Description
In the context of 3GPP architectures, particularly those involving Media Gateways (MGW) and the Media Gateway Control Function (MGCF) or Media Resource Function Controller (MRFC), "RS" stands for Remote Source. This is a protocol-level term inherited from the H.248 (Megaco) protocol, which is the standard interface for controlling media gateways. Within the H.248 model, a Media Gateway is abstracted into a series of terminations and contexts. Terminations are sources or sinks of media streams, and contexts are associations mixing or switching multiple terminations. The "Remote Source" is a specific type of termination descriptor or property that identifies the remote endpoint from which a media stream is originating.
Operationally, when a Media Gateway Controller (MGC) like an MGCF sets up a call traversing a media gateway—for instance, converting between Time-Division Multiplexing (TDM) voice from the legacy PSTN and RTP/IP packets for the IMS core—it uses H.248 commands to configure the terminations within the MGW. For the termination that will receive the incoming media stream (e.g., the RTP stream from the IP side, or the timeslot from the TDM trunk), the controller may specify properties that include identifying it as a Remote Source. This descriptor carries information about the remote party's media parameters, such as the IP address, port number, codec type, and packetization characteristics. It essentially tells the media gateway, "expect to receive a stream from this remote source with these properties."
The RS descriptor is crucial for the MGW to properly configure its receive path. It allows the gateway to open the correct ports, initialize the appropriate jitter buffers, and prepare the necessary transcoding resources if the received codec differs from what needs to be sent out on the other termination. The concept is symmetrical with "Remote Sink," which describes the destination for an outbound stream. Together, they enable the MGC to fully describe a bidirectional media flow using the H.248 protocol's connection model. In 3GPP specifications like TS 29.232 (Mn interface), which profiles H.248 for 3GPP use, these terms are used to define the precise information exchanged between the MGCF and the MGW to establish media bearers for services like voice call continuity between CS and IMS domains.
Purpose & Motivation
The purpose of the "Remote Source" concept within the 3GPP control framework is to provide an abstract, standardized way for a controller to instruct a media gateway about the characteristics of an incoming media stream. This abstraction is fundamental to the decomposition of call control and media processing mandated by the Gateway Control architecture. Historically, in monolithic switches, call control and media switching were tightly integrated. The move to decomposed architectures, with separate MGCFs (for signaling) and MGWs (for media), required a robust protocol—H.248—to allow the controller to command the media plane. The RS descriptor is a key element of this protocol, solving the problem of how to precisely describe a dynamic, remote media endpoint to a relatively dumb media gateway.
Before such abstractions, configuring media paths was often low-level and vendor-specific. The RS term, as part of the H.248 standard, creates a common language. It allows a Media Gateway Controller from one vendor to successfully configure a Media Gateway from another vendor to receive a stream from a third-party endpoint. This interoperability is critical for multi-vendor networks. Its creation was motivated by the need for flexibility and scalability in next-generation networks. An MGCF handling complex call routing (e.g., for a roaming subscriber's call) needs to be able to instruct the MGW to connect a stream from a potentially unknown remote IP address (the RS) to a local TDM circuit. The RS descriptor encapsulates all the necessary IP transport and media encoding information in a structured way.
Furthermore, in 3GPP's evolution towards IMS and SRVCC (Single Radio Voice Call Continuity), the ability to rapidly repoint media streams during handovers is vital. The MGCF uses H.248 commands with RS (and Remote Sink) descriptors to quickly reconfigure the MGW's media connections when a call is handed over from LTE to 2G/3G. Thus, the RS concept exists to enable precise, interoperable, and dynamic control of media flows in a decomposed network architecture, which is a cornerstone of modern telecom networks supporting both legacy and IP-based services.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (90 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the "RS" function saw clarifications and corrections for CSI-RS configuration and triggering procedures, including for aperiodic CSI-RS with different numerology between PDCCH and CSI-RS. It also introduced support for ongoing re-mapping on the source side during SDAP mobility. Furthermore, corrections were made regarding the scaling between CSI-RS and SSB for Beam Failure Recovery (BFR) and the handling of serving cell measurements without SSB or CSI-RS.
- Correction to aperiodic CSI-RS triggering with different numerology between PDCCH and CSI-RS TS 38.214CR0007
- Correction on CSI-RS configuration in 38.214 TS 38.214CR0009
- Removal of "Correction to aperiodic CSI-RS triggering with different numerology between PDCCH and CSI-RS" TS 38.214CR0035
- Support of ongoing re-mapping on source side during SDAP mobility TS 38.300CR0160
- Correction on the scaling between CSI-RS and SSB for BFR TS 38.321CR0423
- Missing need code for refFreqCSI-RS TS 38.331CR0696
+ 7 more changes
In Release 16, the "RS" (Reference Signal) function was enhanced with new capabilities for aperiodic CSI-RS triggering supporting beam switching timings of 224 and 336 symbols, and the introduction of inter-gNB CSI-RS based mobility. It also included corrections and clarifications for multi-TRP operations, cross-carrier scheduling with CSI-RS, and the definition of CSI-RS based intra-frequency and inter-frequency measurements. Furthermore, Release 16 specified the release of the source part of DAPS DRBs upon DAPS handover completion.
- Aperiodic CSI-RS Triggering for UE reporting beamSwitchTiming values of 224 and 336 TS 38.214CR0060
- CR TS 38.300 Remote Interference Management TS 38.300CR0184
- Aperiodic CSI-RS triggering with beam switching timing of 224 and 336 TS 38.331CR1716
- Introduction of Inter-gNB CSI-RS Based Mobility TS 38.300CR0249
- CR to 38.331 on CSI-RS inter-node message TS 38.331CR1354
- 38331 CR for CSI-RS-ResourceConfigMobility TS 38.331CR2250
+ 22 more changes
In Release 17, the RS (Reference Signal) function saw enhancements focused on multi-TRP (mTRP) operations and beam management, including corrections for CSI-RS configurations for inter-cell mTRP, unified TCI states, and slot offsets for resource pairs. It also introduced clarifications and corrections for Beam Failure Detection Reference Signal (BFD-RS) sets, relaxation procedures when two sets are configured, and aspects for remote UE synchronization and power control. Furthermore, specifications were updated for aperiodic CSI-RS timing with mixed numerologies and for Zero-Power (ZP) CSI-RS rate-matching in multi-PDSCH scheduling scenarios.
- CR for CSI-RS power for inter-cell mTRP TS 38.214CR0313
- CR on default QCL for unified TCI state for PDSCH and A-CSI-RS TS 38.214CR0314
- Correction on CSI-RS port restriction for mTRP CSI TS 38.214CR0319
- Correction on slot offsets of CSI-RS resource pairs for MTRP TS 38.214CR0320
- Correction on aperiodic CSI-RS for tracking for fast SCell activation TS 38.214CR0321
- Correction on frequency resource for CSI-RS for tracking in TS 38.214 TS 38.214CR0351
+ 17 more changes
In Release 18, the key new feature for the Remote Source (RS) function was its introduction for pathloss determination of Type 1 Configured Grant PUSCH. This was enhanced by the addition of a MAC Control Element-based mechanism for updating the Pathloss Reference RS. Furthermore, corrections and clarifications were made to the Pathloss RS within the LTM TCI state and to uplink power control procedures for Type-1 CG-PUSCH.
- Introduction of RS for pathloss determination of Type 1 CG PUSCH [PL RS Type 1 CG] TS 38.213CR0567
- Introduction of MAC CE based PL RS updates for Type-1 CG-PUSCH [PL RS Type 1 CG] TS 38.331CR4513
- Downlink positioning support and posSIB request for L2 UE-to-network remote UE [PosL2RemoteUE] TS 38.331CR4066
- Corrections to the Pathloss RS in LTM TCI state TS 38.213CR0633
- Correction on PT-RS Coherence Conditions for 8 Tx TS 38.214CR0591
- Correction on threshold for A-CSI-RS reception for Rel-18 TCI framework TS 38.214CR0613
+ 9 more changes
In Release 19, the Remote Source (RS) function saw enhancements for U2U (UE-to-UE) Relays with the introduction of Peer Remote UE Control Plane Procedures. Furthermore, corrections were made to improve reliability, specifically for SI (System Information) reception by the remote UE in multi-path scenarios and for uplink power control procedures related to Type-1 Configured Grant PUSCH.
- TEI19 Counting of CSI-RS resource referred by N CSI reporting settings [SimCSI_count] TS 38.214CR0681
- TEI19 Simultaneous NZP-CSI-RS resource counting with NES [SimCSI_countNES] TS 38.214CR0689
- Correction on Semi-persistent CSI/Semi-persistent CSI-RS for LTM TS 38.214CR0733
- Correction on association between NZP CSI-RS and CSI-IM TS 38.214CR0744
- Removal of Request for CSI-RS resource configuration for Early CSI acquisition TS 38.300CR1079
- Support for Handover Cancel from the source gNB for LTM TS 38.300CR1119
+ 5 more changes
Explore further
Broader topics and technologies where RS plays a role.
Defining Specifications
3GPP specifications that define or reference RS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 29.238 vj00 | H.248 Profile for IBCF-TrGW Interface | Rel-19 |
| TS 36.104 vj10 | Base Station (BS) radio transmission and reception | Rel-19 |
| TS 36.116 vj00 | E-UTRA Relay RF Requirements | Rel-19 |
| TS 36.117 vj00 | E-UTRA Relay RF Test Methods & Requirements | Rel-19 |
| TS 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 37.104 vj10 | MSR Base Station RF Characteristics | Rel-19 |
| TS 37.141 vj10 | RF Test Methods for Multi-Standard Radio Base Stations | Rel-19 |
| TS 37.145 vj10 | AAS Base Station Conducted Conformance Testing | Rel-19 |
| TS 37.802 va10 | MSR BS RF Requirements for Non-Contiguous Spectrum | Rel-10 |
| TS 37.812 vb30 | Multi-band Multi-standard Radio BS Requirements | Rel-11 |
| TR 37.900 vj00 | Multi-Standard Radio (MSR) Base Station Requirements | Rel-19 |
| TS 38.104 vj20 | NR Base Station RF Requirements | Rel-19 |
| TS 38.141 vj20 | NR Base Station RF Conformance Testing Part 1 | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TR 38.869 vi00 | Study on low-power wake up signal and receiver for NR | Rel-18 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |