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)
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
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
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
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
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
Presentation Layer Functions
 Translates data to a format the receiving node understands (eg, from EBCDIC to ASCII)
 Performs data encryption
 Performs data compression
 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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