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The Download Bandwidth Falls Short of the Expected Values of 40 Mbits and 60 Mbits in the GPON Test of an Office Because the Buffer Value on the FTP Server is not Set Correctly

Publication Date:  2012-07-25 Views:  34 Downloads:  0
Issue Description
In the GPON test of an office, six groups of data (including 100 Mbit/s, 80 Mbit/s, 60 Mbit/s, 40 Mbit/s, 20 Mbit/s, and 10 Mbit/s) are tested. For the test of 40 Mbit/s and 60 Mbit/s, the download rate falls short of the expected values (BW * 80%). For the test of other data, the download rate comes up to the expected values.
OLT version: MA5680T V800R005C32B056
ONT version: HG850 V100R001C02B021
Networking:
PC <====> ONT <=====> OLT <====> Cisco MPLS/IP <====> Internet <====> Oversea FTP Server
                                                                                ||
                                                                                ||
                                                                                ||
                                                                   Local FTP server 
 
Alarm Information
Null
Handling Process

1. For the download BW where the download rate of 40 Mbit/s is open, modify the buffer value of FTP server from 10,240 bytes to 4,096 bytes. The average download rate on the client reaches 32,361.44 Mbit/s ≈ 40 Mbit/s * 80%.
2. For the download BW where the download rate of 60 Mbit/s is open, modify the buffer value of FTP server from 10,240 bytes to 6,144 bytes. The average download rate on the client reaches 56,548.08 Mbit/s ≈ 60 Mbit/s * 80%. 
 

Root Cause
1. View the configuration file fed back by field personnel and check whether this problem is caused by incorrect car settings. Query the traffic-table, ont gemport bind, tcont bind-profile, and dba-profile corresponding to this stream, and determine that the car settings are correct.
2. The first test is conducted through the Oversea FTP server, so engineers preliminarily doubt that there exists a bottleneck in Cisco’s MPLS/IP network or packets are lost at a node. The customer emulates a Local FTP server to test the download test, but the problem persists. This doubt is removed.
3. Subsequently, engineers turn to the OLT. By running a series of commands, engineers determine whether packet loss occurs at this node.
MA5600T(config-if-giu-0/17)#display port traffic 0
In the output information, The received traffic of this port(octets/s) shows the following facts: When the download bandwidth of 40 Mbit/s is open on the upstream port, a data stream of only 21 Mbit/s is received. When the download bandwidth of 60 Mbit/s is open on the upstream port, a data stream of only 32 Mbit/s is received.
Engineers can also view the information repeatedly in the transparent channel mode of the GPBC:
Omitted…
MA5600T(diagnose)%%su
Challenge:MLTQ2XS3
Please input password:
MA5600T(su)%%transparent on 0/4
Serial redirect function is enabled now!
MA5600T(su)%%show tm sta pt
=============== DISCARD ==================================
Upstream:
Drop frame for CAR [120]: 00000712
Drop frame for port-attr-tbl indicates discard [221]: 00000120
Dnstream:
Drop frame for CAR [122]: 00054288
Omitted…
In the Dnstream, the number of "Drop frame for CAR" packets is increased very slowly, but packet loss does not occur on a large scale. The fact proves that the small-scale packet loss is caused because the burst rate of the FTP download exceeds the car value. Packet loss, however, does not occur on a large scale on the GPBC.
The two facts show that the OLT is not faulty and packet loss does not occur on the OLT.
4. Under the same test environment, why does the problem occur to the rate of 40 Mbit/s and 60 Mbit/s rather than the rate of 100 Mbit/s, 80 Mbit/s, 20 Mbit/s, and 10 Mbit/s? Engineers exclude the possibility that the network equipment between the server and the client is faulty. Then, engineers focus their doubt on both ends. FTP is the application-layer protocol that is based on the CS model and TCP. To attain the expected download rate, different buffer values must be set to suit different download BWs on the server and on the client. 
 
Suggestions
FTP download is an end-to-end process. The download rate involves each aspect of the whole end-to-end process, for example, hardware performance of FTP Server and Client, bandwidth and delay of transmission line, algorithm of FTP Server (including the size of the transmitting buffer), and algorithm of Client (including the size of the sliding window). Engineers often encounter the case that the download rate on the client falls short of the expected value due to the incorrect setting of the transmitting buffer on the FTP server. Therefore, it is recommended that different buffer values should be set to suit different download bandwidth on the FTP Server. The recommended proportion of the buffer to the BW is 1:1280”(4096 bytes : 40 *1024 * 1024/8 or 6144 bytes : 60 *1024 * 1024/8 bytes).  

END