Tuesday, May 28, 2013
When
computers were first linked together into networks, moving information
between different types of computers was a very difficult task.
In
the early 1980s, the International Standards Organization (ISO)
recognized the need for a standard network model. This would help
vendors to create interpretable network devices. The Open Systems
Interconnection (OSI) reference model, released in 1984, to addressed
these needs.
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 Need Data 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.
Application
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,
Protocols that operate on this layer include: TELNET, HTTP, FTP, TFTP, SMTP, NTP, SNMP,
Presentation
This
layer 'represents' the data in a particular format to the Application
layer (masks the difference of data format between two dissimilar
systems). It also defines encryption, compression, conversion and other
coding functions.
Specifications defined at this layer include: GIF, TIFF, JPEG, MPEG, MIME, and ASCII.
Specifications defined at this layer include: GIF, TIFF, JPEG, MPEG, MIME, and ASCII.
Session
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.
Transport
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.
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.
A common example of connectionless communication is a Radio City, News paper, Television
Network
This
layer converts the segments from the Transport layer into packets 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 and also this layer 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
Devices that operate on this layer: Routers, Layer 3 Switches.
Data Link
The
Data Links provides transparent network services to the Network layer
so the Network layer can be ignorant about the physical network topology
and provides access to the physical networking media. Responsible for
reassembling 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
Data Link layer has two sub layers: the LLC and the MAC sub layer.
LLC sublayer :
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. This layer is
also responsible for frames sequencing and acknowledgements.
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)
Physical
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: HUB, repeaters and WAN interfaces such as RS-232,, BRI, V.24, V.35, X.25 and Frame Relay.
Summary
What basically happens when data passes from Host A to Host B:
1. Application, Presentation and Session layer take user input and converts it into data,
2. Transport layer adds a segment header converting the data into segments,
3. Network layer adds a network header and converts the segments into packets ,
4. Data Link layer adds a frame header converting the packets into frames,
5. MAC sub layer converts the frames into a bits which the Physical layer can put on the wire.
The steps are known as the 5 steps of data encapsulation. When the bits stream
arrives at the destination, the Physical layer takes it of the wire and converts it
into frames, each layer will remove their corresponding header while the data
flows up the OSI model until it is converted back to data and presented to the
user, this is known as de-capsulation.
