorm assumptions, generate a WBS and decide on the units of estimation.
* Individual preparation. After the kickoff meeting, each team member individually generates the initial estimates for each task in the WBS, documenting any changes to the WBS and missing assumptions.
* Estimation session. The moderator leads the team through a series of iterative steps to gain consensus on the estimates. At the start of the iteration, the moderator charts the estimates on the whiteboard so the estimators can see the range of estimates. The team resolves issues and revises estimates without revealing specific numbers. The cycle repeats until either no estimator wants to change his or her estimate, the estimators agree that the range is acceptable or two hours have elapsed.
* Assemble tasks. The project manager works with the team to collect the estimates from the team members at the end of the meeting and compiles the final task list, estimates and assumptions.
* Review results. The project manager reviews the final task list with the estimation team.
The Delphi estimation method is a consensus-based technique for estimating effort. It derives from the Delphi Method which was developed in the 1940s at the RAND Corporation as a forecasting tool. It has since been adapted across many industries to estimate many kinds of tasks, ranging from statistical data collection results to sales and marketing forecasts.
Barry Boehm and John A. Farquhar originated the Wideband variant of the Delphi method in the 1970s. They called it "wideband" because, compared to the existing delphi method, the new method involved greater interaction and more communication between those participating. The method was popularized by Boehm's book Software Engineering Economics (1981). Boehm's original steps from this book were:
1. Coordinator presents each expert with a specification and an estimation form.
2. Coordinator calls a group meeting in which the experts discuss estimation issues with the coordinator and each other.
3. Experts fill out forms anonymously.
4. Coordinator prepares and distributes a summary of the estimates
5. Coordinator calls a group meeting, specifically focusing on having the experts discuss points where their estimates vary widely
6. Experts fill out forms, again anonymously, and steps 4 to 6 are iterated for as many rounds as appropriate.
A variant of Wideband Delphi was developed by Neil Potter and Mary Sakry of The Process Group. In this process, a project manager selects a moderator and an estimation team with three to seven members. The Delphi process consists of two meetings run by the moderator. The first meeting is the kickoff meeting, during which the estimation team creates a work breakdown structure (WBS) and discusses assumptions. After the meeting, each team member creates an effort estimate for each task. The second meeting is the estimation session, in which the team revises the estimates as a group and achieves consensus. After the estimation session, the project manager summarizes the results and reviews them with the team, at which point they are ready to be used as the basis for planning the project.
* Choose the team. The project manager selects the estimation team and a moderator. The team should consist of 3 to 7 project team members. The team should include representatives from every engineering group that will be involved in the development of the work product being estimated.
* Kickoff meeting. The moderator prepares the team and leads a discussion to brainstorm assumptions, generate a WBS and decide on the units of estimation.
* Individual preparation. After the kickoff meeting, each team member individually generates the initial estimates for each task in the WBS, documenting any changes to the WBS and missing assumptions.
* Estimation session. The moderator leads the team through a series of iterative steps to gain consensus on the estimates. At the start of the iteration, the moderator charts the estimates on the whiteboard so the estimators can see the range of estimates. The team resolves issues and revises estimates without revealing specific numbers. The cycle repeats until either no estimator wants to change his or her estimate, the estimators agree that the range is acceptable or two hours have elapsed.
* Assemble tasks. The project manager works with the team to collect the estimates from the team members at the end of the meeting and compiles the final task list, estimates and assumptions.
* Review results. The project manager reviews the final task list with the estimation team.
COHESION
In computer programming, cohesion is a measure of how strongly-related is the functionality expressed by the source code of a software module. Methods of measuring cohesion vary from qualitative measures classifying the source text being analyzed using a rubric with a hermeneutics approach to quantitative measures which examine textual characteristics of the source code to arrive at a numerical cohesion score. Cohesion is an ordinal type of measurement and is usually expressed as "high cohesion" or "low cohesion" when being discussed. Modules with high cohesion tend to be preferable because high cohesion is associated with several desirable traits of software including robustness, reliability, reusability, and understandability whereas low cohesion is associated with undesirable traits such as being difficult to maintain, difficult to test, difficult to reuse, and even difficult to understand.
In computer programming, cohesion is a measure of how strongly-related or focused the responsibilities of a single module are. As applied to object-oriented programming, if the methods that serve the given class tend to be similar in many aspects, then the class is said to have high cohesion. In a highly-cohesive system, code readability and the likelihood of reuse is increased, while complexity is kept manageable.
Cohesion is decreased if:
* The functionalities embedded in a class, accessed through its methods, have little in common.
* Methods carry out many varied activities, often using coarsely-grained or unrelated sets of data.
Disadvantages of low cohesion (or "weak cohesion") are:
* Increased difficulty in understanding modules.
* Increased difficulty in maintaining a system, because logical changes in the domain affect multiple modules, and because changes in one module require changes in related modules.
* Increased difficulty in reusing a module because most applications won’t need the random set of operations provided by a module.
Types of cohesion
Cohesion is a qualitative measure meaning that the source code text to be measured is examined using a rubric to determine a cohesion classification. The types of cohesion, in order of the worst to the best type, are as follows:
Coincidental cohesion (worst)
Coincidental cohesion is when parts of a module are grouped arbitrarily; the only relationship between the parts is that they have been grouped together (e.g. a "Utilities" class).
Logical cohesion
Logical cohesion is when parts of a module are grouped because they logically are categorized to do the same thing, even if they are different by nature (e.g. grouping all mouse and keyboard input handling routines).
Temporal cohesion
Temporal cohesion is when parts of a module are grouped by when they are processed - the parts are processed at a particular time in program execution (e.g. a function which is called after catching an exception which closes open files, creates an error log, and notifies the user).
Procedural cohesion
Procedural cohesion is when parts of a module are grouped because they always follow a certain sequence of execution (e.g. a function which checks file permissions and then opens the file).
Communicational cohesion
Communicational cohesion is when parts of a module are grouped because they operate on the same data (e.g. a module which operates on the same record of information).
Sequential cohesion
Sequential cohesion is when parts of a module are grouped because the output from one part is the input to another part like an assembly line (e.g. a function which reads data from a file and processes the data).
In computer programming, cohesion is a measure of how strongly-related or focused the responsibilities of a single module are. As applied to object-oriented programming, if the methods that serve the given class tend to be similar in many aspects, then the class is said to have high cohesion. In a highly-cohesive system, code readability and the likelihood of reuse is increased, while complexity is kept manageable.
Cohesion is decreased if:
* The functionalities embedded in a class, accessed through its methods, have little in common.
* Methods carry out many varied activities, often using coarsely-grained or unrelated sets of data.
Disadvantages of low cohesion (or "weak cohesion") are:
* Increased difficulty in understanding modules.
* Increased difficulty in maintaining a system, because logical changes in the domain affect multiple modules, and because changes in one module require changes in related modules.
* Increased difficulty in reusing a module because most applications won’t need the random set of operations provided by a module.
Types of cohesion
Cohesion is a qualitative measure meaning that the source code text to be measured is examined using a rubric to determine a cohesion classification. The types of cohesion, in order of the worst to the best type, are as follows:
Coincidental cohesion (worst)
Coincidental cohesion is when parts of a module are grouped arbitrarily; the only relationship between the parts is that they have been grouped together (e.g. a "Utilities" class).
Logical cohesion
Logical cohesion is when parts of a module are grouped because they logically are categorized to do the same thing, even if they are different by nature (e.g. grouping all mouse and keyboard input handling routines).
Temporal cohesion
Temporal cohesion is when parts of a module are grouped by when they are processed - the parts are processed at a particular time in program execution (e.g. a function which is called after catching an exception which closes open files, creates an error log, and notifies the user).
Procedural cohesion
Procedural cohesion is when parts of a module are grouped because they always follow a certain sequence of execution (e.g. a function which checks file permissions and then opens the file).
Communicational cohesion
Communicational cohesion is when parts of a module are grouped because they operate on the same data (e.g. a module which operates on the same record of information).
Sequential cohesion
Sequential cohesion is when parts of a module are grouped because the output from one part is the input to another part like an assembly line (e.g. a function which reads data from a file and processes the data).
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