The International Organization
introduced the OSI layer for Standardization (ISO) in 1984 in order to provide
a reference model to make sure products of different vendors would interoperate
in networks. OSI is short for Open System Interconnection.
The OSI layer shows WHAT needs to
be done to send data from an application on one computer, trough a network, to
an application on another computer, not HOW it should be done. A layer in the OSI model communicates with
three other layers: the layer above it, the layer below it, and the same layer
at its communication partner. Data transmitted between software programs passes
all 7 OSI layers. The Application, Presentation and Session layers are also
known as the Upper Layers.
The Data Link and Physical layers are often implemented together to
define LAN and WAN specifications.
Application Layer (Layer 7)
Application Layer provides network services directly to
applications. Type of software programs vary a lot: from groupware and web
browser to Tactical Ops (video game). Software programs itself are not part of
the OSI model. It determines the identity and availability of communication
partners, and determines if sufficient resources are available to start
program-to-program communication. This layer is closest to the user. Gateways
operate at this layer. Following are the examples of Application layer
protocols:
i)
Telnet
ii)
SMTP
iii)
FTP
iv)
SNMP
v)
NCP
vi)
SMB
Presentation Layer (Layer 6)
Presentation Layer defines coding and
conversion functions. It ensures that information sent from the application
layer of one system is readable by the application layer of another system. It
includes common data representation formats, conversion of character
representation formats, common data compression schemes, and common data
encryption schemes, common examples of these formats and schemes are:
i)
MPEG, QuickTime
ii)
ASCII, EBCDIC
iii)
GIF, TIFF, JPEG
Gateways operate
at this layer. It transmits data to lower layers.
Session Layer (Layer
5)
The session layer establishes,
manages, maintains and terminates communication channels between software
programs on network nodes. It provides error reporting for the Application and
Presentation layer. Examples of Session layer protocols are:
i)
NFS
ii)
SQL
iii)
RPC
iv)
Zone Information Protocol (ZIP)
Gateways
operate at this layer. It transmits data to lower layers.
Transport Layer
(Layer 4)
The main purpose of this layers is making
sure that the data is delivered error-free and in the correct sequence. It
establishes, maintains and terminates virtual circuits. It provides error
detection and recovery. It is concerned with reliable and unreliable transport.
When using a connection-oriented, reliable transport protocol, such as TCP,
acknowledgments is send back to the sender to confirm that the data has been
received. It provides Flow Control and Windowing. It provides multiplexing; the
support of different flows of data to different applications on the same host.
Examples of Transport layer protocols are:
i)
TCP (connection-oriented,
reliable, provides guaranteed delivery.)
ii)
UDP (connectionless, unreliable,
less overhead, reliability can be provided by the Application layer)
iii)
SPX
Gateways
operate at this layer. It transmits data to lower layers.
Network Layer (Layer
3)
This layer defines logical
addressing for nodes and networks/segments. It enables internetworking, passing
data from one network to another. It defines the logical network layout so
routers can determine how to forward packets trough an internet-work. Routing
occurs at this layer, hence Routed and Routing protocols reside on this layer.
Routed protocols are used to encapsulate data into packets. The header added by
the Network layer contains a network address so it can be routed trough an
internet-work. Examples of Network layer Routed protocols are:
i)
IP
ii)
IPX
iii)
AppleTalk
Routing protocols are used to
create routing tables; routing tables are used to determine the best path /
route. Routing protocols provide periodic communication between routers in an
Internet work to maintain information on network links in a routing table. It
transmits Packets. Routers operate at this layer. Examples of Network layer
Routing protocols are:
i)
OSPF
ii)
IGRP/EIGRP
iii)
RIP
iv)
BGP
v)
NLSP
Data Link Layer
(Layer 2)
It defines
psychical addressing, network topology, and is also concerned with error
notification, sequencing of frames and flow control. Examples of network
topologies are:
i)
Star
ii)
Bus
iii)
Ring
Physical Layer (Layer
1)
The physical layer defines the
electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between
communicating network systems. It transmits and receives bits (bit stream) to
transmission media. Physical layer specifications define characteristics such
as:
- Voltage
levels
- Timing
of voltage changes
- Physical
data rates
- Maximum
transmission distances
- Physical
connectors