Layer 1: Physical Layer
The
physical layer 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, radio frequency).
This
includes the layout of pins, voltages, line impedance, cable specifications,
signal timing and similar characteristics for connected devices and frequency
(5 GHz or 2.4 GHz etc.) for wireless devices.
It
is responsible for transmission and reception of unstructured raw data in a
physical medium. It may define transmission
mode as simplex, half duplex, and full duplex. It defines the network
topology as bus, mesh, or ring being some of the most common.
The
physical layer of Parallel SCSI operates in this layer, as do the physical
layers of Ethernet and other local-area networks, such as token ring, FDDI,
ITU-T G.hn, and IEEE 802.11 (Wi-Fi), as well as personal area networks such as
Bluetooth and IEEE 802.15.4.
The
physical layer is the layer of low-level networking equipment, such as some
hubs, cabling, and repeaters. The physical layer is never concerned with
protocols or other such higher-layer items. Examples of hardware in this layer
are network adapters, repeaters, network hubs, modems, and fiber media
converters.
Layer 2: Data Link Layer
The data link layer provides
node-to-node data transfer—a link between two directly connected nodes. It
detects and possibly corrects errors that may occur in the physical layer. It defines the protocol to
establish and terminate a connection between two physically connected devices.
It also defines the protocol for flow control between them.
IEEE
802 divides the data link layer into two sublayers:
Media
access control (MAC) layer – responsible for controlling how devices in a
network gain access to a medium and permission to transmit data.
Logical
link control (LLC) layer – responsible for identifying network layer protocols
and then encapsulating them and controls error checking and frame
synchronization.
The
MAC and LLC layers of IEEE 802 networks such as 802.3 Ethernet, 802.11 Wi-Fi,
and 802.15.4 ZigBee operate at the data link layer.
The
Point-to-Point Protocol (PPP) is a data link layer protocol that can operate
over several different physical layers, such as synchronous and asynchronous
serial lines.
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
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". 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 the message to the
destination node, possibly routing it through intermediate nodes. If the message
is too large to be transmitted from one node to another on the data link layer
between those nodes, the network may implement message delivery by splitting
the message into several fragments at one node, sending the fragments
independently, and reassembling the fragments at another node. It may, but need
not, report delivery errors.
Message
delivery at the network layer is not necessarily guaranteed to be reliable; a
network layer protocol may provide reliable message delivery, but it need not
do so.
A
number of layer-management protocols, a function defined in the management
annex, ISO 7498/4, belong to the network layer. These include routing
protocols, multicast group management, network-layer information and error, and
network-layer address assignment. It is the function of the payload that makes
these belong to the network layer, not the protocol that carries them.
Layer 5: Session Layer
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 simplex operation, and establishes check pointing,
adjournment, termination, and restart procedures. The OSI model made this layer
responsible for graceful 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
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 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. It is sometimes called the
syntax layer.
The
original presentation structure used the Basic Encoding Rules of Abstract
Syntax Notation One (ASN.1), with capabilities such as converting an
EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and
other data structures from and to XML.
Layer 7: Application Layer
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 resources for the
requested communication are available.
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