SGs interface in OsmoMSC - sysmocom · An SGs location update brings the FSM into the SGs-ASSOCIATED state. (Subscriber is available for CSFB and SMS by the MSC) A 2G location update

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SGs interface in OsmoMSC

Philipp Maier

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Agenda

➔ Circuit switched fallback➔ SGs interface➔ Implementation details

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There is no CS-Domain in LTE

➔ LTE is a pure packet switched network.

– IP only, No voice, No SMS➔ Voice and SMS is provided via IMS

– Stands for “IP Multimedia Subsystem”

– Voice provieded by VoLTE (“Voice over LTE”)

– SMS provided by RCS (“Rich Communication Services”)➔ When IMS is not available, Circuit switched fallback (CSFB) can be used.

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CSFB is a an alternative to IMS

➔ IMS can be complemented with SGs/CSFB

– Conservative approach, old core network components may remain into service➔ SMS delivery is carried out via MSC/VLR (2G/3G)➔ Calls are carried out via 2G/3G RAN.

– only possible when there is 2G/3G coverage available

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The basic idea of CSFB

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➔ Phone is registered at MME/HSS (4G) and MSC/VLR (2G/3G) at the same time➔ SMS messages that arrive at the MSC/VLR (2G/2G) are forwared to the MME (4G) and vice versa➔ MT calls are reported by the VLR/MSC (2G/2G) to the MME (4G), which then sends the Mobile

phone down to 2G/2G RAN for the duration of the call.➔ MO calls are just done like a regular call by the Mobile phone via 2G/3G.

Requires special sigalling between LTE and 2G/3G: SGsAP via SGs interface.

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Agenda


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Add a bridge between LTE and 2G/3G

➔ The SGs interface connects MME and MSC.➔ Very similar to a third RAN besides A and IuCS, but with subtile differences.

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Basic facts about the SGs interface

➔ Specified in 3GPP TS 29.118➔ Transport via SCTP/IP, port 29118, msc = server➔ Protocol is called "SGsAP"➔ TLV messages➔ Signaling only, with the exception of SMS (Unitdata/NAS)➔ Requires an "SGs-Association” in the VLR (and MME) per subscriber

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MME and VLR must co-ordinate➔ Subscriber Coordination between

MME and VLR is needed. A three-state FSM is used to manage an SGs-association.

➔ The SGs-association maintains the subscriber state in relation to the MME

– Location update

– Subscriber availability (CSFB, SMS via SGs)

➔ A similar FSM exists inside the MME to mirror the state.See also: 3GPP TS 29.118, Chapter 4.2.2 States at the VLR

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MME and VLR must co-ordinate➔ An SGs location update brings the

FSM into the SGs-ASSOCIATED state. (Subscriber is available for CSFB and SMS by the MSC)

➔ A 2G location update (A-Interface) sends the FSM into the SGs-NULL state. (Subscriber functions as a normal 2G subscriber)

➔ Any kind of detach invalidates the SGs association. (Detach from EPS is possible, subscriber then lives on as a normal 2G subscriber)

See also: 3GPP TS 29.118, Chapter 4.2.2 States at the VLR

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MME and VLR must co-ordinate➔ The FSM is reset to NULL on any

unexpected situation:

– VLR failure/reset (e.g. restart of osmo-hlr)

– MME failure/reset

– Paging failure

– Alert failure (optional feature, not implemented in osmo-msc)

See also: 3GPP TS 29.118, Chapter 4.2.2 States at the VLR

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Procedures at the SGs interface➔ Paging for non-EPS services procedure➔ Location update for non-EPS services procedure➔ Non-EPS alert procedure➔ Explicit IMSI detach from EPS services➔ Explicit IMSI detach from non-EPS services➔ Implicit IMSI detach from non-EPS services➔ VLR failure procedure➔ MME failure procedure➔ HSS failure➔ MM information procedure➔ Procedure for tunnelling of NAS messages➔ Service request procedure➔ Service abort procedure➔ Implicit IMSI detach from EPS services➔ UE fallback supervision procedure➔ Procedure for MO CSFB indication

➔ Location udate➔ MO/MT SMS➔ MT/MO CSFB call➔ IMSI detach➔ Reset (VLR failure/restart)

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SGs based Location Update

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Now the SGs-associaction isIn state SGs-ASSOCIATEDand the network is ready toperform CSFB calls / SMS.

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MO SMS via SGs

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MT SMS via SGs

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SGs based MT Call

The Sgs-associaction remainsIn state SGs-ASSOCIATED

througout the whole voice call.

RAT change is taking place.

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SGs based MO Call

No MME/VLR communication, technicallyworks like a regular MO call on the 2G side.

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Detach – You can go your own way...Detach

EPS Detach IMSI Detach

GONESGs-NULL

Subscriber continues to live onas a normal 2G/3G subscriber

Subscriber is purged from theVLR (Detached)

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Agenda


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A two-territory implementation

➔ Transmit/Receive of incoming SGs messages, handling of network socket and state.

➔ Message parsing/generation, checking of message contents.

➔ Execution of procedures/actions (e.g. location update, paging, SMS...)

➔ Communication with VLR, create/detach subscribers, request subscriber contexts etc.

MSC VLR

➔ SGs association handling, decision making based on SGs state.

➔ Communication with HLR, execution of LU procedure

➔ Subscriber management (ref counting, SGs state, LU state etc.)

➔ Communication with MSC, so that the MSC part can take appropiate actions

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Implementation in osmo-msc➔ isolate VLR related parts

from MSC related parts. (current principle)

➔ “Work” is done on the MSC side, subscriber handling. is done on the VLR side.

➔ Do not mess with the FSM directly, use an API to perform actions.

➔ Actions may lead to state changes of the SGs association.

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SGs interface in OsmoMSC

Philipp Maier

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Documents

SGs interface in OsmoMSC - sysmocom · An SGs location update brings the FSM into the SGs-ASSOCIATED state. (Subscriber is available for CSFB and SMS by the MSC) A 2G location update