Relationship Between The Number of Switch Ports And The Number Of Cameras  
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Relationship Between The Number of Switch Ports And The Number Of Cameras (Part 2)

Time:2021-03-23 Source:UTEPO


In our last post, we talked about the Relationship Between The Number of Switch Ports And The Number Of Cameras (Part 1). And in this post, we would continue to this topic and provide a case to show you how to choose the switches for a project.


SWITCH SELECTION REQUIREMENTS

1. Access layer switch selection

Check this on last post--->Click


2. Aggregation layer switch selection

If there are a total of 5 switches access, each switch has 20 cameras, the data rate is 4M, then the aggregation layer traffic is: 4Mbps * 20 * 5 = 400Mbps, then the aggregation layer uplink port must be more than 1000M. If 5 IP cameras are connected to a switch, which normally requires an 8-port switch, does this 8-port switch meet the requirements? You can look at the following three aspects.


Bandwidth: Number of ports * port speed * 2 = bandwidth , i.e. 8*100*2 = 1.6Gbps.


Packet Switching Rate: Number of ports*Port Speed/1000*1.488Mpps=Packet Switching Rate, i.e. 8*100/1000*1.488=1.20Mpps.

The packet switching rate of some switches is calculated not to meet this requirement, then it is a non-wire-speed switch, which is prone to cause delays when performing high-volume quantity throughput.


Cascade port bandwidth: the data rate of IPC * number = minimum bandwidth of uplink port, i.e. 4*5 = 20Mbps. usually, when the IPC bandwidth exceeds 45Mbps, it is recommended to use 1000M cascade port.


HERE'S A CASE STUDY TO ILLUSTRATE EXACTLY HOW TO CHOOSE A SWITCH.

There are over 500 HD cameras (Data rate ranges from 3~4M) in a campus, whose network structure contains the access layer, aggregation layer, and core layer. The storage is in the aggregation layer, responsible for aggregating the data from 170 IP cameras.


The difficulties we may face are:

How to choose the equipment?

What is the difference between 100 megabit and gigabit?

What are the factors that affect the transmission of images in the network and which factors are related to the switch?

...


1. BANDWIDTH

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

For example, a switch with up to 48 Gigabit ports should have a full configuration capacity of 48 *1G*2 = 96Gbps to ensure non-blocking wire-speed packet switching when all ports are in full-duplex.


2. PACKET FORWARDING RATE

Full configuration packet forwarding rate (Mbps) = Number of fully configured Gigabit ports × 1.488Mpps + Number of fully configured 100 Megabit ports × 0.1488Mpps, where the theoretical throughput of 1 Gigabit port at a packet length of 64 bytes is 1.488Mpps.


For example, if the claimed packet forwarding rate of a 24 Gigabit ports switch is fewer than 35.71 Mpps (24 x 1.488Mpps = 35.71), it is reasonable to assume that the switch is designed with a blocking architecture. Generally it is the switch with the bandwidth and packet forwarding rate met that is the right switch.


In addition to retaining the ability to upgrade and expand, the switch with large bandwidth and small throughput would have problems with the software efficiency / specific chip circuit design. And the overall performance of switch with a small bandwidth and large throughput is relatively high.


The data rate of the camera, usually the date rate of video transmission, would affect the clarity. It includes the code capabilities of coding sending and receiving devices, etc. And it is the performance of the front-end camera, and nothing to do with the network. The idea of the network causes unclear clarity is a misconception.


According to the above case, the calculation is below

Data rate: 4Mbps

Access: 24*4=96Mbps<1000Mbps<4435.2Mbps

Aggregation: 170*4=680Mbps<1000Mbps<4435.2Mbps


3. ACCESS SWITCH

Mainly considering the link bandwidth between the access layer and aggregation layer, the Uplink capacity of the switch should be greater than the number of cameras (accommodated at the same time) * bit rate, which ensures the real-time video recording. But if you need to view the video in real-time, you should take into account this decoding bandwidth. As the bandwidth of each user to view a video is 4M, the bandwidth for each user viewing each camera at the same time is the number of cameras * bite rate * (1+N), that is, 24*4M*(1+1)=128M.


4. AGGREGATION SWITCH

The aggregation layer needs to simultaneously handle 3-4M data rate from 170 cameras (170*4M=680M), which means the aggregation layer switches need to support simultaneous forwarding of more than 680M switching capacity. Generally, the storage is connecting to the aggregation, so the video is forwarded at wire speed.


Considering the bandwidth of real-time viewing monitoring video, each connection would occupy 4M. Therefore, a 1000M link could support 250 cameras being debugged. Then each access switch connects 24 IP cameras. This is equivalent to a network that can withstand the stress of having 10 users (250/24) per camera viewing in real time.


5. CORE SWITCH

For the core switch, you need to consider its switching capacity and link bandwidth t0 the aggregation. As the storage is connecting to in aggregation layer, there is no pressure of video recording on the core switch. And you only need to consider how many users would simultaneously view how many videos.


Assume that there are 10 people viewing 16 videos at the same time, the switching capacity needs to be greater than 10 * 16 * 4 = 640M.


6. THE KEY FACTORS FOR SELECTION

When choosing switches in the access layer and aggregation layer for a LAN video surveillance system, the factor you only need to consider is the switching capacity, as the clients usually connect to the core switch and get the video.


It is also important to choose a proper aggregation switch. The main pressure is on the aggregation layer switches, bearing the traffic of surveillance storage and real-time view call.

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