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Virtually all networks in use today
are based in some fashion on the Open Systems Interconnection (OSI) standard.
OSI was developed in 1984 by the International Organization for Standardization
(ISO), a global federation of national standards organizations representing
approximately 130 countries.
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The core of this standard is the OSI Reference
Model, a set of seven layers that define the different stages that
data must go through to travel from one device to another over a
network. As you read below you find out all about the OSI
standard.
The Layers - Think of the seven layers as the assembly line in the
computer. At each layer, certain things happen to the data that
prepare it for the next layer. The seven layers, which separate
into two sets, are:
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Application Set
Layer 7: Application - This is the layer that actually interacts with the operating system or application whenever the user chooses to
transfer files, read messages or perform other network-related activities.Layer 6: Presentation - Layer 6 takes the data provided by the
Application layer and converts it into a standard format that the
other layers can understand.
Layer 5: Session - Layer 5 establishes,
maintains and ends communication with the receiving device.
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- Transport Set
Layer 4: Transport - This layer maintains flow control of data and
provides for error checking and recovery of data between the
devices. Flow control means that the Transport layer looks to see
if data is coming from more than one application and integrates
each application's data into a single stream for the physical
network.
Layer 3: Network - The way that the data will be sent to the
recipient device is determined in this layer. Logical protocols,
routing and addressing are handled here.
Layer 2: Data - In this layer, the appropriate physical protocol
is assigned to the data. Also, the type of network and the packet
sequencing is defined.
Layer 1: Physical - This is the level of the actual hardware. It
defines the physical characteristics of the network such as
connections, voltage levels and timing.-
- The OSI
Reference Model is really just a guideline. Actual protocol stacks
often combine one or more of the OSI layers into a single layer.
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- Protocol Stacks
A protocol stack is a group of protocols that all work together to
allow software or hardware to perform a function. The TCP/IP
protocol stack is a good example. It uses four layers that map to
the OSI model as follows:
Layer 1: Network Interface - This layer combines the Physical and
Data layers and routes the data between devices on the same
network. It also manages the exchange of data between the network
and other devices.
Layer 2: Internet - This layer corresponds to the Network layer.
The Internet Protocol (IP) uses the IP address, consisting of a
Network Identifier and a Host Identifier, to determine the address
of the device it is communicating with.
Layer 3: Transport - Corresponding to the OSI Transport layer,
this is the part of the protocol stack where the Transport Control
Protocol (TCP) can be found. TCP works by asking another device on
the network if it is willing to accept information from the local
device.
Layer 4: Application - Layer 4 combines the Session, Presentation
and Application layers of the OSI model. Protocols for specific
functions such as e-mail (Simple Mail Transfer Protocol, SMTP) and
file transfer (File Transfer Protocol, FTP) reside at this level.
As you can see, it is not necessary to develop a separate layer
for each and every function outlined in the OSI Reference Model.
But developers are able to ensure that a certain level of
compatibility is maintained by following the general guidelines
provided by the model. |
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Copyright (c) 1995-2008 On-Call Computer Services of Edmond Oklahoma All Rights Reserved
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- Last update on:
08/15/2008
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