Program Association Table

Description

The Program Association Table (PAT) is a mandatory table defined within the MPEG-2 Systems specification (ISO/IEC 13818-1) and adopted by 3GPP for multimedia broadcast and streaming services, such as MBMS (Multimedia Broadcast Multicast Service) and later enhancements. It resides in packets with Packet ID (PID) 0 within an MPEG-2 Transport Stream (TS). The primary function of the PAT is to act as a root directory for the TS. It provides a mapping between each Program Number (a unique identifier for a program or service within the multiplex) and the PID value of the Transport Stream packets that carry the corresponding Program Map Table (PMT) for that program.

Architecturally, the PAT is the first table a decoder must parse when tuning to a transport stream. The TS is a multiplex of many elementary streams (video, audio, data) belonging to one or more programs. Without the PAT, a receiver would not know where to find the descriptions of these programs. The PAT works by being transmitted periodically within the TS. When a receiver filters for PID 0, it extracts the PAT data. This data is a simple list of Program Numbers and their associated PMT_PIDs. For Program Number 0, the associated PID points to the Network Information Table (NIT), which contains information about the physical network.

Once the PAT is decoded, the receiver can select a desired Program Number and then filter the TS for packets with the PID indicated for that program's PMT. The PMT then lists all the component streams (with their PIDs and stream types) that make up the selected program, allowing the decoder to assemble the audio, video, and other data. In a 3GPP context, this mechanism is crucial for services like eMBMS (evolved MBMS), where broadcast content is delivered over LTE or 5G networks. The PAT enables efficient service discovery and selection within a broadcast/multicast session, allowing user equipment to correctly tune to and decode the intended broadcast service from a multiplexed transport stream delivered over the air interface.

Purpose & Motivation

The PAT exists to solve the fundamental problem of service discovery and demultiplexing in a packetized, multiplexed transport stream environment. In digital broadcasting and streaming, multiple television channels, radio programs, and data services are combined into a single transmission bitstream to maximize bandwidth efficiency. Without a defined indexing mechanism like the PAT, a receiver would have no way to identify which packets belong to which program, making selective tuning impossible. It provides the essential 'table of contents' for the entire multiplex.

Its creation was motivated by the transition from analog to digital broadcasting and the need for a standardized, flexible multiplexing scheme. The MPEG-2 Systems layer, which includes the PAT, was designed to be network-agnostic, working over satellite, cable, terrestrial, and later, mobile networks. 3GPP adopted this model for MBMS to leverage a well-understood, robust mechanism for service description, enabling the delivery of broadcast TV and radio services over cellular networks. This addressed the limitation of earlier mobile broadcast technologies that lacked such a standardized, scalable service layer framework.

In the context of 3GPP, the PAT enables the efficient use of scarce radio resources for broadcast services. By allowing multiple services to be multiplexed into a single broadcast bearer, network capacity is optimized. The PAT allows user equipment to quickly find and decode only the service of interest without processing the entire multiplex, conserving battery power and processing resources. It is a key enabler for linear broadcast services, live event streaming, and software updates over-the-air via broadcast, forming the service layer foundation for point-to-multipoint delivery in mobile networks.

Key Features

• Resides in MPEG-2 Transport Stream packets with PID 0
• Maps Program Numbers to the PID values of their respective Program Map Tables (PMTs)
• Contains an entry for Program Number 0 that points to the Network Information Table (NIT)
• Transmitted periodically to ensure rapid service acquisition by receivers
• Mandatory table for any MPEG-2 Transport Stream
• Enables selective decoding of specific programs from a multiplex

Evolution Across Releases

Defining Specifications