Hello,
Today I would like to share with you the relationship between MSTP time delay and bandwidth and speed.
With the advancement of technology, the development of networks and the increase of users’ dependence on communication, customers have higher and higher requirements for our MSTP circuits. In addition to the original emphasis on reliability and high bandwidth, they also put forward higher requirements on delay- --Low latency. How to ensure low latency, what the latency is related to, and what applications the latency affects do require our attention.
Time delay-The normal PING test time delay should include: sending, receiving, processing and transmitting time delay. The transmission delay should be related to the network elements that the MSTP circuit passes through, the transmission distance, and the processing of Ethernet packets at both ends of the transceiver. According to the optical transmission rate and SDH processing rate, the general transmission delay of MSTP is t=(5us*k+250us*n)*2+1ms. Among them, k represents the distance between the two ends of AB and the unit is km, n represents the number of network elements passing in the middle, and 1ms represents the delay of Ethernet encapsulation and decapsulation by the devices at both ends of the transceiver.
According to the empirical formula, when the transmission distance is less than 100km, the MSTP delay mainly affects the number of network elements passing, that is, the more complex the network, the more network elements pass through the SDH path, and the greater the delay; while the transmission distance is greater than 500km in the long-distance network, the delay is mainly related to the distance. Therefore, the delay in the local network is relatively large, which generally needs to be solved by optimizing the SDH path, while the long-distance usually needs to be solved by selecting a direct route.
What aspects will the delay affect customers at that time, and how does it relate to download speed and throughput? Why is a 100M (using 2 VC3 bundled) point-to-point dedicated circuit opened for customers, and the customer download test rate can only reach 17.0 Mbit/s?
The MSTP circuit needs to encapsulate the Ethernet packet and map it to VC12/VC3/VC4 for transmission. All transmission efficiency, that is, the maximum throughput, is related to the Ethernet packet length. EN=Package length/(Package length+20)*100%, the corresponding relationship between EN and package length is as follows:
Packet length (bytes) | Port rate | Maximum frame rate | Effective bandwidth (Mbit/s) | Transmission efficiency EN(%) |
64 | 100M | 148809 | 76.19 | 76.19 |
128 | 100M | 84459 | 86.49 | 86.49 |
256 | 100M | 45290 | 92.75 | 92.75 |
512 | 100M | 23496 | 96.24 | 96.24 |
1024 | 100M | 11973 | 98.08 | 98.08 |
1280 | 100M | 9615 | 98.46 | 98.46 |
1518 | 100M | 8127 | 98.70 | 98.70 |
If the length of the encapsulated packet is 64 bytes, EN=76.19%, and the packet length is 1518 bytes. The maximum EN is 98.70%. Therefore, the transmission efficiency of MSTP is generally between 85% and 98%. That is, the maximum effective bandwidth of 100M is: 45*2*98.70%=88.8M, and the minimum is: 45*2*76.19=68.57M. The minimum download speed of this circuit is also above 70M, but why is it so?
Originally, according to the principle of data IP, the TCP protocol used for FTP download has an acknowledgement retransmission mechanism. The download rate is related to the sliding window, maximum packet length, confirmation time, and circuit errors. That is, when the circuit is error-free and does not need to be retransmitted and there is no congestion, the sliding window is the largest, that is, the maximum data stream that can be transmitted in a single time is 65500bps. Now the problem arises. Because the circuit has a delay of 30ms, the maximum data stream of 65500bps can be transmitted within 30ms when using FTP single-threaded download, that is, the maximum download rate is 65500*8/30ms=17.466M/s.
To increase the download rate, you should use:
1. It is recommended that customers use multi-threaded downloading or TFTP downloading.
2. Reduce circuit delay.
3. Increase the processing cache.
The delay (more than 20MS) does have a certain impact on the download bandwidth of the network, especially on the high-bandwidth long-distance dedicated line circuit, the impact is more serious.
You are welcome to leave a message and exchange in the comment area. Thank you!
