Saturday, 30 November 2013

Twisted pair cable is just what its name implies; insulated wires housed within a protective casing with a specified number of twist per foot of distance. Twisting of wires reduces the effect of electromagnetic interface which may be generated from nearby electric motors, florescent tube lights etc. Shielded Twisted-Pair(STP) cables are provided with shielding and additional insultation over the cluster of wires to protect it from outside noise. Unsheilded twisted-pair (UTP) cables are mostly used in small office or home LANs which are less costly.
The unshielded twisted-pairs are classified by the by the Electronic Industries Association (EIA) into seven categories of UTP cables.
The most common UTP connector is RJ 45,which can be interested in one way only.
Applications:: Twisted-pair cables are extensively used in telephone lines as voice and data channels. The lines connecting subscribers to the telephone exchange are unshielded twisted-pair cables.
People use twisted-pair cables to connect DSL lines for high speed Internet Connection.
LAN such as 10 Base-T and 100 Base_T uses twisted pair cables.
ADVANTAGES::
Low Cost: the cost of twisted-pair is not so high.
Light weight: Twisted-pair cables are lighter in weight. Hence it dose not require heavy support.
Easy Installation & maintenance: The technology of twisted-pair cable laying is a job of almost any person. Connecting the RJ$% connector to the cable is called crimping and it requires a few hours of practice.
Cheaper equipment: Crimping tools and other tools necessary for twisted-pair cable preparation are cheap. Hence these are easy to purchase.
Disadvantage::
Subceptible to cross talk: Twisted-pair cables are not totally immune to cross talk.

Distance limitation: The maximum length of LAN using twisted-pair cable is short, in comparison to optical fiber.

Monday, 25 November 2013

The first version of Ethernet was based on coxial cable. The original form of Ethernet 10 Base-5 used a thick Coaxial cable that was not directly connected to the computer’s NIC (Network Interface Card). An attachment unit Interface (AUI) ran from a DB15 connector on the back side of the NIC to the thick coax. A “vampire tap” is inserted into the thick coax to which the AUI is connected.
Another version of coaxial cables are there which are thinner in size and hence the name thin coaxial cable 10base-2 Ethernet cards are connected to this type of cable by a BNC (Bayonet-Neill-Concilman)- T connector on the rear side.
The physical construction of a coaxial cable has a central core made of solid or stranded copper which carries the signals. Upon the central conductor there is an insulating layer upon which a braided wire mesh is provided as shield to protect the cable from external noise. Upon this shield another insulating sheath is there enclosing which a plastic cover is provided to complete the construction of the cable.Thick and Thin coax differsin size and cable impedance. Coaxial cables are categorizrd by their Radio Government (RG) ratings each RG number denotes a set of physical specifications uniquely
Application::
Coaxial cable was extensively used in analog telephone networks to carry 10000 voice signals at a time. Later these cables were used in digital telephone networks which could carry digital data upto 600 Mbps. But coaxial cables are almost replaced today by fiber optic cables.
Cable TV networks are still running on coaxial cables in our country. However, in some advance countries these are replaced by fiber optic cables.

Advantages::
1.    Strong construction
2.    Good immunity to noise.
3.    Good LAN length.
Disadvantages::
1.    Heavy weight.
2.    Unrellable tap.

3.    High cost of installation.
Transmission media is the physical path that carry data from transmitter to receiver. The signals Transmitted from one device to another device in the form of electromagnetic energy with the help of transmission media. The Transmission media can be divided into two classes:: Guided Media and Unguided Media.
Gutded transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bounded by the cabling system. Guided media is also known as bound media. “Cabling “ is meant in a generic sense, and is not meant to be interpreted as copper wire cabling only. The followings are the basic guided media used in networking :;

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

Sunday, 17 November 2013

There are not much difference between the OSI and TCP/IP reference model. In both, the concept of independent stack of  protocols is used. The function of the layer are also more or less same. In both model, starting from bottom up to transport layer, the functions of the layers is to provide end-to-end network independent transport service for communicating processes. The layers above the transport layer are application oriented.
On the other hand there are some differences between the two models. In OSI reference model, three concepts are distinct, these are-  

1.       Services
2.     Interface
3.     Protocols
The service tells what a particular layer serves the layer just above it, not how these services are access by the above layer or how these are provided by the layer. The interface tells how the above layer access the services provided by the layer just below it. It also dose not say how these are provided. The protocols used between peer layers are the set of rules agreed upon by both the layers to get the job done. Protocols can be changed without affecting the software in the higher layers.
In TCP/IP model, there is no such distinction between service, Interface and protocol. Therefore, in TCP/IP reference model the protocols are not so hidden as the OSI reference model. As the technology advances protocol between a peer layer can be replaced easily in OSI model then In TCP/IP model.
In OSI model, the layered structure was thought out before the protocols were invited. The designers were new in the network technology and hence in some layer a sub-layer has to be provided latter to accommodate some new model of communication.
The two models also differ in their numbers of layers; in OSI model there are seven layers whether TCP/IP model has four layers. Both have network,transport and application layers in common but the other layers are not same.

In OSI model, network layer supports both connection oriented and connectionless communication and the transport layer supports only connection-oriented communication. On the contrary, in TCP/IP model the network layer supports only connectionless communication but transport layer supports both the mode.

Saturday, 16 November 2013

The internet model of network architecture, also known as TCP/IP reference model, was developed much earlier then the OSI reference model. It was developed from the US department of Defence’s (DoD) research network the ARPANET.
 Eventually the ARPANET connected many universities and other government organizations. During the process of interconnecting all these through the exising telephone lines, satellite link and therefore, a new architecture was needed to overcome that. The architecture thus came out became known as the INTERNET or TCP/IP reference Model. The model has four layers normally.
1.       The Host-to-Network Layer.
2.     The Internet Layer.
3.     The transport Layer.

4.    The application Layer. 
A layer can offers two different types of services to the layer immediately above it – connection-oriented and connectionless service.
In connection oriented service user establish a connection at first, communicates over the connection and at last release the connection. It is similar to a telephone system.
In connectionless service no connection is established beforehand. Instead, like in a postal system, every message caries its full destination address and each routed independently to its destination, then in connectionless service sometime it may happen that the first piece may arrive at the destination after the latter pieces. 
That means at the receiving end the order of delivery may not be same as the  order of transmission. In connection oriented service this never happens

Friday, 15 November 2013

When we study the subject of computer network, we encounter many terminologies associated with the subject. Some  terminologies are briefly described below.
1.       Entity :: The active component in each layer are called entities. Entity can be a software process or hardware like an intelligent input/output device etc.
2.     Peer Entity :: Entities in the same layer running on different machines are called peer entities.
3.     Service provider and service user :: In a layered architecture, layer n provides a service which is used by layer n+1. Here layer n is service provider and layer n+1 is service user.

4.    Service Access Point :: A layer n offers services to layer n+1 at a place which is called service access point. Each SAP has a unique address for identification.
The session layer :: 
This layer offers facility to different users on different computer to establish session between them. A session allow an user to remotely log into a distant machine and transfer file between the two machines. Session layer perform token management to provide unidirectional communication. It also provides a service called synchronization.
The Presentation layer :: 
This layer performs data presentation job by following syntax and semantics rules. Before presenting data to user, it transform data into their acceptable form.. we write date as dd/mm/yyyy, currency as Rs. While in western country it is written as mm/dd/yyyy and $ etc.
The application Layer :: 
This layer is the nearest layer to all the network users it offers variety of protocols that are commonly needed. It helps to transfer file. Different file system have different meaning in different machines with different data format etc. When files are transferred from one machine to another with different file system, the application layer take necessary steps to resolve the abnormalities
Normally, the transport layer creates individual connection for each session. If high throughput is required, the transport layer may establish multiple network connections, dividing the data among individual connection thereby improving the through put.
To reduce cost the transport layer may also multiplex several transport connections on to the network connection. However, multiple connections or multiplexing must not be seen by the session layer.

The transport layer may also determines the type of service given to the user of the network. Error free point-to-point connection is the most popular transport layer service where messages or bytes are delivered in order in which they were sent. Another kind of transport service is the transportation of isolated messages with no guarantee of delivery. The transport layer is a true end layer between source to destination. 
The network layer controls the operation of the subnet. The layer is to determine how packets are routed from source to destination. The routing may be static or dynamic depending on traffic load and availability of channel.
Too many packets may cause congestion and control of such congestion is also a duty of network layer.
The subnet operation require cost, hence some accounting function is also there built into the network layer. When a packet crosses national boundary some other aspects of accounting has to be dealt with by network layer.

Packets have to travel in between heterogeneous network running on different platforms using different network protocol and network layer is also responsible to resolve all the problems arises out of such situations.

Thursday, 14 November 2013

The data link layer used the raw transmission facility and transforms it to an apparently error free facility tobe used inserting appropriate frame boundary, transmit the frames sequentially and process the acknowledgment sent back by the receiving computer. 
If a frame is completely destroyed by noise burst, it is the duty of data link layer to transmit it from the source machine data link layer also ensures that a fast sender be not allowed to swamp a slow receiver. This is called flow control. In a broadcast network it is the duty of MAC sub-layer of data link layer to decide who will access the transmission medium at a particular time.
The physical layer is responsible for transmitting raw bits over the communication channel. This layer is to ensure how to send a 1 bit from the sending computer as 1 bit to the receiving computer, not as 0 bit. The physical layer also deals with the issue of how many bit per second will be transmitted,
what level of voltage will be used to represent 1 and 0 whether transmission will be unidirectional or bidirectional, how the initial connection be established and terminated at the end , how many pins of network connector has and which pin is for what etc etc. Hence the design issues of the physical layer is mostly mechanical, electrical and procedure oriented.
1
.     To interchange data between two process running on two computers there must be some addressing technique for communication correctly.
2.     The communication may be unidirectional or in both direction alternately one after another or bidirectional
3.     Error control is also important issue because physical communication channels are not perfect, rather error pron. Now both communicating parties must agree on which error detecting and correcting code to use. The receiver should also let the sender know which message is received correctly and which is not.
4.     During transfer messages are often divided into pieces and all the pieces may not be transferred in proper order this loss of sequencing has to be taken care of by the protocol concern.
5
.     There must be provision to refrain at fast sender from swamping a slow receiver.
6.     The layer concern may decide to use same connection for several unrelated conversations by multiplexing and de-multiplexing technique.

7.     When there are many paths between sender and receiver, a decision must be taken which route is to take. The decision may be static or dynamic.
Many different types of network protocols and standards are required to ensure that a computer can communicate with another computer located on the next desk or any location around the worlds.

A network protocol is a set of rules that governs the communications between computers and other devices on a network. This rules include guidelines that regulate the characteristics of a network such as access method, allowed physical topologies, types of cabling, and speed of data transfer.some protocols also support message acknowledgement and data compression designed for reliable and/or high-performance network communication. Hundreds of different computer network protocols have been developed each designed for specific purposes and environments. 
The seven-layer OSI model, created by the ISO, defines inter networking environments. It provides a description of how software and hardware interact to permit communication between computers. An interface separates each layer from those above and below it. 
Transmission modes mean the way in which a communication is achieved between two linked device. The device which sends data or information is called the sender and which receives the information is called the receiver. A channel can support either one way communication or two way communication at a time. Based on the way of communication link, transmission modes can be classified as : simplex, half duplex, and full duplex.
Simplex :: 
A simplex connection is a connection in which the data flows in only one direction, from the transmitter to the receive. This type of connection is useful if the data do not need to flow on both direction.
Half-duplex :: 
A half-duplex system provides for communication in both directions, but only one direction at a time. Typically, once a party begins receiving signal, it must wait for the to stop transmitting, before replying. This type of connection makes it possible to have bidirectional communications using the full capacity of the line

Full-duplex :: 
The preferred transmission mode for network communication is full-duplex mode. A full-duplex, allows communication tn both directions simultaneously. Each end of the line can thus transmit and receive at the same time, which means that the bandwidth is divided in two for each direction of data transmission if the same transmission medium is used for both direction of transmission. A  good analogy for a full-duplex system would be a two lane road with one lane for each direction.

Wednesday, 13 November 2013

The term synchronous is used to describe a data transfer method in which a continous stream of data signals is accompanied by timing signals to ensure that the transmitter and the receiver are synchronized with one another. These types of connections are used when large amount of data must be transferred very quickly from one location to the other. The speed of the synchronous connection is attained by transferring data in large blocks instead of individual characters.
A block of bits is transmitted in a steady stream without start and stop bits. Data or information is moved from one place to another at instants in time that are measured against the clock signal being used. This signal is usually comprised of one or more high frequency rectangular shaped waveforms are connected to all the devices that operate synchronously, allowing them to start and stop operations with respect to clock waveform.

In synchronous transmission, each block begins with a preamble bit pattern and generally ends with a postamble bit pattern. In addition, there are some other bits that convey control information. The data with the preamble, postamble, and control information are termed as frame. 
Communication is called asynchronous if the transmitter and receiver do not need to synchronise before each transmission. A sender can wait arbitrarily long between transmission and receiver must be ready to receive data when it arrives most PC serial devices such as mouse, keyboards and modems are asynchronous.
As the name implies, asynchronous communication is performed between two or more devices which operate on independent clocks. Thus there is no guarantee that when Point A begins transmitting, point B will begin receiving, or that point B will continue to sample at the rate point A transmits. In asynchronous transmission, data are transmitted one character at a time, where each character is five to eight bits in length. Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character.
Asynchronously transmitted data is preceded with a start bit which indicates to the receiver that a character is about to begin. The end of a character is followed by a stop bit, which tells the receiver that the character has come to an end, that it should begin looking for the next start bit, and that any bits it receives before getting the start bit should be ignored. To avoid confusion with other bits, the start bit is twice the size of any other bit in the transmission. To ensure data integrity, A party bit is often added between the last bit of data and the stop bit. 
The party bit is set by the transmitter such that the total number of ones in the character, including the party bit, is even or odd, depending on the convention being used. The receiver uses this bit for error detection. The parity bit makes sure that the data to be received is composed of the same number of bits in the same order in which they were sent
Serial transmission sends one bit at atime over a single transmission line. The cost of communication hardware is considerably reduced since only a single wire or channel is require for the serial bit transmission which also slows the speed of transmission. Telephone lines use serial transmission for digital data, thus modems are connected to the computer via a serial port. A serial port is a socket on a computer used to connect the serial interface to a serial line or bus. A serial interface is a data channel that transfers digital data serially; it is typically implemented as a card that plug into a expansion slot on a computer motherboard. Serial interface have multiple lines but only one is used for data. An external serial bus carries serial data to any device connected to it, e.g. Ethernet. Serial transmission can be either synchronous or asynchronous. 
Serial transmission technology is increasingly used for the transmission of digital data. A large number of up-to-date communications networks apply serial transmission. The numerous applications include computer networks for office communications, building and manufacturing automation, and finally, Internet. The transmission of a stream of bits from one device to another across a transmission media involves a great deal of corporation and agreement between the two sides. One of the most fundamental requirements is synchronization. The receiver must know the rate at which bits are being received so that it can sample the medium at appropriate intervals to determine the value of each received bit. 
For achieving the desired synchronization, there are two approaches. Synchronous and asynchronous transmissions are two different methods of transmission synchronization.

Tuesday, 12 November 2013

Parallel transmission communicates bits simultaneously over multiple lines typically the total consists of one or more bytes at a time. Parallel devices have a wider data bus then serial devices and can therefore transfer data in words of one or more bytes at a time. As a result, there is a speedup in parallel transmission bit rate over serial transmission bit rate. The timing for parallel
transmission is provided by a constant clocking signal sent over a separate wire within the parallel cable; thus parallel transmission is considered synchronous. Computers are typically connected to printers and external disk drives via parallel interfaces, ports, and buses
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