Local Descriptor

Description

The Local Descriptor (LD) is a fundamental data structure within the H.248 protocol, also known as the Media Gateway Control Protocol (MGCP) Megaco. H.248 is the standard protocol used for controlling Media Gateways (MGs) from a Media Gateway Controller (MGC) or Softswitch in decomposed network architectures, such as in IMS and VoLTE. A 'Termination' in H.248 represents a logical entity on the MG that sources or sinks media or control streams, like a physical trunk circuit (TDM) or an RTP stream endpoint (IP). Each Termination possesses properties described by Descriptors. The Local Descriptor specifically defines the properties of a Termination that are local to the Media Gateway itself. This typically includes the transport information required to establish a media session. For an IP-based termination, the LD contains critical details such as the local IP address (v4 or v6), the UDP/TCP port number for RTP/RTCP, the supported media codecs (e.g., AMR, G.711), packetization parameters, bandwidth, and other session description protocol (SDP)-like attributes. When an MGC instructs an MG to modify a termination (e.g., to prepare for a call), it sends a command that may include an LD to tell the MG "use this IP address and port with these codecs." The MG responds with a Remote Descriptor (RD) containing the corresponding information from the far end of the session. The LD/RD pair is essential for the MG to correctly configure its media plane. The LD is carried within H.248 transaction messages, most commonly in the 'Modify' command or within the 'LocalControl' and 'Local' descriptors of an 'Add' command. Its structure is flexible, allowing for a wide range of media types and network scenarios, making it the primary mechanism for the MGC to provision the media-bearing resources on the gateway.

Purpose & Motivation

The Local Descriptor exists to provide a standardized, abstracted way for a call control entity (the MGC) to configure the media-specific parameters on a separate media processing device (the MG). Before protocols like H.248, media and call control were often integrated in monolithic switches, making networks inflexible. The drive towards decomposed, next-generation networks (NGN) and the IP Multimedia Subsystem (IMS) required a clean separation of control and media planes. The H.248 protocol was created to manage this separation, and the LD is a core construct within it. It solves the problem of how a controller, which understands the call logic and session negotiation, can precisely instruct a dumb media device on how to send and receive media streams. It addresses the need for interoperability between controllers and gateways from different vendors by providing a standardized syntax for conveying media session parameters. The LD allows the MGC to remain agnostic to the specific hardware or low-level software of the MG, simply providing the necessary session description. This enables scalable, flexible network architectures where powerful controllers can manage large pools of specialized media resources, a cornerstone of modern VoIP and telecommunications networks.

Key Features

• Media Session Configuration: Carries essential parameters for media stream setup, including IP address, port, and codec list.
• H.248 Protocol Element: A defined descriptor type within the H.248/Megaco standard (often referenced in packages).
• Paired with Remote Descriptor: Works in conjunction with the RD to provide complete session information for a bidirectional media stream.
• Supports Various Media Types: Can describe parameters for audio (RTP), video, text, and other media as defined by H.248 packages.
• Used in Modify/Add Commands: Primarily included in H.248 'Modify' and 'Add' commands sent from the MGC to the MG.
• Enables Control/Media Separation: A key enabler for the decomposed architecture by letting the MGC control the MG's media plane.

Evolution Across Releases

Defining Specifications