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Showing posts from 2013


Short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off. CSMA/CD is a type of contention protocol.  Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics. The CSMA/CD method is internationally standardized in IEEE 802.…


Ethernet is a physical and data link layer technology for local area networks (LANs). Ethernet was invented by engineer Robert Metcalfe. A local-area network (LAN) architecture developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology and supports data transfer rates of 10 Mbps. The Ethernet specification served as the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. Ethernet uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards. A newer version of Ethernet, called 100Base-T (or Fast Ethernet), supports data transfer rates of 100 Mbps. And the newest version, Gigabit Ethernet supports data rates of 1 gigabit (1,000 megabits) per second.

Difference between a hub and a switch

Difference between a hub and a switchA Hub is a networking device that allows one to connect multiple PCs to a single network. Hubs may be based on Ethernet, Firewire, or USB connections. A switch is a control unit that turns the flow of electricity on or of in a circuit. It may also be used to route information patterns in streaming electronic data sent over networks. In the context of a network, a switch is a computer networking device that connects network segments.

Hub Switch Physical layer. Hubs are classified as Layer 1 devices per the OSI model. Data Link Layer. Network switches operate at Layer 2 of the OSI model. Hubs always perform frame flooding; may be unicast, multicast or broadcast First broadcast; then unicast & multicast as needed. 4/12 ports Switch is multi port Bridge. 24/48 ports A network hub cannot learn or store MAC address. A network switch stores MAC addresses in a lookup table. Passive Device (Without Software) Active Device (With Software) & Networking devi…
OSI 1)It has 7 layers 2)Transport layer gurantees delivery of packets 3)Horizontal approach 4)Seperate presentation layer 5)Seperate session layer 6)Network layer provides both connectionless and connection oriented services 7)It defines the services,interfaces and protocols very clearly and makes a clear distinction between them 8)The protocol are better hidden and can be easily replaced as the technology changes 9)OSI truly is a general model 10)It has a problem of protocol filtering into a model
TCP/IP 1)Has 4 layers 2)Transport layer does not gurantees delivery of packets 3)Vertical approach 4)No session layer, characteristics are provided by transport layer 5)No presentation layer, characteristics are provided by application layer 6)Network layer provides only connection less services 7)It does not clearly distinguishes between service interface and protocols 8)It is not easy to replace the protocols 9)TCP/IP can not be used for any other application 10)The model does not fit an…

Session Layer

The session layer manages sessions between applications, including initiation, maintenance and termination of information transfer sessions. Usually this is visible to the user by having to log on with a password.

The session layer tracks connections, also called sessions. The session layer should keep track of multiple file downloads requested by a particular FTP application, or multiple telnet connections from a single terminal client, or web page retrievals from a web server.

With TCP/IP this functionality is handled by application software addressing a connection to a remote machine and using a different local port number for each connection. The session layer performs the following functions: Communication with the Presentation layer above.Organize and manage one or more connections per application, between hosts.Communication with the Transport layer below.
Session layer protocols are particularly useful for multimedia applications for which it is necessary to coordinate the timing …


This is a hardware device that is used to network multiple computers together. It is a central connection for all the computers in a network, which is usually Ethernet-based. Information sent to the hub can flow to any other computer on the network. If you need to connect more than two computers together, a hub will allow you to do so. If you only need to network two computers together, a simple crossover Ethernet cable will do the trick.

An Ethernet hubactive hubnetwork hubrepeater hubmultiport repeater or hub is a device for connecting multiple Ethernet devices together and making them act as a single network segment. It has multiple input/output (I/O) ports, in which a signal introduced at the input of any port appears at the output of every port except the original incoming. A hub works at the physical layer (layer 1) of the OSI model. The device is a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if i…

Networking hardware

Networking hardware may also be known as network equipmentcomputer networking devices. Units which are the last receiver or generate data are called hosts or data terminal equipment. All these terms refer to devices facilitating the use of a computer network. Specifically, they mediate data in a computer network.
Specific devicesHub: a device that connects multiple Ethernet segments, making them act as a single segment. When using a hub, every attached device shares the same broadcast domain and the same collision domain. Therefore, only one computer connected to the hub is able to transmit at a time. Depending on the network topology, the hub provides a basic level 1 OSI model connection among the network objects (workstations, servers, etc.). It provides bandwidth which is shared among all the objects, in contrast to switches, which provide a connection between individual nodes. It works on OSI layer 1.Switch: a device that allocates traffic from one network segment to certain line…

Application Layer

In the Internet model, the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols to establish process-to-process connections via ports. In the OSI model, the definition of its application layer is narrower in scope. The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.[2] It separates functionality above the transport layer at two additional levels, the session layer and the presentation layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer. The interface – responsible for displaying the information received to the …

Message encapsulation

Message encapsulation
As data are passed down from an application level through the transport level, the network layer to the
data-link layer they are encapsulated, this is shown in figure 1.4. In order to transmit the characters the
transport layer puts a header on to communicate with its peer module at the remote end. In this header will
be the port number. The transport module passes the data plus header to the network module which puts on
its header containing the remote system address. Finally when this is passed to the data-link code another
header is added.


Set of Rules and procedure under which network governs.

To request any service or exchange any information between 2 programs there must be an agreed set of
commands and data formats, this is a protocol. So, for example, the commands and data sent between
a World Wide Web browser and a remote server are a protocol. The browser (probably) uses the GET
command follow by the name of the required file (page), this protocol is recognised and understood by the
web server program which responds appropriately. Similarly the format of packets sent between Ethernet
cards and their drivers are a protocol. The programs exchanging messages are called peers.

Communication channel

Communication channel Intelecommunicationsandcomputer networking, acommunication channel, orchannel, refers either to a physicaltransmission mediumsuch as a wire, or to alogical connectionover amultiplexedmedium such as a radio channel. Communicating data from one location to another requires some form of pathway or medium. These pathways, called communication channels, use two types of media: cable (twisted-pair wire, cable, and fiber-optic cable) and broadcast (microwave, satellite, radio, and infrared). A channel is used toconveyaninformationsignal, for example a digitalbit stream, from one or severalsenders(or transmitters) to one or severalreceivers. A channel has a certain capacity for transmitting information, often measured by itsbandwidthinHzor itsdata rateinbits per second.
Transmission media are classified as one of the following: ·Guided (or bounded)—waves are guided along a solid medium such as atransmission line.
·Wireless (or unguided)—transmissionandreceptionare achieved b…

Computing Models and Network Development

Computer networking technologies are generally based on the following computing models: ·Centralized computing ·Distributed computing ·Collaborative computing
In addition, the following computing models are used to categorize the way networking services are provided:
·Client /Server ·Client/Network Centralized Computing In a purely Centralized model, all computing resources reside at the primary Datacenter/Mainframes. This includes Domain Authentication Services, Email, Applications, and Share Files. Remote Sites would access these resources using Terminals/Thin Client devices (as opposed to PCs) and bandwidth-friendly enablers such as Citrix XenApp, Microsoft Terminal Services, or VMware Virtual Desktop technologies. In Centralized computing, the mainframes provide all the data storage and computational abilities; the Terminal is simply a remote input/output device. Distributed Computing In distributed computing, personal computers (PCs) have their own processing capabilities. In this model t…

Factors to consider when selecting a transmission medium (TRANSMISSION IMPAIRMENTS)

There are many transmission media available and each media type has certain characteristics, so it’s necessary to aware about all the benefits and shortcomings of each one.
•Cost & Ease of installation •Attenuation •Delay Distortion •Capacity (Bandwidth and throughput) •Transmission delay •Propagation delay •Interference •Noise •Thermal/White Noise •Intermodulation Noise •Crosstalk •Impulse Noise
•Cost & Ease of installation : Costing is an important factors , when we select a media. Because absolute cost and ease of installation data are difficult to provide without referring to specific implementations, one can make relative judgments by comparing each medium to the others. •Attenuation : Attenuation refers to the tendency of electromagnetic waves to weaken or become distorted during transmission. It is loss of energy as the signals propagates outwards. Attenuation increases with distance, as a wave passes through a medium, some of its energy is absorbed or scattered by the medium’s ph…