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NOTE: This page doesn't describe an existing feature in OsmoMSC. Rather, it describes the general nature of a 3GPP feature which we plan to support at some time

SGs Interface

The SGs interface is an optional interface between a 2G (GERAN) / 3G (UTRAN) MSC and an LTE/EPC MME. It's purpose is to facilitate both CSFB (Circuit-Switched Fall Back) and SMSoS (SMS over SGs). It is used for Mobility management (MM) and paging procedures between the EPS (Evolved Packet Services) and CS (Circuit Switched) domain.

It's based on the Gs (VLR-SGSN) interface that was added earlier on to facilitate paging coordination between the CS an PS domain in 2G/3G.

CSFB (Circuit Switched Fall Back)

SMSoS (SMS Over SGs)

In SMSoS, a LTE-attached UE must not fall back to GERAN/UTRAN for MO and MT SMS. This is only used if SMS-over-IP (over IMS) is not configured or if the IMS registration fails.

MO-SMS

MO-SMS are sent by the UE over E-UTRAN and the MME (FIXME: how exactly) and passed over SGs into the MSC. From that point on, they follow normal processing of any other SMS received by that MSC, e.g. over GERAN.

Protocol stacking

The Interface is defined as SGsAP over SCTP over IP, i.e. there is no SCCP/SIGTRAN involved.

SGsAP Protocol encoding

The SGsAP protocol is a successor of the BSSAP+ protocol and specified like a classic GSM protocol with a fixed message header and TLV information elements. There's no abstract syntax or formal definition.

Procedures on SGs

Mobility Management

Attach Procedure

The combined EPS/IMSI attach procedure is based on the combined GPRS+IMSI attach procedure specified earlier for Gs

  • There is a special "attach type" that the UE uses when sending the attach message to the MME. At that point, the UE can also inform the network if it's interested in SMS only or CSFB. Security procedures + registration in the EPS proceed as normal.
  • The MME performs a location update request (LAI, IMSI, MME IP, LU type) via SGs to the MSC/VLR
  • The VLR stores the MME IP address in some internal state and performs a LU procedure in the CS domain towards the HLR
  • The VLR responds on SGs with LU Accept (TMSI) to the MME
  • The EPS Attach procedure completes with LAI+TMSI to the US

Tracking Area / Location Area Update procedure

  • The UE sends a TAU update request to the MME as normal, but indicates combined TA+LA "Update Type"
  • The MME sends a LU Request (LAI, IMSI, MME, LU Type) via SGs to the VLR. The NEW LAI is derived or allocated in the MME based on the GUTI from the UE.
  • The VLR performs a LU procedure in the CS domain towards the HLR
  • The VLR responds on SGs with LU Accept (TMSI) to the MME
  • The TAU procedure completes with LAI+TMSI to the US

Periodic Tracking Area / Location Area Update procedure

  • no periodic LU
  • periodic TA updates are preformed
  • SGs association to MSC/VLR disables implicit detach(!)

CSFB

MO Voice Call

  • under specific circumstances, the UE and eNB + MME will make the UE establish CS connection over GERAN/UTRAN
  • The UE establishes a channel on GERAN/UTRAN and sends a CM Service Request (only after prior combined LA/TA attach)
  • CS call establishment like normal in GERAN/UTRAN

MT Voice Call

  • The MSC receives an incoming voice call for a given subscriber
  • The MSC/VLR determnines this is for a subscriber that is attached via SGs and sends a CS Paging message (IMSI, VLR, TMSI, Location Info, CLID) to the MME
  • The MME forwards the message to the UE, either directly (active) or after paging (idle)
  • ...
  • The UE establishes a channel on GERAN/UTRAN and sends a Paging Response message
  • The MSC/VLR stops the paging timer started when starting to page vis SGs and establishes a CS connection like normal

SMS

Contrary to CSFB, no CS Fallback is performed. SMS are sent over NAS messages between UE and MME

Further Reading

  • 3GPP TS 29.118 - Mobility Management Entity (MME) - Visitor Location Register (VLR) SGs interface specification
  • 3GPP TS 23.272 - Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2
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