93 lines
2.8 KiB
Plaintext
93 lines
2.8 KiB
Plaintext
Osmocom and LTE
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===============
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:author: Harald Welte <laforge@gnumonks.org>
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:copyright: 2019 by Harald Welte (License: CC-BY-SA)
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:backend: slidy
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:max-width: 45em
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== Osmocom and LTE
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* most of Osmocom CNI is written by paid developers these days
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* developments mostly determined by customer requirements
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* sysmocom isn't facing any significnat LTE requirements for years
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* Result: No _native_ LTE support in any Osmocom components
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* However: we do have small LTE related bits
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** SI2quater for broadcasting eUTRAN neighbors over GSM
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** SGs interface for 2G/3G side of CSFB and for SMS-over-SGs
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== Irrespective of Osmocom...
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What do we need to run a combined 2G/4G network?
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1. shared subscriber database (HLR + HSS)
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2. advertisement of respective neighbor cells
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3. shared PS tunnel endpoint (GGSN + P-GW)
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optionally:
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* SGs interface in MME and MSC for CSGB + SMS-over-SGs
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* Inter-RAT PS handover
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* SRVCC (Single Radio Voice Call Continuity)
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== Shared Subscriber Database
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* Required mainly for Authentication
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* poor man's hack: Simply install same IMSI/Key material in both HLR and HSS
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** will result in auth re-sync on every authentication due to non-synchronized SEQ/SQN of UMTS AKA
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* proper solution:
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** Single HLR with both GSUP and DIAMETER, or
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** DIAMETER-to-GSUP translator (to make LTE use OsmoHLR), or
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** GSUP-to-DIAMETER translator (to make OsmoMSC/SGSN use HSS)
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== Advertisement of neighbor cells
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* GSM must advertise eUTRAN neighbors
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** we have SI2quater support in osmo-bts
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** TEMS phones show proper/valid decode
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* LTE must advertise GSM neighbors
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** no "OML" for S1-AP
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** configuration mechanism is vendor-proprietary
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== Shared PS tunnel endpoint
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* if the MS/eUE moves from GPRS to LTE, it wants to keep PDP/PDN contexts
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* solution: implementing P-GW and GGSN functionality in one element
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** ergw might be able to do that (untested, complex)
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** OsmoGGSN doesn't speak GTPv2C (required for P-GW)
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** nextepc P-GW doesn't speak GTPv1C (required for GGSN)
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Solution:
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* play with ergw, or
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* adopt nextepc GTPv2C code generator to use msgb/libosmo* and add GTPv2C to OsmoGGSN
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== SGs interface
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* Osmocom already implements full SGs interface
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* Interaction with MME on LTE side needs only interop testing
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== Inter-RAT PS handover
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* inter-RAT HO offers seamless migration between GERAN + eUTRAN
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* uses S3 interface between MME and SGSN (GTPv2C based)
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* uses S4 interface between S-GW and SGSN (GTPv2C based)
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* tbh, I don't know the details here.
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* any help appreciated
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* https://www.tech-invite.com/3m23/toc/tinv-3gpp-23-401_zh.html
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* 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.
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== Conclusions
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The logical steps to improve our LTE interworking are:
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1. split HLR/HSS and split GGSN/P-GW (today)
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2. GSUP-to-DIAMETER translator for shared HLR/HSS
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3. shared GGSN/P-GW for
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== EOF
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End of File
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