Classic GSM User Plane
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user plane joins signaling plane at all times
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voice circuit between two phones in same BTS
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goes from BTS to BSC (Abis) where a BSC-colocated TRAU is located
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goes from BSC to MSC (A, typiclaly G.711))
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returns from MSC to same BSC
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returns from same BSC to same BTS
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this is of course far from optimal
Problems with classic approach
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works great with low-latency, land-based/wired TDM network
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works not well with remote sites connected via satellite (VSAT)
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maritime sector
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rural BTSs in areas where no other back-haul is available
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various vendors have implemented proprietary solutions
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conveniently creating vendor lock-in
3GPP LCLS to the rescue
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official 3GPP standard appears to solve the problem in interoperable fashion
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underlying principle quite simple:
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each call gets a Global Call Reference on A inerface
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this allows BSC to identify two legs of same call
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this enables BSC to locally switch the user plane between such calls
3GPP LCLS problem domains
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early media / alerting phase
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what about playback of voice announcements ("your prepaid account is low")?
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what about lawful interception?
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what about hand-over implications (intra-BSC, inter-BSC, inter-MSC, inter-RAT, …)?
3GPP LCLS tone / announcement
3GPP LCLS: Negotiation in CN
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don’t blindly activate LCLS, but negotiate it
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all calls start non-locally switched
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LCLS negotiation on Nc interface (ISUP, BICC, SIP-I) between MSCs
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LCLS negotiation only succeeds if all CN (MSC) support it
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LCLS-Negotiation-Request IE
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LCLS-Negotiation-Response IE
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LCLS-Configuration-Preference IE
3GPP LCLS Negotiation results
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variety of different possible media configurations (examples)
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classic non-local (non-LCLS)
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connected both-way in BSS (basic LCLS)
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connected both way in BSS and bi-casted UL to CN
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connected both way in BSS and send access DL from CN
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BSS may combine or replace local DL data with DL data from CN
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connected both way in BSS and send access DL from CN, block local DL
3GPP LCLS: Implications on A / BSSMAP
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BSS is instructed about media configuration in BSSMAP
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during call establishment
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BSSMAP ASSIGNMENT REQ with GCR + LCLS-Config IEs
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BSSMAP ASSIGNMENT COMPLETE with LCLS-BSS-Status IE
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during handover
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BSSMAP HANDOVER COMPLETE with LCLS-BSS-Status IE
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at any other time
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BSSMAP LCLS-NOTIFICATION from BSS → MSS
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BSSMAP LCLS-CONNECT-CONTROL message MSS → BSS
3GPP LCLS: Osmocom Plans
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Plan: support minimal sub-set without bi-casting
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in OsmoBSC
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requires us to add BSSMAP LCLS IEs/Messages
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will use MGCP to re-configure the CN-side RTP connection on OsmoMGW
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development will be test-driven (develop TTCN-3 tests first)
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in OsmoMSC (only among RANs connected to single MSC)
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requires us to permit/enable LCLS on AoIP
