Cellular Network Infrastructure » Core Network (CN) » OsmoBSCNAT

this is becoming more relevant again after 4 years of being on hold...

laforge wrote:

In order to be able to use it also with 3GPP AoIP, we will probably need to

• make sure SCCPlite/IPA support in libosmo-sigtran is complete + validated

this part is done, as we support sccplite/ipa from osmo-bsc via that path

• port osmo-bscnat over to use libosmo-sigtran as transport interface on both BSC and MSC facing interfaces
• run an instance of osmo-mgw next to osmo-bsc_nat for handling RTP and OSMUX media streams to/from BSC and RTP to the (3rd party) MSC

those two parts are missing. This will inevitably end up being a relatively major re-architecture of osmo-bscnat. To reduce the complexity: If there are "odd" use casese in the existing code base, we can certainly review those and drop them in case no known user for them exists anymore.

Inserting osmo-mgw into the user plane (and modifying the control plane IP/Port as per the mgw-allocated connections) will be similar to how osmo-mgw is already used co-located to the BSC, and will be used co-located to BSCNAT (#5152).

flow of events¶

individual procedures related to MGW insertion¶

BSSMAP Assignment¶

The failure case[s] of course will need to release the endpoint on the MGW

handover request from MSC to BSC¶

This is an inbound handover into the RAN, processing should be similar to Assignment Req above.

INTERNAL HANDOVER REQUIRED¶

This is sent by the BSC to the MSC in case the codec/codec-config is changed as part of a BSS-internal handover. We need to handle related signaling with MGW and MSC.

hiding intra-BSCNAT handover from MSC¶

If there are inter-BSC hand-overs behind one BSC-NAT, it would probably confuse the MSC quite a bit if they were brought up to the MSC as inter-BSC HO, while the BSCs view is that they are infact intra-BSC (as the BSCNAT appears as one BSC to it).

Hence, we will need to handle inter-BSC handovers within one BSC-NAT internally, and only bring those to the MSCs attention which actually are beyond the service area of the BSC-NAT.

as discussed today, osmith and Hoernchen will be working at this

Sorry, I've only briefly updated a related SYS issue and not this one.

create specification here in the wiki

Looks like that's done with what laforge added here, so marking as complete (or should there be more? in that case, please correct me):
https://osmocom.org/projects/osmo-bscnat/wiki/AoIP_OsmoBSCNAT

create ladder diagrams for handling of key prcedures

Done for calls, which is the most important one:
https://osmocom.org/projects/osmo-bscnat/wiki/Ladder_diagrams_for_key_procedures

(Again, if there should be more to complete this checkbox, please correct me.)

create TTCN3 test suite covering key procedures

Hoernchen is on this, since I had started with the implementation part he suggested that he takes over the testsuite.

rewrite osmo-bscnat using libosmo-sigtran (for AoIP support)

I've started this, WIP branch is in osmith/wip of osmo-bsc-nat.git. This needs some more work, but I've planned to to submit first patches to gerrit tomorrow.

EDIT: urgent tasks came up, but I should be able to submit first patches by the start of next week.

Submitted new patches. Very basic forwarding of messages between osmo-bsc and osmo-msc is now implemented (with point codes for MSC and BSC in the config, not saving/using a mapping yet, and without modifying the messages). Sending RESET and RESET ACK works, OsmoBSC reports that the connection to MSC is established.

I've submitted patches to map connections between MSC and the BSCs. With them it's possible to do a successful Location Updating procedure between MS2 and MSC in the above network structure.

Patches are again here:
https://gerrit.osmocom.org/q/topic:bscnat

Next up is making more of the message forwarding work, until calls between MS1 and MS2 work.

With latest patches: calls work between MS1 and MS2 in the following network structure, with RTP traffic going through MGW-BSCNAT:

      MS1 --- BTS1 --- BSC1 --.
               |        |     BSCNAT ----------- MSC
               |        |     |   |              |
               '-- MGW-BSC1 --|-- MGW-BSCNAT --- MGW-MSC
                              |   |
      MS2 --- BTS2 --- BSC2 --'   |
               |        |         |
               '-- MGW-BSC2 ------'