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diff --git a/2018/osmocon2018-lcls/lcls-announcement.png b/2018/osmocon2018-lcls/lcls-announcement.png Binary files differnew file mode 100644 index 0000000..f28e534 --- /dev/null +++ b/2018/osmocon2018-lcls/lcls-announcement.png diff --git a/2018/osmocon2018-lcls/lcls-conn_model_1.png b/2018/osmocon2018-lcls/lcls-conn_model_1.png Binary files differnew file mode 100644 index 0000000..ee778d5 --- /dev/null +++ b/2018/osmocon2018-lcls/lcls-conn_model_1.png diff --git a/2018/osmocon2018-lcls/lcls-conn_model_2.png b/2018/osmocon2018-lcls/lcls-conn_model_2.png Binary files differnew file mode 100644 index 0000000..371d2c9 --- /dev/null +++ b/2018/osmocon2018-lcls/lcls-conn_model_2.png diff --git a/2018/osmocon2018-lcls/lcls-overview.png b/2018/osmocon2018-lcls/lcls-overview.png Binary files differnew file mode 100644 index 0000000..585ecbe --- /dev/null +++ b/2018/osmocon2018-lcls/lcls-overview.png diff --git a/2018/osmocon2018-lcls/osmocon2018-lcls.adoc b/2018/osmocon2018-lcls/osmocon2018-lcls.adoc new file mode 100644 index 0000000..1f3ee24 --- /dev/null +++ b/2018/osmocon2018-lcls/osmocon2018-lcls.adoc @@ -0,0 +1,120 @@ +3GPP LCLS (Local Call, Local Switch) +==================================== +:author: Harald Welte <laforge@gnumonks.org> +:copyright: 2018 by Harald Welte (License: CC-BY-SA) +:backend: slidy +:max-width: 45em + +== Classic GSM User Plane + +* user plane joins signaling plane at all times +* voice circuit between two phones in same BTS +** goes from BTS to BSC (Abis) where a BSC-colocated TRAU is located +** goes from BSC to MSC (A, typiclaly G.711)) +** returns from MSC to same BSC +** returns from same BSC to same BTS +* this is of course far from optimal + +== Problems with classic approach + +* works great with low-latency, land-based/wired TDM network +* works not well with remote sites connected via satellite (VSAT) +** maritime sector +** rural BTSs in areas where no other back-haul is available +* various vendors have implemented proprietary solutions +** conveniently creating vendor lock-in + +== 3GPP LCLS to the rescue + +* official 3GPP standard appears to solve the problem in interoperable fashion +* underlying principle quite simple: +** each call gets a _Global Call Reference_ on A inerface +** this allows BSC to identify two legs of same call +** this enables BSC to locally switch the user plane between such calls + +image::lcls-overview.png[width=100%] + +== 3GPP LCLS problem domains + +* early media / alerting phase +* what about playback of voice announcements ("your prepaid account is low")? +* what about lawful interception? +* what about hand-over implications (intra-BSC, inter-BSC, inter-MSC, inter-RAT, ...)? + +== 3GPP LCLS early media + +image::lcls-conn_model_1.png[width=100%] + +== 3GPP LCLS early media + +image::lcls-conn_model_2.png[width=100%] + +== 3GPP LCLS tone / announcement + +image::lcls-announcement.png[width=100%] + +== 3GPP LCLS: Negotiation in CN + +* don't blindly activate LCLS, but _negotiate_ it +** all calls start non-locally switched +* LCLS negotiation on Nc interface (ISUP, BICC, SIP-I) between MSCs +* LCLS negotiation only succeeds if all CN (MSC) support it +** LCLS-Negotiation-Request IE +** LCLS-Negotiation-Response IE +** LCLS-Configuration-Preference IE + +== 3GPP LCLS Negotiation results + +* variety of different possible media configurations (examples) +** classic non-local (non-LCLS) +** connected both-way in BSS (basic LCLS) +** connected both way in BSS _and_ bi-casted UL to CN +** connected both way in BSS _and_ send access DL from CN +*** BSS may combine or replace local DL data with DL data from CN +** connected both way in BSS _and_ send access DL from CN, block local DL + +== 3GPP LCLS: Implications on A / BSSMAP + +* BSS is instructed about media configuration in BSSMAP +** during call establishment +*** BSSMAP ASSIGNMENT REQ with GCR + LCLS-Config IEs +*** BSSMAP ASSIGNMENT COMPLETE with LCLS-BSS-Status IE +** during handover +*** BSSMAP HANDOVER COMPLETE with LCLS-BSS-Status IE +** at any other time +*** BSSMAP LCLS-NOTIFICATION from BSS -> MSS +*** BSSMAP LCLS-CONNECT-CONTROL message MSS -> BSS + +== 3GPP LCLS: Osmocom Support + +. minimal sub-set without bi-casting +.. in OsmoBSC +... added BSSMAP LCLS IEs/Messages +... use MGCP to re-configure the CN-side RTP connection on OsmoMGW +... development was test-driven (TTCN-3 tests written first) +.. in OsmoMSC +... currently not implemented yet + +== 3GPP LCLS: Osmocom Restrictions + +. RTP audio looping is done in BSC-colocated MGW +.. saves RTP on A interface between BSC and MSC +.. doesn't save RTP round-trip on Abis between BTS and BSC +. In deployments with a centralized BSC, that's insufficient +.. you want true RTP loop-back betwen the BTSs involved in locally switched call +. implementation pending, should happen rather soon + + +== Further Reading + +* 3GPP TS 23.284 http://www.3gpp.org/DynaReport/23284.htm +* 3GPP WI 430001 (LCLS-CN) http://www.3gpp.org/DynaReport/WiCr--440021.htm +* 3GPP WI 440021 http://www.3gpp.org/DynaReport/WiCr--440021.htm +* http://osmocom.org/projects/cellular-infrastructure/wiki/Local_Call_Local_Switch +* Osmocom tickets +** http://osmocom.org/issues/1602 +** http://osmocom.org/issues/2487 + +== EOF + +End of File |
