Basic knowledge
网络延迟：网络延时指一个数据包从用户的计算机发送到网站服务器，然后再立即从网站服务器返回用户计算机的来回时间。通常使用网络管理工具PING(Packet Internet Grope)来测量网络延时。
抖动：抖动是QOS里面常用的一个概念，其意思是指分组延迟的变化程度。网络延时随时都在不停的变化称为抖动（延时的变化成为抖动）。
丢包：丢包率，是一个比率，网络中数据的传输是以发送和接收数据包的形式传输的，理想状态下是发送了多少数据包就能接收到多少数据包，但是由于信号衰减、网络质量等等诸多因素的影响下，并不会出现理想状态的结果，就是不会发多少数据包就能接收到多少。
MTU：因为协议数据单元的包头和包尾的长度是固定的，MTU越大，则一个协议数据单元的承载的有效数据就越长，通信效率也越高。MTU越大，传送相同的用户数据所需的数据包个数也越低。
Key points of weak network test
Charles capture packets for weak network testing, suitable for PC and mobile (android / ios)
1. Open Proxy-> Throttle Settings
2. The specific interface of opening Throttle Settings is introduced as follows
3. The presets of charles already have common network speed simulation settings. You can set them according to your needs, or you can add presets yourself. Specifically, you can study
Bandwidth (Bandwidth) [throughput]
bandwidth definition data can be transmitted beyond the upper limit of time, which is specified in kilobits per second. You can specify different bandwidth limits for upload and download links.
Utilisation (use)
utilization is the percentage of the total bandwidth may be used at any one time. It only serves as a scaling factor for available bandwidth. For most modern Internet connections the utilization rate is always 100%.
Round-trip Latency [Delay] Round-trip
latency measures the millisecond delay of the first round-trip communication between the client and the remote server. It is used for every request from client to server and server to client.
MTU (Maximum Transmission Unit)
The maximum size of any TCP packet transmitted. Specifying the MTU does not change the available bandwidth, but allows Charles to allocate bandwidth-sized blocks at the MTU, resulting in a realistic level of splitting in each transmission packet.
Reliability [packet loss]
Reliability is a measure of the likelihood of a complete connection failure. This is very useful for simulating unreliable network conditions. Reliability is specified as the probability of successfully transmitting a 10kib message, so a value of 50% means that half of all 10kib transmissions will succeed. Larger messages or smaller messages are more or less likely to fail, so 20kib transmission will only have a 25% success rate and 5kib transmission success rate is about 70%.
Stability [Jitter]
Stability is a measure of the probability that a connection is unstable, thus reducing quality. This is a very useful analogue network, such as a mobile network, where the connection quality is poor on a regular basis. If the connection is unstable, the quality of the connection will randomly drop within the unstable quality range. This quality value is then applied to the available bandwidth as another scaling factor.
unstable quality range
The following configuration is used to simulate a 2G cellular network that is not very good.
Bandwidth 256 kbps (upload and download)
Utilize 100%
delay 300 ms
MTU is 1500 bytes
Reliability 75%
stable 75%
unstable quality range is 20% to 25%