Electric Echo Loss

Description

Electric Echo Loss (EEL) is a key transmission performance parameter defined in 3GPP specifications, primarily concerned with voice and multimedia telephony services. It quantifies the loss applied to an electrical echo signal traveling back towards its source. Echo in telephony occurs when a speaker's voice signal is reflected back from a hybrid circuit (which converts between 2-wire and 4-wire lines) or other impedance mismatches in the network path. If not sufficiently attenuated, this echo becomes audible and degrades call quality, especially with long transmission delays common in mobile and international calls.

Architecturally, EEL is measured and controlled at specific points in the voice transmission chain. In a traditional circuit-switched mobile network, it is relevant at the interface between the Mobile Switching Center (MSC) and the Public Switched Telephone Network (PSTN), or within the MSC itself where echo control devices are located. In the IP Multimedia Subsystem (IMS) for Voice over LTE (VoLTE) or Voice over NR (VoNR), echo control is handled by the Media Resource Function (MRF), which includes a Media Resource Function Processor (MRFP) capable of acoustic and electric echo cancellation. The EEL parameter defines a minimum required loss that must be imposed on any electrical echo path to ensure it is below the perceptible threshold.

The metric works by defining a test signal (a specific tone or speech-like signal) injected at the receive path (e.g., from the far-end talker) and measuring the level of that signal returning on the corresponding send path (towards the far-end talker) after traversing the potential echo path within the network equipment. EEL is expressed in decibels (dB). A higher EEL value indicates better echo suppression. For example, a network node might be required to provide an EEL of 46 dB or greater, meaning the echoed signal is attenuated by at least 46 dB. This loss can be achieved through a combination of inherent hybrid loss and active echo cancellation performed by digital signal processors.

Its role in the network is critical for maintaining high Mean Opinion Score (MOS) for voice quality. In modern networks employing IMS and rich communication services, EEL requirements are specified for media gateways and MRFPs to ensure consistent performance regardless of the underlying access technology (2G, 3G, 4G, 5G). It is part of a suite of voice quality metrics that also includes parameters like delay, packet loss, and acoustic echo loss. Compliance with EEL specifications ensures that echo is managed network-side, reducing the burden on user equipment to perform echo cancellation and providing a uniform quality of experience for all users on the network.

Purpose & Motivation

Electric Echo Loss was introduced in early 3GPP releases (Rel-5) to address the persistent problem of talker echo in digital telecommunication networks. As networks evolved from analog to digital and incorporated longer transmission paths (e.g., via satellites or international gateways), the round-trip delay increased. Even a small amount of echo becomes distinctly audible and annoying with delays above about 20-30 milliseconds. The purpose of defining EEL was to establish a standardized, measurable performance requirement for network equipment to suppress this electrical echo, thereby guaranteeing a minimum baseline voice quality for end-users.

The problem it solves is the degradation of conversational speech quality due to impedance mismatches, primarily at 2-wire to 4-wire conversion points (hybrids). Without sufficient loss, the far-end speaker hears their own voice reflected back after a delay. Prior to standardized metrics like EEL, echo control was implemented in an ad-hoc manner, leading to inconsistent voice quality across different network segments and operators. The standardization of EEL allowed for the objective testing and type-approval of switching equipment, media gateways, and later IMS media resources, ensuring interoperability and a predictable user experience.

The historical context for its continued relevance through to Release 20 is the transition to all-IP networks and the convergence of services. While early releases focused on circuit-switched voice, later releases applied EEL principles to the IMS-based packet-switched voice (VoLTE/VoNR). It addresses the limitation that purely packet-based networks do not inherently solve analog line echo problems; echo can still originate from legacy PSTN interconnections or from analog terminals connected via adapters. Therefore, EEL remains a crucial requirement in the end-to-end media path to ensure that high-definition voice and video telephony services are not marred by echo, regardless of the complexity or heterogeneity of the underlying network interconnections.