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Data link Layer ​

  • Framing
  • Flow control
  • Error control

1. Framing ​

  • Character Count: The frame's size is specified in the header.
  • Flag Bytes with Byte Stuffing
  • Starting and ending flags with bit stuffing

2. Error Control ​

异常检验

  • Parity Checks(奇偶校验): 多一个校验位表示1的奇偶
  • Two-Dimensional Parity(二维奇偶校验)
  • Cyclic Redundancy Check (CRC): Treats the data as a polynomial, divides it by a predefined polynomial, and checks for remainders to detect errors. 文字描述过程比较复杂,看道题懂了,需要双方先预设一个校验的多项式

3. Error Correction ​

异常修正

  • Hamming Codes(汉明编码): A more advanced error correction method that uses redundant bits to detect and correct single-bit errors. These codes are designed to correct errors based on the "Hamming Distance," which measures how many bit positions differ between two codewords.
    • 2k>=n+k+1
    • 我们以从左到右(从小到大)的偶校验做题
    • 校验位即为2的幂

4. Flow Control ​

流量控制

  • Feedback-based flow control
    • Stop-and-Wait Protocol: The sender waits for acknowledgment before sending the next frame. It’s a simple but inefficient method due to potential delays.This kind of protocols is called Automatic Repeat reQuest (ARQ) 一个一个的发
    • Sliding Window Protocol: A more efficient approach that allows the sender to transmit multiple frames before needing an acknowledgment, adjusting based on network conditions. 滑窗,重点是失败了如何回退
      • Go-Back-N Protocol: Requires retransmission of a set of frames if an error is detected.
      • Selective Repeat: Only the erroneous frames are retransmitted, making it more efficient.
  • Rate-based flow control

Multiple-access Protocols ​

5. Channelization Protocols ​

  • FDMA (Frequency-Division Multiple Access): Divides the available bandwidth into frequency bands for different stations.
  • TDMA (Time-Division Multiple Access): Allocates time slots to stations for transmission.
  • CDMA (Code-Division Multiple Access): Uses unique codes to distinguish between signals sent by different stations.

扩展频谱: 防窃听? 码分地址?

  • Frequency-hopping spread spectrum(跳频广播)
  • Direct Sequence Spread Spectrum(直接序列扩展频谱): 基于码片序列的异或

6. Random Access Protocols ​

  • Aloha: Stations send data whenever they need to, but this can lead to collisions.(直接发)
  • Slotted Aloha: Time is divided into slots, which reduces collisions.(分时直接发)
  • Carrier Sense Multiple Access (CSMA): Stations sense the channel before transmitting, reducing collisions.(听听有没有人在发送)
    • Persistent: 一直监听信道,一旦信道空闲就尝试发消息
    • Non: 信道忙,会随机休息一段时间,再重新监听
    • p-persitent: 信道空闲有p的概率发送/等待
    • CSMA with Collision Detection (CSMA/CD): Sends data while monitoring the channel; if a collision is detected, the transmission is stopped and retried after a random time.与上面的分类不同

7. Controlled-Access Protocols ​

  • Polling: A master node polls each station to give it permission to transmit.
  • Token Passing: A token is passed around, and only the station holding the token can transmit, preventing collisions.