Download - Open System Interconnection (OSI) Model
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Open System Interconnection (OSI) Model
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Objectives• Explain the OSI reference model, which
sets standards for LAN and WAN communications
• Discuss communication between OSI stacks when two computers are linked through a network
• Apply the OSI model to realistic networking situations
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OSI Reference Model• Foundation that brings continuity to LAN
and WAN communications• Product of two standards organizations:
• ISO• ANSI
• Developed in 1974• Set of communication guidelines for
hardware and software design
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OSI Guidelines Specify…• How network devices contact each other;
how devices using different protocols communicate
• How a network device knows when to transmit and not transmit data
• How physical network network devices are arranged and connected
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OSI Guidelines Specify…• Methods to ensure that network
transmissions are received correctly• How network devices maintain a consistent
rate of data flow• How electronic data is represented on
network media
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OSI Layers
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OSI Layers• Bottom layers
• Support for physical connectivity, frame formation, encoding, and signal transmission
• Middle layers• Establish and maintain a communication session
between two network nodes• Monitor for error conditions
• Uppermost layers• Application/software support for encrypting data and
assuring interpretation/presentation of data
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Physical Layer Functions• Provides transfer medium (eg, cable)• Translates data into a transmission signal• Sends signal along the transfer medium• Includes physical layout of network• Monitors for transmission errors• Determines voltage levels for data signal
transmissions and to synchronize transmissions• Determines signal type (eg, digital or analog)
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Analog Signals
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Digital Signals
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Data Link Layer Functions• Constructs data frames• Creates CRC information; checks for errors• Retransmits data if there is an error• Initiates communications link; makes sure it
is not interrupted (ensures node-to-node physical reliability)
• Examines device addresses• Acknowledges receipt of a frame
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Data Link Layer• Data link frame contains fields consisting of
address and control information• Two important sublayers
• Logical link control (LLC)• Media access control (MAC)
• Connectionless service versus connection-oriented service
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Network Layer Functions• Determines network path for routing
packets• Helps reduce network congestion• Establishes virtual circuits• Routes packets to other networks,
resequencing packet transmissions when needed
• Translates between protocols
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Transport Layer Functions• Ensures reliability of packet transmissions• Ensures data is sent and received in the same order• Sends acknowledgement when packet is received• Monitors for packet transmission errors and
resends bad packets• Breaks large data units into smaller ones and
reconstructs them at the receiving end for networks using different protocols
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Session Layer Functions• Establishes and maintains communications
link• Determines which node transmits at any
point in time• Disconnects when communication session
is over• Translates node addresses
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Presentation Layer Functions• Translates data to a format the receiving
node understands (eg, from EBCDIC to ASCII)
• Performs data encryption• Performs data compression
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Application Layer Functions• Enables sharing remote drivers and printers• Handles e-mail messages• Provides file transfer services• Provides file management services• Provides terminal emulation services
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Communicating Between Stacks
• OSI model provides standards for:• Communicating on a LAN• Communicating between LANs• Internetworking between LANs and WANs and
between WANs and WANs
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Peer Protocols
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Applying the OSI Model