Remote Source

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.

Key Features

• H.248/Megaco protocol descriptor for an inbound media stream endpoint
• Used by Media Gateway Controllers (MGCF/MRFC) to configure Media Gateways
• Encapsulates remote endpoint details like IP address, port, and codec information
• Enables dynamic setup and modification of media paths in a decomposed architecture
• Critical for interoperability between controllers and gateways from different vendors
• Supports services requiring media interworking, such as CS-IMS call continuity and SRVCC

Evolution Across Releases

Defining Specifications