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The Open Systems Interconnect (OSI) model has seven layers
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The layers are stacked this way:
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Application
Presentation
Session
Transport
Network
Data Link
Physical

PHYSICAL LAYER
The physical layer, the lowest layer of the OSI model, is concerned with the transmission and
reception of the unstructured raw bit stream over a physical medium
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It provides:
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Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to
better accommodate the characteristics of the physical medium, and to aid in bit and
frame synchronization
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Physical medium transmission: transmits bits as electrical or optical signals appropriate
for the physical medium, and determines:
o
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What physical medium options can be used
How many volts/db should be used to represent a given signal state, using a given
physical medium

DATA LINK LAYER:
The data link layer provides error-free transfer of data frames from one node to another over the
physical layer, allowing layers above it to assume virtually error-free transmission over the link
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Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are
available
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Frame acknowledgment: provides/expects frame acknowledgments
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Frame delimiting: creates and recognizes frame boundaries
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Media access management: determines when the node "has the right" to use the physical
medium
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It provides:
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Routing: routes frames among networks
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Frame fragmentation: if it determines that a downstream router's maximum transmission
unit (MTU) size is less than the frame size, a router can fragment a frame for
transmission and re-assembly at the destination station
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Subnet usage accounting: has accounting functions to keep track of frames forwarded by
subnet intermediate systems, to produce billing information
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This layer relieves the upper layers of the need to know anything about the data transmission and
intermediate switching technologies used to connect systems
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In the network layer and the layers below, peer protocols exist between a node and its immediate
neighbor, but the neighbor may be a node through which data is routed, not the destination
station
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TRANSPORT LAYER:
The transport layer ensures that messages are delivered error-free, in sequence, and with no
losses or duplications
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The size and complexity of a transport protocol depends on the type of service it can get from the
network layer
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If the network layer is unreliable and/or only supports datagrams, the transport
protocol should include extensive error detection and recovery
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The transport layer at the destination station reassembles the
message
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Message traffic control: tells the transmitting station to "back-off" when no message
buffers are available
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Typically, the transport layer can accept relatively large messages, but there are strict message
size limits imposed by the network (or lower) layer
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The transport layer header information must then include control information, such as message
start and message end flags, to enable the transport layer on the other end to recognize message
boundaries
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End-to-end layers:
Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the
transport layer and the layers above are true "source to destination" or end-to-end layers, and are
not concerned with the details of the underlying communications facility
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SESSION LAYER:
The session layer allows session establishment between processes running on different stations
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Session support: performs the functions that allow these processes to communicate over
the network, performing security, name recognition, logging, and so on
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It can be viewed
as the translator for the network
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The presentation layer provides:
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Character code translation: for example, ASCII to EBCDIC
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Data compression: reduces the number of bits that need to be transmitted on the network
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For example, password encryption
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This layer contains a variety of commonly needed functions:
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Resource sharing and device redirection
Remote file access
Remote printer access
Inter-process communication
Network management
Directory services
Electronic messaging (such as mail)
Network virtual terminals



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