计算机网络第6章：OSI层次：会话、表示、应用层.ppt

* One way that communication processing takes place: When a browser opens, it is connected to the default page and the files of the page are transferred to the client. After the processing is completed, the connection is broken The second way: As Telnet and FTP, establish a connection to the server and maintain that connection until all processing is performed. The client terminates the connection when the user determines that he/she has finished. All communication activity falls into one of these two categories. Application Layer: Communication Ways * * * * * * * * * If you have any question about EBCDIC and ASCII, please refer to /computing/asciiebcdic.html * * * * * * * * * * 对网络管理很重要的一种应用程序 * 大致描述email收发过程，client—serverserverclient.回忆一下使用的端口smtp25 pop3110 * * * * * * * * * * OSI Layer 5-7:Application Layers OSI Layer 5-7: Application Layers The Session Layer The Presentation Layer The Application Layer Layer 5: The Session Layer This includes starting, stopping, and resynchronizing two computers who are having a rap session. two-way simultaneous communication? two-way alternate control? have synchronized the subjects of your conversations? The Session Layer Checkpoint is used to separate parts of a session, previously referred to as dialogues Dialogue separation is the orderly initiation, termination, and managing of communication. The Session Layer Some Applications of Layer 5 The Application Layers The Session Layer The Presentation Layer The Application Layer Layer 6 - The Presentation Layer The presentation layer is responsible for presenting data in a form that the receiving device can understand. The presentation layer has 3 main functions: Data formatting Data compression Data encryption Data Formatting Imagine two dissimilar systems. One uses Extended Binary Coded Decimal Interchange Code (EBCDIC) to format text The other uses American Standard Code for Information Interchange (ASCII) to format text Layer 6 provides the tra