OSI七层模型续.ppt

Lesson Aim Enter lesson aim here. Lesson Aim Enter lesson aim here. Lesson Aim Enter lesson aim here. Lesson Aim Enter lesson aim here. Lesson Aim Enter lesson aim here. Module 2, Lesson 3 – Replace the existing graphic on slide 14 with this new graphic (align items properly) Reference – SG 55 * Purpose: This figure describes a simple way to determine whether one should use a crossover or straight thru cable. Lesson Aim Enter lesson aim here. 在数据通信中，表示计算机二进制的比特序列的数字信号是典型的矩形脉冲信号，人们将基本频率分量对应的矩形脉冲信号称为基本频带（简称基带）。这种矩形脉冲信号称为基带信号。在数字信道上直接传送基带信号的方法称为基带传输。 * Lesson Aim Enter lesson aim here. * Lesson Aim Enter lesson aim here. * ?CSMA也叫做LBT（先听后说）， 它是希望传输数据的站点首先对媒体上有无载波进行监听，以确定是否有别的站点在传输数据。 如果媒体空闲，该站点便可传输数据；否则，该站点将使用某种退避算法避让一段时间后再做尝试。 常用的退避算法有非坚持 、1－坚持 、P－坚持 三种。 * Lesson Aim Enter lesson aim here. There are three unlicensed bands: 900 MHz, 2.4 GHz, and 5.7 GHz. The 900-MHz and 2.4?GHz bands are referred to as the Industrial, Scientific, and Medical (ISM) bands, and the 5?GHz band is commonly referred to as the Unlicensed National Information Infrastructure (UNII) band. Frequencies for these bands are as follows: 有三个未经许可的频带： 900兆赫， 2.4 GHz和5.7 GHz的。 900 MHz和2.4 - GHz的频带被称为工业，科学和医疗协会（ ISM ）频段，和5 - GHz频段通常被称为无牌国家信息基础设施（ UNII ）频段。 这些频段的频率如下： 900-MHz band: 902 MHz to 928 MHz. 2.4-GHz band: 2.400 MHz to 2.483 GHz. (In Japan, this band extends to 2.495 GHz.) 5-GHz band: 5.150 MHz to 5.350 MHz, 5.725 MHz to 5.825 MHz, with some countries supporting middle bands between 5.350 MHz and 5.825 MHz. Not all countries permit 802.11a, and the available spectrum varies widely. The list of countries that permit 802.11a is changing. The figure shows WLAN frequencies. Next to the WLAN frequencies in the spectrum are other wireless services such as cellular phones and NPCS (Narrowband Personal Communication Services). The frequencies used for WLAN are ISM bands. Unlicensed frequency bands do not require a license to operate wireless equipment. However, there is no exclusive use of a freque