Feature: Communications
Figure 1: The protocol structure of voice over NR
Protecting voice quality in 5G network setups By Reiner Stuhlfauth, Technology Manager Wireless, Rohde & Schwarz
T
echnological advances have brought on a major change in communications, from circuit-switched 2G networks with initial focus on telephony,
to fully packet-switched 4G networks for Internet data transfer. Today we are talking 5G technology, with its flexible and sophisticated architecture, pitched to provide enhanced data services for mobile devices. Despite the focus on the exchange of high
volumes of data, voice services remain a key element, with two major considerations: First, there’s radio access network (RAN) – i.e., whether 5G new radio (NR) is offered in addition to LTE as non-standalone access (NSA, option 3 deployment), or whether there is a 5G standalone network (SA mode, or option 2 deployment). Te NSA mode enables dual-connectivity scenarios with either LTE (EN-DC) or 5G as primary radio access technology (NE-DC). Te second consideration is the type of
network used – EPC or 5G core (5GC). In a dual-connectivity scenario, there can be a voice-service restriction, indicated by the radio access technology (RAT). It concentrates on voice or speech services, although 5G may certainly offer video or communication services, so-called “Rich
Communications Services”, which are managed in a similar way to voice. Te only difference here is in the support of emergency services, where the network distinguishes between emergency or general voice calls through the signalling. Regarding protocol and transport, emergency and voice are handled in a similar way, except for quality of service (QoS) profiles, but a network may support both services as separate offerings. Tere is a difference between legacy
networks and a 5G network offering voice services, as the latter exchanges connection parameters and service access policies during the registration procedure. Te user equipment will indicate its capabilities to the network and, in the other direction, the network offers subscribed services, i.e., voice or video calls.
VoNR Voice over New Radio (VoNR) is voice over IP, incorporating the IP multimedia subsystem (IMS) infrastructure, already part of legacy technologies like LTE; see VoNR’s protocol structure in Figure 1. It offers a management and orchestration system that guarantees end-to-end QoS for each application, compared to VoIP with its traffic-channel-only approach. IMS establishes, controls and maintains
22 November 2021
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a packet data unit session, including its data bearers, with corresponding QoS flow for best end-user experience. Tere will be at least two data bearers established: one for content (speech packets containing the encoded audio itself) and one for IMS signalling. As in VoLTE, there is a major difference
between voice over IMS in 5G systems (5GS) and voice services offered by external applications, e.g., so-called over-the-top (OTT) speech services. Tis is because OTT speech may operate transparently over the connectivity network and there is no IMS management to ensure QoS, which raises the question of how to connect IMS to the 5G core network. For reasons such as fast market adoption,
network deployments, disaggregation of network entities and coexistence with legacy technologies, there is no single 5G deployment scenario; see Figure 2. Te evolutionary paths describe if an NSA connection voice is supported by E-UTRA only, or if the simultaneous NR data connection can be sustained or suspended, an option referred to as “voice over LTE in EN-DC”. Te EPS fallback is for scenarios where
5GC doesn’t provide voice services; if needed, the voice call will be transferred to an EPS connection (VoLTE), also including
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