Sunday, 4 October 2015

MME pool inbalance subscriber distribution

Problem Description
OPERATOR X network consist of two USN and they both form a pool.
After commercial for sometime, notice subscriber imbalance issue for both USN inside a pool although USN set relative
capabilities 100:100 and already delivered to enode b during S1_setup_response.

USN02(Tenalai) will always have more attach users compare to USN 01(HQ).

No alarm trigger.
Handling Process
1. Perform below test to verify S1-flex feature.
a) Use 5 testing UE, purge all the UE GUTI record in MME. Find an enode b where currently no user, and let all the testing UE attach to LTE network one by one.
Monitor the users UE status in M2000.
b) Notice after MME purge all the GUTI record in MME, UE attach in balance manner according to the relative capacity that set in MME.
c) However, after all the users fall back to 3G network, and TAU(tracking area update) back to 4G network, notice most of the UE will attach to same MME with MMEC=02,
and causing users number imbalance between two MME in the pool. From the tracing of Enode B side, notice Enode b allocate 

d) According to 3GPP for S1 flex in Enode side, if S-TMSI is available, Enode B will use S-TMSI to allocate UE. S-TMSI only available in 4G
network. S-TMSI (SAE Temporary Mobile Subscriber Identity) is used to locally identify a UE in short within a MME group (Unique within a MME Pool).
The reason why S-TMSI is used to minimize inter-MME signalling where non first time attach users in LTE network should attach to previous attach MME based on MMEC available in S-TMSI.
          S-TMSI= MMEC + MTMSI

e) When UE TAU from 3G to 4G, TAU request does not carry S-TMSI 
However, there are two type of information available, Old GUTI/Mapped GUTI and Additional GUTI.
Old GUTI/Mapped GUTI is mapped from 3G network PTMSI, 
Additional GUTI carry UE 4G information when UE first attach to 4G network.
Based on the tracing below, MMEGI=0x8001 and MMEC=01 is 4G network planning. While in 3G network, MMEGI=0x324 and MMEC=02. 
d) Back to the MME user inbalance problem,  Enode B tracing show all the UE carry MMEC=02, therefore they are allocated to MME02,
causing the imbalance. This MMEC=02, reflect that Enode B get this information from Old GUTI, in this case is the 3G network information to do the allocation.

e) 3G network consist of 2 SGSN each with MMEC=02 and MMEC=03 while 4G network consist of two USN each with MMEC=01 and MMEC=02.
Whenever UE fall back to 3G network, SGSN will allocate a PTMSI to the UE and this PTMSI will mapped to Old GUTI that carry SGSN information that UE attach in 3G network.
This explain why whenever user TAU back to 4G network it will relocated to USN with MMEC=02, although the UE is attach to USN with MMEC=01 previously before
fall back to 3G network and causing USN with MMEC=02 has more attach users than the other.

f) Based on the logic of S1 flex, when UE attach in 4G network, in order to minimize inter-MME signalling, Enode B will use MMEC information to allocate
the user back to same MME. However same theory apply to this case, Enode B will need to use additonal GUTI for the allocation instead of Old GUTI. 
Additonal GUTI carry MMEC that UE first attach to 4G network.
Root Cause
1) Solution A:
Identify from 3GPP, study how Enode B perform UE allocation. Based on the information, when an Enode B selects an MME, the Enode B may use a selection function which
distinguishes if the GUMMEI is mapped from P-TMSI/RAI or is a native GUMMEI.  The differentiation between a GUMMEI mapped from P-TMSI/RAI and a native
GUMMEI may be performed based on the value of most significant bit of the MME Group ID. In this case, if the MSB is set to "0" then the GUMMEI is mapped from P-TMSI/RAI and if
MSB is set to "1", the GUMMEI is a native one.
In 4G network, MMEGI is planned as 0x8001, where most significant bit is "1" while in  3G network, MMEGI is planned as 0x324. The most significant bit is "0".

This can be concluded where Enode B able to identify LTE and Non-LTE attached users, however the proper handling method for scenario TAU from 3G back to 4G network is not define by 3GPP.

Currently Enode B will consider only network planning where both 3G and 4G network are using same USN equipment,
it is valid where Enode B using Old GUTI MMEC information to relocate attach users after TAU back to 4G.

However, in Operator x, both 3G and 4G is multivendor with different MMEGI and MMEC planning. Submitted request to Enode team to handle
multivendor situation and treat all the users that TAU from 3G to 4G network as new users and allocate them as according to MME relative capacity using S1 flex.

2) Solution B
Changing MMEC planning in core network. Changing MMEC consider as a high risk operation.

a) Both USN changing MMEC code, need to restart both MME when operation complete. Expected downtime.
b) MME DNS hostfile need modification accordingly.
c) SGSN s10 need modification accordingly.
d) USN Integrate to HSS DM link need medification accordingly.
e) HSS diameter link need modification accordingly.

Suggestions and Summary
USN version V900R011C01SPC300.

Budi Prasetyo

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