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Location Services

NOTE: This page describes our current understanding of the 3GPP specification; none of this is implemented so far in Osmocom!

LCS includes a variety of different methods, such as
  • Timing Advance
  • E-OTD
  • GNSS
  • U-TDOA
  • Multilateration TA
  • Multilateration OTD

The most simple and hence most wide-spread approach is Timing Advance

Relevant specs

  • 3GPP TS 43.059 "Functional stage 2 description of Location Services (LCS) in GERAN"
  • 3GPP TS 48.031 "SMLC-SMLC Protocol" (SMLCPP)
    • Based on ASN.1 PER; transferred via BSSAP-LE
  • 3GPP TS 49.031 BSSAP LCS extension (BSSAP-LE)
    • spoken on Lb (BSC-SMLC) and Lp (SMLC-SMLC) interface
      • Lp uses connectionless SCCP only
      • Lb uses connection-oriented SCCP and is very much like BSSAP/BSSMAP/DTAP on A interface
  • 3GPP TS 48.071 "SMLC-BSS interface Layer 3" (BSSLAP)
    • On Lb interface, encapsulated as BSSLAP APDU IE

Procedures

See 8.5.3 of TS 43.059

TA based positioning in CS domain for A interface

See Section 9.3 of 3GPP TS 43.059

MS in idle state

  • MSC sends BSSAP Perform Location Request
  • BSC sends SCCP-CR(BSSAP-LE Perform Location Request) to SMLC
  • SMLC asks BSC to determine TA of MS via SCCP-DT1(BSSMAP-LE Connection Oriented Info (TA Request))
  • BSC sends PAGING REQUEST via BTS to MS
  • MS sends PAGING RESPONSE via BTS to BSC; BTS adds timing offset information
  • BSC reports CellID + TA value to SMLC BSSMAP-LE Connection Oriented Info (TA Response)
  • SMLC returns BSSAP-LE Perform Location Response to BSC
  • BSC returns result to MSC in BSSAP Perform Location Response

MS in dedicated state

  • MSC sends BSSAP Perforl Location Request
  • BSC sends SCCP-CR(BSSAP-LE Perform Location Request), including a BSSLAP APDU TA Layer 3 with current TA
  • SMLC may decide to use the received TA, or to do another TA request:
    • SMLC asks BSC to determine TA of MS via SCCP-DT1(BSSMAP-LE Connection Oriented Info (TA Request))
    • BSC checks current TA (as per measurement processing) for lchan of MS
    • BSC reports CellID + TA value to SMLC BSSMAP-LE Connection Oriented Info (TA Response)
  • SMLC returns BSSAP-LE Perform Location Response to BSC
  • BSC returns result to MSC in BSSAP Perform Location Response

Functionality in BSC

  • BSC receives requests for LCS information from CN via A interface
  • BSC passes those requests to the SMLC via Lb interface
    • BSSAP-LE protocol stack (on top of SCCP) implementatioon

Functionality in SMLC

Protocol stack looks like this:
  • BSSLAP (TS 48.071)
  • BSSAP-LE

Communication with MS

Protocol stack looks like this:
  • RRLP (TS 48.031)
  • BSSLAP (TS 48.071)
  • BSSAP-LE (TS 49.031)

Communication with peer SMLC

  • SMLCPP (TS 48.031)
  • BSSAP-LE (TS 49.031)

Message Segmentation

LCS messages can be segmented either
  • at the SMLCPP layer
  • At RR layer (TS 44.018) and BSAP-LE Layer (TS 4.031) for segmentation of RRLP (TS 44.031) for legacy MS (<= Release 4)

Functionality in SMLC

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