How many cameras can a switch carry? Choose according to the stream and number of cameras

 

1. Camera code stream

 

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

2. The number of cameras must figure out 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. For example:

 

Look at a single stream according to the brand of the network camera you use, and then estimate how many cameras a switch can connect to.

 

For example: ①1.3 million: a single bit stream of a 960p camera is usually 4M, if you use a 100M switch, you can connect 15 units (15×4=60M); if you use a gigabit switch, you can connect 150 (150×4=600M).

 

② 2 million: 1080P camera single bit stream is usually 8M, with 100M switch, you can connect 7 units (7×8=56M); with Gigabit switch, you can connect 75 units (75×8=600M). These are all mainstream H.264 camera as an example to explain to you, H.265 is halved.

 

The collection layer and core layer are calculated based on how many images the switch has aggregated.

 

The calculation method is as follows: if you connect to a 960P network camera, generally use a 100M switch within 15 channels of images; if you exceed 15 channels, use a gigabit switch; if you connect a 1080P network camera, generally use a 100M switch within 8 channels of images, which exceeds Gigabit switches are used for 8 channels.

 

2,the selection requirements of the switch

 

The monitoring network has a three-layer architecture: the core layer, the convergence layer, and the access layer.

1. Selection of access layer switch Condition 1:

 

Camera code stream: 4Mbps, 20 cameras is 20*4=80Mbps.

 

In other words, the upload port of the access layer switch must meet the transmission rate requirement of 80Mbps/s, considering the actual transmission rate of the switch (usually 50% of the nominal value, 100M is about 50M), so the access layer The switch should be a switch with 1000M upload port.

 

Condition 2:

 

The backplane bandwidth of the switch, if you choose a 24-port switch with two 1000M ports, a total of 26 ports, the backplane bandwidth of the switch at the access layer requires: (24*100M*2+1000*2*2)/1000 =8.8Gbps backplane bandwidth.

 

Condition 3:

 

Packet forwarding rate: The packet forwarding rate of a 1000M port is 1.488Mpps/s, then the exchange rate of the switch at 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 demand.

 

2. The choice of aggregation layer switch

 

If there are a total of 5 switches connected, each switch has 20 cameras, and the code stream is 4M, then the flow of the aggregation layer is: 4Mbps*20 *5=400Mbps, then the upload port of the aggregation layer must be above 1000M.

 

If 5 IPCs are connected to a switch, an 8-port switch is generally required. Does this 8-port switch meet the requirements? You can look at the following three aspects:

 

Backplane bandwidth: Number of ports*port speed*2=backplane bandwidth, that is, 8*100*2=1.6Gbps.

 

Packet exchange rate: number of ports * port speed/1000*1.488Mpps = packet exchange rate, that is, 8*100/1000*1.488=1.20Mpps.

 

The packet exchange rate of some switches is sometimes calculated to be unable to meet this requirement, then it is a non-wire-speed switch, which is likely to cause delay when handling large-capacity quantities.

 

Cascade port bandwidth: IPC code stream * quantity = minimum bandwidth of the upload port, that is, 4.*5=20Mbps. Generally, when the IPC bandwidth exceeds 45Mbps, it is recommended to use the 1000M cascade port.

 

Third, how to choose a switch

For example, there is a campus network with more than 500 high-definition cameras with a code stream of 3 to 4 megabytes. The network structure is divided into access layer-convergence layer-core layer. Stored in the convergence layer, each convergence layer corresponds to 170 cameras.

 

The problem faced: how to choose the product, the difference between 100M and Gigabit, what are the reasons that affect the image transmission in the network, and which factors are related to the switch...

 

1. Backplane bandwidth

 

Two times the sum of the capacity of all ports x the number of ports should be less than the nominal backplane bandwidth, which can realize full-duplex non-blocking wire-speed switching, which proves that the switch has the conditions for maximum data exchange performance.

 

For example: a switch that can provide up to 48 Gigabit ports, its fully configured capacity should reach 48 × 1G × 2 = 96Gbps, to ensure that all ports are in full duplex, providing non-blocking wire-speed packet switching .

 

2. Packet forwarding rate

 

Full configuration packet forwarding rate (Mbps) = number of fully configured GE ports × 1.488 Mpps + number of fully configured 100M ports × 0.1488 Mpps, of which the theoretical throughput of one gigabit port when the packet length is 64 bytes is 1.488 Mpps.

 

For example: if a switch can provide up to 24 Gigabit ports, and the declared packet forwarding rate is less than 35.71 Mpps (24 x 1.488Mpps = 35.71), then there is reason to believe that the switch uses a blocking structure design.

 

Generally, a switch that satisfies the backplane bandwidth and packet forwarding rate is the appropriate switch.

 

Switches with relatively large backplanes and relatively small throughput, in addition to retaining the ability to upgrade and expand, are software efficiency/dedicated chip circuit design problems; switches with relatively small backplanes and relatively large throughput have higher overall performance.

 

The camera stream affects the definition, usually the stream setting of the video transmission (including the encoding and decoding capabilities of the encoding sending and receiving equipment, etc.). This is the performance of the front-end camera and has nothing to do with the network.

 

Generally, users think that the clarity is not high, and the idea that it is caused by the network is actually a misunderstanding.

 

According to the above case, calculate:

 

Stream: 4Mbps Access: 24*4=96Mbps<1000Mbps<4435.2Mbps Convergence: 170*4=680Mbps<1000Mbps<4435.2Mbps

 

3. Access switch

 

The main consideration is the link bandwidth between the access to the convergence, that is, the uplink link capacity of the switch needs to be greater than the number of cameras that can be accommodated at the same time * bit rate.

 

In this way, there is no problem with real-time video recording, but if a user sees the video in real time, this bandwidth needs to be considered. The bandwidth occupied by each user to view a video is 4M. If each camera connected to the switch has If one person is watching, the number of cameras * bit rate * (1+N) bandwidth is needed, that is, 24*4*(1+1)=128M.

 

4. Convergence switch

 

The aggregation layer needs to process the 3-4M stream (170* 4M=680M) of 170 cameras at the same time, which means that the aggregation layer switch needs to support simultaneous forwarding of more than 680M switching capacity. Generally, storage is connected to the convergence, so the video recording is forwarded at line speed.

 

However, considering the bandwidth of real-time viewing and monitoring, each connection occupies 4M, and a 1000M link can support 250 cameras to be debugged and called. Each access switch is connected to 24 cameras, 250/24, which is equivalent to the network can withstand the pressure of 10 users viewing each camera simultaneously in real time.

 

5. Core switch

 

The core switch needs to consider the switching capacity and the link bandwidth to the aggregation. Because storage is placed at the aggregation layer, the core switch does not have the pressure of video recording, that is, just consider how many people watch how many channels of video at the same time.

 

Assuming that in this case, there are 10 people watching at the same time, each watching 16 channels of video, that is, the exchange capacity needs to be greater than 10*16*4=640M.

 

6. Key points of switch selection When selecting switches for video surveillance in a local area network, the choice of access layer and aggregation layer switches 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 aggregation switch, it is very important to choose the appropriate aggregation switch because it is necessary to undertake the monitoring of the storage traffic and the pressure of real-time viewing and calling monitoring.