第4章-多路复用技术.ppt

* * In Synchronous TDM many time slots in a frame are wasted With statistical TDM, time slots are not pre-assigned to particular data sources. Rather, time slots are allocated dynamically based on demand. User data are buffered and transmitted as rapidly as possible using available time slots. For multiplexer, there are n I/O input lines, but only k (kn) time slots available on the TDM frame. So data rate on line lower than aggregate rates of input lines. On input, multiplexer scans input buffers and collects data until a frame is fulled. On output, multiplexer receives a frame and distributes the slots of data to the appropriate output buffers. * * * * Only three lines sending data * * Only four lines sending data * * c. All five lines sending data * * In synchronous TDM, the device to which the data in a time slot belong is indicated by the position of the time slot in the frame. But in asynchronous TDM, data from a given device might be in the first slot of one frame and in the third of the next frame. In the absence of fixed positional relationships, each time slot must carry an address telling the demultiplexer how to direct the data. This address, for local use only, is attached by the multiplexer and discarded by the demultiplexer once it has been read. Because address is overhead, asynchronous TDM is efficient only when the size of the time slots is kept relatively large. b. 只有四条输入线路发送数据 帧和地址Frames and Addresses c. 所有输入线路都发送数据 帧和地址Frames and Addresses 莆田学院现代教育技术中心 2009年3月 STDM Addressing寻址问题 在同步时分复用中，在某一时隙中的数据属于哪个设备由时隙在帧中的位置指明。但是在异步时分复用中，从同一个设备发出的数据在一帧中可能位于第一个时隙，而在另一帧中却可能位于第三个时隙。 由于缺乏固定的位置关系，每个时隙必须携带一个地址来告诉解复用器如何为其中的数据定向。这个地址只用于局部，由复用器添加其中，而一旦数据被读入就由解复用器去除。 由于地址是一种额外开销，所以异步时分复用只有在传输保持较大时隙的时候才有较高的效率。 本章推荐阅读 吕梁高等专科学校--计算机系 课程网站 /jsjx/kj/network/netpage/html/UNIT2/U24/U24.htm (有课前练习、课后习题、应用案例等） 网狐教程“多路复用技术的基本概念 ” /book/chapter.php?id=538volume=2chapter=9 全国高等教育自学考试指导委员会 组编《计算机网络与通信》 第3.3节 多路复用技术（全国高等教育自学考试指定教材-计算机及应用专业独立本科段用） 主编 冯