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<sect1 id="OSI">
The OSI (Open Systems Interconnect) reference model, developed by the ISO
(International Standards Organisation) in 1977, is a conceptual model for
network communications. The model consists of seven layers with each layer
consisting of a number of protocols that are used to communicate with
protocols at the same layer on other nodes of the network. Upper-layer
protocols use the services offered by lower-layer protocols to transmit
and process data. The seven layers are outlined below:
<varlistentry><term>Layer 1 - Physical</term>
This layer deals with the network media and the hardware that supports it:
repeaters, hubs, connectors, network interface cards, etc.... The
transmission media, connectors, and topologies used by various network
architectures (such as Ethernet) are defined at the physical layer.
This layer is responsible for translating bits of binary data from the
upper layers into signals to be sent over the transmission media, and
converting incoming signals into bits to be sent to the data link layer.
This layer also defines signalling methods to be used on the network media.
Analog, or broadband, signalling modulates the signals into frequencies;
different frequencies can be used simultaneously as data channels. Digital,
or baseband, signalling uses high and low voltage levels to represent binary
1s and 0s. Most LAN architectures use baseband signaling.
The physical layer also deals with bit synchronisation. Communications can
be asynchronous (bits are sent in a single channel, and transitions from
high to low voltage levels can indicate the division between bits) or
synchronous (a seperate clock signal is sent to indicate when each bit
arrives in the data channel). Fast Ethernet, RS232, and ATM are protocols
with physical layer components.
<varlistentry><term>Layer 2 - Data Link</term>
Whereas the physical layer deals strictly with bits of data, the data
link data organises data into groups called frames. Frames include a
header that defines the hardware address of the node. This address is
also called a MAC (Media Access Control) address, or physical address.
Hardware addresses are set in network interface cards (NICs). Most NICs
have a unique hardware address preprogrammed into them by the manufacturer.
Some models allow you to set the hardware address using switches or a
configuration utility. Although some network cards allow you to change
the MAC address, this is rarely necessary. In addition, changing MAC
addresses may result in duplicated addresses which cause network
communication problems. The data link layer is also responsible for
error and flow control between nodes. Error control involves adding
a checksum (Cyclic Redundancy Check, or CRC) to the data so that it
can be verified at the other end. Flow control ensures that data is
sent only when the receiving device is ready to receive it.
The data link layer is further subdivided by the IEEE 802 standards. A
bridge is a device that works with the data link layer to filter
information and send it between network segments.
<varlistentry><term>Layer 3 - Network</term>
The network layer adds additional headers to the frames from the data
link layer, forming packets. The network layer headers define a logical
address (such as an IP address of IPX address). This layer also
translates between physical (MAC) and logical (network) addresses.
The other major responsibility of the network layer is routing. Routing
consists of forwarding packets to the network segment of their destination,
possibly through one or more intermediate nodes. Routers are hardware
devices that work at the network layer.
<varlistentry><term>Layer 4 - Transport</term>
The transport layer is responsible for assembling packets into their proper
sequence, checking them for errors, and passing them on the session layer.
Aacknowledgements are sent to indicate that the data has been received, and
retransmissions are requested for objects that are bit received correctly.
When transmitting, this layer breaks large messages into packets of the
appropiate size for the network, and passes them on the network layer.
<varlistentry><term>Layer 5 - Session</term>
This layer maintains a session, or connection, between two nodes on the
network. This layer is responsible for requesting connections, sending
periodic messages to maintain the connection, and tearing down the
connection when communication is finished. Protocols at the session layer
also determine which nodes are currently allowed to send data. Services
are required to establish connections, such as name resolution and
security, are also handled by the session layer.
<varlistentry><term>Layer 6 - Presentation</term>
The presentation layer is responsible for tranlating data sent by the
application layer into the proper format for network communication, and
translating data received from the network into a format the application
can understand. If used, compression and encryption are also handled at
this level.
<varlistentry><term>Layer 7 - Application</term>
The application layer is the interface network-aware applications use to
access the network. This layer controls access by applications to the
network, and is responsible for informing an application when a network
error has occured. Certain higher-level protocols that can be used by
simple client applications (such as FTP and NFS) also operate at this
level. A user is most likely to recognise this layer through the programs
that they use to interface to the network. For example, through a web
browser such as Microsoft Internet Explorer or its open source alternative,