1 The application layer is the only

1 The application layer is the only part of a communications process that a user sees, and even then, the user doesn't see most of the work that the application does to prepare a message for sending over a network. The layer converts a message's data from human-readable form into bits and attaches a header identifying the sending and receiving computers.

2 The presentation layer ensures that the message is transmitted in a language that the receiving computer can interpret (often ASCII). This layer translates the language, if necessary, and then compresses and perhaps encrypts the data. It adds another header specifying the language as well as the compression and encryption schemes.

3 The session layer opens communications and has the job of keeping straight the communications among all nodes on the network. It sets boundaries (called bracketing) for the beginning and end of the message, and establishes whether the messages will be sent half-duplex, with each computer taking turns sending and receiving, or full-duplex, with both computers sending and receiving at the same time. The details of these decisions are placed into a session header.

4 The transport layer protects the data being sent. It subdivides the data into segments, creates checksum tests - mathematical sums based on the contents of data - that can be used later to determine if the data was scrambled. It can also make backup copies of the data. The transport header identifies each segment's checksum and its position in the message.

5 The network layer selects a route for the message. It forms data into packets, counts them, and adds a header containing the sequence of packets and the address of the receiving computer.

6 The data-link layer supervises the transmission. It confirms the checksum, then addresses and duplicates the packets. This layer keeps a copy of each packet until it receives confirmation from the next point along the route that the packet has arrived undamaged.

7 The physical layer encodes the packets into the medium that will carry them - such as an analogue signal, if the message is going across a telephone line - and sends the packets along that medium.

8 An intermediate node calculates and verifies the checksum for each packet. It may also reroute the message to avoid congestion on the network.

9 At the receiving node, the layered process that sent the message on its way is reversed. The physical layer reconverts the message into bits. The data-link layer recalculates the checksum, confirms arrival, and logs in the packets. The network layer recounts incoming packets for security and billing purposes. The transport layer recalculates the checksum and reassembles the message segments. The session layer holds the parts of the message until the message is complete and sends it to the next layer. The presentation layer expands and decrypts the message. The application layer converts the bits into readable characters, and directs the data to the correct application.

Kết quả (Trung) 3:

1应用层的通信过程，用户看到的只是一部分，即使这样，用户不会看到大部分的工作，应用程序编写的一个消息发送到网络。该层的消息转换的数据从人类可读的形式成小块，将头标识发送和接收计算机

。2表示层保证消息被接收计算机可以解释语言的传播（通常是ASCII）。这层转换的语言，如果有必要，然后压缩或加密数据。它增加了一个表头指定语言以及压缩和加密方案

。3会话层打开通讯具有保持直的网络上的所有节点之间的通信工作。这集的边界（称为包围）的开始和结束的信息，建立信息是否会发送半双工，每个计算机轮流发送和接收，或全双工，随着计算机发送和接收在同一时间。这些决定的细节都放在一个会话报头。

4传输层保护被传输的数据。它将数据分成段，创建校验和试验数据的基础上，以后可以用来确定数据是否是加密的内容的数学总和。它也可以让数据备份。传输报头标识每个段的校验和消息中的位置。

5网络层路由选择信息。它的形式的数据包，统计，并增加了一个表头含有数据包的顺序和接收计算机的地址。

6数据链路层负责传输。这证实了校验和，然后地址和重复的数据包。这层保持复制的每个数据包，直到它接收到沿着包完好无损地运到了路线下点确认。

7物理层的编码数据包为媒介，将它们如模拟信号，如果信息是要隔着电话线将沿介质

包。8中间节点计算和验证每个数据包的校验和。它也可能把消息以避免网络拥塞。

9在接收节点，分层过程中发送消息的方式是相反的。物理层再把信息分成小块。数据链路层重新计算校验和，确认到达，并在包的日志。网络层安全和计费的目的了传入的数据包。传输层重新计算校验和，将报文段。会话层保持消息的部分直到消息是完整的并且将它发送到下一层。表示层的扩展和解密消息。应用层将比特转换成可读的字符，并将数据正确应用。