Osmocom and LTE

most of Osmocom CNI is written by paid developers these days
developments mostly determined by customer requirements
sysmocom isn’t facing any significnat LTE requirements for years
Result: No native LTE support in any Osmocom components
However: we do have small LTE related bits
- SI2quater for broadcasting eUTRAN neighbors over GSM
- SGs interface for 2G/3G side of CSFB and for SMS-over-SGs

Irrespective of Osmocom…

What do we need to run a combined 2G/4G network?

shared subscriber database (HLR + HSS)
advertisement of respective neighbor cells
shared PS tunnel endpoint (GGSN + P-GW)

optionally:

SGs interface in MME and MSC for CSGB + SMS-over-SGs
Inter-RAT PS handover
SRVCC (Single Radio Voice Call Continuity)

Shared Subscriber Database

Required mainly for Authentication
poor man’s hack: Simply install same IMSI/Key material in both HLR and HSS
- will result in auth re-sync on every authentication due to non-synchronized SEQ/SQN of UMTS AKA
proper solution:
- Single HLR with both GSUP and DIAMETER, or
- DIAMETER-to-GSUP translator (to make LTE use OsmoHLR), or
- GSUP-to-DIAMETER translator (to make OsmoMSC/SGSN use HSS)

Advertisement of neighbor cells

GSM must advertise eUTRAN neighbors
- we have SI2quater support in osmo-bts
- TEMS phones show proper/valid decode
LTE must advertise GSM neighbors
- no "OML" for S1-AP
- configuration mechanism is vendor-proprietary

Shared PS tunnel endpoint

if the MS/eUE moves from GPRS to LTE, it wants to keep PDP/PDN contexts
solution: implementing P-GW and GGSN functionality in one element
- ergw might be able to do that (untested, complex)
- OsmoGGSN doesn’t speak GTPv2C (required for P-GW)
- nextepc P-GW doesn’t speak GTPv1C (required for GGSN)

Solution:

play with ergw, or
adopt nextepc GTPv2C code generator to use msgb/libosmo* and add GTPv2C to OsmoGGSN

SGs interface

Osmocom already implements full SGs interface
Interaction with MME on LTE side needs only interop testing

Inter-RAT PS handover

inter-RAT HO offers seamless migration between GERAN + eUTRAN
uses S3 interface between MME and SGSN (GTPv2C based)
uses S4 interface between S-GW and SGSN (GTPv2C based)
tbh, I don’t know the details here.
any help appreciated
https://www.tech-invite.com/3m23/toc/tinv-3gpp-23-401_zh.html
probably a long way for Osmocom to support it, given that we don’t really do any inter-SGSN HO even inside a single RAT.

Conclusions

The logical steps to improve our LTE interworking are:

split HLR/HSS and split GGSN/P-GW (today)
GSUP-to-DIAMETER translator for shared HLR/HSS
shared GGSN/P-GW for

EOF

End of File