OSI

OSI model
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The Open Systems Interconnection model (OSI Model) will group
communication functions into seven logical layers. A layer serves the layer
above it and is served by the layer below it. For example, a layer that
provides error-free communications across a network provides the path needed by
applications above it, while it calls the next lower layer to send and receive
packets that make up the contents of that path.


Layer 1: Physical Layer
(cable)

The physical layer has the following major functions:

It defines the electrical and physical specifications of the
data connection. It defines the relationship between a device and a physical
transmission medium (e.g., a copper or fiber optical cable). This includes the
layout of pins, voltages, line impedance, cable specifications, signal timing,
hubs, repeaters, network adapters, host bus adapters (HBA used in storage area
networks) and more.

It defines the protocol to establish and terminate a connection
between two directly connected nodes over a communications medium.

Layer 2: Data Link
Layer
(switches operates on)

The data link layer provides node-to-node data transfer -- a
reliable link between two directly connected nodes, by detecting and possibly
correcting errors that may occur in the physical layer. The data link layer is
divided into two sub layers:

The Point-to-Point Protocol (PPP) is an example of a data link
layer in the TCP/IP protocol stack.

The ITU-T G.hn standard, which provides high-speed local area
networking over existing wires (power lines, phone lines and coaxial cables),
includes a complete data link layer that provides both error correction and
flow control by means of a selective-repeat sliding-window protocol.

Layer 3: Network Layer(router works on, ip address)

The network layer provides the functional and procedural means
of transferring variable length data sequences (called datagrams) from one node
to another connected to the same network. It translates logical network address
into physical machine address.

A network is a medium to which many nodes can be connected, on
which every node has an address and which permits nodes connected to it to
transfer messages to other nodes connected to it by merely providing the
content of a message and the address of the destination node and letting the
network find the way to deliver ("route") the message to the
destination node. In addition to message routing, the network may (or may not)
implement message delivery by splitting the message into several fragments,
delivering each fragment by a separate route and reassembling the fragments,
report delivery errors, etc.


Layer 4: Transport Layer
(deals with the data that back and
forth)

The transport layer provides the functional and procedural means
of transferring variable-length data sequences from a source to a destination
host via one or more networks, while maintaining the quality of service
functions.

An example of a transport-layer protocol in the standard
Internet stack is Transmission Control Protocol (TCP), usually built on top of
the Internet Protocol (IP).

The transport layer controls the reliability of a given link through
flow control, segmentation/desegmentation, and error control. Some protocols
are state- and connection-oriented. This means that the transport layer can
keep track of the segments and retransmit those that fail. The transport layer
also provides the acknowledgement of the successful data transmission and sends
the next data if no errors occurred. The transport layer creates packets out of
the message received from the application layer. Packetizing is a process of
dividing the long message into smaller messages.

Layer 5: Session Layer(create session bw your computer and the computer you are connection )

The session layer controls the dialogues (connections) between
computers. It establishes, manages and terminates the connections between the
local and remote application. It provides for full-duplex, half-duplex, or
simple x operation, and establishes check pointing, adjournment, termination,
and restart procedures. The OSI model made this layer responsible for close of
sessions, which is a property of the Transmission Control Protocol, and also
for session check pointing and recovery, which is not usually used in the
Internet Protocol Suite. The session layer is commonly implemented explicitly
in application environments that use remote procedure calls.

Layer 6: Presentation
Layer
(operation system works on)

The presentation layer establishes context between
application-layer entities, in which the application-layer entities may use
different syntax and semantics if the presentation service provides a big
mapping between them. If a mapping is available, presentation service data
units are encapsulated into session protocol data units, and passed down the
protocol stack.

This layer provides independence from data representation (e.g.,
encryption) by translating between application and network formats. The
presentation layer transforms data into the form that the application accepts.
This layer formats and encrypts data to be sent across a network

Layer 7: Application Layer(Firefox, chrome, Skype)

The application layer is the OSI layer closest to the end user,
which means both the OSI application layer and the user interact directly with
the software application. This layer interacts with software applications that
implement a communicating component. Such application programs fall outside the
scope of the OSI model. Application-layer functions typically include
identifying communication partners, determining resource availability, and
synchronizing communication.
When identifying communication partners, the application layer determines the
identity and availability of communication partners for an application with
data to transmit. When determining resource availability, the application layer
must decide whether sufficient network or the requested communication exists.

时间: 2024-10-16 04:47:23

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