manuals/gbproxy: Add osmo-bsc MSC pooling chapter from Neels as a base
Related: SYS#5115, SYS#5005 Change-Id: I930f5df847ea6f40d0ee0c75dd25b6dd20e00839
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== MSC Pooling
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MSC pooling is described in 3GPP TS 23.236 <<3gpp-ts-23-236>>, and is supported
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by OsmoBSC since mid 2020.
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The aim of MSC pooling is to distribute load from a BSC across multiple MSCs,
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which are equivalent and redundant infrastructure for the same core network.
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The main mechanism for MSC pooling is the TMSI identity, which an MSC hands out
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to its attached subscribers. Typically 10 bits of the TMSI are designated as a
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Network Resource Identifier (NRI) that identifies the originating MSC, and
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allows OsmoBSC to direct a subscriber back to the same MSC instance that
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previously negotiated the IMSI Attach procedure. Typically, the full NRI value
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range available is divided into N even ranges, where each MSC is assigned one
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NRI range.
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Subscribers attaching without a TMSI identity, or those with unknown NRI value,
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are evenly distributed across MSC instances. OsmoBSC uses a round-robin
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approach to distribute load across all connected MSCs.
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A Paging Response from a subscriber is always returned back to whichever MSC
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initiated the Paging, regardless of the Mobile Identity used.
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Finally, a NULL-NRI is a special NRI value that indicates that the MSC wishes
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to offload this subscriber to a different MSC. A NULL-NRI is an arbitrary NRI
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value that is chosen distinctly for each PLMN served by a BSC, so that a
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subscriber can be reassigned within that PLMN. Upon (periodic) Location
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Updating, an offloading MSC hands out a NULL-NRI value in the assigned TMSI,
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along with a non-broadcast LAI. The subscriber will notice the LAI mismatch,
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and immediately re-attempt the attach using the TMSI containing the NULL-NRI.
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The BSC recognises the NULL-NRI and redirects the subscriber to one of the
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other MSCs. A prerequisite for this to work well is that the particular MSC is
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previously marked as not accepting new subscribers, in the BSC's configuration.
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The mechanisms described above make up the NAS node selection function
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implemented in the BSC.
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3GPP TS 23.236 also defines that an offloading MSC hands subscriber information
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to the newly assigned MSC, which takes place outside the scope of the BSC.
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=== Configuring MSC Pooling
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The NRI ranges assigned to each MSC must match in the BSC and the MSC
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configuration. If MSC and BSC had inconsistent NRI value ranges configured,
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attached subscribers would be redirected MSC instances that did not perform the
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attach, possibly rendering the core network unusable.
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==== Connecting Multiple MSCs
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The `cs7 instance` configuration defines the SCCP addresses to reach the MSCs
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at. In addition, each MSC is configured by its own `msc` section in the
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configuration. An example osmo-bsc.cfg serving three MSCs:
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----
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cs7 instance 0
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# SCCP address book entries for the three MSCs
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sccp-address my-msc-0
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point-code 0.23.0
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sccp-address my-msc-1
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point-code 0.23.1
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sccp-address my-msc-2
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point-code 0.23.2
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# assign each MSC configuration its remote SCCP address
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msc 0
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msc-addr my-msc-0
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msc 1
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msc-addr my-msc-1
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msc 2
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msc-addr my-msc-2
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# configure NRI value ranges
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network
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nri bitlen 10
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nri null add 0
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msc 0
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nri add 1 341
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msc 1
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nri add 342 682
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msc 2
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nri add 683 1023
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----
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==== NRI Value Bit Length
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In OsmoBSC, the NRI value's bit length is freely configurable from 1 to 15
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bits. 3GPP TS 23.236 suggests a typical bit length of 10, which is OsmoBSC's
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default. The NRI bit length must be identical across the entire MSC pool.
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Change the NRI value bit length in OsmoBSC's VTY configuration like this:
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----
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network
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nri bitlen 10
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----
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In the TMSI bits, regardless of the NRI bit length, the NRI value always starts
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just after the most significant octet of a TMSI (most significant bit at TMSI's
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bit 23).
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==== NULL-NRI
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Since OsmoBSC supports serving only one PLMN, NULL-NRI are configured globally.
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Even though 3GPP TS 23.236 indicates that there is a single NULL-NRI per PLMN,
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OsmoBSC allows configuring multiple NULL-NRI values.
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----
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network
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nri null add 0
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nri null add 423
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----
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==== Assigning NRI Ranges to MSCs
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Each MSC configured in OsmoBSC must be assigned a distinct NRI value range.
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Overlapping NRI value ranges will cause failure to serve subscribers.
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NRI values are typically configured in ranges, here dividing a 10bit range
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(0..1023) into three equal ranges, while leaving 0 available to be configured
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as NULL-NRI:
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----
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msc 0
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nri add 1 341
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msc 1
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nri add 342 684
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msc 2
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nri add 685 1023
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----
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NRI can also be assigned in single values:
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----
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msc 0
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nri add 23
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----
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Ranges can be constructed arbitrarily by a sequence of `add` and `del`
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configurations, here a contrived example:
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----
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msc 0
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nri add 0 342
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nri del 23
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nri del 42 235
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nri add 1000 1023
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----
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To view the current NRI config in a running OsmoBSC instance, use the
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`show nri` command, here showing the result of the contrived example:
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----
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OsmoBSC(config-msc)# show nri
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msc 0
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nri add 0 22
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nri add 24 41
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nri add 236 342
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nri add 1000 1023
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----
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On the VIEW and ENABLE VTY nodes, `show nri` shows all MSCs:
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----
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OsmoBSC> show nri
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msc 0
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nri add 1 341
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msc 1
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nri add 342 684
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msc 2
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nri add 685 1023
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----
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When configuring overlapping NRI value ranges across MSCs, the telnet VTY warns
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about it, and starting OsmoBSC with such a configuration will fail:
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----
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msc 0
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nri add 1 511
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msc 1
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nri add 512 1023
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msc 2
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nri add 500 555
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----
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This results in:
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----
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$ osmo-bsc
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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
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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
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----
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==== MSC Offloading
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To effectively offload a particular MSC, it must be marked as no longer taking
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new subscribers in OsmoBSC. This can be achieved in the telnet VTY by:
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----
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msc 0
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no allow-attach
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----
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This MSC will, as long as it is connected, continue to serve subscribers
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already attached to it: those that yield an NRI matching this MSC, and those
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that are being paged by this MSC. But OsmoBSC will no longer direct new
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subscribers to this MSC.
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To re-enable an MSC for attaching new subscribers:
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----
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msc 0
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allow-attach
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----
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