Cellular Network Infrastructure » OsmoSMLC

h1. 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_

h2. 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_

h2. Procedures

See 8.5.3 of TS 43.059

h3. TA based positioning in CS domain for A interface

See Section 9.3 of 3GPP TS 43.059

!location_services_ta.png!

h4. 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@

h4. 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@

h2. 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

h2. Functionality in SMLC

Protocol stack looks like this:

* BSSLAP (TS 48.071)

* BSSAP-LE

h3. Communication with MS

Protocol stack looks like this:

* RRLP (TS 48.031)

* BSSLAP (TS 48.071)

* BSSAP-LE (TS 49.031)

h3. Communication with peer SMLC

* SMLCPP (TS 48.031)

* BSSAP-LE (TS 49.031)

h3. 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) 

h2. Functionality in SMLC

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