The OSI (Open System Interconnection) model is
developed by ISO(International Organization for standardization) in 1984 to provide a reference model for the complex aspects
related to network communication. It divides the different functions and
services provided by network hardware and software in 7 layers. This
facilitates modular engineering, simplifies teaching and learning network technologies,
helps to isolate problems and allows vendors to focus on just the layer(s) in
which their hardware or software is implemented and be able to create products
that are compatible, standardized and interoperable.
The diagram below shows the 7 layers of the OSI
Model, to remember them in the correct order a common mnemonic is often used: All People Seem To NeedData Processing.
The Application, Presentation and Session layer
are known as the Upper Layer and
are implemented in software. The Transport and Network layer are mainly
concerned with protocols for delivery and routing of packets to a destination
and are implemented in software as well. The Data Link is implemented in hard-
and software and the Physical layer is implemented in hardware only, hence its
name. These last two layers define LAN and WAN specifications.
7.APPLICATION LAYER
The Application layer provides network services directly to the user's
application such as a web browser, email software and Windows Explorer. This
layer is said to be "closest to the user".
Protocols that operate on this layer include: TELNET, HTTP, FTP, TFTP, SMTP,
NTP, SNMP, EDI.
6.PRESENTATION LAYER
This layer 'represents' the data in a particular format to the Application
layer. It defines encryption, compression, conversion and other coding
functions.
Specifications defined at this layer include: GIF, TIFF, JPEG, MPEG, MIME, and
ASCII.
5.SESSION LAYER
Establishes, maintains and terminates end-to-end connections (sessions) between
two applications on two network nodes. It controls the dialogue between the
source and destination node, which node can send when and how long. Also
provides error reporting for the Application, Presentation and Session layer.
Protocols/API's that operate on this layer include: RPC, SQL, NETBIOS.
4.TRANSPORT LAYER
This layer converts the data received from the upper layers into segments. The
Transport layer is responsible for end-to-end (also called
source-to-destination) delivery of entire messages. Provides end-to-end
connectivity, it allows data to be transferred reliably and sequencing to
guarantee that it will be delivered in the same order that it was sent.
Provides services such as error checking and flow control (software).
Protocols that operate on this layer: TCP, UDP, NETBEUI, SPX.
These protocols are either connectionless or connection-oriented:
Connection-oriented means
that a connection (a virtual link) must be established before data can be
exchanged. This can guarantee that data will arrive, and in the same order it
was sent. It guarantees delivery by sending acknowledgements back to the source
when messages are received. TCP is an example of an connection-oriented
transport protocol.
A common example of connection-oriented communication is a telephone call: you
call, the 'destination' picks up the phone and acknowledges and you start
talking (sending data). When a message or a piece of it doesn't arrive, you
say: "What!?" and the sender will retransmit the data.
Connectionless is the opposite of connection-oriented; the
sender does not establish a connection before it sends data, it just sends
without guaranteeing delivery. UDP is an example of an connectionless transport
protocol.
3.NETWORK LAYER
This layer converts the segments from the Transport layer into packets (or
datagrams) and is responsible for path determination, routing, and the delivery of
these individual packets across multiple networks without guaranteed delivery.
The network layer treats these packets independently, without recognizing any
relationship between those packets, it relies on upper layers for reliable
delivery and sequencing.
Also this layer is is responsible for logical
addressing (also known as network addressing or Layer 3
addressing) for example IP addresses
Examples of protocols defined at this layer: IP, IPX, AppleTalk, ICMP, RIP,
OSPF, BGP, IGRP, EIGRP, NLSP, ARP, RARP, X.25
Devices that operate on this layer: Routers, Layer 3 Switches.
Network layer
addresses
Also known as Layer 3 or Logical addresses. These type of addresses are
protocol-dependent, for example if the network protocol is IP, IP addressing
will be used which is made up of a network part and a host part and needs a
subnet mask to determine the boundaries of these parts. An example of an IP
address is: 172.16.0.1 and a subnet mask: 255.255.0.0
Another example is Novell's IPX addressing, which uses a combination of a
hexadecimal network address + the layer 2 MAC address to form a network layer
address, for example" 46.0010E342A8BC
2.DATA LINK LAYER
The Data Links provides transparent network services to the Network layer so
the Network layer can be ignorant about the physical network topology and and
provides access to the physical networking media. Responsible for reassambling
bits taken of the wire by the Physical layer to frames, makes sure they are in
the correct order and requests retransmission of frames in case an error
occurs. Provides error checking by adding a CRC to the frame, and flow control. Examples of devices that operate on this layer
are switches, bridges, WAPs, and NICs.
DATA LINK LAYER SUB DIVIDED INTO TWO SUB LAYER:-
LLC sublayer
LLC is short for Logical Link Control. The
Logical Link Control is the upper sublayer of the Data Link layer. LLC masks
the underlying network technology by hiding their differences hence providing a
single interface to the network layer. The LLC sublayer uses Source Service
Access Points (SSAPs) and Destination Service Access Points (DSAPs) to help the
lower layers communicate to the Network layer protocols acting as an
intermediate between the different network protocols (IPX, TCP/IP, etc.) and
the different network types (Ethernet, Token Ring, etc.) This layer is also
responsible for frames sequencing and acknowledgements.
The LLC sublayer is defined in the IEEE standard 802.2.
MAC sublayer
The Media Access Control layer takes care of physical addressing and allows
upper layers access to the physical media, handles frame addressing, error
checking. This layer controls and communicates directly with the physical
network media through the network interface card. It converts the frames into
bits to pass them on to the Physical layer who puts them on the wire (and vice
versa)
IEEE LAN standards such as 802.3, 802.4, 802.5 and 802.10 define standards for
the MAC sublayer as well as the Physical layer.
Other standards on this layer include: X.25 and Frame Relay
Data Link layer
addresses
Also known as layer 2 addresses, BIAs (Burned-in Address), physical address and
most commonly referred to as MAC address. This is a fixed address programmed
into a NIC or a router interface for example.
00-10-E3-42-A8-BC is an example of a MAC address. The first 6 hexadecimal
digits (3 bytes) specify the vendor/manufacturer of the NIC, the other 6 digits
(3 bytes) define the host.
The layer 2 broadcast address is FF-FF-FF-FF-FF-FF.
1.PHYSICAL LAYER
This layer communicates directly with the physical media, it is responsible for
activating, maintaining and deactivating the physical link. It handles a raw
bits stream and places it on the wire to be picked up by the Physical layer at
the receiving node. It defines electrical and optical signaling, voltage
levels, data transmission rates and distances as well as mechanical
specifications such as cable lengths and connectors, the amount of pins and
their function.
Devices that operate on this layer: HUBs/concentrators, repeaters, NICs, and
LAN and WAN interfaces such as RS-232, OC-3, BRI, V.24, V.35, X.25 and Frame
Relay.
