Local Source

Description

Within the 3GPP architecture, particularly in specifications related to the Media Gateway Control Function (MGCF) and IP Multimedia Subsystem (IMS), the term "Local Source" (LS) is adopted from the ITU-T H.248 (Megaco) protocol naming convention. H.248 is a master-slave protocol used by a Media Gateway Controller (MGC), such as an MGCF, to control the resources and media processing within a Media Gateway (MGW). The protocol models a Media Gateway as containing a set of Terminations and Contexts. Terminations represent physical or logical endpoints that source or sink media streams (e.g., a TDM timeslot or an RTP port), while Contexts are associations that mix, switch, or connect multiple Terminations.

The "Local Source" is a specific role assigned to a Termination within a Context. In a typical two-termination Context representing a point-to-point call, one Termination is designated as the Local Source (LS) and the other as the Remote Source (RS). The Local Source Termination is the entity that is physically located within the Media Gateway being controlled and is the source of the media stream *from the perspective of that specific MGW*. For example, if an MGW is converting a TDM circuit from the PSTN side to an RTP stream on the IP side, the TDM termination would be configured as the Local Source for the media flowing from the PSTN into the gateway, and the RTP termination would be the Remote Source for that same media flow direction.

This distinction is crucial for the Media Gateway Controller to correctly command media processing operations like echo cancellation, transcoding, or packetization. Commands sent via H.248, such as Modify, can include descriptors (e.g., LocalControl, LocalDescriptor) that contain parameters specific to the Local Source termination, such as the codec to use, packetization period, and sending IP address/port for the RTP stream. The LS/RS model allows the MGC to unambiguously define the topology and flow of media within a Context, enabling precise control over complex scenarios involving multiple media streams, conferencing, or announcements. In 3GPP specs like TS 29.230 and TS 29.238, which define H.248 profiles for IMS and PSTN/CS interworking, the use of Local Source is specified to ensure interoperable control of Media Gateways across different vendor equipment.

Purpose & Motivation

The concept of Local Source (and its counterpart, Remote Source) was introduced in the H.248 protocol to solve the problem of unambiguous media stream control in a decomposed gateway architecture. Traditional monolithic switches handled switching and media processing internally. With the advent of softswitch and IMS architectures, the control plane (MGC/MGCF) was separated from the media plane (MGW). This required a standardized protocol, H.248, for the controller to instruct the gateway on how to process media. A key challenge was for the controller to describe bidirectional media flows and processing requirements without inherent knowledge of the gateway's internal physical mappings.

The LS/RS terminology provides this necessary frame of reference. By defining a "local" perspective (the MGW itself) and a "remote" perspective (the entity on the other side of the media stream), the protocol commands become relative and portable. The same logical command sequence can be used by an MGC to control different types of MGWs (e.g., one with a DS1 interface, another with an E1 interface) because it operates on the abstract roles of LS and RS rather than absolute physical ports. This abstraction is vital for multi-vendor interoperability and for simplifying the control logic in the MGC.

In the context of 3GPP's adoption of H.248 for IMS Media Gateway Control, specifying the use of Local Source was motivated by the need for precise control in interworking scenarios. When an MGCF sets up a call between the PSTN and IMS, it must create Contexts in the MGW that bridge TDM and IP terminations. Using the LS designation allows the MGCF to correctly configure which termination provides the incoming media to be packetized (the LS on the TDM side) and which termination provides the incoming packets to be converted to TDM (the LS on the IP side). This ensures proper media flow, supports advanced features like tone injection and law conversion, and forms the foundation for reliable voice and video interworking between circuit-switched and packet-switched domains.