[[sgs]]
== SGs interface

OsmoMSC offers an SGs interface using the SGsAP protocol. The SGs interface is an
optional interface between a 2G (GERAN) / 3G (UTRAN) MSC and an 4G (EUTRAN) MME.
Its 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.

=== VTY configuration

The SGs interface implementation in OsmoMSC is automatically active and requires
only minimal configuration. When no specific configuration is provided OsmoMSC
will listen on 0.0.0.0:29118 (SCTP) for incoming connections.

This is sufficient in the most configurations, but in larger installations,
where services are either tied to specific interfaces and/or more instances of
OsmoMSC run in parallel, a custom configuration is necessary.

The user has the option to configure the IP address (`local-ip`) and the SCTP
port (`local-port`) and also the `vlr-name` that OsmoMSC uses to identify itself
towards the MME. It should be noted that the `vlr-name` is usually the DNS name
for the IP address of the VLR/MSC, so IP address used and the `vlr-name` should
match the DNS server entry.

----
sgs
 local-ip 127.0.0.1
 local-port 29118
 vlr-name vlr.example.net
----

In order to fine tune the behavior of the SGs interface the user also has
control over the relevant timers (`ts5`, `ts6-2`, `ts7`, `ts11`, `ts14`, `ts15`)
and counters (`ns7`, `ns11`). Each timer and counter has to be configured
separately. In the following example we change the value of ts and ns11.

----
sgs
 timer ts7 23
 counter ns11 100
----


[NOTE]
====
In case multiple instances of OsmoMSC run in parallel, it is advised to use a
different `vlr-name` for each instance. In any case it must be ensured that the
SGs interface of each instance is bound to a different IP address and/or port.
====

=== Connection monitoring

The user can use the VTY command `show sgs-connections` to list the MMEs that
are currently connected to OsmoMSC.