Computer Network (Third Edition) Hu Liang Chapter 4 Summary of Knowledge Points
study guide
1. Master the usage occasions and working methods of inquiry/confirmation and polling/selection modes
2. Understand the flow control and error control of the stop-and-wait protocol, and be able to perform relevant quantitative calculations.
3. Master the working methods of fallback N and selection control in sliding window protocol
4. Understand the relevant concepts of HDLC protocol, up to the three types of frames
5. Understand the bridge routing algorithm and master the spanning tree algorithm
Data link layer: line discipline, error control, flow control
(1) Line regulations
Function: Coordinate the work of both communication parties and solve the problem of "who should send now". There are two ways to implement it: inquiry/confirmation mode and polling/selection mode.
Query/confirm mode
Usage occasions:
There is a link between two devices with the same level and a session is taking place.
Way of working:
Polling/selection mode
Usage occasions:
A multi-point connection system used in a topology where one device acts as the master device and the other devices are slave devices.
Way of working:
The master device controls the link. Slave devices obey the instructions of the master device.
Polling: The master device wants to receive data and asks the slave devices one by one whether there is data. If the device being queried has no data to send, it will send a denial frame. If it has data, it will send data. The master device will send a confirmation frame.
Select: The master device sends data to inform the target slave device that it is ready to receive data. The master device generates and sends a selection frame (SEL), fills in the frame with the address of the slave device that receives the data, the slave device in the link, checks the address, if so, reads the data, and returns an acknowledgment frame (ACK).
Address question:
Each device has an independent and different address, which exists in a specific field in the frame.If it comes from the master device, the address specifies the recipient of the data. If it comes from the slave device, , the address indicates the data generator.
(2) Flow control and error control
Flow control: Control how much data can be sent and how much data can be received to prevent data loss and channel congestion caused by the sending speed exceeding the receiving end's processing speed. Features: The data flow cannot overload the receiver; the receiver confirms the data.
Error Control: Error detection and retransmission methods. If an error occurs, the receiver returns a denial frame and the error frame is retransmitted. There are three situations of being retransmitted: Frame corruption, frame loss, and confirmation frame loss.
Flow control and error control are implemented together.There are two main technologies: stop waiting protocol and sliding window protocol.
stop waiting for agreement
Stop waiting for flow control in protocols
The sender sends a frame and waits for an acknowledgment frame. The next frame will not be sent until the acknowledgment frame is received. Repeat until the sender sends an End of Transmission (EOT) frame
Quantitative analysis in the stop-and-wait protocol (calculation questions)
I won’t list symbols for now, I will summarize and write calculation questions later.