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daniel gerrit-no-reply at lists.osmocom.orgdaniel has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-sgsn/+/22494 ) Change subject: manuals/gbproxy: Add osmo-bsc MSC pooling chapter from Neels as a base ...................................................................... manuals/gbproxy: Add osmo-bsc MSC pooling chapter from Neels as a base Related: SYS#5115, SYS#5005 Change-Id: I930f5df847ea6f40d0ee0c75dd25b6dd20e00839 --- A doc/manuals/chapters/gbproxy-sgsnpool.adoc 1 file changed, 213 insertions(+), 0 deletions(-) git pull ssh://gerrit.osmocom.org:29418/osmo-sgsn refs/changes/94/22494/1 diff --git a/doc/manuals/chapters/gbproxy-sgsnpool.adoc b/doc/manuals/chapters/gbproxy-sgsnpool.adoc new file mode 100644 index 0000000..923a088 --- /dev/null +++ b/doc/manuals/chapters/gbproxy-sgsnpool.adoc @@ -0,0 +1,213 @@ +== 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 +---- -- To view, visit https://gerrit.osmocom.org/c/osmo-sgsn/+/22494 To unsubscribe, or for help writing mail filters, visit https://gerrit.osmocom.org/settings Gerrit-Project: osmo-sgsn Gerrit-Branch: master Gerrit-Change-Id: I930f5df847ea6f40d0ee0c75dd25b6dd20e00839 Gerrit-Change-Number: 22494 Gerrit-PatchSet: 1 Gerrit-Owner: daniel <dwillmann at sysmocom.de> Gerrit-MessageType: newchange -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.osmocom.org/pipermail/gerrit-log/attachments/20210127/f1587a6d/attachment.htm>