<p> OSI Layers</p><p>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</p><p>1 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</p><p>2</p>