1、 Select according to the code stream and number of cameras

1. Camera bitstream

Before choosing a switch, you should first figure out how much bandwidth each image occupies.

2. Number of cameras
Make sure the bandwidth capacity of the switch. Commonly used switches include 100m switches and Gigabit switches. Their actual bandwidth is generally only 60-70% of the theoretical value, so the available bandwidth of their ports is roughly 60mbps or 600mbps.
give an example:
Look at the code stream of a single webcam according to the brand of the webcam you use, and then estimate how many cameras can be connected to a switch.
For example, 1.3 million: a single 960p camera usually has a code stream of 4m. If you use a 100m switch, you can connect 15 (15 × 4=60M）；
With Gigabit switch, you can connect 150 (150 × 4=600M）
2million: a single 1080p camera usually has a code stream of 8m. With a 100m switch, seven (7) cameras can be connected × 8=56M）；
With Gigabit switches, you can connect 75 (75 × 8=600M）
These are explained by taking the mainstream H.264 camera as an example. Halve the h.265.
In terms of network topology, a LAN is usually a two-tier to three-tier structure. The end connected to the camera is the access layer. Generally, a 100m switch is enough, unless you connect many cameras to a switch.
The collection layer and core layer are calculated according to the number of images gathered by the switch.

The calculation method is as follows: if you connect a 960p webcam, generally within 15 channels of images, use a 100m switch; Gigabit switches are used for more than 15 channels; If you connect a 1080p network camera, generally, use a 100m switch for images within 8 channels, and use a Gigabit switch for more than 8 channels.

2、 Switch selection requirements
The monitoring network has three layers: core layer, convergence layer and access layer.
1. Selection of access layer switch
Condition 1: camera code stream: 4Mbps, 20 cameras is 20*4=80mbps.
In other words, the uplink port of the access layer switch must meet the transmission rate requirements of 80mbps / s. considering the actual transmission rate of the switch (usually 50% of the nominal value, and about 50 m for 100m), the access layer switch should choose a switch with a 1000m uplink port.
Condition 2: backplane bandwidth of the switch. If a 24 port switch with two 1000m ports and a total of 26 ports is selected, the backplane bandwidth requirements of the switch in the access layer are: (24*100m*2+1000*2*2) /1000=8.8gbps backplane bandwidth.
Condition 3: packet forwarding rate: if the packet forwarding rate of a 1000m port is 1.488mpps/s, the switch switching rate of the access layer is: (24*100m/1000m+2) *1.488=6.55mpps.

According to the above conditions, when 20 720p cameras are connected to a switch, the switch must have at least one 1000m upload port and more than 20 100m access ports to meet the requirements.

2. Selection of convergence layer switch
If a total of five switches are connected, each switch has 20 cameras, and the code stream is 4m, then the flow of the convergence layer is 4mbps*20 *5=400mbps, then the upload port of the convergence layer must be more than 1000m.
If five IPCs are connected to a switch, an 8-port switch is generally required. Does this 8-port switch meet the requirements? We can see the following three aspects:
Backplane bandwidth: number of ports * port speed *2= backplane bandwidth, that is, 8*100*2=1.6gbps.
Packet switching rate: number of ports * port speed /1000*1.488mpps= packet switching rate, that is, 8*100/1000*1.488=1.20mpps. Sometimes the packet switching rate of some switches can not meet this requirement, that is, non line speed switches, which are prone to delay when carrying out large-scale throughput.
Cascade port bandwidth: the code stream of IPC * number = the minimum bandwidth of the upload port, that is, 4.*5=20mbps. Generally, when the IPC bandwidth exceeds 45mbps, it is recommended to use a 1000m cascade port.

3、 How to choose a switch?
give an example:
There is a campus network, more than 500 high-definition cameras, 3-4 megabytes of code stream, and the network structure is divided into access layer - convergence layer - core layer. Stored in the convergence layer, each convergence layer corresponds to 170 cameras.
Problems: how to choose products, the difference between 100m and Gigabit, what are the reasons that affect the transmission of images in the network, and what are the factors related to switches

1. Backplane bandwidth
The capacity of all ports x twice the sum of the number of ports should be less than the nominal backplane bandwidth, which can realize full duplex non blocking line speed switching, proving that the switch has the conditions to give full play to the maximum data exchange performance.
For example, a switch that can provide up to 48 Gigabit ports should have a full configured capacity of 48 × 1G × 2 = 96gbps, which can ensure the non blocking line speed packet switching when all ports are in full duplex.

2. Packet forwarding rate
Full configuration packet forwarding rate (Mbps) = full configuration Ge ports × 1.488mpps+ 100 megabyte ports in full configuration ×  0.1488mpps, of which one Gigabit port has a theoretical throughput of 1.488mpps when the packet length is 64 bytes.
For example, if a switch can provide up to 24 Gigabit ports, and the claimed packet forwarding rate is less than 35.71 MPPs (24 x 1.488mpps = 35.71), then it is reasonable to think that the switch adopts a blocking structural design.
Generally, the switch that meets both backplane bandwidth and packet forwarding rate is the appropriate switch.
The switch with relatively large backplane and relatively small throughput, in addition to retaining the ability to upgrade and expand, has software efficiency / dedicated chip circuit design problems; Switches with relatively small backplanes and large throughput have relatively high overall performance.
Camera bitstream affects the definition, which is usually the bitstream setting of video transmission (including the encoding and decoding capabilities of coding transmission and receiving equipment), which is the performance of front-end cameras and has nothing to do with the network.
Usually users think that the definition is not high, and the idea that it is caused by the network is actually a misunderstanding.
According to the above case, calculate:
Code stream: 4Mbps
Access: 24*4=96mbps<1000mbps<4435.2mbps
Convergence: 170*4=680mbps<1000mbps<4435.2mbps

3. Access switch
Mainly considering the link bandwidth between access and convergence, that is, the uplink link capacity of the switch needs to be greater than the number of cameras * code rate held at the same time.
In this way, there is no problem with real-time video recording, but if a user sees the video in real time, it also needs to consider this bandwidth. The bandwidth used by each user to view a video is 4m. If each camera connected to the switch has a person watching, it needs the bandwidth of camera number * code rate * (1+n), that is, 24*4* (1+1) =128m.

4. Convergence switch
The convergence layer needs to process the 3 ‐ 4m code stream (170 * 4m=680m) of 170 cameras at the same time, which means that the convergence layer switch needs to support the switching capacity of forwarding more than 680m at the same time. Generally, the storage is connected to the convergence, so the video recording is transmitted at line speed.
However, considering the bandwidth of real-time monitoring, each connection occupies 4m, and a 1000m link can support 250 cameras to be debugged. Each access switch is connected with 24 cameras, 250/24, which is equivalent to that the network can withstand the pressure of 10 users viewing each camera in real time.

5. Core switch
The core switch needs to consider the switching capacity and the link bandwidth to the convergence. Because the storage is placed in the convergence layer, the core switch has no pressure of video recording, that is, it only needs to consider how many people watch how many videos at the same time.
Suppose that in this case, there are 10 people watching at the same time, and each person watches 16 channels of video, that is, the exchange capacity needs to be greater than 10*16*4=640m.

6. Switch selection focus
When selecting switches for video monitoring in LAN, the selection of switches in access layer and convergence layer usually only needs to consider the factor of switching capacity, because users usually connect and obtain video through core switches.
In addition, since the main pressure is on the convergence layer switch, it is very important to choose a suitable convergence switch because it is not only responsible for monitoring the flow of storage, but also for real-time viewing and calling monitoring.