CCNA-OSI参考模型问题：

Which layer of the 7 layer model provides services to the application layer over the Session layer connection？

A.Network B.Transport C.Session D.Presentation E.Application F.Datalink

Select the 1 best answer（选择一个最佳答案）

在7层OSI模型中为应用层连接会话层及提供服务的是那一层？

解答：

在7层模型中分部情况是：物理层－数据链路层－网络层－传输层－会话层－表示层－应用层，在会话层和应用层之间是表示层，表示层提供的服务是：为异种机通信提供一种公共语言，以便能进行互操作。这种类型的服务之所以需要，是因为不同的计算机体系结构使用的数据表示法不同。对于用户数据来说，可以从两个侧面来分析：一个是数据含义被称为语义同，另一个是数据的表示形式，称做语法，像文字，图形，声音，文种，压缩，加密等都属于语法范畴。

例如：ASCⅡ，EBCDIC，JPEG，GIF，PICT，MIDI，MPEG

本题是讲在会话层和应用层之间的层是什么层：就是表示层。

记忆7层模型的方法：物－数－网络－传－会－表－应。

正确答案：D

参考资料：

Sybex CCNA 4.0 - Chapter 1 ：Internetworking - The OSI Reference Model

The Presentation Layer

The Presentation layer gets its name from its purpose： It presents data to the Application layer and is responsible for data translation and code formatting.

This layer is essentially a translator and provides coding and conversion functions. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading （for example， EBCDIC to ASCII）。 By providing translation services， the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one.

The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression， decompression， encryption， and decryption are associated with this layer. Some Presentation layer standards are involved in multimedia operations too. The following serve to direct graphic and visual image presentation：

PICTA picture format used by Macintosh programs for transferring QuickDraw graphics.

TIFFTagged Image File Format； a standard graphics format for high-resolution， bitmapped images.

JPEGPhoto standards brought to us by The Joint Photographic Experts Group.

Other standards guide movies and sound：

MIDIMusical Instrument Digital Interface （sometimes called Musical Instrument Device Interface）， used for digitized music.

MPEGIncreasingly popular Moving Picture Experts Group standard for the compression and coding of motion vdeo for CDs. It provides digital storage and bit rates up to 1.5Mbps.

QuickTimeFor use with Macintosh programs； manages audio and video applications.

RTFRich Text Format， a file format that lets you exchange text files between different word processors， even in different operating systems.