Effective File Size

Description

Effective File Size (EFS) is a parameter defined within the context of 3GPP's Multimedia Broadcast/Multicast Service (MBMS) and potentially other data delivery mechanisms. It is not the literal size of a file in bytes, but rather a derived metric that represents the total volume of data that must be transmitted over the radio interface to deliver the original file content with a specified level of reliability. The EFS calculation incorporates the original file size and adds all necessary overheads required for transmission.

The primary components added to the original file size to compute EFS include Forward Error Correction (FEC) parity data and various protocol layer headers. In MBMS, files or data units are often delivered using the File Delivery over Unidirectional Transport (FLUTE) protocol atop the User Datagram Protocol (UDP) and Internet Protocol (IP). Each layer adds its own header (FLUTE, UDP, IP). Furthermore, to combat errors in the broadcast channel, Application Layer Forward Error Correction (AL-FEC) is applied, typically using Raptor or RaptorQ codes as specified in 3GPP TS 26.346. The AL-FEC scheme generates repair symbols that are transmitted alongside the source symbols, increasing the total data volume. The EFS is the sum of the source symbols (derived from the original file and protocol headers) and the repair symbols.

EFS is a critical parameter for network planning and operation. For MBMS scheduling, the network needs to know the EFS to determine how much radio resource (e.g., how many transmission time intervals) must be allocated to broadcast a file within a desired time window. It directly relates to the required data rate and the coverage area achievable for a given modulation and coding scheme. By using EFS, network planners can accurately dimension the capacity of an MBMS bearer and create efficient transmission schedules that ensure reliable reception at the cell edge, where channel conditions are poorest and the most FEC overhead is needed.

Purpose & Motivation

EFS was defined to address the practical challenge of resource allocation for reliable broadcast/multicast file delivery in mobile networks. Simply using the original file size is insufficient for radio planning because the system must add significant overhead for error correction and protocol encapsulation to ensure files are received correctly by all users in a coverage area, despite varying radio conditions. Without a standardized way to account for this overhead, scheduling would be inefficient or unreliable.

The concept emerged with the standardization of MBMS, which aimed to deliver content like software updates, news feeds, or video clips to many users simultaneously. Traditional point-to-point (unicast) retransmission protocols are inefficient for this. MBMS uses one-to-many broadcast with AL-FEC for reliability. EFS provides a unified metric that encapsulates the total radio resource cost of delivering a file, enabling automated and optimal scheduling algorithms. It solves the problem of translating a service layer requirement (deliver file X) into a radio resource requirement (allocate Y physical resource blocks).

It allows content providers and network operators to have a common understanding of the resource consumption for a given piece of content. This is essential for capacity management, service level agreements, and efficient use of the broadcast carrier. EFS bridges the gap between the application layer and the radio resource layer in broadcast services.

Key Features

• Represents total radio transmission size including FEC and protocol overhead
• Key input parameter for MBMS broadcast scheduling algorithms
• Incorporates overhead from AL-FEC schemes like Raptor codes
• Accounts for protocol headers from FLUTE, UDP, and IP layers
• Enables accurate radio resource allocation for file delivery services
• Standardized metric for capacity planning of broadcast bearers

Evolution Across Releases

Defining Specifications