In the network of the video surveillance system, a large amount of continuous video data needs to be transmitted, which requires the switch to have the ability to transmit data stably. The more cameras connected to the switch, the greater the amount of data flowing through the switch. We can think of code flow as water flow, and switches are water conservancy hubs one by one. Once the flowing water flow exceeds the load, the dam will burst. Similarly, if the amount of data forwarded by the camera under the switch exceeds the forwarding capacity of a certain port, this port will also cause a large amount of data to be discarded, causing problems.
Let’s talk about the four main aspects of switch selection.
01 Choose Gigabit or 100M?
In the network of the video surveillance system, a large amount of continuous video data needs to be transmitted, which requires the switch to have the ability to transmit data stably. The more cameras connected to the switch, the greater the amount of data flowing through the switch. We can think of code flow as water flow, and switches are water conservancy hubs one by one. Once the flowing water flow exceeds the load, the dam will burst. Similarly, if the amount of data forwarded by the camera under the switch exceeds the forwarding capacity of a certain port, this port will also cause a large amount of data to be discarded, causing problems.
For example, if a 100M switch forwards more than 100M of data, it will cause a large amount of packet loss, resulting in a blurry screen.
So, how many cameras do you need to choose a Gigabit switch?
There is a standard, which depends on the amount of data forwarded by the camera’s uplink port: if the amount of data forwarded by the uplink port is greater than 70M, choose a Gigabit port, that is, choose a Gigabit switch or a Gigabit uplink switch.
Here’s a quick way to calculate and select:
Bandwidth value = (sub stream + main stream) * number of channels * 1.2<br>
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Bandwidth value > 70M, use Gigabit
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If the bandwidth value is <70M, use 100M
Let me give an example to illustrate: there is a switch connected to 20 H.264 200W cameras (4 + 1M), then according to this calculation, the forwarding rate of the uplink port is (4 + 1)*20* 1.2=120M>70M, in this case a Gigabit switch is required. In some scenarios, only one port of the switch needs to be Gigabit, but if the system structure cannot be optimized to balance traffic, then a Gigabit switch or a Gigabit uplink switch is required.
Question 1: The code stream calculation process is very clear, but why multiply it by 1.2?
Because according to the principle of network communication, the encapsulation of data packets also follows the TCP/IP protocol, and the data part needs to be marked with the header fields of each protocol layer to be transmitted smoothly, so the header will also occupy a certain proportion of overhead.
We often say that the camera 4M bit rate, 2M bit rate, etc., actually refer to the size of the data part. According to the proportion of data communication, the head overhead accounts for about 20%, so the formula should be multiplied by 1.2.
▲The data header accounts for about 20% of the overhead
Question 2: Why is it 70M instead of 100M?
Mainly to consider burst traffic. The video data stream is composed of many frames. The seemingly flat data stream actually produces many instantaneous bursts of data. In this case, the switch needs to be able to buffer and rectify the data fluctuations.
The switch stores-forward-store-forward these data, so it is recommended to have a certain amount of reservation. When designing the switching network, there can be 30%~40% of the reservation. For a 100M port, it is recommended that the forwarding traffic should not exceed 70M.
Cameras commonly used in engineering mainly have two bit rates: H.264 and H.265. Calculate according to the bit rate:
We use the H.264 200W camera (the main sub-stream is calculated as 4 + 1M), the bandwidth calculation and switch selection in the common serial network:
The star network structure is as follows:
02 How to choose a core switch?
Large and medium-sized monitoring networks are usually designed according to the access-aggregation-core three-layer structure. The core switch is the data forwarding center of the entire network and carries a large amount of data flow. Therefore, it is necessary to ensure that there is no bottleneck in the forwarding of each port of the core switch.
Some people have some misunderstandings about the choice of core switches. For example, if there are 200 or 500 cameras, if calculated according to the method of 500*5M=2500M, the result is far greater than the forwarding rate of Gigabit ports. Does the project have to use 10 Gigabit switches?
Not necessarily, in fact, in a typical large-scale monitoring network, the traffic will not be concentrated on one port, but will be distributed on multiple ports and forwarded by multiple gigabit ports.
As shown below:
It can be seen that each port in the figure does not exceed 1000M, and any two Gigabit ports of a full Gigabit switch can achieve 1000M bidirectional transmission, and the total throughput (full load) is generally less than or equal to the backplane of the switch bandwidth.
Therefore, when selecting a core switch, according to the number of IPCs, the following suggestions are recommended:
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100~200 units, Gigabit managed switches are recommended
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200~500 units, three layer managed switches are recommended
At present, the second/third layer managed full Gigabit switch is suitable for the core exchange of the monitoring network and undertakes large-capacity data exchange. Form various networks.
For a large or very large (300~1000) monitoring network, it is necessary to use a three-layer switch to divide the network segment, and it is recommended to use a three-layer switch. Here are the networking schemes for 100, 300, and 500 points.
100 IPC networking scheme
About 100 points, the design focus is on the non-blocking forwarding core.
Networking scheme of 300 IPC
About 300 points, the design focuses on multiple network segments and smooth forwarding.
500 IPC networking scheme
The scale of 500 points requires redundant design, which is very suitable for large parks such as government enterprises.
03How to choose a PoE switch?
PoE is a technology for power supply and data transmission through network cables. Only one network cable is needed to connect to a PoE camera point, and no additional power wiring is required.
What should be considered when choosing a PoE switch?
01 Single port power
Whether the power of a single port can meet the maximum power of any IPC connected to the switch, that is, select the specification of the switch according to the maximum power of the IPC.
The normal PoE IPC power will not exceed 10W, so the switch only needs to support 802.3af. However, the power requirement of some high-speed domes is about 20W, or the power of some wireless access APs will be higher, so the switch needs to support 802.3at.
The output power corresponding to the two technologies is as follows:
02 The maximum power supply of the whole machine
Confirm that the maximum power supply of the whole machine meets the requirements, and the power of all IPCs needs to be taken into consideration during design. The maximum output power of the switch must be greater than the sum of the power of all IPCs.
03 power supply type
There is no need to consider the use of eight-core network cables for transmission.
If it is a four-core network cable, you need to confirm whether the switch supports Class A power supply.
When choosing, you can choose according to the advantages and cost considerations of various PoE:
04 How to choose a fiber optic switch?
In the monitoring of long-distance points, fiber optic transceivers and fiber optic switches are often used. In the following example, a relatively comprehensive fiber optic switching network equipment is included, such as transceivers, switches, modules, etc.
Optical switches, fiber optic transceivers, and optical modules can be used in conjunction with each other. When choosing, pay attention to using them in pairs, and make sure that the A-B ends match.
The A/B end is the two ends of optical fiber transmission. No matter whether the two ends are switches, optical modules or fiber optic transceivers, the two ends must be A and B respectively to be paired and used (the A or B end is marked on the product model) ).
The operating wavelength of the A-end device is 1310nm (RX), 1550nm (TX), and must be used with the B-end fiber optic transceiver (RX1550nm, TX1310nm).
Finally, port speed, fiber type, and dual or single fiber also need to be considered.