Real Time

Description

Within the 3GPP architecture, 'Real Time' (RT) is not a single protocol or interface but a fundamental Quality of Service (QoS) characteristic and service classification. It defines a category of traffic where the information value is critically dependent on the timeliness of delivery. The end-to-end transfer delay must be bounded and predictable to be acceptable for the application. This contrasts with Non-Real Time (NRT) or background traffic, where throughput or error rate may be more important than strict delay. The RT classification permeates multiple network layers and specifications, influencing protocol design, resource reservation, and scheduling algorithms.

How RT requirements are implemented involves several key components across the Radio Access Network (RAN) and Core Network (CN). In the CN, for a service like Voice over IP (VoIP), the IP Multimedia Subsystem (IMS) and the Policy and Charging Control (PCC) architecture are involved. The PCC rules, defined by the Policy and Charging Rules Function (PCRF), specify that a bearer for a VoIP call must be established with a QoS Class Identifier (QCI) value associated with conversational voice (e.g., QCI 1), which has strict guarantees for priority, packet delay budget, and packet error loss rate. This QoS profile is then enforced by the Packet Data Network Gateway (PGW) and the serving gateway. In the RAN, the real-time nature dictates the operation of the MAC and physical layers. For example, in UMTS, the Radio Access Bearer (RAB) for a voice call is configured with specific attributes like guaranteed bit rate, transfer delay, and traffic handling priority. The Node B scheduler must prioritize radio resources for these RT bearers over NRT data to meet the delay budget.

Its role is absolutely central to the core function of mobile networks: enabling interactive communication. The entire air interface design, from frame structure in GSM (TDMA timeslots) to Transmission Time Intervals (TTI) in UMTS/LTE, and mini-slots in 5G NR, is optimized to support low-latency RT services. Scheduling algorithms in the base station (eNodeB/gNB) constantly make millisecond-level decisions to allocate physical resource blocks (PRBs) to UEs with active RT sessions. Admission control mechanisms also use RT service requirements to decide whether a new call can be accepted without degrading the performance of existing calls. Furthermore, mobility procedures like handover are optimized for RT services, often using faster, network-controlled handovers to prevent call drops, unlike the more measurement-driven handovers possible for best-effort data.

Purpose & Motivation

The concept of Real Time service classification exists to address the fundamental challenge of supporting delay-sensitive human communication, primarily voice, over shared, packet-switched networks. Early mobile telephony (GSM) was inherently circuit-switched, dedicating a physical channel (a timeslot) for the duration of a call, which naturally guaranteed timing. With the evolution towards packet-switched networks in UMTS and especially LTE/5G, all traffic, including voice, became packetized data. This introduced the problem of jitter and variable delay due to statistical multiplexing and queueing. The RT QoS class was created to ensure that these voice/video packets receive prioritized treatment through the entire network path, mimicking the guarantees of a circuit-switched channel.

Historically, the move to All-IP networks (AIPN) in 3GPP Release 5 and beyond made explicit QoS mechanisms imperative. Without them, voice packets could be delayed behind large file downloads, rendering conversation impossible. The RT classification, along with associated QoS parameters (like QCI, ARP, GBR), provides a standardized framework for networks to distinguish this critical traffic. It solves the problem of service degradation in a converged network, allowing operators to offer traditional telephony services (via VoLTE, VoNR) with carrier-grade reliability over an IP-based infrastructure. This enabled the sunset of legacy circuit-switched cores while maintaining, and even enhancing, the user experience for real-time communication.

Key Features

• Defines traffic with bounded end-to-end delay requirements
• Maps to specific QoS Class Identifiers (QCIs) for bearer setup
• Influences RAN scheduling and priority algorithms
• Triggers specific admission control and resource reservation policies
• Essential for conversational voice (VoLTE/VoNR) and live video streaming
• Requires optimized mobility management (e.g., seamless handover)

Evolution Across Releases

Defining Specifications