Development of the Open Standards Interconnection (OSI) reference model was started in
1983 by an number of major computer and telecommunications companies. It was eventually
adopted as an international standard by the International Standards Organisation (ISO) and is
currently embodied within the ITU-TS X.200 Recommendation.
The model comprises 7 layers which define various functions involved in establishing and
servicing end-to-end communications circuits across a network. These 7 layers are generally
viewed in two blocks;
· Application Functional Layers. These are layers 4-7 of the OSI Model and relate to
the end-to-end functions between two or more users at the periphery of a network.
· Network Functional Layers. These are layers 1-3 of the OSI Model and refer to the
functions required to transport data across a network.
Layer 7: The application layer...This is the layer at which communication partners are
identified, quality of service is identified, user authentication and privacy are considered, and
any constraints on data syntax are identified. (This layer is not the application itself, although
some applications may perform application layer functions.)
Layer 6: The presentation layer...This is a layer, usually part of an operating system, that
converts incoming and outgoing data from one presentation format to another (for example,
from a text stream into a popup window with the newly arrived text). This layer is sometimes
called the syntax layer.
Layer 5: The session layer...This layer sets up, coordinates, and terminates conversations,
exchanges, and dialogs between the applications at each end. It deals with session and
connection coordination.
Layer 4: The transport layer...This layer manages the end-to-end control (for example,
determining whether all packets have arrived) and error-checking. It ensures complete data
transfer.
Layer 3: The network layer...This layer handles the routing of the data (sending it in the right
direction to the right destination on outgoing transmissions and receiving incoming
transmissions at the packet level). The network layer does routing and forwarding.
Layer 2: The data-link layer...This layer provides synchronization for the physical level and
does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge
and management.
Layer 1: The physical layer...This layer conveys the bit stream through the network at the
electrical and mechanical level. It provides the hardware means of sending and receiving data
on a carrier.
1983 by an number of major computer and telecommunications companies. It was eventually
adopted as an international standard by the International Standards Organisation (ISO) and is
currently embodied within the ITU-TS X.200 Recommendation.
The model comprises 7 layers which define various functions involved in establishing and
servicing end-to-end communications circuits across a network. These 7 layers are generally
viewed in two blocks;
· Application Functional Layers. These are layers 4-7 of the OSI Model and relate to
the end-to-end functions between two or more users at the periphery of a network.
· Network Functional Layers. These are layers 1-3 of the OSI Model and refer to the
functions required to transport data across a network.
Layer 7: The application layer...This is the layer at which communication partners are
identified, quality of service is identified, user authentication and privacy are considered, and
any constraints on data syntax are identified. (This layer is not the application itself, although
some applications may perform application layer functions.)
Layer 6: The presentation layer...This is a layer, usually part of an operating system, that
converts incoming and outgoing data from one presentation format to another (for example,
from a text stream into a popup window with the newly arrived text). This layer is sometimes
called the syntax layer.
Layer 5: The session layer...This layer sets up, coordinates, and terminates conversations,
exchanges, and dialogs between the applications at each end. It deals with session and
connection coordination.
Layer 4: The transport layer...This layer manages the end-to-end control (for example,
determining whether all packets have arrived) and error-checking. It ensures complete data
transfer.
Layer 3: The network layer...This layer handles the routing of the data (sending it in the right
direction to the right destination on outgoing transmissions and receiving incoming
transmissions at the packet level). The network layer does routing and forwarding.
Layer 2: The data-link layer...This layer provides synchronization for the physical level and
does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge
and management.
Layer 1: The physical layer...This layer conveys the bit stream through the network at the
electrical and mechanical level. It provides the hardware means of sending and receiving data
on a carrier.
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