网络层2015全解.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Data communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. However, it is impractical for two devices to be directly, point-to-point connected because of : - The devices are very far apart. - Some device may require a link to many of the others at various time. The solution is to attach each device to a communication network. The devices are refered to network stations. The stations may be computer, terminal, telephones, or other comunicating devices. Each station attaches to a network node. The set of nodes to which stations attach is a boundary of a communication network that is capable of tranferring data between pairs of stations. * * * * * * * * 15-745 Lecture 4b ? Seth Copen Goldstein 2001-2 * * * The OSI Layers 1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium. 2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control. 3.Network - Provides upper layers with independence from the data transmission and switching technoloogies used to connect systems; responsible for establishing, maintaining, and terminating connections. 4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control. 5.Session - Provides the control structure for communication bewteen applications; establishs, manages, and terminates connections (session) between cooperating applications. 6.Presentation - Provides undependence to the application processes from differrence in data representation (syntax). 7.Application - Provide acces