← Back to Hub ← 返回主页

Week 13: LAN Topologies & Project Submission 第 13 周：局域网拓扑与项目提交

Core Concepts 核心概念

1. LAN Topologies (9.1 - 9.2)

  • Bus Topology: All nodes connect to a single shared central cable. Requires terminators at each end. Simple but vulnerable to backbone cable failure and packet collisions.
  • Ring Topology: Nodes connect in a closed loop. Data passes token-by-token sequentially in one direction. Stable under heavy loads but a single node failure can break the ring.
  • Star Topology: Nodes connect to a central active Hub or Switch. The switch routes frames directly to target ports, eliminating collisions. Highly scalable and most common LAN topology today.

2. Media Access Control (MAC) (9.3)

  • CSMA/CD (Carrier Sense Multiple Access with Collision Detection): Used in wired Ethernet. Devices listen to the wire before sending. If two send at once, they detect the collision, send a jam signal, and back off using an exponential random delay.
  • CSMA/CA (Collision Avoidance): Used in wireless networks (Wi-Fi). Since devices cannot listen while transmitting, they attempt to avoid collisions entirely by using Request to Send/Clear to Send (RTS/CTS) handshakes and ACK validation frames.

1. 局域网拓扑 (9.1 - 9.2)

  • 总线型拓扑 (Bus)： 所有节点连接到单根共享的总线电缆。两端需要终端匹配器。简单，但极易受到主干电缆故障和数据包碰撞的影响。
  • 环型拓扑 (Ring)： 节点连接成闭环。数据通过令牌在单向上依次传递。在高负载下较稳定，但单个节点损坏可能会破坏整个环路。
  • 星型拓扑 (Star)： 节点连接到中央集线器或交换机。交换机将帧直接路由到目标端口，消除碰撞。扩展性极高，是当今最常用的局域网拓扑。

2. 介质访问控制 (MAC) (9.3)

  • CSMA/CD（载波监听多路访问/冲突检测）： 用于有线以太网。设备在发送前监听导线。若两台设备同时发送，它们会检测到冲突，发送干扰信号，并使用指数退避算法进行随机等待。
  • CSMA/CA（冲突避免）： 用于无线网络 (Wi-Fi)。由于设备无法在发送时同时监听信道，因此通过使用请求发送/允许发送 (RTS/CTS) 握手和 ACK 确认帧来尽量避免冲突发生。

Concept Visualization: Star vs. Bus Topology 概念可视化：星型与总线型拓扑

Star Topology Switch Bus Topology Terminator Terminator

Tips 提示

  • CSMA/CD vs CSMA/CA: Remember CD is for Collision Detection (Wired, e.g. IEEE 802.3 Ethernet). CA is for Collision Avoidance (Wireless, e.g. IEEE 802.11 Wi-Fi).
  • Switch forwarding: A network Switch operates at Layer 2 (Data Link) and builds a MAC address table to forward frames selectively, unlike a passive Hub which floods all ports.
  • CSMA/CD 与 CSMA/CA： 记住 CD 代表冲突检测（有线网络，如 IEEE 802.3 以太网）。CA 代表冲突避免（无线网络，如 IEEE 802.11 Wi-Fi）。
  • 交换机转发原理： 网络交换机工作在第二层（数据链路层），并通过构建 MAC 地址表来进行选择性地帧转发，而不像无源集线器 (Hub) 那样广播泛洪到所有端口。

Continuous Assessment: Group Project Solution (30% Weight) 持续性评估：小组项目方案 (权重 30%)

Scenario: You are Network Consultants designing a new branch network layout for 'NexGen Logistics'. The branch contains 30 employees across 3 departments: Administration (10 users), Operations (15 users), and IT Support (5 users). Given the address space 192.168.50.0/24, design a Variable Length Subnet Mask (VLSM) scheme.

场景背景： 你们是为 'NexGen Logistics' 设计新分部网络的网络顾问。分部内共有 30 名员工，分布在 3 个部门：行政部 (10人)、业务部 (15人) 和 IT 支持部 (5人)。给定 IP 地址空间 192.168.50.0/24，设计可变长子网掩码 (VLSM) 划分方案。

Step-by-Step VLSM Subnetting Scheme Calculation VLSM 子网划分步骤与计算

1. Sort subnets from largest host requirement to smallest: Operations (15 users) → Admin (10 users) → IT Support (5 users). 1. 按照主机数需求由大到小对子网进行排序：业务部（15台） → 行政部（10台） → IT 支持部（5台）。

Department Hosts Req Subnet Size Subnet Mask Network ID Usable Host Range Broadcast Address
Operations 15 32 (/27) 255.255.255.224 192.168.50.0 192.168.50.1 - .30 192.168.50.31
Administration 10 16 (/28) 255.255.255.240 192.168.50.32 192.168.50.33 - .46 192.168.50.47
IT Support 5 8 (/29) 255.255.255.248 192.168.50.48 192.168.50.49 - .54 192.168.50.55

Cloud Computing & Network Strategy Recommendation 云计算与网络策略建议

  • Cloud Model Selection (SaaS): We recommend a SaaS (Software as a Service) model for general business applications (e.g. cloud CRM, email, file storage). This minimizes the initial capital cost for NexGen Logistics since they do not need to purchase servers or hire large maintenance crews.
  • VoIP Solution: Using VoIP over internet connections enables cheap, scalable branch telephony without telephone copper runs.
  • VPN Secure Access: Implement an IPsec client-to-site VPN to allow administrators and managers to securely access the private company intranet resources when working remotely.
  • 云模型选择 (SaaS)： 我们建议对于通用业务应用采用 SaaS（软件即服务） 模型（例如云端 CRM、电子邮件、文件协作）。这最大程度地减少了 NexGen Logistics 新分部的初始硬件投资成本，无需本地购置昂贵服务器和配置专属运维。
  • VoIP 方案： 使用基于互联网的 VoIP 服务可以实现廉价、高扩展的分部电话通信，避免铺设专门的模拟电话线缆。
  • VPN 安全访问： 实施 IPsec 客户端到站点 VPN，以允许行政管理人员在远程办公时安全访问公司私有的内网文件资源。

Practical Exercises & Step-by-Step Guide 实践练习与分步指南

Exercise 1: Inspecting Switch MAC Address Table 练习 1：查看交换机 MAC 地址表

Steps to analyze:

  1. In Cisco Packet Tracer, connect PC0, PC1, and PC2 to Switch0's FastEthernet 0/1, 0/2, and 0/3 ports.
  2. Configure PC0 with IP `192.168.1.10`, PC1 with `192.168.1.11`, and PC2 with `192.168.1.12`.
  3. Open PC0 command prompt. Send a ping: ping 192.168.1.11.
  4. Go to Switch0 -> click the **CLI** tab -> press Enter.
  5. Type enable to enter privileged mode, then type show mac address-table.
  6. Note how the switch has dynamically associated the MAC addresses of PC0 and PC1 with ports Fa0/1 and Fa0/2. Note that PC2's MAC address is NOT in the table yet because it has not transmitted any frames.

分析步骤：

  1. 在 Cisco Packet Tracer 中，将 PC0、PC1 和 PC2 分别连接到 Switch0 的 FastEthernet 0/1、0/2 和 0/3 接口。
  2. 为 PC0 配置 IP `192.168.1.10`，PC1 配置 `192.168.1.11`，PC2 配置 `192.168.1.12`。
  3. 打开 PC0 命令提示符。执行 ping 命令： ping 192.168.1.11。
  4. 进入 Switch0 -> 点击 **CLI** 选项卡 -> 按回车。
  5. 输入 enable 进入特权模式，然后输入 show mac address-table。
  6. 观察交换机如何自动将 PC0 和 PC1 的物理 MAC 地址关联至 Fa0/1 和 Fa0/2 端口。注意，PC2 的 MAC 地址尚未列出，因为其尚未向网络发送任何帧数据。

Self-Check Practice Questions 自测练习题

1. Which LAN topology connects all client devices to a single shared central backbone cable? 哪种局域网拓扑将所有客户端设备连接到单根共享的中央主干电缆？

2. What is the media access control mechanism used in wired IEEE 802.3 Ethernet? 有线 IEEE 802.3 以太网中使用的介质访问控制机制是什么？

3. A network Switch operates at which Layer of the OSI Model? 网络交换机工作在 OSI 模型的哪一层？

4. Switches make frame forwarding decisions based on which address? 交换机基于什么地址做出数据帧转发决策？

5. Why does a star topology offer higher operational reliability than a bus topology? 为什么星型拓扑比总线型拓扑具有更高的运行可靠性？

6. How does an Ethernet switch dynamically build its MAC address table? 以太网交换机如何动态构建其 MAC 地址表？

7. Describe how a sender transmits under the CSMA/CD protocol. 描述发送端在 CSMA/CD 协议下如何传输。

8. How does the CSMA/CA protocol attempt to avoid collisions in wireless networks? CSMA/CA 协议如何试图避免无线网络中的碰撞？

Vocabulary Matching Game 词汇配对游戏

Match the LAN concept on the left with its definition on the right. (Click a term, then click its matching definition). 将左侧的局域网概念与右侧的定义进行配对。（点击术语，然后点击匹配的定义）。

Star Topology
Bus Topology
CSMA/CD
CSMA/CA
Switch Forwarding
Collision detection mechanism used in wired Ethernet lines 用于有线以太网线路的碰撞检测多路介质访问机制
Nodes connect to a central active Switch routing frames directly 所有子节点连接到一个中央活动交换机设备以路由帧
Nodes connect to a single central shared cable with terminators 所有端节点连接到带有物理终结匹配器的单根总线电缆
Uses a dynamic MAC address table to forward frames selectively 利用动态维护的 MAC 地址表对网络帧进行选择性独立转发
Collision avoidance mechanism used in wireless Wi-Fi networks 用于无线 Wi-Fi 频段的碰撞避免多路介质访问机制

Review Questions 复习问答题

Q1. Describe the Bus topology and Ring topology, highlighting one advantage and one disadvantage for each.

Q2. Explain the main characteristics of Ethernet networks and how they handle access to the shared transmission medium.

Q3. Design and contrast Star and Mesh topologies, discussing their redundancy and deployment tradeoffs.