计算机网络笔记第二章

1. 物理层基本概念

• 物理层要干的事
  • 解决如何在连接各种计算机的传输媒体上传输数据比特流，而不是指具体的传输媒体
  • 确定传输媒体接口有关的一些特性-》定义标准
• 物理层特性
  • 机械特性
    • 规格
    • 接口形状
    • 引线数目
    • 引脚数量
    • 排列情况
  • 电气特性
    • 电压范围
    • 阻抗匹配
    • 传输速率
    • 距离限制
  • 功能特性
    • 指明某一电平表示何种意义
    • 接口不见的信号线的用途
  • 规程特性
    • 规定各条物理线路的工作规程和时序关系

1. 数据通信基础知识

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   • 调制解调器

     • 转换数字信号和模拟信号
     • 可以作为发送器和接收器

   • 信源

     • 发送信息的源头

   • 信宿

     • 信息的终点

   • 

   • 数据

     • 传送信息的实体，通常是有意义的符号序列

   • 信号

     • 数据的电气/电磁的表现，是数据在传输过程中的存在形式
     • 数字信号
       • 代表消息的参数取值是离散的
     • 模拟信号
       • 代表消息的参数取值是连续的

     

   • 信道

     • 信号的传输媒介。
     • 一般用来表示向某一个方向传送信息的介质，因此一条通信线路往往包含一条发送信道和一条接收信道
     • 

   • 三种通信方式

     • 单工通信
       • 只有一个方向的通信而没有反方向的交互
       • 仅需要一条信道
     • 半双工通信
       • 通信的双方都可以发送或接收信息，但不能同时发送和接收
       • 需要两条信道
     • 全双工
       • 通信双方可以同时发送和收受信息
       • 需要两条信道

   • 两种数据传输方式

     • 串行传输
       • 速度慢，费用低，适合远距离
     • 并行传输
     • 速度快，费用高，适合近距离

   • 码元

     • 指一个固定时长的信号波形（数字脉冲）
     • 代表不同离散数值的基本波形
     • 是数字通信中数字信号的计量单位
     • 1码元可以携带多个比特的信息量
     • 
     • 传输k种离散状态的码元叫做k进制码元
     • 码元宽度
     • 四进制码元
     • 4进制码元波形
     • 码元速率和进制数无关

   • 速率

     • 数据的传输速率
     • 表示单位时间内传输的数据量
     • 可以用
       • 码元传输速率
         • 码元速率
         • 波形速率
         • 调制速率
         • 符号速率
         • 单位时间内数字通信系统所传输的码元个数（可称为脉冲个数或信号变化的次数）
         • 单位是波特Baud
         • 1波特表示数字通信系统每秒传输一个码元
       • 信息传输速率
         • 别名信息速率
         • 比特率
         • 表示单位时间内数字传输系统传输的二进制码元个数（即比特数）
         • 单位是比特/秒（bit/s）
       • 表示
       • 若一个码元携带mbit信息量，则nBaud的码元传输速率所对应的信息传输速率为mxnbit/s

   • 带宽

     • 某点到零一点所能通过的“最高数据率”
     • 单位是b/s
     • 理想状态
     • 
     • 2000Baud 8000bit/s 1200Baud 23200bit/s 第二个
     • 2000Baud 4000bit/s 1200Baud 4800bit/s 第二个
     • m进制码元携带的比特为log₂（m）个不是m个

   • 信道上传送信号

     • 基带信号
       • 数字1 0 以不同的电压表示
       • 再送到数字信道上去传输（基带传输）
       • 来自信源的信号都属于基带信号
     • 带宽信号
       • 将基带信号进行调制后形成的频分复用模拟信号，在传送到模拟信道上去传输（宽带传输）
     • 
     • 距离近时，信号的衰减不大，可使用基带传输
     • 距离远时，可使用带宽传输

   • 编码与调制

     • 
     • 
     • 编码方式
       • 
       • 
       • 归零编码
         • 一个码元中，前半段为正常，后半段归零
       • 反向不归零
         • 遇到0改变， 1不变，照着前一个
         • 对全为1不太行
       • 
       • 曼彻斯特编码
         • 前高后低为1，前低后高为0
         • 看题目定义
         • 也可以换
         • 
       • 差分曼彻斯特编码
         • 同1异0
         • 
         • 抗干扰强于曼特斯特编码
         • 
       • 4B/5B编码
       • 
       • 

   • 数字数据调制为模拟信号

     • 
     • 4x1200

   • 

   • 

   • 

2. 两个公式limit

   • 失真
     • 现实中信息受到干扰产生变化
     • 有的可以识别有的不能了
     • 影响失真程度的因素
       • 码元传输速率
       • 信号传输距离
       • 噪声干扰
       • 传输媒体质量
     • 失真现象
       • 码间串扰
         • 接收端收到的信号波形失去了码元之间清晰界限的现象
         • 信道带宽是信道能通过的最高频率和最低频率之差
         • 3300Hz-300Hz = 3000Hz
   • 奈氏准则（奈奎斯特定理）内忧
     • 在理想低通（无噪声，带宽受限）条件下
     • 为了避免码间串扰
     • 极限码元传输速率为2W Baud， W是信道带宽，单位是Hz（只有在这两个公式才用带宽 ）
     • 极限数据率
     • 限制码元传输速率上限(不产生码间串扰)
     • 理想低通信道下的极限数据传输率=2Wlog₂V(b/s) V为几种码元/码元的离散电平数目
     • 信道的频带越宽，就可以用更高的速率进行码元的有效传输
     • 没有对比特率进行限制
     • 在奈氏准则下，要提高数据的传输速率，就必须使用多元制调制方法
     • 
     • 3kHz * log （4 * 4） = 24 kb/s
   • 香农定理 外患
     • 对极限比特传输速率进行限制
     • 噪声存在于所有的电子设备和通信信道中
     • 噪声会影响接收端对码元的判决
     • 所以使用信噪比
       • 信噪比 = 信号平均功率 / 噪声的平均功率
       • 记为S/N
       • 并用dB 作为度量单位
       • 即 信噪比 （dB） = 10log₁₀(S/N)
     • 使用条件
       • 在带宽受限
       • 有噪声的信道
     • 为了不产生误差
       • 信息的传输速率有上限值
       • 信道的极限数据传输速率 = Wlog₂(1+S/N) (b/s)
       • W为带宽， 单位为Hz
       • S/N为信噪比
     • 信道的带宽或信道中的信噪比越大，则信息的极限传输速率就越高
     • 对一定的传输带宽和一定的信噪比，信息传输速率就确定了
     • 只要信息的传输速率低于信道的极限传输速率，就一定能找到某种方法来实现无差错的传输
     • 香农定理得出的极限信息传输率，实际信道能达到的传输速率要比它低不少
     • 
     • W = 3kHz
     • 30dB = 10log₁₀(1+S/N)
     • S/N = 10³ = 1000
     • 最大数据传输速率为: 3kHz * log₂(1 + S/N) 约等于 30kb/s 2¹⁰ 约等于1001
   • 
   • 奈氏：2Wlog₂V
   • 香农
     • 信噪比 = 10log₁₀S/N
       • 要想提高数据率，就要提高带宽/采用更好的编码技术
     • 极限数据传输速率 = Wlog₂(1+S/N)
       • 要想提高数据率，就要提高带宽/信噪比
       • [1583136074239][link1]
       • S/N = 127
       • 最大数据速率 = 4kHz * log₂(1+127) = 28kb/s
       • 2 * 4kHzlog₂2 = 8kb/s
       • 所以是8kb/s

3. 看图说话

4. 传输介质

   • 无线信道
   • 有线信道

5. 物理层设备
   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计算机网络笔记-物理层(一)

原文：https://www.cnblogs.com/hamusuta/p/12398958.html