Themyscira Wireless Technical Specification TW-TS-003 Version 1.0.1 BSSMAP extension for selection of enhanced RTP transport formats 0. Foreword This Technical Specification (TS) has been produced under the authority of the Presiding Sisterhood (government) of the Women's Republic of Themyscira as part of Themyscira Wireless technical initiative. Author: Mother Mychaela N. Falconia, High Priestess of Telecommunications As an official publication of Themyscira government, this document is not subject to copyright. 1. Scope The BSSMAP extension defined in this TS is applicable to the AoIP interface (MSC-BSS interface) in any IP-based GSM network whose operator wishes to use the enhanced RTP transport facilities defined in other Themyscira Wireless Technical Specifications. The intent of the present TS is to make it possible to add support for Themyscira enhanced RTP transport to mainline OsmoBSC and OsmoBTS, a course that requires a mechanism for selecting between standard 3GPP-compliant operation and Themyscira extensions. 2. References The following documents contain provisions which, through reference in this text, constitute provisions of the present document. [1] 3GPP TS 48.008: "Mobile Switching Centre - Base Station system (MSC-BSS) interface; Layer 3 specification". [2] 3GPP TS 48.103: "Base Station System - Media GateWay (BSS-MGW) interface; User Plane transport mechanism". [3] TW-TS-001: "Enhanced RTP transport of FR and EFR codec frames in an IP-based GSM RAN". [4] TW-TS-002: "Enhanced RTP transport of HRv1 codec frames in an IP-based GSM RAN". 3. Definitions, conventions and abbreviations The terms BSS, BSSAP, BSSMAP, AoIP and MSC are used as defined in [1]. CN stands for Core Network. RTPext is the short name for BSSMAP information element GSM0808_IE_THEMWI_RTP_EXTENSIONS, defined in chapter 5 of the present TS. Hexadecimal numbers are given in C language notation. 4. Extensions to BSSMAP procedures 4.1. Extension to assignment procedure If a given CN implementation wishes to use enhanced RTP transport formats defined in the related family of Themyscira Wireless specifications, and thereby intentionally deviate from the standard RTP formats prescribed in [2], it shall indicate this special requirement by adding the RTPext IE, defined in chapter 5 of the present TS, to the ASSIGNMENT REQUEST message it sends to the BSS. Bits within this RTPext IE indicate exactly which TW-TS enhanced RTP transport formats the CN wishes to use. If the BSS understands the RTPext IE and is able to support the requested extensions, it shall include an RTPext IE in its ASSIGNMENT COMPLETE response, with individual bits in the RTPext IE response indicating which specific RTP extensions have been applied and will be in use. The MSC shall consider the presence or absence of RTPext IE in the BSS response to determine whether or not its request to use Themyscira extensions has been granted. In a given CN implementation, the use of TW-TS enhanced RTP transport formats may be an absolute requirement, such that the CN in question cannot operate with a BSS that lacks the corresponding extension. In such circumstanes, if the MSC receives an ASSIGNMENT COMPLETE response that lacks the RTPext IE, or if individual bits within the RTPext IE response are cleared where the corresponding formats are strictly required, the MSC may declare a fatal error condition to its local O&M and terminate the call attempt. 4.2. Extension to handover procedures It is anticipated that certain handover procedures will require extensions similar to the one specified in section 4.1 for assignment. This work is currently left for further study. 5. Themyscira RTP Extensions information element One non-standard information element is added to BSSMAP by the present TS: Themyscira RTP Extensions IE, symbolic name GSM0808_IE_THEMWI_RTP_EXTENSIONS, also referred to as RTPext IE for short. This IE can be optionally added to some BSSMAP request messages as detailed in chapter 4, and it also appears in responses to those extended requests. This chapter defines the coding of this RTPext IE. 5.1. Information element identifier The IEI assigned to GSM0808_IE_THEMWI_RTP_EXTENSIONS is 0xF2, allocated from the range that is already used for Osmocom extensions prior to the present work. 5.2. Fixed length IE The RTPext IE defined here is one of fixed length, consisting of just two octets: the IEI and one octet of flag bits. This format allows for a maximum of 8 communicable flags, while only 2 of these bits are defined by the present TS - hence this format is deemed to be sufficient for anticipated future additions. 5.3. Detailed coding of the IE The bit numbering convention and the format structure drawing convention used in this section are the same as in 3GPP TS 48.008 clause 3.2.2. The complete format of RTPext IE is as follows: +---+---+---+---+---+---+---+---+ | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | +---+---+---+---+---+---+---+---+ | IEI 0xF2 | octet 1 +---+---+---+---+---+---+---+---+ | r | r | r | r | r | r |HR |FR | octet 2 +---+---+---+---+---+---+---+---+ The bit labeled "FR" indicates that the use of enhanced RTP transport for FRv1 and EFR codec frames per TW-TS-001 is requested or accepted. The bit labeled "HR" indicates that the use of enhanced RTP transport for HRv1 codec frames per TW-TS-002 is requested or accepted. All bits labeled "r" shall be set to 0 by senders and ignored by receivers; they may be assigned meaning in future versions of the present TS. 5.4. Semantic meaning of "requested or accepted" The phrasing "requested or accepted" in the definition of specific bits in section 5.3 has the following semantic meaning: a) In request messages from the MSC to the BSS, presence of RTPext IE with this bit set indicates that the CN wishes to use the RTP extension in question. b) In response messages from the BSS to the MSC, presence of RTPext IE with this bit set indicates that the BSS supports the RTP extension in question and will use it on the call leg in question as requested by the CN. Annex A (informative): Specification change history Version 1.0.1: initial publication for review by Osmocom community.