【正文】 rastructure Module ICND—219 Hierarchical Campus Model ICND—220 Enterprise Composite Network Model ICND—221 OSI Model Overview Data Flow Layers Transport Layer Data Link Network Layer Physical Application (Upper) Layers Session Presentation Application ICND—222 Keeping different applications’ data separate User Interface ?How data is presented ?Special processing such as encryption Tel HTTP ASCII EBCDIC JPEG Operating System/ Application Access Scheduling Transport Layer Data Link Network Layer Physical EXAMPLES Session Presentation Application Role of Application Layers ICND—223 TCP UDP SPX / HDLC EIA/TIA232 IP IPX Presentation Application Session EXAMPLES Role of Data Flow Layers ? Reliable or unreliable delivery ? Error correction before retransmit ? Combines bits into bytes and bytes into frames ? Access to media using MAC address ? Error detection not correction ? Move bits between devices ? Specifies voltage, wire speed and pinout cables Transport Data Link Physical Network Provide logical addressing which routers use for path determination ICND—224 Transport Data Link Physical Network Upper Layer Data Upper Layer Data TCP Header Data IP Header Data LLC Header 0101110101001000010 Data MAC Header Presentation Application Session Segment Packet Bits Frame PDU FCS FCS Encapsulating Data ICND—225 Upper Layer Data LLC Hdr + IP + TCP + Upper Layer Data IP + TCP + Upper Layer Data TCP+ Upper Layer Data Upper Layer Data 0101110101001000010 Transport Data Link Physical Network Presentation Application Session Deencapsulating Data