== MSC Pooling

MSC pooling is described in 3GPP TS 23.236 <<3gpp-ts-23-236>>, and is supported
by OsmoBSC since mid 2020.

The aim of MSC pooling is to distribute load from a BSC across multiple MSCs,
which are equivalent and redundant infrastructure for the same core network.

The main mechanism for MSC pooling is the TMSI identity, which an MSC hands out
to its attached subscribers. Typically 10 bits of the TMSI are designated as a
Network Resource Identifier (NRI) that identifies the originating MSC, and
allows OsmoBSC to direct a subscriber back to the same MSC instance that
previously negotiated the IMSI Attach procedure. Typically, the full NRI value
range available is divided into N even ranges, where each MSC is assigned one
NRI range.

Subscribers attaching without a TMSI identity, or those with unknown NRI value,
are evenly distributed across MSC instances. OsmoBSC uses a round-robin
approach to distribute load across all connected MSCs.

A Paging Response from a subscriber is always returned back to whichever MSC
initiated the Paging, regardless of the Mobile Identity used.

Finally, a NULL-NRI is a special NRI value that indicates that the MSC wishes
to offload this subscriber to a different MSC. A NULL-NRI is an arbitrary NRI
value that is chosen distinctly for each PLMN served by a BSC, so that a
subscriber can be reassigned within that PLMN. Upon (periodic) Location
Updating, an offloading MSC hands out a NULL-NRI value in the assigned TMSI,
along with a non-broadcast LAI. The subscriber will notice the LAI mismatch,
and immediately re-attempt the attach using the TMSI containing the NULL-NRI.
The BSC recognises the NULL-NRI and redirects the subscriber to one of the
other MSCs. A prerequisite for this to work well is that the particular MSC is
previously marked as not accepting new subscribers, in the BSC's configuration.

The mechanisms described above make up the NAS node selection function
implemented in the BSC.

3GPP TS 23.236 also defines that an offloading MSC hands subscriber information
to the newly assigned MSC, which takes place outside the scope of the BSC.

=== Configuring MSC Pooling

The NRI ranges assigned to each MSC must match in the BSC and the MSC
configuration. If MSC and BSC had inconsistent NRI value ranges configured,
attached subscribers would be redirected MSC instances that did not perform the
attach, possibly rendering the core network unusable.

==== Connecting Multiple MSCs

The `cs7 instance` configuration defines the SCCP addresses to reach the MSCs
at. In addition, each MSC is configured by its own `msc` section in the
configuration. An example osmo-bsc.cfg serving three MSCs:

----
cs7 instance 0
 # SCCP address book entries for the three MSCs
 sccp-address my-msc-0
  point-code 0.23.0
 sccp-address my-msc-1
  point-code 0.23.1
 sccp-address my-msc-2
  point-code 0.23.2

# assign each MSC configuration its remote SCCP address
msc 0
 msc-addr my-msc-0
msc 1
 msc-addr my-msc-1
msc 2
 msc-addr my-msc-2

# configure NRI value ranges
network
 nri bitlen 10
 nri null add 0
msc 0
 nri add 1 341
msc 1
 nri add 342 682
msc 2
 nri add 683 1023
----

==== NRI Value Bit Length

In OsmoBSC, the NRI value's bit length is freely configurable from 1 to 15
bits. 3GPP TS 23.236 suggests a typical bit length of 10, which is OsmoBSC's
default. The NRI bit length must be identical across the entire MSC pool.

Change the NRI value bit length in OsmoBSC's VTY configuration like this:

----
network
 nri bitlen 10
----

In the TMSI bits, regardless of the NRI bit length, the NRI value always starts
just after the most significant octet of a TMSI (most significant bit at TMSI's
bit 23).

==== NULL-NRI

Since OsmoBSC supports serving only one PLMN, NULL-NRI are configured globally.
Even though 3GPP TS 23.236 indicates that there is a single NULL-NRI per PLMN,
OsmoBSC allows configuring multiple NULL-NRI values.

----
network
 nri null add 0
 nri null add 423
----

==== Assigning NRI Ranges to MSCs

Each MSC configured in OsmoBSC must be assigned a distinct NRI value range.
Overlapping NRI value ranges will cause failure to serve subscribers.

NRI values are typically configured in ranges, here dividing a 10bit range
(0..1023) into three equal ranges, while leaving 0 available to be configured
as NULL-NRI:

----
msc 0
 nri add 1 341
msc 1
 nri add 342 684
msc 2
 nri add 685 1023
----

NRI can also be assigned in single values:

----
msc 0
 nri add 23
----

Ranges can be constructed arbitrarily by a sequence of `add` and `del`
configurations, here a contrived example:

----
msc 0
 nri add 0 342
 nri del 23
 nri del 42 235
 nri add 1000 1023
----

To view the current NRI config in a running OsmoBSC instance, use the
`show nri` command, here showing the result of the contrived example:

----
OsmoBSC(config-msc)# show nri
msc 0
 nri add 0 22
 nri add 24 41
 nri add 236 342
 nri add 1000 1023
----

On the VIEW and ENABLE VTY nodes, `show nri` shows all MSCs:

----
OsmoBSC> show nri
msc 0
 nri add 1 341
msc 1
 nri add 342 684
msc 2
 nri add 685 1023
----

When configuring overlapping NRI value ranges across MSCs, the telnet VTY warns
about it, and starting OsmoBSC with such a configuration will fail:

----
msc 0
 nri add 1 511
msc 1
 nri add 512 1023
msc 2
 nri add 500 555
----

This results in:

----
$ osmo-bsc
DMSC ERROR msc 2: NRI range [500..555] overlaps between msc 2 and msc 0. For overlaps, msc 0 has higher priority than msc 2
DMSC ERROR msc 2: NRI range [500..555] overlaps between msc 2 and msc 1. For overlaps, msc 1 has higher priority than msc 2
----

==== MSC Offloading

To effectively offload a particular MSC, it must be marked as no longer taking
new subscribers in OsmoBSC. This can be achieved in the telnet VTY by:

----
msc 0
 no allow-attach
----

This MSC will, as long as it is connected, continue to serve subscribers
already attached to it: those that yield an NRI matching this MSC, and those
that are being paged by this MSC. But OsmoBSC will no longer direct new
subscribers to this MSC.

To re-enable an MSC for attaching new subscribers:

----
msc 0
 allow-attach
----