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Networking Models

Networking Models

Networking Models 

In a network, we work over numbers of machines, adapters etc., that are manufactured by different- different vendors.

That’s why we need some standards. Standards organization such as the International Standards Organization (ISO)
the Institute of Electrical and Electronics Engineers, Inc (IEEE) have developed models that have become globally recognized and accepted as standards for designing Computer Networks.
The OSI Model—
As the concept of networking became more widespread in the business world, the idea of being able to connect networks and disparate systems became a necessity.
For this type of communication to take place, however, there needed to be a standard.
The solution came in 1878 when the ISO released an architecture that would allow achieve this goal.
These specifications were revised in 1884 and became international standards for networked communication and connections.
it is important for network administrators to know the history and understand the function of this specification, which is called the OSI Reference Model.
The OSI model presents a layered approach to networking. Each layer of the model handles a different portion of the communications process.
By separating such communications into layers, the OSI model simplified how network hardware and software work together, as well as eased troubleshooting by providing a specific method for how components should function.
Now that we know why the model was implemented, let’s move on to explore just how it works. Keep in mind that the OSI model is a completely conceptual reference.
The OSI model
7.Application
6.Presentation
5.Session
4.Transport
3.Network
2.Data link
1.Physical
Layers in the OSI model-
Physical Layer:
Layer 1:
The Physical layer converts bits into signals (like electromagnetic pulse etc.) for outgoing messages, and signals into bits for incoming ones.
This layer arranges the transmission of a data frame’s bits when they are dispatched across the network.
The Physical layer manages the interface between a computer and the network medium, and instructs the driver software and the network interface as to what needs to be sent across the medium.
Physical layer is concerned with the following.
1) Physical characteristic of interfaces and media.
2) Representation of bits (encoding).
3) Data rate (Transmission rate).
4) Synchronization of bits.
5) Physical topology.
6) Transmission mode.
Data Link Layer
Layer2 is the data link layer.
This layer places data frames from the network layer onto the physical medium, the data link layer is responsible for providing the error free transfer of these frames from one computer to another through the physical layer.
Destination ID Sender ID Control Data CRC
Destination ID-
the unique identifier of the workstation to which the information is being sent.
Sender ID-
The sender ID contains the unique identifier of the workstation sending the information.
Control-
The control information is used for frame type, routing, and segmentation information.
Data-
The data is the information itself.
CRC-
The cyclical redundancy check (CRC) contains error correction and verification information to ensure that the data frame is received properly.
Specific responsibilities of the data link layer include the following:
1. Framing
2. Physical addressing
3. Flow control
4. Error control
5. Access control
Network Layer-
Layer3, the network layer, is responsible for addressing messages and translating logical addresses and names into physical addresses.
This layer determines which path the data should take from the source to the destination computer based on network conditions priority of service, and other factors.
Specification responsibilities of the network layer include the following
1) Logical Addressing
2) Routing
Transport Layer
The transport layer ensures that messages are delivered error free, in sequence and with no losses or duplications.
Layer 4 repackages messages dividing long messages into several packets and collecting messages together in one package, to provide for their efficient transmission over the network.
Responsibilities of Transport layer
1) Service-point addressing.
2) Segmentation and reassembly.
3) Connection control
4) Flow control
5) Error control.
Session Layer
Layer 5, The Session layer allows two networked resources to hold ongoing communications, called a session, across a network.
In other words, applications on each end of the session are able to exchange data for the duration of the session (connection).
This layer manages session setup, information or message exchanges, and turn off when the session ends.
It is also responsible for identification so that only designated parties can participate in the session, and handles security services for controlling access to session information.
The Session layer furnishes synchronization services between tasks at each end of the session. This layer places checkpoints in the data stream so if communications fail, only data after the most recent checkpoint need be retransmitted.
The Session layer also manages issues such as who may transmit data at a certain time and for how long, and maintains a connection through transmission of messages that keep the connection active; these messages are designed to keep the connection to be closed down due to inactivity.
Works of Session layer
1) Dialog control.
2) Synchronization.
Presentation Layer
Layer 6, The Presentation layer manages data-format information for networked communications.
. Also called the network’s translator, it converts outgoing messages into a generic format that can be transmitted across a network; then, it converts incoming messages from that generic format into one that makes sense to the receiving application.
This layer is also responsible for protocol conversion, data encryption and decryption, and graphics commands.
Information sent by the Presentation layer may sometimes be compressed to reduce the amount of data to be transferred (this also requires decompression on the receiving end).
It is at this layer that a special software facility known as a redirector operates.
The redirector intercepts requests for service and redirects requests that cannot be resolved locally to the networked resource that can handle them.
A utility called the redirector operates at this level. Its purpose is to redirect Input/Output operations to resources outside the local computer.
Works of presentation layer
1) Translation.
2) Encryption.
3) Compression.
Application Layer
Layer 7 of the OSI model, The Application layer is referred to as the top layer of the OSI Reference Model.
This layer allows access to network services—such as networked file transfer, message handling, and database query processing—that support applications directly.
This layer also controls general network access, the transmission of data from sender to receiver (called flow control), and error recovery for applications when appropriate.
Works of Application layer
1) Network virtual terminal.
2) File transfer, access and management.
3) Mail services.
4) Directory services.
In the OSI model the purpose of each layer is to provide services to the next higher layer and shield the upper layer from the details of how the services are actually implemented.
Relationship between OSI Model layers.
The layers are setup in such a way that each layer acts as though it were communicating with its associated layer on the other computer.
This is logical communication between peer layers.
Physical communication takes place between adjacent layers on one computer. Only the lowest layer in the networking model can pass information directly to its counterpart on one computer.
Information on the sending computer passes through all the lower layers.
The information that moves across the networking cable to the receiving computer and up that computer’s networking layers until arriving at the same level that sent the information.
An exchange of data using the OSI Model:
As data passes from the application through the OSI layers, each layer wraps the data with layer-specific information.
This information, in the form of headers and/or trailers, is read later by the corresponding layer on the receiving computers.
The physical layer adds header information called the frame preamble to the outside of the frame and adds trailer information called the frame post amble to the outside of the frame, but it does not add to the data with in the frame.
Following figure gives an overall view of the OSI layer, l7 data means the data unit at layer 7, then moves from layer to layer in descending, sequential order at each layer (except layer 7 and 1), a header is added to the data unit , when the formatted data unit passes through the physical layer (layer 1),.
It is changed into an electromagnetic signal and transported along a physical link.
Upon reaching its destination, the signal passes into digital form.
The data units then move backup through the OSI layers. As each block of data reaches the next higher layer.
The headers and trailers attached to it at the corresponding sending layer are removed.
IEEE 802 Model:
Another networking model was developed by the Institute of Electrical and Electronic Engineer Inc.
IEEE’s project 802 defined LAN standards for the physical and data link layers of the OSI midel.the 802 project divided the Data-Link layer into the logical link control(LLC) and the Media access control (MAC)sublayers.
The 802 specifications fall into 12 different categories, each of which has its own number, as described in the following:
802.1 Internetworking
802.2 Logical Link Control (LLC)
802.3 Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) LANs (Ethernet)
802.4 Token Bus LAN
802.5 Token Ring LAN
802.6 Metropolitan Area Network (MAN)
802.7 Broadband Technical Advisory Group
802.8 Fiber Optic Technical Advisory Group
802.8 Integrated Voice and Data Networks
802.9 Network Security
802.10 Wireless Networks
802.12 Demand Priority Access LAN, 90BaseVG-AnyLAN
The LLC sublayer manages data link communication and defines the use to transfer information from the LLC sublayer to the upper OSI layers.
The MAC sublayer provides shared access between the computers’ network adapters and the physical layer.
The MAC sublayer is responsible for delivering error-free data between computers on a network.