OSI模型概述 OSI Model Overview

为什么网络模型要分层? Why a Layered Network Model? Reduces complexity 减少复杂度 Standardizes interfaces 标准化接口 Facilitates modular engineering 便于模块化设计 Ensures interoperable technology 提供兼容性 Accelerates evolution 推动网络的发展 Simplifies teaching and learning 方便网络知识的教学和学习 在70年代Internet网络得到了迅速的发展和应用在当时的 情况下已经变成一个非常巨大的网络但是由于各种原因许多网 络的设计采用不同的硬件和软件造成一个必然结果就是不同的 网络之间互相不兼容互相不能通信为了解决这个问题国际标 准化组织ISO International Organization for Standardization 认识到只有制定一个网络模型才能让所有网络设计人员设计出的 网络能够互相通信协同工作基于此 ISO于1984年提出OSI参 考模型即我们常讲到的OSI七层协议 2

数据封装(Encapsulating Data) 发送端 Application Presentation PDU Session Upper Layer Data Segment Transport TCP Header Upper Layer Data Network Packet IP Header Data Purpose: This figure illustrates encapsulation. Emphasize: The protocol data units (PDUs) are the terms used in the industry and in this course to describe data at the different layers. Encapuslation is a key concept that illustrates how data is formatted prior to being sent across a link. This example is an illustration is Ethernet (or token ring) at the data link and physical layer and TCP/IP at the network and transport layers. Transition: The following discusses de-encalsulation. MAC Header Data FCS Data Link Frame 0101110101001000010 Physical Bits 3

LLC Hdr + IP + TCP + Upper Layer Data 数据解封装(De-encapsulating Data) 接收端 Application Presentation Session Upper Layer Data Transport Upper Layer Data TCP Header Network TCP+ Upper Layer Data IP Header Purpose: This figure illustrates de-encapsulation. Emphasize: At the destination, the headers at each layer are stripped off as the data moves back up the stack. LLC Hdr + IP + TCP + Upper Layer Data Data Link MAC Header 0101110101001000010 Physical 4

点到点的通讯过程 Peer-to-Peer Communication Lesson Aim <Enter lesson aim here.> 5

Physical Layer Functions 物理层功能 Physical Layer Functions Defines 介质类型 Media type 连接器类型 Connector type 信号类型 Signaling type Ethernet 802.3 EIA/TIA-232 V.35 Physical Purpose: This section describes the physical layer in more detail. Note: 802.3 is responsible for LANs based on the carrier sense multiple access collision detect (CSMA/CD) access methodology. Ethernet is an example of a CSMA/CD network. EIA/TIA-232 and V.35 are physical standards that support synchronous serial. 6

思科光缆GBIC接口 Cisco Fiber-Optic GBICs Lesson Aim <Enter lesson aim here.> 7

UTP实现直通线接法 UTP Implementation Straight-through Cable 10BaseT/ 100BaseTx Straight-through Straight-through Cable 8 1 Hub/Switch Server/Router 8 Pin Label 1 RD+ 2 RD- 3 TD+ 4 NC 5 NC 6 TD- 7 NC 8 NC Pin Label 1 TD+ 2 TD- 3 RD+ 4 NC 5 NC 6 RD- 7 NC 8 NC 1 8 1 w g b o br 8 1 w g b o br 568A：白绿 | 绿 | 白橙 | 蓝 | 白蓝 | 橙 | 白棕 | 棕 直通线 568B：白橙 | 橙 | 白绿 | 蓝 | 白蓝 | 绿 | 白棕 | 棕 568A：白绿 | 绿 | 白橙 | 蓝 | 白蓝 | 橙 | 白棕 | 棕 直通线 8

UTP实现交叉线接法 UTP Implementation Crossover Cable 10BaseT/ 100BaseT Crossover Crossover Cable Hub/Switch Hub/Switch 8 1 Pin Label 1 RD+ 2 RD- 3 TD+ 4 NC 5 NC 6 TD- 7 NC 8 NC Pin Label 1 RD+ 2 RD- 3 TD+ 4 NC 5 NC 6 TD- 7 NC 8 NC 1 8 8 1 w o o b w g w g br w br 8 1 Emphasize: Make sure each student has seen a Cat 5 cable. Describe the test for crossover cable determination. If you hold the two ends of an RJ-45 cable side by side, you'll see eight colored strips, or pins, at each end. If the order of the colored pins is the same at each end, then the cable is straight through. If the order of the colors is reversed at each end, then the cable is crossed over or rolled. Ethernet uses pins 1, 2, 3, and 6. For crossover, pin 1 is connected to 3, and oin 2 is connected to 6. Crossover cables are used for switch to switch cabling, PC to PC cabling, switch to hub cabling, hub to hub cabling, or router to router cabling. br w g w br w o w b b o g 568B：白橙 | 橙 | 白绿 | 蓝 | 白蓝 | 绿 | 白棕 | 棕 交叉线 9

各种UTP的使用 Using Varieties of UTP Lesson Aim <Enter lesson aim here.> 10

Modular Interfaces Serial WAN ports can be modular WAN Interface Card Serial WAN ports can be modular 1603 Router—rear view Module Ethernet 10BaseT Ethernet AUI ISDN BRI S/T Console 3640 Router— rear view Purpose: This figure shows modular interface routers. Emphasize: A modular interface router requires additional hardware network modules and/or WAN interface cards. This generally costs more than a fixed interface router. However, you have the flexibility to change the interface components as your network scales. When configuring an interface on a fixed configuration router, some interfaces must be called out with the slot and port. Note: The figure displays a 1603 router with a 60-pin WAN interface card. It also displays a 3640 with an 8 port serial network module. The laboratory exercise suggests the use of a 3640 as the core router. If you use this router, show the students the various network modules you have installed. 11

Hubs Operate at Physical layer D 所有设置都在相同冲突域 All devices in the same collision domain 所有设置都在相同广播域 All devices in the same broadcast domain 设备共享相同的带宽 Devices share the same bandwidth HUB实际上是多端口中继器，接收数字型号并进行放大和整形，然后将数字信号转发到所有活动端口上，这表示接入到HUB的所有设备在同一个冲突域内 Emphasize: All devices attached to a hub are on the same collision and broadcast domain. A hub is a layer one device. 12

Hubs: 一个冲突域 Hubs: One Collision Domain More end stations means more collisions CSMA/CD is used 带冲突检测的载波侦听多路访问 Purpose: This figure compares hubs in a CSMA/CD environment to a highway with multiple access points. Emphasize: The more entrance points onto the highway, the greater the likelihood for a collision to occur. Likewise, the more end stations on a hub trying to access the line, the more collisions occur. 13

CSMA/CD Lesson Aim <Enter lesson aim here.> 14

Data Link layer Functions 数据链路层功能 Data Link layer Functions Defines Physical source and destination addresses 将数据包组合为字节，字节 组合成帧 使用MAC地址实现对设备的访问 执行差错检测，但不纠正 802.2 Data Link Frame Relay HDLC Ethernet 802.3 Purpose: This figure compares physical standards to data link standards. Emphasize: As illustrated, certain physical standards are associated with certain data link standards. For example, 802.3 is used with data link standard 802.2 for Ethernet. It is not used in WAN connections. EIA/TIA-232 v.35 Physical 15

Data Link Layer Functions (cont.) 数据链路层功能 Data Link Layer Functions (cont.) MAC Layer - 802.3 # Bytes 8 6 6 2 Variable 4 Preamble Dest add Source add PID Data FCS 0000.0C xx.xxxx Emphasize: All MAC addresses on a LAN must be unique. Two devices with the same MAC address cannot be on the same data link. Note: The IEEE assigned vendor code is just one of Cisco’s many MAC vendor codes. Cisco has many more, many of which came with Cisco’s acquisitions. LLC子层负责向其上层提供服务；MAC子层的主要功能包括数据帧的封装/卸装，帧的寻址和识别，帧的接收与发送，链路的管理，帧的差错控制等。MAC子层的存在屏蔽了不同物理链路种类的差异性。 在每种格式的以太网帧的开始处都有64比特（8字节）的前导字符，其中，前7个字节称为前同步码（Preamble），内容是16进制数0xAA，最后1字节为帧起始标志符0xAB，它标识着以太网帧的开始。前导字符的作用是使接收节点进行同步并做好接收数据帧的准备。 IEEE assigned Vendor assigned MAC Address IEEE ：电子和电气工程师协会 16

Switches and Bridges Operate at Data Link Layer 运行在数据链层的交换机和桥 Switches and Bridges Operate at Data Link Layer Data Link Emphasize: To reduce the number of collisions, a switch can be split into multiple segments, each in a separate collision domain. Note that all segments are in the same broadcast domain. OR 1 2 3 4 1 2 每个 segment 有自已的 collision domain 所有 segments 是在相同的 broadcast domain 17

多个冲突域 Multiple Collision Domains Lesson Aim <Enter lesson aim here.> 18

Network Layer Functions 网络层功能 Network Layer Functions Defines logical source and destination addresses associated with a specific protocol Defines paths through network Interconnects multiple data links 互联多条数据线路 Network IP, IPX 802.2 Data Link HDLC Frame Relay Purpose: This figure maps the lower layers to the network layer. Emphasize: Routing occurs at the network layer. Most of this course uses the IP network layer protocol. It supports multiple lower layer protocols. IPX will also be covered in a single chapter in this course. Routers are network layer devices. Ethernet 802.3 EIA/TIA-232 v.35 Physical 19

Network Layer End Station Packet 网络层功能 Network Layer Functions (cont.) Network Layer End Station Packet Source address Destination address IP Header Data Logical Address 172.15.1.1 Emphasize: Route determination occurs at this layer so a packet must include a source and destination address. Network layer addresses have two components, a network component for internetwork routing and a node number for a device specific address. The example in the figure is an example of an IP packet and address. Network Node 20

运行于网络层的Router Routers: Operate at the Network Layer Broadcast control Multicast control Optimal path determination Traffic management Logical addressing Connects to WAN services Emphasize: Routers perform the network layer functions depicted in the figure. 21

Network Device Domains Hub Bridge Switch Router Collision Domains: Emphasize: This figure summarizes the different collision domains and broadcast domains on various network devices. It is important that students understand the difference between the two domains. 1 4 4 4 Broadcast Domains: 1 1 1 4 22