COMP211-W5

Lec6-Link Layer 链路层和局域网

Goals

  • Understand principles behind link layer services:
    • error detection, correction
    • sharing a broadcast channel: multiple access(多点接入)
    • link layer addressing
    • local area networks(局域网): Ethernet, WLAN
    • 可靠数据传输,流量控制
  • instatiation, implementation of various link layer technologies

    Review:

    • 网络层以一个子网为单位作为路由
    • 最长路由匹配
    • 子网内部:主机和主机如何沟通? —— 链路层来解决

      特例

    • 点到点链路(广域) & 多点链路(局域) 多个设备连接一个ip — 引出多点接入的问题
    • 无线链路出错率要比有线链路出错率要高
    • 局域网的访问控制
    • 多点链路会在广域上造成碰撞最后损失

Introduction

terminology:

  • hosts and routers: nodes
  • communication channels that connect adjacent nodes along communication path: links
    • wired
    • wireless
    • LANs
  • layer-2 packet: frame, encapsulates datagram

link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

  • datagram transferred by different link protocols over different links:
    • e.g., WiFi on first link, Ethernet on next link
  • each link protocol provides different services
    • e.g., may or may not provide reliable data tranasfer over link

transportation analogy:

  • trip from Princeton to Lausanne
    • limo: Princeton to JFK
    • plane: JFK to Geneva
    • train: Geneva to Lausanne
  • tourist = datagram
  • transport segment = communication link
  • transport mode = link-layer protocol
  • travel agent = routing algorithm
  • framing, link access:
    • encapsulate datagram into frame, adding header, trailer
    • channel access if shared medium
    • “MAC” addresses in frame headers identify source, destination (different from IP address!)
  • reliable delivery between adjacent nodes
    • we already know how to do this!
    • seldom used on low bit-error links
    • wireless link: high error rates
      • Q: why both link-level and end-end reliability?

  • flow control:
    • pacing between adjacent sending and receiving nodes
  • error detection:
    • errors caused by single attenuation, noise.
    • receiver detects errors, signals retransmission, or drops frame
  • error correction:
    • receiver identifies and corrects bit error(s) without retransmission
  • in each-and-every host
  • link layer implemented in network interface card(NIC) or on a chip
    • Ethernet, WiFi card or chip —— (CPU)
    • implements link, physical layer
  • attaches into host’s system buses
  • combination of hardware, software, firmwawre

Interfaces communicating

Error Detection, correction

Error detection

EDC: error detection and correction bits (e.g., redundancy)
D: data protected by error checking, may include header fields

数据检查并非百分百可靠

Parity checking

Internet checksum (review)

Cyclic Redundancy Check(CRC)

CRC example


3. 两边同除G得到余数 «R=

Mulitple access protocols

A day in the life of a web request

Synthesis: a day in the life of a web request

A day in the life: scenario

A day in the life: connecting to the Internet


A day in the life … ARP(befor DNS, before HTTP)

uisng DNS

TCP connection carrying HTTP

HTTP request/reply

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    缺失模块。
    1、请确保node版本大于6.2
    2、在博客根目录（注意不是yilia根目录）执行以下命令：
    npm i hexo-generator-json-content --save

    3、在根目录_config.yml里添加配置： 

      jsonContent:
    meta: false
    pages: false
    posts:
      title: true
      date: true
      path: true
      text: false
      raw: false
      content: false
      slug: false
      updated: false
      comments: false
      link: false
      permalink: false
      excerpt: false
      categories: false
      tags: true

很惭愧<br><br>🌳就是最菜的<br>谢谢大家