August 8, 2022
A netizen asked for help and said that the image of the network surveillance camera will be stuck, but after calculating the bandwidth, one network camera occupies 8M bandwidth, the switch is 100M, and 11 sets are connected. In theory, 100÷8=12.5 sets, a total of 11 sets are connected. If the network bandwidth is enough, but the network is still stuck, what is the reason?
We have already written relevant articles about the reasons for Caton, and those who are interested can look through the historical articles. The cause can usually be found externally, but what should you do if the external factor is ruled out?
In response to these confusions, we must first understand the basic concepts of network monitoring and related equipment.
What is video stream and frame rate?
The video stream is the data flow used in unit time after the video file is encoded. Also called video bitrate. It is the most important part of the video picture quality (from Baidu Encyclopedia).
The unit time of the code stream in our surveillance video is 1 second. 1 second = 1000 milliseconds (used below).
So will the video image flow through at a constant speed like flowing water? Certainly not. It is passed through frames, which are the unit of data transmission, and the more frames per second (expressed in fps), the smoother the displayed action will be. Higher frame rates result in smoother, more realistic video animations.
If the video is 25 frames per second, there is a time gap between each frame. The time interval can be calculated as 40 milliseconds.
Then we can know that the average bit rate of sending and sending interval is smaller than the bit rate when sending video frames.
Frames are also divided into I-frames, P-frames, and B-frames. The I frame is usually the first frame of each video clip, and its occupancy is the largest, which can be understood as follows with a schematic diagram.
What are full duplex and half duplex?
Full-duplex transmission means that the switch can also receive data while sending data, and the two are synchronized. This is like a wide road running east to west, allowing two carriages to pass head-on. When carriage A travels from east to west, carriage B When traveling from west to east, two vehicles can travel at the same time without affecting each other. The wide road in this example represents a full-duplex link. Current switches support full duplex. The advantage of full duplex is that the delay is small and the speed is fast.
Half-duplex means that only one action occurs in a period of time. For example, a single-plank bridge can only allow one person to pass at the same time. When two people, A and B, come across from both ends of the river bank, this situation occurs. Only one person can stop at the bridge head first, wait for another person to come over, and then continue to walk over. In this example, the single-plank bridge represents a half-duplex link. Early walkie-talkies and early hubs were all half-duplex products. When the data traffic is large, the link working in the half-duplex mode will have conflicts and wrong packets, which will eventually affect the work performance. Therefore, half-duplex has gradually withdrawn from the stage of history.
The difference between full-duplex mode and half-duplex mode in network cards
1. In full-duplex mode, the 8 mesh wires must be connected to the corresponding line sequence positions of the crystal head respectively;
2. In half-duplex mode, only need to connect 4 wires, refer to T568B standard, generally use 4 wires in 1, 2, 3, 6 line sequence positions, namely: orange-white, orange, green-white, green. Four wires. Orange-white and orange are used to send data, and green-white and green are used to receive data.
3. In full-duplex mode, the maximum speed of the network cable can reach more than 100 megabits. In half-duplex mode, the speed that the network cable can reach is usually around 10 megabits.
4. The network cable in full-duplex mode can work in both full-duplex mode and half-duplex mode; but the network cable in half-duplex mode cannot work in full-duplex mode.
5. In half-duplex mode, the extra two sets of lines can be used to connect telephone lines, for example, separate the "white-blue, blue" sets of lines for use as telephone lines. In the LAN network wiring, this method can save the wiring cost, and a twisted pair can be used as a network cable and a telephone line at the same time.
What is the network port rate?
The network interfaces we are using today are all Ethernet interfaces. The interface rate of the network port determines the bandwidth of the interface to transmit data. The commonly used interfaces are 10Mbps, 100Mbps, 1000Mbps and other rate types.
Common Ethernet interface types include RJ-45 interface, RJ-11 interface, SC fiber interface, FDDI interface, AUI interface, BNC interface, and Console interface. But we usually say that the speed of the network port is the RJ-45 interface, which is the crystal head of the network cable.
Interfaces with different rates may also be successfully connected, and their working rates need to be negotiated eventually. If a 100 Mbps auto-negotiation interface is interconnected with a 10 Mbps auto-negotiation interface, the negotiated working rate is 10 Mbps. Therefore, the interface rate is not necessarily the working rate. Generally, the working rate is less than or equal to the interface rate.