INROAMER GENERATES GGSN CDR IN VPLMN (SUSPECTED LOCAL BREAK ROAMING SCENARIO) IN A NETWORK WHICH SUPPORTS HOME ROUTED ROAMING SCENARIO.

A case study involving the Huawei MME (USN9810 V9R11) & Huawei PDN-GW (UGW9811 V9R13)

PREAMBLE:

During the usual network checks to see the contents of all CDR's generated, there was one unusual scenario that was noted where an inroamer had generated ggsn-cdr (Local breakout roaming scenario), Usually, we expect the inroamer to generate sgsn-cdr, and the ggsn-cdr must be generated in his home GGSN and not the visited GGSN (Home routed roaming scenario).

LIST OF ACRONYMS USED IN THIS WRITE UP:

1). VSGSN-Visiting SGSN

2). VGGSN-Visiting GGSN

3). HGGSN-Home GGSN

4). BG-Border Gateway

5). GRX-GPRS Roaming Exchange

BASIC STRATEGIES OF GPRS ROAMING:

STRATEGY 1: Access Data Service through the VSGSN and HGGSN (Home Routed).

In this model roaming subscribers attach to the SGSNs of visit PLMN, but subscribers can only resolve IP address of their home GGSN through APN, so the last data tunnel is set between SGSN of visit PLMN (flagged as VSGSN) and GGSN of home PLMN (flagged as HGGSN). Subscribers access data networks through this channel.

The technical features of this strategy are as follows:

1). Because subscribers use the APN of home PLMN to start services,GPRS networks must be interconnected.

2). Subscribers obtain dynamic IP address distributed by the GGSN of home PLMN or static IP address subscribed by the HLR of home PLMN.

3). Subscribers access data networks through the GGSN of home PLMN.

4). Use BG between VSGSN and HGGSN.

5). Need recursive resolutions of multi-level DNSs and interconnections among DNSs.

In this state of roaming strategy, subscribers cannot use the APN and GGSN of visit PLMN. For international roaming, the subscriber data flag VPLMN-ALLOWED in HLR must be FALSE that indicates that subscribers cannot use the OI of visit PLMN.

A SIMPLE TOPOLOGY FOR THIS FLOW IS AS SHOWN BELOW:

STRATEGY 2: Access Data Service through the VSGSN and VGGSN (Local Break Out)

In this model roaming subscribers attach to the SGSNs of visit PLMN and resolve IP address of GGSN through APN. The last data tunnel is set between SGSN ofvisit PLMN (flagged as VSGSN) and GGSN of visit PLMN (flagged as VGGSN). Subscribers access data networks through this channel.

The technical features of this strategy are as follows:

1). Because subscribers use the APN of visit PLMN to start services, GPRS networks dont need to be interconnected.

2). Subscribers obtain dynamic IP address distributed by the GGSN of visit PLMN or static IP address subscribed by the HLR of home PLMN.

3). Subscribers access data networks through GGSN of visit PLMN.

4). There is no need for DNS interconnection and recursive resolution.

In this state of roaming strategy, subscribers cannot use the APN of home PLMN, For international roaming, the subscriber data VPLMN-ALLOWED in HLR must be TRUE that indicates subscribers can use the OI of roaming network.

NOTE:

There are some more details about this roaming scenario, i have not presented here, in the even that you are interested in more details, you may need to read a little bit more, there could even be introduction of the S9 interface interconnecting the HPCRF to the VPCRF to be able to download charging policies etc etc..

A SIMPLE TOPOLOGY FOR THIS FLOW IS AS SHOWN BELOW:

ISSUE ISOLATION AND ANALYSIS:

With the above preamble in my mind the issue of why a ggsn-cdr of an inroamer was being generated in the VGGSN was almost clarified because by now we knew all the flags to be checked.

STEP1: We need to check the Insert Subscriber Data (ISD) Flag, this is sent into the VSGSN from HLR during the Location update procedure and we check the status of the VPLMN-ALLOWED flag.

As seen from above, this flag is set as Allowed. This went a long way in directing my troubleshooting and i was almost sure this was a closed case. But the questions that rung in my head were:

1). If the HPLMN has incorrectly set the VPLMN-ALLOWED flag, is there a way we can stop this CDR generation in the VPLMN? Because for most operators, they prefer the home routed because of charging issues and it will also offer a reliable point of reconciliation for the Visited Network in case of sgsn-cdr and Home Network incase of the ggsn-cdr if there disputes in charging between roaming operators.

CONCLUSION:

FEW QUESTIONS THAT COULD ARISE IN THE ENGINEER'S HEAD:

1). How we can restrict it on our network?

ANS: We can use below command to limit visiting users on VGGSN.

Change current visiting-user-access enable to disable under the APN configurations.Just in case the Home HLR has wrong configuration as in this scenario.

 2). How could PDP context activation be successful yet in SGSN, we don’t allow this MCC/MNC?

ANS: When the HLR carries vplmnAddressAllowed flag, The SGSN will use local MCCMNC, then select the local GGSN, it will not check user’s IMSI MCCMNC information.

3). How did VGGSN assign IP?

ANS: Because of configuration I have shown in 1 above, GGSN will use bound IP pool to allocate IP address to visiting user, this is almost the same as a local user.

As i sign out, i will say a few things below:

For the case of Home routed roaming scenario:

1). HLR must set VPLMN-ALLOWED as FALSE. Am sure even this operator just made a mistake as roaming agreement with them was Home Routed.

2). In Local GGSN visiting-user-access must be set to disable. Usually, it is enable by default.

HAPPY HOLIDAYS && I WISH YOU ALL A FRUITFUL 2018...LET'S MEET AGAIN IN THE NEW YEAR!!! BLESS!!!!