Intro

OpenBSC (later OsmoNITB) started with E1 BTS
until NITB Split, E1 support remained present
even old OsmoBSC (sccplite) never had E1 BTS suport
new OsmoBSC also has no E1 BTS support
let’s change that!

Classic OsmoNITB with E1-BTS + ext MNCC

Abis/E1 with 16k sub-slots in 64k slots
E1 device offers 64k timeslots only
- 16k sub-channel mux/demux done in software
TRAU frames in 16k sub-slots decoded (to FR/EFR codec frames)
passed next to MNCC signaling over MNCC socket

Classic OsmoNITB with E1-BTS + ext MNCC + RTP

just like previous example, but we generate RTP from TRAU frames
MNCC interface is signaling only
user voice frames handled via RTP

Classic OsmoNITB with IP-BTS + ext MNCC + RTP proxy

Abis/IP BTS use RTP transport for speech frames
OsmoNITB is asked to provide RTP proxy functionality

Classic OsmoNITB with IP-BTS + ext MNCC + direct RTP

Abis/IP BTS use RTP transport for speech frames
OsmoNITB rtp_proxy is disabled
RTP data passes directly from BTS to external PBX and vice-versa

IP-BTS User Plane in post-NITB

When using IP-based BTS like OsmoBTS, this looks like this:

RTP to/from the BTS is handled by the BSC-colocated OsmoMGW

E1-BTS User Plane in post-NITB

When using E1-based BTS, it should look like this:

TRAU frames in 16k E1 sub-slots to/from the BTS are handled by the BSC-colocated OsmoMGW
OsmoMGW needs real media gateway functionality from E1 sub-slots to RTP
E1 driver needs to support single E1 span (line) that
- opens RSL/OML LAPDm signaling on one TS from OsmoBSC
- opens TRAU on other TS from OsmoMGW
osmo-bsc.cfg states which Um TS maps to which E1 TS/SS
osmo-bsc uses MGCP EP naming scheme like E1/Line1/TS4/SS2@mgw

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