SYMmetric conditions

Description

SYMmetric conditions (SYM) is a defined set of network transmission parameters used as a standardized test condition within 3GPP specifications, most notably in TS 26.935 for the evaluation of speech and audio codec performance under packet-switched network conditions. It is not a network function or protocol, but a laboratory reference model. The SYM condition creates a simulated network environment where identical, or symmetric, impairments are applied to the media stream in both the uplink and downlink directions between two endpoints (e.g., two user equipment devices). This controlled environment is crucial for conducting reproducible and comparable subjective or objective quality tests.

The condition is characterized by specific, quantified values for key network impairment parameters. These typically include a defined end-to-end packet loss rate (e.g., a specific percentage of packets are randomly or burstly discarded), a fixed network delay (one-way delay), and may also include specifications for jitter (delay variation) and packet loss distribution (e.g., random loss vs. burst loss as defined by a Gilbert-Elliott model). The 'symmetric' aspect means that if a 3% packet loss is applied on the path from Endpoint A to B, an identical 3% loss is applied on the path from B to A. This creates a balanced stress test for the codec's error concealment and robustness algorithms.

In practice, test equipment or network simulators are configured to impose these SYM conditions on the Real-time Transport Protocol (RTP) streams carrying the encoded speech or audio. Codecs under test, such as AMR, AMR-WB, or EVS, are then subjected to these conditions. The resulting output audio is evaluated using subjective listening tests (like Mean Opinion Score - MOS) or objective perceptual quality algorithms (like POLQA). By using a common set of SYM conditions, different codecs, or different modes of the same codec, can be fairly compared against each other, as they are all tested under the same, precisely defined network adversity. This allows 3GPP to standardize codecs with known performance characteristics under specific network quality scenarios.

Purpose & Motivation

SYMmetric conditions were defined to establish a common, reproducible baseline for evaluating and comparing the robustness of speech and audio codecs to packet network impairments. Before such standardized test conditions, codec evaluations could use arbitrary or vendor-specific impairment profiles, making it difficult to objectively compare performance claims and select the best codec for standardization. The creation of SYM (and other conditions like ASYM - asymmetric) provides a controlled 'worst-case' or typical scenario that reflects real network issues like congestion and loss.

Its purpose is to ensure technical rigor in the 3GPP codec selection and characterization process. By defining specific symmetric impairment levels, it allows for consistent testing across different laboratories and equipment vendors. This solves the problem of inconsistent test methodologies and enables the creation of reliable performance data sheets for codecs. The results under SYM conditions help network planners and equipment manufacturers understand how a codec will perform in degraded but balanced network situations, informing decisions on codec mode adaptation and network quality of service (QoS) planning. It is a fundamental tool for quality assurance in voice over LTE (VoLTE) and voice over NR (VoNR) services.