Hello,
Today, I would like to share with you this topic which describes the abnormal explanation of the CAR speed limit on the OLT side.
The GPON service test is carried out through the dedicated line, and the phenomenon that the actual measurement rate is much smaller than the configured rate during single-thread download is observed.
Networking: Test PC-OT925G-MA5680T-S8512-metropolitan area network.
There is no rate limit on the upstream of the ONU side, and several traffic of different rates (2M, 40M, 10M) are used on the OLT side to set the downstream rate of the service flow.
The ONU side performs a single-threaded download test. The measured rate is much smaller than the configured rate. When the configured rate is 2M, the measured rate is 150kBps.
On the ONU side, the multi-thread download test is performed. The measured rate is equivalent to the configured rate, reaching the expected value, and does not exceed the required range. When the configured rate is 2M, the measured rate is 220kBps.
The CAR limit used by GPON equipment will be discarded when downloading burst data (not cached when downloading as ADSL). The HTTP and FTP downloads used for testing use the TCP protocol. TCP will use the congestion avoidance algorithm and slow start to adjust the transmission Strategy to avoid data congestion on transmission lines. Once congestion is found (timeout or duplicate acknowledgment is received), the sender will reduce the sending speed, and the impact will be greatly reduced when downloading in multiple threads.
In the test case, the rate negotiated by the general port is greater than the CAR-restricted rate. Therefore, the data packet transmission rate must also be greater than the CAR-restricted rate.
After the device receives the data packet, the rate will exceed the CAR rate. Packet discarding (relatively speaking, ADSL and other port rate-limiting buffers) therefore cause the problem that the single-thread download rate is lower than the actual required rate.
In the actual test, the rate test is performed by multi-threading or using a special test device as much as possible. At this time, the device bandwidth can reach a predetermined value and meet the required range.
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