This layer allows access to the net work services, such as printing, or messaging
(Services not to be confessed with application software; spreadsheets, data base programs, etc.)
This is the layer that is directly used by the work station user.
Also controls data flow and date error recovery.
All virtual terminal Software is handled here, by an editor.
Handles incompatibilities of file transfer.
Handles functions such as electronic mail, remote jobs entry, directory, lookup
This layer makes the transmission more manageable.
This layer arranges data into frames.
Transmits the frames and processes the acknowledgment frames sent by receiver.
Allows for error checking with the data frames; lost frames or damaged frames.
Each frame includes control information and indicate the beginning and end of the frame.
Provides service interface to the upper layer protocol.
Deals with buffering issues and variable speed issues between the nodes.
The OSI MODEL
The Open Systems Interconnection Model was developed by the International Standards Organization as a way to provide a common way of describing network protocols. Remember that this is a model not a protocol. The Model consists of 7 layers which are:
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Purpose of the OSI Model
To break down the complexity of a network into smaller and more manageable parts.
Provide standard interfaces between network parts or functions.
For symmetry in the functions performed by network nodes. This means each that performs the same function as the node’s counter part on another network.
Makes change the soft ware easier because of the flexibly provided by the network logic.
Provides standard jargon and terminology for software designers, network designers, technical support technicians users, manufactures, students, etc.
Below is a quick review of each followed by a few acronyms to help you remember them. This should suffice to refresh you memory, if you have already studied in class, or be enough knowledge for the network section of the A+ Examination. Keep in mind this files is intended as a quick reminder or general overview. Volumes of detailed information have been written on each one of these seven layers. People have gotten rich by merely specializing in just one them.
(Details are listed below)
Application Layer
Presentation Layer
Layer is responsible for the presentation or format of the data.
Layerwhere the character set information is determined.
Concerned with syntax and semantics of message being transmitted.
This layer also determines network security, file transfers.
It performs protocol conversion and manages data compression.
Session Layer
A session is all the procedures needed to transfer a singe file. One that session is over the next one begins.
This layer:
Makes, maintains, and ends communication sessions between applications on different computers.
In other words Administers by establishing connection, transferring data, releasing connections.
Connection Establishment:
Verifies user login and passwords.
Establish Connection ID numbers.
Agrees which services are required and the duration of.
Determines which node begins the conversation.
Coordinates numbering and re-transmission procedures.
Allows for Dialogue Control; Simples, Half-duplex, Full-duplex.
Transferring of Data:
Actual Transfer.
Acknowledgment of data receipt or negative acknowledgment
Resumes interrupted communications.
Transport Layer
This is top of the lower layers in the OSI Model.
This layer Accepts data from the Session Layer, splits it into chunks called datagrams.
Passes datagrams to the Network Layer.
Informs upper layers if data is not passed to the receiving node correctly.
This layer is responsible for reliable end to end communication of data.
Makes sure data frames are error free, controls data flow.
Trouble shoots any problems with sending or receiving data frames.
Provides for error correction.
Can also take several smaller messages, combining them into a larger single message.
Creates a distinct connection for each session sent by Session Layer.
May choose multiple network connections to speed throughput.
Network Layer
Provides necessary conditions for exchange of data between two specific points over the network.
Addresses messages and translates logical address in to physical addresses.
Allows node to node communications where intermediate systems may only be used only to pass the data.
Manages traffic through packet switching, routing and controlling the congestion of data.
Queries the intermediate nodes to determine best route between hosts that are not directly connected.
Judges the best network path for the data based on priority, network conditions and other variables.
Some writers liken this layer to a sort of traffic cop.
Provides a delivery service to the upper layers if the are unable to transfer with out Network Layer help.
Data link Layer
Physical Layer
The bottom layer of the OSI is the physical layer.
Provides the physical connection between the systems.
Determines whether data travels in a single direction or bi-directional.
Deals with electrical, optical, functional, and mechanical interfaces to the cable.
Defines how long each piece of data is, and the translation of data into the electrica.l pulses that are sent.
Notifies the data Link layer of any faults in the transmission.
Ways to Remember This Model
From top to bottom:
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Here is a way to remember form the bottom up...
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(a quick review in simple language.)
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Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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