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manuals/gbproxy: MSC -> SGSN for pooling chapter 

Mostly just change the chapter so it makes sense for gbproxy. Some todos
are still left

Change-Id: I905835c2be7be43fe376fbc9d743107948c7e6d4
Related: SYS#5115, SYS#5005
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Daniel Willmann 2021-01-27 19:24:41 +01:00
parent 22248f33df
commit ae303b40fc
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189
doc/manuals/chapters/gbproxy-sgsnpool.adoc
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@ -1,96 +1,87 @@
== MSC Pooling
== SGSN Pooling
MSC pooling is described in 3GPP TS 23.236 <<3gpp-ts-23-236>>, and is supported
by OsmoBSC since mid 2020.
SGSN pooling is described in 3GPP TS 23.236 <<3gpp-ts-23-236>>, and is supported
by OsmoGbProxy since early 2021.
The aim of MSC pooling is to distribute load from a BSC across multiple MSCs,
The aim of SGSN pooling is to distribute load from a BSS across multiple SGSNs,
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
The main mechanism for SGSN pooling is the TLLI/P-TMSI, which an SGSN hands out
to its attached subscribers. Typically 10 bits of the P-TMSI are designated as a
Network Resource Identifier (NRI) that identifies the originating SGSN, and
allows OsmoGbProxy to direct a subscriber back to the same SGSN instance that
previously negotiated the Attach procedure. Typically, the full NRI value
range available is divided into N even ranges, where each SGSN 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.
Subscribers attaching without a TLLI, or those with unknown NRI value,
are evenly distributed across SGSN instances. OsmoGbProxy uses a hash-based
approach to distribute load across all connected SGSNs.
A Paging Response from a subscriber is always returned back to whichever MSC
A Paging Response from a subscriber is always returned back to whichever SGSN
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
Finally, a NULL-NRI is a special NRI value that indicates that the SGSN wishes
to offload this subscriber to a different SGSN. A NULL-NRI is an arbitrary NRI
value that is chosen distinctly for each PLMN served by a BSS, 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,
Updating, an offloading SGSN hands out a NULL-NRI value in the assigned TLLI,
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.
and immediately re-attempt the attach using the TLLI containing the NULL-NRI.
OsmoGbProxy recognises the NULL-NRI and redirects the subscriber to one of the
other SGSNs. A prerequisite for this to work well is that the particular SGSN is
previously marked as not accepting new subscribers, in OsmoGbProxy's configuration.
The mechanisms described above make up the NAS node selection function
implemented in the BSC.
implemented in OsmoGbProxy.
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.
3GPP TS 23.236 also defines that an offloading SGSN hands subscriber information
to the newly assigned SGSN, which takes place outside the scope of OsmoGbProxy.
=== Configuring MSC Pooling
=== Configuring SGSN 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
The NRI ranges assigned to each SGSN must match in the OsmoGbProxy and the SGSN
configuration. If inconsistent NRI value ranges are configured,
attached subscribers would be redirected to SGSN 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:
==== Connecting Multiple SGSNs
----
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 the Network Service
ns
bind udp sgsn
listen 10.0.0.1 23000
nse 1
ip-sns 10.0.1.1 23000
ip-sns 10.0.1.1 23001
nse 2
ip-sns 10.0.1.2 23000
nse 3
ip-sns 10.0.1.3 23000
# configure NRI value ranges
network
gbproxy
nri bitlen 10
nri null add 0
msc 0
sgsn 1
nri add 1 341
msc 1
sgsn 2
nri add 342 682
msc 2
sgsn 3
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.
In OsmGbProxy, the NRI value's bit length is freely configurable from 0 to 15
bits. 3GPP TS 23.236 suggests a typical bit length of 10. Setting the length
to 0 disables SGSN pooling, this is also the default.
The NRI bit length must be identical across the entire SGSN pool.
Change the NRI value bit length in OsmoBSC's VTY configuration like this:
Change the NRI value bit length in OsmoGbProxy's VTY configuration like this:
----
network
gbproxy
nri bitlen 10
----
@ -100,9 +91,9 @@ bit 23).
==== NULL-NRI
Since OsmoBSC supports serving only one PLMN, NULL-NRI are configured globally.
Since OsmoGbProxy 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.
OsmoGbProxy allows configuring multiple NULL-NRI values.
----
network
@ -110,9 +101,9 @@ network
nri null add 423
----
==== Assigning NRI Ranges to MSCs
==== Assigning NRI Ranges to SGSNs
Each MSC configured in OsmoBSC must be assigned a distinct NRI value range.
Each SGSN configured in OsmoGbProxy 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
@ -120,18 +111,18 @@ NRI values are typically configured in ranges, here dividing a 10bit range
as NULL-NRI:
----
msc 0
sgsn nsei 1
nri add 1 341
msc 1
sgsn nsei 2
nri add 342 684
msc 2
sgsn nsei 3
nri add 685 1023
----
NRI can also be assigned in single values:
----
msc 0
sgsn nsei 1
nri add 23
----
@ -139,75 +130,65 @@ Ranges can be constructed arbitrarily by a sequence of `add` and `del`
configurations, here a contrived example:
----
msc 0
sgsn nsei 1
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:
On the VIEW and ENABLE VTY nodes, `show nri all` shows all SGSNs:
----
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
OsmoGbProxy> show nri all
sgsn nsei 1
nri add 1 341
msc 1
sgsn nsei 2
nri add 342 684
msc 2
sgsn nsei 3
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:
When configuring overlapping NRI value ranges across SGSNs, the telnet VTY warns
about it, and starting OsmoGbProxy with such a configuration will fail:
----
msc 0
sgsn nsei 1
nri add 1 511
msc 1
sgsn nsei 2
nri add 512 1023
msc 2
sgsn nsei 3
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
$ osmo-gbproxy
% Warning: NSE(00003/SGSN): NRI range [500..555] overlaps between NSE 00003 and NSE 00001. For overlaps, NSE 00001 has higher priority than NSE 00003
% Warning: NSE(00003/SGSN): NRI range [500..555] overlaps between NSE 00003 and NSE 00002. For overlaps, NSE 00002 has higher priority than NSE 00003
----
==== MSC Offloading
==== SGSN 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:
To effectively offload a particular SGSN, it must be marked as no longer taking
new subscribers in OsmoGbProxy. This can be achieved in the telnet VTY by:
----
msc 0
sgsn nsei 1
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.
This SGSN will, as long as it is connected, continue to serve subscribers
already attached to it: those that yield an NRI matching this SGSN, and those
that are being paged by this SGSN. But OsmoGbProxy will no longer direct new
subscribers to this SGSN.
To re-enable an MSC for attaching new subscribers:
TODO: Is paging response relevant for SGSN?
To re-enable an SGSN for attaching new subscribers:
----
msc 0
sgsn nsei 1
allow-attach
----