First, the three basic issues of the data link layer

basic concept

Nodes: hosts, routers

Link: A physical channel between two nodes in a network

Data link: A logical channel between two nodes in a network

Frame: The protocol data unit of the link layer, encapsulating the network layer datagram

Encapsulation and framing

Transparent transmission

character counting

character padding

error control

Second, the calculation of CRC

3. Bit stuffing algorithm of PPP protocol

Four, CSMA/CD protocol (find the shortest effective frame length)

1. Basic concepts of CSMA/CD

CSMA/CD (Carrier Sense Multiple Access with Collision Detection): Carrier Sense Multiple Access/Collision Detection.
MA: Multi-point access: It means that this is a bus-type network. Many computers are connected to a bus in a multi-point manner.
CS: Carrier Sense: That is, "send while listening". Each station must constantly monitor the channel, whether it wants to send data or in the middle of sending data.
CD: Collision detection: The adapter detects the change of the signal voltage on the channel while sending data. When the voltage swing value exceeds a certain threshold, it is considered that at least two stations on the bus are sending data at the same time, indicating that a collision (or conflict) has occurred

2. Effect of propagation delay

eg：

Example 1:

Answer: choose a random number between 0~1023

3. The shortest effective frame problem

Five, Ethernet, network card function, MAC address, MAC frame number format

ethernet

Ethernet provides connectionless and unreliable services.
Connectionless : There is no handshake process between the sender and the receiver
. Unreliable: The data frame number checked, the receiver does not confirm , and the error frame is directly discarded. Correction is the responsibility of the upper layer.
Ethernet only achieves error-free reception, not reliable transmission.

Adapters and MAC addresses

类型字段用来标志上一层使用的是什么协议，
以便把收到的 MAC 帧的数据上交给上一层的这个协议。

数据字段的正式名称是 MAC 客户数据字段。
最小长度 64 字节 - 18 字节的首部和尾部 = 数据字段的最小长度（46字节）

FCS:当数据字段的长度小于 46 字节时，应在数据字段的后面加入整数字节的填充字段，以保证以太网的 MAC 帧长不小于 64 字节。