The OSI model is a standardized framework for network functions and schemes. It breaks down otherwise complex network interactions into simple elements, allowing developers to modularize design efforts. This method allows many independent developers to work on separate network functions that can be applied in a “plug-and-play” manner. | Page 406 Tuesday February 19 2002 3 48 PM General Concepts Defining Networks Several different types of users access the network from many locations Main office Most corporate information is located here. Everyone is connected to the LAN. Branch office Remote sites with a separate LAN access the main office through the WAN. Private residences Many employees work out of their homes which become part of the network. Other sites Mobile users can con nect from virtually anywhere. Hierarchical Model Cisco uses a hierarchical network model. High traffic loads create a need for efficient routing and switching techniques. Defining a Network s Key Points Cisco uses a hierarchical network model. The three layers are the access layer the distribution layer and the core layer Access layer Provides user connectivity to the network. Distribution layer Responsible for routing filtering and WAN access. Core layer Responsible for fast-switching services. OSI Model The OSI model is a standardized framework for network functions and schemes. It breaks down otherwise complex network interactions into simple elements allowing developers to modularize design efforts. This method allows many independent developers to work on separate network functions that can be applied in a plug-and-play manner. OSI Model Application User interface Telnet HTTP Presentation Encryption and other processing ASCII jpeg Session Manages multiple applications Operating systems Scheduling Page 407 Tuesday February 19 2002 3 48 PM OSI Model Continued Transport Provides reliable or unreliable delivery and some error correction TCP UDP SPX Network Provides logical addressing used by routers IP IPX Data link Creates frames from bits of data Uses MAC addresses to access endpoints Provides error detection but no correction HDLC Physical Specifies voltage wire speed and pinout cables EIA HA Protocol data units PDUs are used to communicate between layers. Encapsulation is the method
