Tuesday, July 27, 2010

Must Read!

A book that explains SAE packet core in a nice way! A must read for engineers working in mobile packet core.

Amazon link: http://amzn.com/0123748267

Tuesday, July 20, 2010


Quick thought regarding the NAS layers between UE and MME. Since UE and MME directly cannot talk to each there could be a disconnect between the both. Consider this. When UE goes into idle mode, it is not known to the network until the network receives a TAU. In the mean time imagine that network initiated a bearer deletion procedure. If eNB doesn’t find the UE then bearer is implicitly detached on MME and eNB, but the bearer information is still present with the UE. At this point there could be a disconnect between UE NAS layer and MME NAS layer.

Assume another scenario where handover is in progress. Because of some reason the target eNB has not accepted a particular bearer. The same is conveyed to network in S1AP message and to UE in RRC messages. But NAS layers in MME and UE still have the bearer information. There could be several cases like this where there can be different NAS states in UE and MME. The question here is will RRC and NAS layers in UE be in sync. That is if a Data radio bearer for a particular bearer is not established, will the NAS layer be intimated the same by RRC and UE will delete the bearer from its NAS layer?


Wednesday, July 7, 2010

3G to 4G

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)

GGSN & PGW integration:- This will be a big change but shouldn't be that complicated to an operator. I am assuming that LTE subscribers will be given a new APN that can be accessed over a 3G or 4G network. This would need GGSN and PGW to co-exist as there is need to share the APN along with user context
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?