[[gb-sgsn-pool]]
== Gb interface in SGSN Pooling

SGSN Pooling is a modern optional extension of the 3GPP GERAN
architecture.  It is officially referred-to as _Intra Domain Connection
of RAN Nodes to Multiple CN Nodes_.  It requires The use of IP-SNS and
does not support legacy or non-standard Gb variants.

Without this optional feature, a given PCU/BSS always connects to one
SGSN via a Gb interface.  All traffic is handled through that one
interface.

While the NS-level load sharing function and the _weights_ of the IP-SNS
allow for load distribution between different user plane entities, there
was demand for additional scalability and fail-over capabilities leading
to the SGSN pooling feature.

The major changes introduced to the Gb interface by SGSN pooling are as
follows:

* There is a separate NSE in the BSS for each of the SGSNs in the pool,
  creating a 1:1 relationship between BSS-NSE and SGSN.
* Each of those per-SGSN NSEs has it's own NS-VCGs and NS-VCs, unrelated
  to those of the other NSEs
* Each of those per-SGSN NSEs has it's own Signaling BVC
* Each Cell in the BSS has one PTP BVC _per SGSN in the pool_

This relationship is illustrated in <<fig-gb-pool>> below.

[[fig-gb-pool]]
.Gb interface concepts when SGSN pooling feature is used (from 3GPP TS 48.016)
image::./common/images/gb-concepts-pool.pdf[]

Looking strictly at the Gb interface, this means that there is a completely
separate Gb interface between each BSS and each pool member SGSN.  All of the
procedures explained in <<gb-ip-sns>> hence occur N number of times to N number
of SGSN pool members.

=== Status of SGSN Pool support in Osmocom

==== osmo-pcu

There is currently no direct support for SGSN pooling in `osmo-pcu`
itself.  However, as of December 2020, `osmo-gbproxy` is being extended
with SGSN pooling support.

This means that SGSN pooling can be added to any `osmo-pcu` based
deployment by introducing `osmo-gbproxy` between `osmo-pcu` and the SGSN.

The use of `osmo-gbproxy` also has the added benefit that all Gb
interfaces from various PCUs are aggregated into one Gb interface
towards (each) SGSN.  Most deployments are used to such a _one interface
per BSS_ approach as they are used to the BSC-colocated PCU architecture
and not to the BTS-colocated PCU architecture as implemented in Osmocom.

==== osmo-gbproxy

FIXME

==== osmo-sgsn

FIXME