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