Themyscira Wireless Technical Memorandum TW-TM-004
Version 1.0.1

Description of RSL_IE_OSMO_RTP_EXTENSIONS defined in Osmocom Abis

0. Foreword

This document is a Technical Memorandum (TM) and does not carry the normative
power of a Themyscira Wireless Technical Specification (TW-TS).  It has been
produced in order to assist in the ongoing cooperation between Themyscira
Wireless and Osmocom.

Author: Mother Mychaela N. Falconia, High Priestess of Telecommunications

This document has been produced in conjunction with a Themyscira public works
project; as a public work, it is not subject to copyright.

1. Application in question

Themyscira Wireless CN (core network) implementation is designed to work
together with Osmocom BSS, with 3GPP-defined AoIP interface serving as the
interconnection point between ThemWi and Osmocom realms.  However, ThemWi
requires certain modifications to 3GPP-defined user plane transport; the use of
these modifications is communicated via a BSSMAP extension defined in TW-TS-003.
Furthermore, these modifications to RTP payload encodings are extensions to the
information content being carried, and therefore must originate at the BTS -
hence a mechanism is needed inside the BSS to communicate the use of these
Themyscira-defined RTP payload formats from the BSC to the BTS.

Unlike the AoIP/BSSMAP interface that is almost entirely defined by 3GPP and
has only a few Osmocom and Themyscira extensions, the Abis interface between
OsmoBSC and OsmoBTS is essentially private to Osmocom, defined and maintained
there.  Therefore, the necessary addition to Abis RSL is being created by way
of ThemWi contributing the necessary patches to Osmocom and advocating for their
inclusion.

2. RSL_IE_OSMO_RTP_EXTENSIONS definition

This new IE is given IEI code point 0x64, allocated directly after the last
Osmocom-private IEI assignment of RSL_IE_OSMO_OSMUX_CID = 0x63.

The coding format of this new IE is TLV, as indicated by Osmocom reviewers for
all newly defined IEs.

Only one value octet is currently defined; any additional octets are for future
expansion.

The single value octet currently in use has the following bit definitions:

Bit mask 0x01: if set, instructs the BTS to use TW-TS-001 format for FR & EFR
Bit mask 0x02: if set, instructs the BTS to use TW-TS-002 format for HR
Remaining 6 bits: currently unused

3. Applicability and use of the new IE

RSL_IE_OSMO_RTP_EXTENSIONS is to be added to RSL_MT_IPAC_CRCX and
RSL_MT_IPAC_MDCX messages (from OsmoBSC to OsmoBTS) when OsmoBSC is fulfilling
a request on the part of the CN to use otherwise non-standard Themyscira RTP
formats.  Furthermore, OsmoBSC shall check BTS feature flags and confirm that
the BTS supports these extensions before adding this non-standard IE.

4. BTS feature flags

Alongside with the definition of RSL_IE_OSMO_RTP_EXTENSIONS, two new BTS feature
flags are defined:

BTS_FEAT_TWTS001 indicates that the BTS supports TW-TS-001 RTP format (FR & EFR)
BTS_FEAT_TWTS002 indicates that the BTS supports TW-TS-002 RTP format (HR)