Overlay network

Overlay network

An overlay network is a virtual computer network that is built on top of another network. Nodes in the overlay are connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one.

A sample overlay network: IP over SONET over Optical

For example, many peer-to-peer networks are overlay networks because they are organized as nodes of a virtual system of links run on top of the Internet. The Internet was initially built as an overlay on the telephone network.
The most striking example of an overlay network, however, is the Internet itself: At the network layer, each node can reach any other by a direct connection to the desired IP address, thereby creating a fully connected network; the underlying network, however, is composed of a mesh-like interconnect of subnetworks of varying topologies (and, in fact, technologies). Address resolution and routing are the means which allows the mapping of the fully connected IP overlay network to the underlying ones.
Overlay networks have been around since the invention of networking when computer systems were connected over telephone lines using modems, before any data network existed.
Another example of an overlay network is a distributed hash table, which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a map) indexed by keys.
Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees to achieve higher-quality streaming media. Previous proposals such as IntServ, DiffServ, and IP Multicast have not seen wide acceptance largely because they require modification of all routers in the network. On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers. The overlay has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes a message traverses before reaching its destination.
For example, Akamai Technologies manages an overlay network that provides reliable, efficient content delivery (a kind of multicast). Academic research includes end system multicast and overcast for multicast; RON (resilient overlay network) for resilient routing; and OverQoS for quality of service guarantees, among others.

Intranets and extranets

Intranets and extranets

Intranets and extranets are parts or extensions of a computer network, usually a LAN.
An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that are under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.

Extranet

An extranet is a network that is limited in scope to a single organization or entity and also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities—a company's customers may be given access to some part of its intranet—while at the same time the customers may not be considered trusted from a security standpoint. Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although an extranet cannot consist of a single LAN; it must have at least one connection with an external network.

VPN (Virtual private network)

Virtual private network

A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of the virtual network are said to be tunneled through the larger network when this is the case. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

VPN may have best-effort performance, or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a topology more complex than point-to-point.

Ethernet

Ethernet

Ethernet is a family of protocols used in LANs, described by a set of standards together called IEEE 802 published by the Institute of Electrical and Electronics Engineers. It has a flat addressing scheme and is mostly situated at levels 1 and 2 of the OSI model. For home users today, the most well-known member of this protocol family is IEEE 802.11, otherwise known as Wireless LAN (WLAN). However, the complete protocol suite deals with a multitude of networking aspects not only for home use, but especially when the technology is deployed to support a diverse range of business needs. MAC bridging (IEEE 802.1D) deals with the routing of Ethernet packets using a Spanning Tree Protocol, IEEE 802.1Q describes VLANs, and IEEE 802.1X defines a port-based Network Access Control protocol, which forms the basis for the authentication mechanisms used in VLANs, but it is also found in WLANs – it is what the home user sees when the user has to enter a "wireless access key".

Differentiate between system software and application software

Q. Differentiate between system software and application software

System Software:

System software is a set of programs to control all components of computer and to manage overall operations of computer system. System software is used as a base to install and run all application software. Examples of system software include:

1. Operating Systems, like Microsoft Windows, DOS, Unix and Linux
2. Device Drivers like Device driver software of a Printer (found on CD normally provided with the printer)
3. Utility Programs like AVAST anti virus, Disk Scanners and File Viewers etc.

Application Software:

Users can use Application Software to perform different applications on computer. Application Software generally performs a specific task for a computer user. Examples of Application software include:

1. Word processors like Microsoft Word
2. Spreadsheet Software like MS Excel
3. Database software like Microsoft Access etc.

Difference between System & Application Software

System Software	Application Software
1. It is a set of programs to control computer components and operations.	1. It is used to solve particular problems of user.
2.  It is general-purpose software.	2. It is specific purpose software.
3. It executes all the time in computer. Because it controls overall operations and components of computer.	3. It executes as and when required. For example, if we need to write a letter, we will open Microsoft Word - an application software. And after we have written and printed the letter, we will close Microsoft Word.
4. The number of system software is less than application software.	4. The number of application software is much more than system software.
5. System software is essential for a computer to work. Without System software (like operating systems), a computer is useless. We cannot use computer hardware without system software. That is why we install an operating system first of all, after buying a new computer.	5. Application software is not essential for a computer to work. For example, if we need to play some games then we buy a game CD or download games from internet, and install in our computer.
6. Examples are: Windows, Dos, Unix, Linux, Norton Antivirus etc.	6. Examples are: MS Word, MS Excel, MS Power point etc.

 

Differences between LAN and WAN

The differences between LAN and WAN is as follows:

LAN (Local Area Network)
1.	LAN is used to connect computers at one place, in one room or a building.
2.	LAN covers small and limited area.
3.	Data transfer speed is very fast in LAN because of small distance.
4.	Local Area network speed is normally form 1 to l00 Mbps.
5.	LAN is less costly.
6.	LAN is usually connected through wires.
7.	Normally, the   connection    in    a    LAN    is permanent using wires.
8.	LAN is used for sharing data, information, files and hardware like printers, hard disk, modem etc.
9.	LAN has   less   possibility   of data transmission error.
10.	LAN is used to transfer data over small distances.
11	LAN is easy to install and configure.
12	LAN is easy to troubleshoot.
WAN (Wide Area Network)
1	WAN is used to connect computers anywhere in the world.
2	WAN can cover larger area. It can cover cities, countries and even continents.
3	Data transfer speed is slower in WAN.
4	Transmission     speeds     are     normally 56 kbps to 45 Mbps.
5	The hardware needed for a WAN is more expensive.
6	WAN is usually connected through telephone lines, fiber optic cable or through wireless media.
7	The    connection    in    WAN    may not be permanent.
8	WAN is used to share only data and information like Email and file transfer
9	WAN has more possibility of data transmission error because of long distance.
10	WAN is used to transfer data over long distances.
11	WAN is difficult to install and configure due to connecting computers at multiple and distant sites.
12	WANs are difficult to troubleshoot.

 

ADVANTAGES OF COMPUTER NETWORKS

ADVANTAGES OF COMPUTER NETWORKS

Following are some important advantages of computer networks:

1. Data and Information Sharing

Different employees of an organization can share common information with the help of computer network.

2. Software Sharing

Different software can be shared with the help of computer networks.

3. Hardware Sharing

Different hardware can be shared with the help of computer networks. For example, if there are five computer users in an office, we can save cost by using only one printer with the help of computer networking. In a computer network, only one computer will manage all printing needs of the computer users in a small office. In addition, we can share other hard ware devices like hard disks, CD-Rom Drives / Writers, modems, routers and scanners etc.

4. Money Saving

We can save a lot of money by using computer networking, because of sharing the same software and hardware in a network.

5. Internet Sharing

We can buy only one DSL internet connection and share it with in whole organization with the help of computer networking.

6. Easy Communication Within Organization

We can send and receive messages and files through network. In this way, a file can be moved to one place to another within an organization without the need of a physical worker to take and carry the files to a destination. With the help of a command in a computer we can transfer file within seconds from one room to another.

7. Easy Communication Outside Organization

We can send and receive emails and share files with other people outside organization using internet.

Bus Topology

Bus Topology

Bus topology is the simplest network topology. In bus topology, all computers in network are connected to a common communication wire. This wire is called BUS. Terminators are used at both ends of wire.

 

 

 

 

 

Advantages of Bus Network Topology

1. It is very simple topology.
2. It is easy to use.
3. It needs small amount of wire for connecting computers.
4. It is less expensive due to small wire needed.
5. If one computer fails, it does not disturb the other computers in network. Other computers will continue to share information and other resources with other connected computers.

Disadvantages of Bus Network Topology

1. Only small number of computers can be connected in a bus network.
2. Network speed slows down as the number of computer increases in bus topology.
3. Finding a fault is difficult in bus topology.

 

Star Topology

Star Topology

Star Topology is the most popular and widely used network topology in local area networks. In star topology, all computers are connected with a central device known as HUB or Switch. Now-a-days Hubs are often replaced by Fast Network Switches. The sender computer sends data to the hub. The hub sends it to the destination computer. So, all data communication is managed through HUB or Switch.
Note: The major difference between a Networking Hub and Networking Switch is that HUB sends data to all computers, and the destination computer will accept it while other computers will not. On the other hand, Switch sends data only to the destination computer. Therefore, switch can reduce network traffic and hence provides fast transmission speed.

Advantages of Star Network Topology

1. Easy Installation and maintenance

It is easy to maintain network. That is why it is so popular.

2. Adding or removing computers

Adding or removing computers can be done without disturbing the network. We connect the new computer with the HUB by means of a networking cable. One end of the cable (RJ 45 connector) is inserted in computer’s Network Interface Card and the other end (RJ 45 connector) is plugged into the HUB, and that’s it.

3. Fault Diagnosis

In a star network topology, finding faults is easy. If a computer is no more connected with your network, you can check its cable and connectors or network settings in its Operating system.

4. Network Reliability

Single computer failure will not disturb whole network, since all other computers are connected with separate links ( wires ) to HUB. Definitely, they will work fine.

5. Better performance

Star topology can prevents the passing of data through an excessive number of nodes. By using a Switch, at most, 3 devices and 2 links are involved in any communication between any two devices.
6. Device Isolation
Each device is separately connected to HUB or Switch and is isolated. This is why each device works independently.

Disadvantages of Star Network Topology

1. In star network topology, data communication depends on HUB. If central hub fails, then whole network fails.
2. Since each computer will be connected with HUB by means of a separate wire, star network topology needs more cable to connect computers.
3. It is more expensive due to more wires.

 

Ring Topology

Ring Topology

In Ring topology, each computer is connected to the next computer such that last computer is connected to the first. Every computer is connected to next computer in the ring. Each computer retransmits what it receives from the previous computer. Suppose, computer A needs to send data to computer D. Now the computer A sends data to computer B. As computer B is not the destination computer, so it will retransmit data to computer C. Finally, Computer C will transfer data to computer D, the destination computer. When a node sends a message, the message is processed by each computer in the ring. If a computer is not the destination node, it will pass the message to the next node, until the message arrives at its destination.

Advantages of Ring Network Topology

1. It is relatively less expensive than a star topology network.
2. In a Ring topology, every computer has an equal access to the network.
3. Performs better than a bus topology under heavy network load

Disadvantages of Ring Network Topology

1. Failure of one computer in the ring can affect the whole network.
2. It is difficult to find faults in a ring network topology.
3. Adding or removing computers will also affect the whole network since every computer is connected with previous and next computer.
4. Sending a message from one computer to another takes time according to the number of nodes between the two computers. Communication delay is directly proportional to number of nodes in the network.