Sunday, 4 October 2015

LTE Upload Problem with SGW on SPUe GGSN

Problem Description
LTE Upload issue from a particular vendor eNB areas when using the SGW function on SPUe GGSN.
There was a major degradation of throughput on UL data rates. Customer expectation was more than 35Mbps UL data rate and there was only 8Mbps UL data rate possible 

There was no alarm information, the issue was found during drive tests and field testing

Handling Process
From the S1U and SGi interface wireshark traces we could see a high number of TCP Fragment packets and also many TCP out-of.order packets on the uplink.
 This issue was seen on the UL tests from another vendor eNB areas and on Huawei eNB areas there was no issue.
It was determined later from the other vendor eNB that it has a different mechanism of IP handling and hence the high number of fragments. Also this issue was seen
only when SGW functionality was used on SPUe GGSN. Software version on GGSN was PS10.0 SPC100. This issue is seen only on SPUe
GGSN because of the way IP packet handling is implemented on SPUe. In SPUe by default the packet can be handled by any CPU
on the SPUe card and hence can result in out-of-order packets.

Traces cannot be attached as they contain user plane data...

Root Cause
After analysis it was found that the UL data degradation was because of the way the SPUe was handling the IP reassembly.
The softpara dword51 with bit 5 set to 1 solved the issue.DWORD51 Controlling Whether to Enable Functions Related to the Forwarding Plane (2)
If it is set to 0, each CPU on the SPUe reassembles IP fragments. This reassembly method enables higher throughput, but cannot ensure the
incoming and outgoing sequence of IP fragments, and therefore applies to sites that are not sensitive to out-of-order packets.
If it is set to 1, only one CPU on the SPUe reassembles IP fragments. This reassembly method ensures the incoming and outgoing sequence of IP fragments,
but decreases the SPUe throughput, and therefore applies to sites that are sensitive to out-of-order packets.

Suggestions and Summary
Before using the dword51 setting we need to understand the Site behaviour and the number of out-of-order packets. In some cases there might be a degration in the overall throughout,
but in this particular case we saw an improvement in UL data throughput from 8Mbps to 40Mbps, exceeding customer expectation.

Budi Prasetyo

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