== Location Services: Lb interface to SMLC

OsmoBSC and OsmoSMLC support positioning by Timing-Advance (TA), since October
2020.

A Perform Location Request is initiated by the MSC via BSSMAP on the
A-interface, for a specific subscriber. The request is typically passed on via
BSSMAP-LE on the Lb-interface to the SMLC. If required, the SMLC may request the
subscriber's Timing Advance (a.k.a. Access Delay) from the BSC via BSSLAP
(encapsulated BSSLAP APDU in a BSSMAP-LE Connection Oriented Information
message). The SMLC may combine several location and velocity estimate methods to
form a GAD PDU containing the resulting geographic location information. In
TA-based positioning, the Timing-Advance information from the BSC is combined
with the preconfigured latitude and longitude of the serving cell to form a
location estimate. This is returned to the BSC via the Lb-interface, and in turn
to the MSC via the A-interface.

[mscgen]
----
include::{srcdir}/message-sequences/location_services_ta.msc[]
----

Location Services (LCS) are described in 3GPP TS 43.059 <<3gpp-ts-43-059>>.
Messages for LCS on the A-interface (BSSMAP, between BSC and MSC) are described
in 3GPP TS 48.008 <<3gpp-ts-48-008>>, on the Lb-interface (BSSMAP-LE between BSC
and SMLC) in 3GPP TS 49.031 <<3gpp-ts-49-031>>. The resulting geographic
location and possibly velocity information is encoded in GAD, described in 3GPP
TS 23.032 <<3gpp-ts-23-032>>.

[[smlc-config]]
=== Configure Lb-interface

All Lb-interface related configuration is found in the `smlc` section of
OsmoBSC's configuration.

By default, the Lb-interface is disabled in OsmoBSC. It is started by `enable`.

----
smlc
 enable
----

On the Lb-interface, OsmoBSC always uses SSN "BSC (BSSMAP-LE)" (SSN code 250)
and contacts the remote SMLC on SSN "SMLC (BSSMAP-LE)" (SSN code 252).

The point-codes are configurable, and default to OsmoBSC's local point-code
0.23.3 (187), and remote SMLC point-code 0.23.6 (190).

Typically, multiple BSCs connect to the same SMLC, so that each BSC needs to
have a distinct point-code, while the SMLC has a single, fixed point-code.

To configure a different remote SMLC point-code, first configure an arbitrarily
named SCCP address in the `cs7` address book, and then apply that to the
`smlc-addr` configuration:

----
cs7 instance 0
 sccp-address my-smlc
  point-code 0.42.6
smlc
 enable
 smlc-addr my-smlc
----

For the BSC side, it suffices to configure a point-code in the `cs7` section,
so that the BSC typically uses the same point-code on A and Lb interfaces. In
this example, the BSC has point-code `1.2.3` on the Lb interface:

----
cs7 instance 0
 point-code 1.2.3
smlc
 enable
----

It is also possible to configure a distinct BSC's point-code on Lb, using the
`bsc-addr` configuration. In the following example, the BSC uses point-code
`0.42.3` only on the Lb interface, while the A interface remains unchanged:

----
cs7 instance 0
 sccp-address my-bsc-on-lb
  point-code 0.42.3
smlc
 enable
 bsc-addr my-bsc-on-lb
----

The geographic locations of individual cells are configured at the SMLC. See
for example OsmoSMLC's user manual <<userman-osmosmlc>>.