DIFFERENCE BETWEEN CSMA/CD AND CSMA/CA:

DIFFERENCE BETWEEN CSMA/CD AND CSMA/CA:

By Mehak Cloth's   May 30, 2016   No comments:

CSMA/CD:
Carrier sense multiple access with collision detection (CSMA/CD) is one of the most popular access methods, with CSMA/CD, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. If a host wishes to place data on the wire, it will “sense” the wire and determine whether there is a signal already on the wire. If there is, the host will wait to transmit the data; if the wire is free, the host will send the data, as shown in Figure 1.

Figure 1: A host "sensing" the wire to see if it is free of traffic.

The problem with the process just described is that, if there are two systems on the wire that “sense” the wire at the same time to see if the wire is free, they will both send data out at the same time if the wire is free. When the two pieces of data are sent out on the wire at the same time, they will collide with one another, and the data will be destroyed. If the data is destroyed in transit, the data will need to be retransmitted. Consequently, after a collision, each host will wait a variable length of time before retransmitting the data (they don’t want the data to collide again), thereby preventing a collision the second time. When a system determines that the data has collided and then retransmits the data, that is known as collision detection.

 To summarize, CSMA/CD provides that before a host sends data on the network, it will “sense” (CS) the wire to ensure that the wire is free of traffic. Multiple systems have equal access to the wire (MA), and if there is a collision, a host will detect that collision (CD) and retransmit the data.

CSMA/CA:

Carrier sense multiple access with collision avoidance (CSMA/CA) is not as popular as CSMA/CD and for good reason. With CSMA/CA, before a host sends data on the wire, it will “sense” the wire as well to see if the wire is free of signals. If the wire is free, it will try to “avoid” a collision by sending a piece of “dummy” data on the wire first to see whether it collides with any other data. If it does not collide, the host in effect assumes “If my dummy data did not collide, then the real data will not collide,” and it submits the real data on the wire.

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NETWORK MEDIA AND CONNECTORS

NETWORK MEDIA AND CONNECTORS

By Mehak Cloth's   May 30, 2016   No comments:

Networks are built using a topology of bus, star, or ring, but how the systems will be connected in the topology that you choose. Cabling is the medium for the transmission of data between hosts on the LANs. LANs can be connected together using a variety of cable types, such as unshielded twisted-pair, coax, or fiber. Each cable type has its own advantages and disadvantages.

There are three primary types of cable media that can be used to connect systems to a network:

1. Coaxial cable
2. Twisted-pair cable
3. Fiber-optic cable

Transmission rates that can be supported on each of these physical media are measured in millions of bits per second, or megabits per second (Mbps).

1. COAXIAL CABLE:

Coaxial, or coax, cable looks like the cable used to bring the cable TV signal to your television. One strand (a solid-core copper wire) runs down the middle of the cable. Around that strand is a layer of insulation, and covering that insulation is braided wire and metal foil, which shields against electromagnetic interference. A final layer of insulation covers the braided wire. Because of the layers of insulation, coaxial cable is more resistant to outside interference than other cabling, such as unshielded twisted-pair (UTP) cable. Figure 1 shows a coaxial cable with the copper core and the layers of insulation.

Figure 1: Coaxial Cable

There are two types of coax cabling: thinnet and thicknet. The two differ in thickness and maximum cable distance that the signal can travel.

THINNET:

This refers to RG-58 cabling, which is a flexible coaxial cable about ¼-inch thick. Thinnet is used for short-distance communication and is flexible enough to facilitate routing between workstations. Thinnet connects directly to a workstation’s network adapter card using a British naval connector (BNC) and uses the network adapter card’s internal transceiver. The maximum length of thinnet is 185 meters. Figure 1.1 displays thinnet coaxial cabling and the BNC connector on the end.

Figure 1.1: Thinnet Coaxial Cable with a BNC Connector

THICKNET:
This coaxial cable, also known as RG-8, gets its name by being a thicker cable than thinnet. Thicknet cable is about ½-inch thick and can support data transfer over longer distances than thinnet. Thicknet has a maximum cable length of 500 meters and usually is used as a backbone to connect several smaller thinnet-based networks. Due to the thickness of ½ inch, this cable is harder to work with than thinnet cable. A transceiver often is connected directly to the thicknet cable using a connector known as a vampire tap. Connection from the transceiver to the network adapter card is made using a drop cable to connect to the adapter unit interface (AUI) port connector.

2. TWISTED-PAIR CABLE:
Twisted-pair cabling gets its name by having four pairs of wires that are twisted to help reduce crosstalk or interference from outside electrical devices. (Crosstalk is interference from adjacent wires.) Figure 2 shows a twisted-pair cable. 

There are two forms of twisted-pair cabling unshielded twisted-pair (UTP) and shielded twisted-pair (STP).

Figure 2: Twisted-Pair Cable

I) UNSHIELDED TWISTED-PAIR (UTP) CABLE:

Unshielded twisted-pair (UTP) cables are familiar to you if you have worked with telephone cable. The typical twisted-pair cable for network use contains four pairs of wires. Each member of the pair of wires contained in the cable is twisted around the other. The twists in the wires help shield against electromagnetic interference. The maximum distance of UTP is 100 meters.

UTP cable uses small plastic connectors designated as registered jack 45, or most often referred to as RJ-45. RJ-45 is similar to the phone connectors, except that instead of four wires, as found in the home system, the network RJ-45 connect to contains eight contacts, one for each wire in a UTP cable. The bottom cable in Figure 2.1 is an RJ-45 connector.

It can be easy to confuse the RJ-45 connector with the RJ-11 connector. The RJ-11 connector is a telephone connector and is shown in Figure 2.1 (the cable on the top). In an RJ-11 connector, there are four contacts; hence there are four wires found in the telephone cable. With RJ-45 and RJ-11, you will need a special crimping tool when creating the cables to make contact between the pins in the connector and the wires inside the cable.

UTP cabling has different flavors, known as grades or categories. Each category of UTP cabling was designed for a specific type of communication or transfer rate.

Figure 2.1: RJ-11 Connector and an RJ-45 Connector

i) STRAIGHT-THROUGH CABLES:
CAT 5 UTP cabling usually uses only four wires when sending and receiving information on the network. The four wires of the eight that are used are wires 1, 2, 3, and 6. Figure shows the meaning of the pins on a computer and the pins on a hub (or switch), which is what you typically will be connecting the computers to. When you configure the wire for the same pin at either end of the cable, this is known as a straight-through cable.

Figure : Pinout Diagram for a Straight-Through Cable

You will see in the figure that wires 1 and 2 are used to transmit data (TX) from the computer, while wires 3 and 6 are used to receive information (RX) on the computer. You will also notice that the transmit pin on the computer is connected to the receive pin (RX) on the hub via wires 1 and 2. This is important because we want to make sure that data that is sent from the computer is received at the network hub. We also want to make sure that data sent from the hub is received at the computer, so you will notice that the transmit pins (TX) on the hub are connected to the receive pins (RX) on the computer through wires 3 and 6. This will allow the computer to receive information from the hub. The last thing to note about Figure 1-18 is that pin 1 on the computer is connected to pin 1 on the hub by the same wire, thus the term straight-through. You will notice that all pins are matched straight through to the other side in Figure.

Table: Different UTP Category Cabling

ii) CROSSOVER CABLES:

At some point, you may need to connect two computer systems directly together without the use of a hub, from network card to network card. To do this, you would not be able to use a straight-through cable because the transmit pin on one computer would be connected to the transmit pin on another computer, as shown in Figure a. How could a computer pick up the data if it was not sent to the receive pins? This will not work, so we will need to change the wiring of the cable to what is known as a crossover cable. In order to connect two systems directly together without the use of a hub, you will need to create a crossover cable by switching wires 1 and 2 with wires 3 and 6 at one end of the cable, as shown in Figure b. You will notice that the transmit pins on Computer A are connected to the receive pins on Computer B, thus allowing Computer A to send data to Computer B. The same applies for Computer B to send to Computer A—pins A and B on Computer B are wired to pins 3 and 6 on Computer A so that Computer A can receive data from Computer B.

Figure. 1

Figure. 2

iii) ROLLOVER:

A rollover cable is a popular cable type in the networking world and is used to connect to a Cisco device such as a router or a switch. Also known as a console cable, this cable connects from the computer’s serial port to the console port of the router or switch. Once the network administrator connects to the console port, he or she is then able to configure the router or switch.

II) SHIELDED TWISTED-PAIR (STP) CABLE:

Shielded twisted-pair (STP) cable is very similar to UTP cabling, but it differs from UTP in that it uses a layer of insulation within the protective jacket, which helps maintain the quality of the signal. Figure 1-22 shows the size of STP cabling as compared to UTP.

3. FIBER-OPTIC CABLE:

The third type of cabling is fiber-optic cabling. Fiber-optic cabling is unlike coax and twisted-pair, because both of those types have a copper wire that carries the electrical signal. Fiber-optic cables use optical fibers that carry digital data signals in the form of modulated pulses of light. An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, known as the cladding. There are two fibers per cable one to transmit and one to receive. The core also can be an optical-quality clear plastic, and the cladding can be made up of gel that reflects signals back into the fiber to reduce signal loss. Figure shows fibers in a fiber-optic cable.

There are two types of fiber-optic cables: single-mode fiber (SMF) and multimode fiber (MMF).

SINGLE-MODE FIBER: Uses a single ray of light, known as a mode, to carry the transmission over long distances.

MULTIMODE FIBER: Uses multiple rays of light (modes) simultaneously, with each ray of light running at a different reflection angle to carry the transmission over short distances.

Fiber-optic cable supports up to 1000 stations and can carry the signal up to and beyond 2 kilometers. Fiber-optic cables are also highly secure from outside interference, such as radio transmitters, arc welders, fluorescent lights, and other sources of electrical noise. On the other hand, fiber-optic cable is by far the most expensive of these cabling methods, and a small network is unlikely to need these features. Depending on local labor rates and building codes, installing fiber-optic cable can cost as much as $500 per network node.

Figure: Fiber-Optic Cable

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TYPES OF NETWORKS

TYPES OF NETWORKS

By Mehak Cloth's   May 29, 2016   No comments:

Organizations of different sizes, structures, and budgets need different types of networks. A local newspaper company has needs for its network that would be different from the needs of a multinational company. Networks can be divided into one of two categories: peer-to-peer or server-based networks.

PEER-TO-PEER NETWORK:

A peer-to-peer network has no dedicated servers; instead, a number of workstations are connected together for the purpose of sharing information or devices. When there is no dedicated server, all workstations are considered equal; any one of them can participate as the client or the server. Peer-to-peer networks are designed to satisfy the networking needs of home networks or of small companies that do not want to spend a lot of money on a dedicated server but still want to have the capability to share information or devices. For example, a small accounting firm with three employees that needs to access customer data from any of the three systems or print to one printer from any of the three systems may not want to spend a lot of money on a dedicated server. A small peer-to-peer network will allow these three computers to share the printer and the customer information with one another. The extra cost of a server was not incurred because the existing client systems were networked together to create the peer-to-peer network. The Microsoft term for a peer-to-peer network is a workgroup. Be aware that peer-to-peer networks typically consist of fewer than 10 systems. Most of the modern operating systems such as Windows XP and Windows Vista already have built-in peer-to-peer networking capabilities, which is why building a peer-to-peer network would be a “cheap” network solution. The disadvantage of a peer-to-peer network is the lack of centralized administration—with peer-to-peer networks, you need to build user accounts and configure security on each system.

Figure: Peer-To-Peer Network

It is important to note that peer-to-peer networks are designed for fewer than 10 systems, and with Microsoft client operating systems such as Windows XP Professional, only 10 concurrent network connections to those clients are allowed. This means that if you have 15 or 20 employees, you eventually will need to implement a server-based network.

SERVER-BASED NETWORKS:
A big disadvantage of peer-to-peer networking is that you can’t do your day-today administration in a single place. With peer-to-peer networking, user accounts typically are created on all the systems, and data files are stored throughout all the systems. This leads to a more complicated environment and makes your job harder as a network administrator. Usually after four or five systems have been networked, the need for a dedicated server to store all of the user accounts and data files becomes apparent this is a server-based network.

The advantage of a server-based network is that the data files that will be used by all of the users are stored on the one server. This will help you by giving you a central point to set up permissions on the data files, and it will give you a central point from which to back up all of the data in case data loss should occur. With a server-based network, the network server stores a list of users who may use network resources and usually holds the resources as well.

Figure: Server-Based-Network

The server in a server-based network may provide a number of different services. The services it will offer to the network usually are decided by the server’s role.

There are a number of different roles that a server could play on a network:

• File and print servers 
• Application servers 
• Web servers 
• Directory servers 

File and print servers control and share printers and files among clients on the network. File and print servers were the original reason to have a network; a large number of users needed access to the same files, so the files were placed on a server, and all clients were connected to the server when they needed to work with the files.

File servers often have the following characteristics:

• Large amounts of memory 
• Fast hard disks 
• Multiple CPUs 
• Fast I/O buses 
• High-capacity tape drives 
• Fast network adapters 
• Redundant power supplies 
• Hot-swappable hard disks and power supplies 

File and print servers also check the access control list (ACL) of each resource before allowing a user to access a file or use a printer. If the user or a group to which the user belongs is not listed in the ACL, the user is not allowed to use the resource, and an “access denied” message appears on the user’s screen. Application servers are servers that run some form of special program on the server. A good example of an application server is a server that runs the company’s e-mail server. The e-mail server software is special software that can be run on a server operating system. Another example of software that would run on an application server is a database server product such as Microsoft SQL Server. A database server is a server that holds the company’s core business data and typically gives this data to custom applications that run on the workstations. These are some applications that you might find on an application server:

• Microsoft SQL Server 
• Oracle 
• Microsoft Exchange Server 
• IBM Lotus Domino 

Web servers are servers that run the Hypertext Transfer Protocol (HTTP) and are designed to publish information on the Internet or the corporate intranet. Web servers are popular in today’s businesses because they host web applications (web sites) for the organization. These web applications could be designed for internal use, or they could be used to publish information to the rest of the world on the Internet. Examples of web server software are Microsoft’s Internet Information Services that runs on Windows or Apache web server software that runs on UNIX/Linux, Novell NetWare, and Windows. Directory servers hold a list of the user accounts that are allowed to log on to the network. This list of user accounts is stored in a database (known as the directory database) and can store information about these user accounts such as address, city, phone number, and fax number. A directory service is designed to be a central database that can be used to store everything about such objects as users and printers. In a server-based network environment, the centralized administration comes from the fact that the directory server stores all user accounts in its directory database. When a user sits at a client machine to log on to the network, the logon request is sent to this directory server. If the username and password exist in the directory database, the client is allowed to access network resources. It is important to note that a server can have numerous roles at the same time. A server can be a file and print server, as well as an application server, or it can be a file, print, and directory server all at the same time. Because a single server can perform multiple roles, a company will not need to purchase an additional server every time a new product (or feature) is implemented on the network, and this fact reduces the cost of a server-based network.

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WHAT IS BROADBAND AND BASEBAND

WHAT IS BROADBAND AND BASEBAND

By Engr. Aneel Kumar   May 27, 2016   No comments:

There are two different techniques that may be used to transmit the signal along the network wire which are base-band communication and broadband communication.

BASEBAND
Sends digital signals through the media as a single channel that uses the entire bandwidth of the media. The signal is delivered as a pulse of electricity or light, depending on the type of cabling being used. Baseband communication is also bidirectional, which means that the same channel can be used to send and receive signals. BROADBAND
Sends information in the form of an analog signal, which flows as electromagnetic waves or optical waves. Each transmission is assigned to a portion of the bandwidth, so unlike with baseband communication, it is possible to have multiple transmissions at the same time, with each transmission being assigned its own channel or frequency. Broadband communication is unidirectional, so in order to send and receive, two pathways will need to be used. This can be accomplished either by assigning a frequency for sending and assigning a frequency for receiving along the same cable or by using two cables, one for sending and one for receiving.

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HYBRID TOPOLOGY AND ITS ADVANTAGES AND DISADVANTAGES

HYBRID TOPOLOGY AND ITS ADVANTAGES AND DISADVANTAGES

By Mehak Cloth's   May 27, 2016   No comments:

It is typical for networks to implement a mixture of topologies to form a hybrid topology. For example, a very popular hybrid topology is a star-bus topology, in which a number of star topologies are connected by a central bus. This is a popular topology because the bus will connect hubs that are spread over distance. Another very popular hybrid topology is the star-ring topology. The star-ring topology is popular because it looks like a star but acts as a ring. For example, there is a network architecture known as Token Ring (more on this later, in the section “Network Architectures”) that uses a central “hub” type device, but the internal wiring makes a ring. Physically it looks like a star, but logically it acts as a ring topology.

Figure: Hybrid Topology.

Advantages of Hybrid Network Topology
1) Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network.
2) Scalable: Its easy to increase the size of network by adding new components, without disturbing existing architecture.
3) Flexible: Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high.
4) Effective: Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology.
Disadvantages of Hybrid Topology
1) Complexity of Design: One of the biggest drawback of hybrid topology is its design. Its not easy to design this type of architecture and its a tough job for designers. Configuration and installation process needs to be very efficient.
2) Costly Hub: The hubs used to connect two distinct networks, are very expensive. These hubs are different from usual hubs as they need to be intelligent enough to work with different architectures and should be function even if a part of network is down.
3) Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a lot of cables, cooling systems, sophisticate network devices, etc.

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OPERATING PRINCIPLE OF THE DSTATCOM

OPERATING PRINCIPLE OF THE DSTATCOM

By Engr. Aneel Kumar   May 27, 2016   No comments:

Basically, the DSTATCOM system is comprised of three main parts: a Voltage Source Converter (VSC), a set of coupling reactors and a controller. The basic principle of a DSTATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source inverter (VSI) connected to a dc capacitor (energy storage device). The ac voltage source, in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the DSTATCOM is caused by the voltage difference across this reactance. The DSTATCOM is connected to the power networks where the voltage-quality problem is a concern. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches (IGBT’s, which are used at the distribution level) of the power converter accordingly.

Figure: Block Diagram of the voltage source converter based DSTATCOM

The AC voltage control is achieved by firing angle control. Ideally the output voltage of the VSI is in phase with the bus (where the DSTATCOM is connected.) voltage. In steady state, the dc side capacitance is maintained at a fixed voltage and there is no real power exchange, except for losses. The DSTATCOM differs from other reactive power generating devices (such as shunt Capacitors, Static VAR Compensators etc.) in the sense that the ability for energy storage is not a rigid necessity but is only required for System unbalance or harmonic absorption.

There are two control objectives implemented in the DSTATCOM. One is the ac voltage regulation of the power system at the bus where the DSTATCOM is connected. And the other is dc voltage control across the capacitor inside the DSTATCOM. It is widely known that shunt reactive power injection can be used to control the bus voltage. In conventional control scheme, there are two voltage regulators designed for these purposes. AC voltage regulator for bus voltage control and dc voltage regulator for capacitor voltage control. In the simplest strategy, both the regulators are proportional integral (PI) type controllers.

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IDENTIFYING NETWORK TOPOLOGIES

IDENTIFYING NETWORK TOPOLOGIES

By Engr. Aneel Kumar   May 27, 2016   No comments:

A network topology is the physical layout of computers, cables, and other components on a network. There are a number of different network topologies, and a network may be built using multiple topologies. The different types of network layouts are

1. Bus topology
2. Star topology
3. Mesh topology
4. Ring topology
5. Hybrid topology
6. Wireless topology

1) BUS TOPOLOGY

A bus topology uses one cable as a main trunk to connect all of the systems together. A bus topology is very easy to set up and requires no additional hardware such as a hub. The cable is also called a trunk, a backbone, or a segment.

With a bus topology, when a computer sends out a signal, the signal travels the cable length in both directions from the sending computer. When the signal reaches the end of the cable length, it bounces back and returns in the direction it came from. This is known as signal bounce. Signal bounce is a problem, because if another signal is sent on the cable length at the same time, the two signals will collide and be destroyed and then must be retransmitted. For this reason, at each end of the cable there is a terminator. The terminator is designed to absorb the signal when the signal reaches the end, preventing signal bounce. If there is no termination, the entire network fails because of signal bounce, which also means that if there is ever a break in the cable, you will have unterminated ends and the entire network will go down.

A bus is a passive topology, which means that the workstations on the bus are not responsible for regenerating the signal as it passes by them.

Since the workstations do not play an active role, the workstations are not a requirement of a functioning bus, which means that if a workstation fails, the bus does not fail. But if there is an unterminated end in the bus, the entire network will fail.

2) STAR TOPOLOGY

In a star topology, all computers are connected through one central device known as a hub or a switch. Each workstation has a cable that goes from the network card to the hub device. One of the major benefits of a star topology is that a break in the cable causes only the workstation that is connected to the cable to go down, not the entire network, as with a bus topology. Star topologies are very popular topologies in today’s networking environments.

3) MESH TOPOLOGY

A mesh topology is not very common in computer networking today. In a mesh topology, every workstation has a connection to every other component of the network.

4) RING TOPOLOGY
In a ring topology, all computers are connected via a cable that loops in a ring or circle. A ring topology is a circle that has no start and no end. Because there are no ends, terminators are not necessary in a ring topology.

Signals travel in one direction on a ring while they are passed from one computer to the next, with each computer regenerating the signal so that it may travel the distance required.

5) HYBRID TOPOLOGY

It is typical for networks to implement a mixture of topologies to form a hybrid topology. For example, a very popular hybrid topology is a star-bus topology, in which a number of star topologies are connected by a central bus. This is a popular topology because the bus will connect hubs that are spread over distance.

Another very popular hybrid topology is the star-ring topology. The star-ring topology is popular because it looks like a star but acts as a ring. For example, there is a network architecture known as Token Ring that uses a central “hub” type device, but the internal wiring makes a ring. Physically it looks like a star, but logically it acts as a ring topology.

6) WIRELESS TOPOLOGY

A wireless topology is one in which few cables are used to connect systems. The network is made up of transmitters that broadcast the packets using radio frequencies. The network contains special transmitters called cells, or wireless access points, which extend a radio sphere in the shape of a bubble around the transmitter.

This bubble can extend to multiple rooms and possibly floors in a building. The PCs and network devices have a special transmitter-receiver, which allows them to receive broadcasts and transmit requested data back to the access point. The access point is connected to the physical network by a cable, which allows it, and any wireless clients, to communicate with systems on the wired network.

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RING TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

RING TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

By Mehak Cloth's   May 27, 2016   No comments:

In a ring topology, all computers are connected via a cable that loops in a ring or circle. As shown in Figure 1, a ring topology is a circle that has no start and no end. Because there are no ends, terminators are not necessary in a ring topology. Signals travel in one direction on a ring while they are passed from one computer to the next, with each computer regenerating the signal so that it may travel the distance required.

Figure 1: Ring Topology

ADVANTAGES OF RING TOPOLOGY:

A major advantage of a ring topology is that signal degeneration is low because each workstation is responsible for regenerating or boosting the signal. With the other topologies, as the signal travels the wire, it gets weaker and weaker as a result of outside interference: eventually, it becomes unreadable if the destination system is too far away. Because each workstation in a ring topology regenerates the signal, the signal is stronger when it reaches its destination and seldom needs to be retransmitted.

DISADVANTAGES OF RING TOPOLOGY:

The biggest problem with ring topologies is that if one computer fails or the cable link is broken, the entire network could go down. With newer technology, however, this isn’t always the case. The concept of a ring topology today is that the ring will not be broken when a system is disconnected; only that system is dropped from the ring. Isolating a problem can be difficult in some ring configurations. (With newer technologies, a workstation or server will put out a beacon if it notices a break in the ring.) Another disadvantage is that if you make a cabling change to the network or move a workstation, the brief disconnection can interrupt or bring down the entire network

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MESH TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

MESH TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

By Mehak Cloth's   May 26, 2016   No comments:

A mesh topology is not very common in computer networking today. In a mesh topology, every workstation has a connection to every other component of the network, as illustrated in Figure.

Figure: Mesh Topology

ADVANTAGES OF MESH TOPOLOGY:

The biggest advantage of a mesh topology is fault tolerance, meaning that, if there is a break in a cable segment, traffic can be rerouted through a different pathway because there are multiple pathways to send data from one system to another. This fault tolerance means that it is almost impossible for the network to go down due to a cable fault.

DISADVANTAGES OF MESH TOPOLOGY:

A disadvantage of a mesh topology is the cost of the additional cabling and network interfaces to create the multiple pathways between each system. A mesh topology is very hard to administer and manage because of the numerous connections.

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STAR TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

STAR TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

By Mehak Cloth's   May 26, 2016   No comments:

In a star topology, all computers are connected through one central device known as a hub or a switch, as illustrated in Figure 1. Each workstation has a cable that goes from the network card to the hub device. One of the major benefits of a star topology is that a break in the cable causes only the workstation that is connected to the cable to go down, not the entire network, as with a bus topology. Star topologies are very popular topologies in today’s networking environments.

Figure 1: Star Topology

ADVANTAGES OF A STAR TOPOLOGY:

One advantage of a star topology is scalability and ease of adding another system to the network. If you need to add another workstation to the network with a star topology, you simply connect that system to an unused port on the hub. Another benefit is the fact that if there is a break in the cable it affects only the system that is connected to that cable. Figure 1-7 shows a hub with a few ports available. Centralizing network components can make an administrator’s life much easier in the long run. Centralized management and monitoring of network traffic can be vital to network success. With a star configuration, it is also easy to add or change configurations because all of the connections come to a central point.

Figure 2: Hub

DISADVANTAGES OF A STAR TOPOLOGY:

On the flip side, if the hub fails in a star topology, the entire network comes down, so we still have a central point of failure. But this is a much easier problem to troubleshoot than trying to find a cable break with a bus topology. Another disadvantage of a star topology is cost. To connect each workstation to the network, you will need to ensure that there is a hub with an available port, and you will need to ensure you have a cable to go from the workstation to the hub. Today, the cost is increasingly less of a disadvantage because of the low prices of devices such as hubs and switches.

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BUS TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

BUS TOPOLOGIES AND ITS ADVANTAGES AND DISADVANTAGES

By Mehak Cloth's   May 26, 2016   No comments:

A bus topology uses one cable as a main trunk to connect all of the systems together (shown in Figure.1). A bus topology is very easy to set up and requires no additional hardware such as a hub. The cable is also called a trunk, a backbone, or a segment. With a bus topology, when a computer sends out a signal, the signal travels the cable length in both directions from the sending computer. When the signal reaches the end of the cable length, it bounces back and returns in the direction it came from. This is known as signal bounce. Signal bounce is a problem, because if another signal is sent on the cable length at the same time, the two signals will collide and be destroyed and then must be re transmitted. For this reason, at each end of the cable there is a terminator. The terminator is designed to absorb the signal when the signal reaches the end, preventing signal bounce. If there is no termination, the entire network fails because of signal bounce, which also means that if there is ever a break in the cable, you will have unterminated ends and the entire network will go down, as shown in Figure 2. A bus is a passive topology, which means that the workstations on the bus are not responsible for regenerating the signal as it passes by them. Since the workstations do not play an active role, the workstations are not a requirement of a functioning bus, which means that if a workstation fails, the bus does not fail. But if there is an unterminated end in the bus, the entire network will fail.

Figure 1: Bus Topology

Figure 2: A break in the cable with the bus topology

ADVANTAGES OF BUS TOPOLOGY:

One advantage of a bus topology is cost. A bus topology uses less cable than a star topology or a mesh topology, and you do not need to purchase any additional devices such as hubs. Another advantage of a bus topology is the ease of installation. With a bus topology, you simply connect the workstation to the cable segment or backbone. You need only the amount of cable to connect the workstation to the backbone. The most economical choice for a network topology is a bus topology, because it is easy to work with and a minimal amount of additional devices are required. Most importantly, if a computer fails, the network stays functional.

DISADVANTAGES OF BUS TOPOLOGY:

The main disadvantage of a bus topology is the difficulty of troubleshooting it. When the network goes down, it is usually due to a break in the cable segment. With a large network, this problem can be tough to isolate. Scalability is an important consideration in the dynamic world of networking. Being able to make changes easily within the size and layout of your network can be important in future productivity or downtime. The bus topology is not very scalable.

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WHAT IS INTERNET AND INTRANET AND EXTRANET

WHAT IS INTERNET AND INTRANET AND EXTRANET

By Mehak Cloth's   May 26, 2016   No comments:

Internet, intranet, and extranet are three terms that describe “Internet-type” applications that are used by an organization.

INTERNET:

If you wish to expose information to everyone in the world, then you would build an Internet type application. An Internet-type application uses Internet protocols such as HTTP, FTP, or SMTP and is available to persons anywhere on the Internet. We use the Internet and web applications as ways to extend who the application can reach. For example, I no longer need to go to the bank to transfer funds. Because the bank has built a web site on the Internet, I can do that from the comfort of my own home.

Figure 1: Internet

INTRANET:

An application is considered to be on the company’s intranet if it is using Internet-type protocols such as HTTP or FTP but the application is available only within the company. The information on a company’s intranet would not be accessible to persons on the Internet because it is not for public use. For example, a few years ago I was sitting with my banking officer going over my account and noticed that the bank had moved all of its customer account information to a web site and that the banking officer was using a web browser to retrieve my account details. Although the application was being used by a web browser, it was still an “internal” application meant only for banking officers.

Figure 2: Intranet

EXTRANET:

From time to time, an application that has been built for the company’s intranet and used by internal employees will need to be extended to select business partners or customers. If you extend your intranet out to select business partners or customers, you have created an extranet. An extranet cannot be used by anyone else external to the company except for those selected individuals. Figure displays the basic configurations of Internet, intranet, and extranet.

Extranet

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TYPES OF DATA FLOW IN COMMUNICATION SYSTEM

TYPES OF DATA FLOW IN COMMUNICATION SYSTEM

By Mehak Cloth's   May 26, 2016   No comments:

Communication between two devices can be simplex, half-duplex, or full-duplex.

SIMPLEX:

In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.

HALF-DUPLEX:

In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.

Figure. Data flow (simplex, half-duplex, and full-duplex)

FULL-DUPLEX:

In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously. The full-duplex mode is like a two-way street with traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction. This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. One common example of full-duplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time. The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.

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DATA COMMUNICATIONS AND ITS COMPONENTS

DATA COMMUNICATIONS AND ITS COMPONENTS

By Mehak Cloth's   May 26, 2016   No comments:

When we communicate, we are sharing information. This sharing can be local or remote. Between individuals, local communication usually occurs face to face, while remote communication takes place over distance.

Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter.

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.

2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.

3. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission.

4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms delay, an uneven quality in the video is the result.

COMPONENTS OF DATA COMMUNITATION:

A data communications system has five components

1. Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

2. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

3. Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

4. Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5. Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices.Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese.

Figure: Five components of data communication.

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WHAT IS SERVERS AND WORKSTATIONS AND HOSTS

WHAT IS SERVERS AND WORKSTATIONS AND HOSTS

By Engr. Aneel Kumar   May 26, 2016   No comments:

A typical network involves having users sit at workstations, running such applications as word processors or spreadsheet programs. The workstation also is known as a client, which is just a basic computer running a client operating system such as Windows XP or Linux. These users typically store their files on a central server so that they can share the files with other users on the network. The server is a special computer that contains more disk space and memory than are found on client workstations. The server has special software installed that allows it to function as a server. This special software can provide file and print services (to allow sharing of files and printers), provide web pages to clients, or provide e-mail functionality to the company.

The term host refers to any computer or device that is connected to a network and sends or receives information on that network. A host can be a server, a workstation, a printer with its own network card, or a device such as a router. We can summarize by saying that any system or device that is connected to the network is known as a host.

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