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Friday, November 22, 2013

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:
  1. Communication with the Presentation layer above.
  2. Organize and manage one or more connections per application, between hosts.
  3. Communication with the Transport layer below.

Session layer protocols are particularly useful for multimedia applications for which it is necessary to coordinate the timing of two or more types of data, such as voice and moving images, with a high degree of precision. Examples include video conferencing and streaming.
Examples of session layer protocols include DLC (data link control), PAP (printer access control), SMB (server message block), ASP (AppleTalk session protocol), NetBIOS (network basic input/output system) and ZIP (zone information protocol).

TYPES of Session:

Simplex: Transmission from one side only. eg TV, Radio.
Half Duplex: Transmission from both side but onc side at a time. eg Walkie Takie, Chatting.
Full Duplex: Transmission from both side simultaneously. eg Telephoe, Mobile, Video Chat.

Hub

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 it detects a collision.

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 devices

  • Hub: 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 lines (intended destination(s)) which connect the segment to another network segment. Unlike a hub, a switch splits the network traffic and sends it to different destinations rather than to all systems on the network. It works on OSI layer 2.
  • Repeater: a device which amplifies or regenerates digital signals received while sending them from one part of a network into another. It works on OSI layer 1.
  • Router: a specialized network device that determines the next network point to which it can forward a data packet towards the ultimate destination of the packet. Unlike a gateway, it cannot interface different protocols. It works on OSI layer 3.
  • Bridge: a device that connects multiple network segments along the data link layer. It works on OSI layer 2.
  • Gateway: this device is placed at a network node and interfaces with another network that uses different protocols. It works on OSI layers 4 to 7.
  • Network interface controller: a piece of computer hardware allowing the attached computer to communicate by network.
  • Wireless network interface controller: a piece of computer hardware allowing the non-attached computer to communicate by LAN.
  • Modem: device that modulates an analog "carrier" signal (such as sound) to encode digital information, and that also demodulates such a carrier signal to decode the transmitted information, such as a computer communicating with another computer over a telephone network.

Thursday, November 21, 2013

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 user.

The application layer consists of what most users think of as programs. The application does the actual work at hand. Although each application is different, some applications are so useful that they have become standardized.
The Internet has defined standards for:
File transfer (FTP):
Connect to a remote machine and send or fetch an arbitrary file. FTP deals with authentication, listing a directory contents, ascii or binary files, etc.
Remote login (telnet):
A remote terminal protocol that allows a user at one site to establish a TCP connection to another site, and then pass keystrokes from the local host to the remote host.
Mail (SMTP):
Allow a mail delivery agent on a local machine to connect to a mail delivery agent on a remote machine and deliver mail.
News (NNTP):
Allows communication between a news server and a news client.
Web (HTTP):
Base protocol for communication on the World Wide Web.

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.

Protocols

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
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium 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 to convey an information signal, for example a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

Transmission media are classified as one of the following:
·         Guided (or bounded)—waves are guided along a solid medium such as a transmission line.

·         Wireless (or unguided)—transmission and reception are achieved by means of an antenna.

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 the application is divided into tasks, and each task is assigned to a computer for processing. The results of the processing can be sent as data to other computers.
Collaborative computing
Collaborative computing (also called cooperative processing) is a type of distributed computing using networked computers that “collaborate” by sharing processing abilities. It is aform of Distributed computing that allows task to be shared by by computers as needed. In  Collaborative computing model two or more computers can share the same task, where as Distributed computing assigns each task to a single computer.

Client/Server Computing

In Client/server computing model, several clients (PCs) are connected to a server
In this model:
·         Processing capabilities are distributed across multiple machines.
·         Clients request services from server.
·         The server performs some of processing for the client.
Client/Network Computing


In Client/Network computing model, users lg in to a network and connect to a set of services rather than to a specific server. These services can be provided through a directory services approach.

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 physical properties. The loss is measured is decibels per kilometer (db/km).attenuation is higher at higher frequencies.
          Delay Distortion: one property of signal propagation is that the speed of travel of frequency is highest at the centre of bandwidth and lowest at both the ends. Delay Distortion is caused by the fact that the signals of varying frequencies travel at different speeds along the medium.
          Capacity: the capacity of a transmission medium is often stated as bandwidth. Bandwidth is the range of cycle frequencies, measured in hertz (Hz) or cycle per second, that a transmission medium can physically accommodate.
 Throughput is a measurement of the amount of data transmitted within a specified time period, usually measured in bits per second (bps).
          Transmission delay: it is the delay, which is present due to link capacity
          Propagation delay: Propagation delay is the time between the last bit transmitted at the head node of the link and the last bit received at the tail node.
          Interference: interference occurs when undesirable electromagnetic waves affect the signal. Interference can be caused by many factors, including
          Electromagnetic Interference (EMI)
          Radio wave interference (RFI)

          Noise : Noise is unwanted energy from sources other than transmitter. Thermal noise is caused by random motion of the electrons in a wire and is unavoidable. Intermodulation Interference Occurs whenever signals of different frequencies share the same medium. Cross Talk is caused by inductive coupling between two wires. Impulse noise caused by spikes on the power line or other causes.