Conversational Service

Description

In 3GPP specifications, a conversational service is defined as an interactive service that provides for bi-directional communication by means of real-time (no store-and-forward) end-to-end information transfer from user to user. This definition is sourced from ITU-T Recommendation I.113. Conversational services are one of the four traffic classes identified for QoS in UMTS and LTE: conversational, streaming, interactive, and background. The conversational class is the most delay-sensitive, as it supports applications such as voice over IP (VoIP), video telephony, and interactive gaming. These services require stringent delay and jitter constraints to maintain acceptable quality of experience.

The conversational service class is characterized by low end-to-end delay, low delay variation (jitter), and a guaranteed bit rate. The typical maximum tolerable delay for voice is around 100-150 ms one-way, while video telephony may allow slightly higher. The network must prioritize these flows to ensure timely delivery. In UMTS, conversational services are typically carried over dedicated channels (DCH) or high-speed downlink packet access (HSDPA) for the downlink, and enhanced dedicated channel (E-DCH) for the uplink. In LTE, they are carried over guaranteed bit rate (GBR) bearers with QoS class identifier (QCI) values such as QCI 1 for voice and QCI 2 for video telephony. In 5G, they are mapped to 5G QoS identifiers (5QI) with similar characteristics.

The conversational service class is essential for real-time communication. It ensures that voice and video packets are delivered within strict time bounds, preventing noticeable degradation. The network achieves this through resource reservation, priority scheduling, and careful admission control. For example, when a user initiates a voice call, the network establishes a dedicated bearer with GBR and low latency. The scheduler in the base station prioritizes these packets over non-real-time traffic. If resources are insufficient, the call may be blocked or dropped to maintain QoS for existing calls.

The definition of conversational service has remained unchanged since Rel-4, indicating its stability and fundamental nature. It is specified in 3GPP TS 21.905 (Vocabulary) and TS 22.060 (General Packet Radio Service (GPRS); Service description; Stage 1). The concept is also used in other specifications for QoS negotiation and policy control. The conversational class is a key element in the overall QoS architecture of 3GPP networks, enabling operators to offer differentiated services and ensure customer satisfaction.

In summary, conversational service is a real-time, bi-directional communication service that is critical for voice and video telephony. It is the most demanding QoS class, requiring low delay and jitter, and is supported by dedicated bearers with guaranteed bit rates. Its definition has been stable across all 3GPP releases, reflecting its importance and maturity.

Purpose & Motivation

The conversational service class was defined to address the specific needs of real-time, bi-directional communication applications, such as voice and video telephony. Traditional circuit-switched voice services provided dedicated resources and guaranteed quality, but with the advent of packet-switched networks, new mechanisms were needed to ensure that voice and video could be delivered with acceptable quality over a shared, best-effort infrastructure. The conversational service class was created to provide a framework for prioritizing and managing these real-time flows, ensuring that they receive the necessary network resources and treatment.

Without a dedicated QoS class for conversational services, real-time traffic would compete with non-real-time traffic on an equal basis, leading to unacceptable delays and jitter. The conversational class solves this by defining strict performance requirements and enabling the network to reserve resources and schedule packets accordingly. This allows operators to offer voice over IP and video telephony with quality comparable to circuit-switched services, while benefiting from the efficiency and flexibility of packet-switched networks. The conversational service class has been a cornerstone of 3GPP QoS architecture since its inception.