• Minimum ethernet frame is 64 bytes
  • Measured in byte Measured in byte


  • Size: 64 - 1518 bytes
    • Minimum: an 18-byte header and a 46-byte payload.
    • Maximum: a 1,500-byte payload.
  1. Preamble: Informs the receiving system that a frame is starting and enables synchronization.
  2. Start Frame Delimiter (SFD): Signifies that the Destination MAC Address field begins with the next byte.
  3. Destination MAC Address: The MAC address of the destination device.
  4. Source MAC Address: The MAC address of the source device.
  5. EtherType (in Ethernet II): In Ethernet II, this field identifies the protocol carried in the payload of the frame (e.g., IPv4, IPv6, or ARP).
    • A value of 0x0800 indicates that the payload is an IP packet
    • A value of 0x0806 indicates that the payload is an ARP packet.
  6. Data and Pad: Contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).
  7. Frame Check Sequence (FCS): Contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data


  1. 准备发送: 适配器从网络层获得一个分组,加上首部和尾部,组成以太网帧,放入适配器的缓存中;
  2. 检测信道: 若检测到信道忙,则不停检测,直到信道空闲。若检测到信道空闲,并在 96 比特时间内仍保持空闲,则发送帧;
  3. 在发送过程中不停检测:
    • 发送成功: 在争用期内未检测到碰撞,则帧肯定发送成功。发送后,回到(1)
    • 发送失败: 在争用期内检测到碰撞,停止发送数据,发送干扰信号。执行二进制 指数退避 算法,等待 r 倍 512 比特后,返回步骤(2),继续检测信道。若重传 16 次仍不成功,则停止重传并向上报错误。
  • 争用期长度为 2t,即端到端传播时延的两倍。检测到碰撞后不发送干扰信号。