Its interesting to look at how operators would be migrating from 3G/2G to 4G. 3G/2G networks are here to stay for a long time. 4G is the one which is supposed to co-exist with the existing 3G/2G networks. 3G/2G inter-working with 4G might need few tweaks on the packet core of exiting 3G/2G networks and also of-course the whole new 4G network deployment itself. Below are few such change I can think of from packet core network perspective. (I am assuming here that there are no major upgrades done on 3G network, which means 3G network is still pre Rel 8)
HLR/HSS:- HLR is database from 2G/3G. HSS is the new database in 4G. I would like these two entities to co-exist and share the data. We dont want to SGSN to contact one database and MME to contact other for authenticating the same imsi.
DNS integrations:- The same dns servers in 3G network should help MME in selecting SGW and PGW. Few entries need to be added and MME should be pointed to that DNS
SGSN configuration changes:- SGSN shoud be configured to point to new GGSN if users from LTE are moving to 3G network. This should not be a big problem as new subscribers mean new imsi and imsi can be mapped to a APN in SGSN (?) which in turn identifies the GGSN
Gn interface:- There is need for Gn interface between MME and SGSN to provision inter RAT handovers. If UE is moving from 4G to 3G then the Gn SGSN should be able to contact MME and pull out the UE context and vice versa.
Each point can taken up for discussion. Any more scenarios you can think of?
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for the HLR and HSS part, there is a product already where HLR supports Diameter interface, so I guess that problem is solved....
You wrote: "SGSN configuration changes:- SGSN shoud be configured to point to new GGSN if users from LTE are moving to 3G network. This should not be a big problem as new subscribers mean new imsi and imsi can be mapped to a APN in SGSN (?) which in turn identifies the GGSN"
Why do we need that? If the bearer/PDP context is already established in LTE, then the SAE-GW/GGSN has already been chosen and shall not change during inter-RAT handover; this is similar to a handover between 3G and 2G: the SGSN may change, but not the GGSN. Isn't this standard procedure?
This is a misunderstanding! When I said new GGSN, I meant GGSN that is co-located with PGW. So if a user from LTE (PGW/GGSN) is moving to 3G, the SGSN should contact the GGSN (PGW/GGSN). This needs a configuration update in SGSN, doesnt it?
Is there really a config change required in SGSN? As PGW/GGSN would be well equipped to talk to SGSN like GGSN was doing it before (in 3G), I don't think any config change is required in SGSN as such....or is it?
If you move the existing GGSN to PGW or add a new GGSN in PGW then SGSN should know it right?
Some vendors, to reuse investments in 2G/3G, are offering features in SGSN to route towards correct GGSN/PDN-GW based on IMSI series, etc.
So subscribers with 2G/3G subscription would land in GGSN and LTE subscribers would remain hooked to PDN-GW even performing IRAT to SGSN.
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