Computer networking is an important section in many govt. exams like IBPS PO, IBPS SO, IBPS Clerical, SBI PO, SBI clerical and others. It becomes very significant to have knowledge of computer networking for Bank Exams.
- What is a Computer Network?
Simple, simple , a NETWORK by connecting two or more computers. These connections or arrangements are called Computer Networks. These computers in the Computer Network may attached with cables, telephone lines, radio waves, satellites or infrared light,beams, wireless etc.
A Computer Network is a collection of Computers and Devices which are connected via communicating devices and transmission media to enable transmission of meaningful data and information between them.
. What Are Networks Used For?
A simplified but worthwhile description of the uses of computer networks might be as follows:
Sharing of hardware: For example, several PCs might be networked together in a wired or wireless local area network (LAN) to share a printer.
Sharing of information: Distributed databases, e-mail, the World Wide Web and so on are examples of this. Here the sharing involves both LANs and wide area networks (WANs), especially the latter.
OSI (Open Systems Interconnection) is
reference model for how applications can communicate over a network. A reference model is a conceptual framework for understanding relationships. The purpose of the OSI reference model is to guide vendors and developers so the digital
communication products and software programs they create willinteroperate
, and to facilitate clear comparisons among communications tools. Most vendors involved in telecommunications make an attempt to describe their products and services in relation to the OSI model. And although useful for guiding discussion and evaluation, OSI is rarely actually implemented, as few network products or standard tools keep all related functions together in well-defined layers as related to the model. The TCP/IP protocols, which define the Internet, do not map cleanly to the OSI model.
2. Overview of the Layers
The layers collectively are often referred to as the protocol stack.
The Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application). The layers are stacked this way:
- Data Link
a. Physical Layer
This is concerned with the nature of the physical media (metal or optical cable, free-space microwave, etc.) used to send signals, the nature of the signals themselves, and so on.
There is also the question of signal form; the signals themselves may be in the form of pure 0-1 bits, or may be in the form of certain frequencies. In addition there are questions concerning how a receiver distinguishes two bits which are adjacent in time.
A major issue is the form of the medium, both in terms of the materials it uses and its topology. A basic wired Ethernet, for example, consists of cable conducting electrical signals; the connections could also be wireless. More complicated networks, including Ethernets, may consist of more than one cable, with all of them connected via a hub. The latter has become common even at the household level.
b. Data Link Layer
For example, in an Ethernet, this layer is concerned with ensuring that two network stations connected to the same cable do not try to access the line at the same time. For this reason the Ethernet operation is an example of what is called a Medium Access Control (MAC) Protocol.
Here is an overview of how the Ethernet MAC protocol works, using a “listen before talk” approach. When a network node has a message ready to send, it first senses the cable to see if any node is currently sending. If so, it generates a random backoff time, waiting this amount of time before trying again. If the node does not “hear” any other node sending, it will go ahead and send.
There is a small chance that another node actually had been sending but due to signal propagation delay the transmission had not yet reached the first node. In that case a collision will occur, destroying both messages. Both nodes will sense the collision, and again wait random amounts of time before trying again.
This layer also does the setting up of frames of bits (i.e. sets of consecutive bits sent along the wire), which not only include the message itself but also information such as (say, in the Ethernet case) the Ethernet ID number of the destination machine.
Messages may be broken up into pieces before being sent. This may be handled at the transport level (see below), but may also be done at the data link level
c. Network Layer
This is the routing layer. Questions addressed in this layer include: If in our example above saturn wants to send a message to holstein, how is that accomplished? Obviously its first step is to send the message to mars; how does saturn know this? How can alternate routes be found if traffic congestion occurs?
- Routing: routes frames among networks.
- Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
- Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
- Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
- Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.
d. Transport Layer
Suppose saturn’s message to holstein consists of a large file transfer, say 100 megabytes. This transfer will take a long time (by network standards), and we certainly don’t want it to monopolize the network during that time. We also must deal with the fact that the buffer space at holstein won’t be large enough to deal with a 100-megabyte message. Also, one 100-megabyte message would have a sizable probability of having at least one bit in error, and if so, we would have to retransmit the entire message!
So, the file transfer must be done in pieces. But we don’t want to burden the user at saturn with the task of breaking up the 100 megabytes into pieces, nor do we want to burden the user at holstein with the reassembly of the messages. Instead, the network software (again, typically in the OS) should provide these services, which it does at the transport layer, as for example is the case with TCP.
The transport layer provides:
- Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
- Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
- Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
- Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
e. Session Layer
This layer is concerned with management of a session, i.e. the duration of a connection between two network nodes. The word connection here does not mean something physical, but rather refers to an agreement between two nodes that some chunks of data with some relation to each other will be exchanged for some time. Actually, TCP does this in some senses, as does the socket interface to TCP, which is very much like the interfaces for reading or writing a file (described in more detail later).
f. Presentation Layer
This layer deals with such matters as translating between character codes, if the source uses one and the destination the other. In the old days, this could mean ASCII at one end and EBCDIC on the other end. Today, though, it could mean for example two different coding systems for Chinese characters, Big 5 and GB.
g. Application Layer
You can write programs at the application layer yourself, and of course you use many programs written by others, such as ftp, Web browsers, e-mail utilities, and so on.
- Resource sharing and device redirection
- Remote file access
- Remote printer access
- Inter-process communication
- Network management
- Directory services
- Electronic messaging (such as mail)
- Network virtual terminals
3. Network Security
System and network technology is a key technology for a wide variety of applications. Security is crucial to networks and applications. Although, network security is a critical requirement in emerging networks, there is a significant lack of security methods that can be easily implemented.
There exists a “communication gap” between the developers of security technology and developers of networks. Network design is a well-developed process that is based on the Open Systems Interface (OSI) model. The OSI model has several advantages when designing networks.
It offers modularity, flexibility, ease-of-use, and standardization of protocols. The protocols of different layers can be easily combined to create stacks which allow modular development. The implementation of individual layers can be changed later without making other adjustments, allowing flexibility in development. In contrast to network design, secure network design is not a well-developed process.
When considering network security, it must be emphasized that the whole network is secure. Network security does not only concern the security in the computers at each end of the communication chain. When transmitting data the communication channel should not be vulnerable to attack.
A possible hacker could target the communication channel, obtain the data, decrypt it and re-insert a false message. Securing the network is just as important as securing the computers and encrypting the message.
4. Internet Attack Methods
Internet attacks methods are broken down into categories. Some attacks gain system knowledge or personal information, such as eavesdropping and phishing. Attacks can also interfere with the system’s intended function, such as viruses, worms and trojans. The other form of attack is when the system’s resources are consumes uselessly, these can be caused by denial of service (DoS) attack. Other forms of network intrusions also exist, such as land attacks, smurf attacks, and teardrop attacks. These attacks are not as well known as DoS attacks, but they are used in some form or another even if they aren’t mentioned by name.
Interception of communications by an unauthorized party is called eavesdropping.
Passive eavesdropping is when the person only secretly listens to the networked messages. On the other hand, active eavesdropping is when the intruder listens and inserts something into the communication stream. This can lead to the messages being distorted. Sensitive information can be stolen this way.
Viruses are self-replication programs that use files to infect and propagate . Once a file is opened, the virus will activate within the system.
A worm is similar to a virus because they both are self-replicating, but the worm does not require a file to allow it to propagate . There are two main types of worms, mass-mailing worms and network-aware worms. Mass mailing worms use email as a means to infect other computers. Network-aware worms are a major problem for the Internet
A network-aware worm selects a target and once the worm accesses the target host, it can infect it by means of a Trojan or otherwise.
A worm is similar to a virus because they both are self-replicating, but the worm does not require a file to allow it to propagate . There are two main types of worms, mass-mailing worms and network-aware worms. Mass mailing worms use email as a
means to infect other computers. Network-aware worms are a major problem for the Internet. A network-aware worm selects a target and once the worm accesses the target host, it can infect it by means of a Trojan or otherwise.
Trojans appear to be benign programs to the user, but will actually have some malicious purpose. Trojans usually carry some payload such as a virus.
Phishing is an attempt to obtain confidential information from an individual, group, or organization. Phishers trick users into disclosing personal data, such as credit card numbers, online banking credentials, and other sensitive information.
g. IP Spoofing Attacks
Spoofing means to have the address of the computer mirror the address of a trusted computer in order to gain access to other computers. The identity of the intruder is hidden by different means making detection and prevention difficult. With the current IP protocol technology, IP-spoofed packets cannot be eliminated.
h. Denial of Service
Denial of Service is an attack when the system receiving too many requests cannot return communication with the requestors. The system then consumes resources waiting for the handshake to complete. Eventually, the system cannot respond to any more requests rendering it without service