Custom Search

Free GRE words

Tuesday, June 2, 2009

network tutorials and peer to peer communication

Basic Network tutorial

Networking evolves to support current and future application. The OIS reference model organizes network functions into seven categories called layers. Data flows from upper-level user applications to lower-level bits transmitted over network media. Peer-to-peer functions use encapsulation and de-encapsulation to layer interfaces. Most network manager tasks configure the lower three layers. The enterprise is a corporation, agency, school, or other ogranization that will tie together its data, communication, computing, and file servers.
Developments on the enterprise network include:
• LANs interconnected to provide access to computers or file servers in other locations.
• End-user needs for higher bandwidth on the LANs.
• Relaying technologies for WAN service.

Layer Functions
Each layer of the OSI model serves a specific function. Those functions are defined by the OSI and can be used by any network products vendor. The functions are:
1. Application – The application layer provides network services to user applications. For example, a word processing application is serviced by file transfer services at this layer.
2. Presentation – This layer provides data representation and code formatting. It ensures that the data that arrives from the network can be used by the application, and it ensures that information sent by the application can be transmitted on the network.
3. Session – This layer establishes, maintains, and manages sessions between applications.
4. Transport – This layer segements and reassembles data into a data stream. This layer uses the TCP protocol.
5. Network – This layer determines the best way to move data from one place to another. The router operates at this layer. This layer uses the Internet Protocol addressing scheme.
6. Data link – This layer provides physcial transmission across the medium. It handles error notification, network topology, and flow control. This layer uses the Media Access Control (MAC) address.
7. Physical – This layer provides the electrical, mechanical, procedural, and functional means for activating and maintaining the physical link between systems. This layer uses the physical media like twisted pair, coaxial, and fiber-optic cable.
Host Layers: provide accurate data delivery between computers.
Media Layers: control physcial delivery of messages over the network.

Peer to Peer Communication
Each layer uses its own layer protocol to communicate with its peer layer in the other system. Each layer’s protocol exchanges information, called protocol data units (PDUs), between peer layers. A given layer can use a more specific name for its PDU. For example, in TCP/IP the transport layer of TCP communicates with the peer TCP function using segments. This peer-layer protocol communication is achieved by using the services of the layers below it. The layer below any current layer provides its services to the current layer. Each lower-layer service takes upper-layer information as part of the lower layer PDUs it exchanges with its layer peer.
Thus, the TCP segments become part of the network layer packets (also called datagrams) exchanged between IP peers. In turn, the IP packets must become part of the data link frames exchanged between directly connected devices. Ultimately, these frames must become bits as the data is finally transmitted by the physical-layer protocol using hardware.

Data Encapsulation
Each layer depends on the service function of the ISO/OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it can add whatever headers and trailers the layer will use to perform its function.
For example, the network layer provides a service to the transport layer, and the transport layer presents “data” to the internetwork subsystem. The network layer has the task of moving that data through the internetwork. It accomplishes this task by encapsulating the data within a header. This header contains information required to complete the transfer, such as source and destination logical addresses.
The data link layer in turn provides a service to the network layer. It encapsulates the network layer information in a frame. The frame header contains information required to complete the data link functions.  For example, the frame header contains physcial addresses.
The physcial layer also provides a service to the data link layer. This service includes encoding the data link frame into a pattern of ones and zeros for transmission on the medium (usually a wire).
Data Encapsulation Example
As networks perform services for users, the flow and packaging of the information changes. In this example of internetworking, five conversion steps occur:
Step 1: As a user sends an email message, its alphanumeric characters are converted to use the internetwork. This is the data.
Step 2: One change packages the message “data” for the internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.
Step 3: The next change prepares tha data so they can use the internetwork by putting the data into a packet or datagram that contains a network header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.
Step 4: Each network device must put the packet into a frame. The frame allows connection to the next directly connected network device on the link. Each device in the chosen network path requires framing to connect to the next device.
Step 5: The frame must be converted into a pattern of 1s and 0s for transmission on the medium (usually a wire). Some clocking function enables the devices to distinguish these bits as they tranverse the medium.The medium on the physical internetwork can vary along the path used. For example, the email message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN.

No comments: