Layered Coding Transport

Description

Layered Coding Transport is a 3GPP standardized mechanism for the delivery of media, particularly video, over broadcast/multicast and unicast networks. It is designed to work with layered video codecs, most notably Scalable Video Coding (SVC), an extension of the H.264/AVC standard. The core principle of LCT is to transport the different layers of a scalable video bitstream as separate logical channels or transport sessions. The base layer, containing the essential video information, is sent independently from one or more enhancement layers, which add spatial resolution (size), temporal resolution (frame rate), or quality (Signal-to-Noise Ratio). This separation allows receivers to subscribe to and decode only the layers they can successfully receive and process, based on factors like available radio bandwidth, device capability, and subscription level.

From a transport perspective, LCT typically utilizes the File Delivery over Unidirectional Transport (FLUTE) protocol, which itself is built on top of the Asynchronous Layered Coding (ALC) protocol suite. FLUTE/ALC provides reliable file delivery over IP multicast. In LCT, each video layer (base or enhancement) is packaged as a separate FLUTE file delivery session. These sessions are announced via a Session Description Protocol (SDP) file or a Media Presentation Description (MPD) in the context of Dynamic Adaptive Streaming over HTTP (DASH). A client device receives the announcement, identifies the available layers and their corresponding transport sessions (identified by specific Transport Session Identifiers - TSIs), and joins the multicast groups or establishes the unicast connections necessary to receive the layers it selects.

The architecture involves several key components: the Broadcast Multicast Service Centre (BM-SC) in the network, which acts as the source and controller for the FLUTE sessions; the LTE/5G broadcast entities like eMBMS (evolved Multimedia Broadcast Multicast Service) or 5G MBS (Multicast Broadcast Service) for distribution; and the client device with a media player and a FLUTE/ALC receiver. The client performs adaptive streaming logic by monitoring reception conditions (e.g., packet loss rate, signal strength) and dynamically adding or dropping enhancement layer sessions. If an enhancement layer is lost due to poor reception, the base layer can still be decoded, providing a lower-quality but continuous video experience—a key feature for robust mobile broadcast. LCT is thus a hybrid approach, combining the efficiency of multicast for widespread distribution with the flexibility of client-driven adaptation.

Purpose & Motivation

Layered Coding Transport was developed to address the challenges of delivering high-quality video streaming services efficiently over mobile networks, particularly in broadcast/multicast scenarios like Mobile TV. Traditional unicast streaming is inefficient for popular live events, as it duplicates the same stream to every user, consuming excessive network resources. Broadcast/multicast (e.g., MBMS) solves the efficiency problem but traditionally delivered a single, fixed-quality stream, which was unsuitable for the diverse and fluctuating reception conditions of mobile users.

LCT was created to combine the spectrum efficiency of broadcast with the adaptability of unicast streaming. Its primary purpose is to enable Quality of Experience (QoE)-driven adaptive streaming over broadcast channels. By splitting the video into independent layers, it allows a single broadcast transmission to serve users with different device capabilities (e.g., smartphones vs. tablets) and different radio conditions (e.g., cell-edge vs. cell-center). Users experiencing poor signal quality can decode only the robust base layer, while users with excellent reception can add enhancement layers for high definition. This maximizes service coverage and user satisfaction. Furthermore, LCT facilitates service tiering, where operators can offer basic (base layer only) and premium (base + enhancement layers) subscriptions over the same broadcast infrastructure. It was a foundational technology for eMBMS-based services and influenced later adaptive streaming standards like DASH, where the layered concept evolved into representations segmented over HTTP.