| == 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 |
| ---- |