Enhancing XR Application Awareness for optimized user experience
Truly immersive Extended Reality (XR) experiences require media with increased dimensions, spatial resolutions and frame rates as well as highly responsive interactivity metadata. This, in turn, increases the requirements on network bandwidth and latency compared to traditional media services. Additionally, remote rendering, which is used to address the processing and energy constraints in XR end devices, heavily depends on the network performance to deliver a desirable user experience.
The Power of Protocol Data Unit (PDU) Sets
In the development work for Release 18 – the next update of the global telecommunications standard by the 3rd Generation Partnership Project (3GPP) – the need for enhanced application awareness within the network has been recognized. The aim was to create a tighter integration between the application layer, the Core Network (CN), and the Radio Access Network (RAN) to address the requirements for XR experiences. This is because a network that is more aware of the application needs can provide a better experience. As a pivotal step towards achieving this goal, 3GPP introduced the Protocol Data Unit (PDU) Set framework.
A PDU is a unit of information at a given protocol layer. At the IP layer, a PDU is an IP packet. 3GPP defined a PDU Set as one or more PDUs carrying the payload of one unit of information generated at the application layer. For example, a PDU Set can be a video frame or slice that the application can decode. An important aspect of XR application awareness is the PDU Set based Quality of Service (QoS) handling. This allows the network to handle all PDUs within a Set in an integrated manner and to apply differentiated handling across PDU Sets.
To understand the significance, let’s consider how the loss of video packets impacts an application. If a video frame is missing a PDU, the application may no longer be able to decode it, unless there are mechanisms in place for concealing packet loss or forward error correction. Also, loss of certain video frames, like I-frames, can have a greater impact on the user experience. To address this, the network was given the ability to prioritize certain PDU Sets that are more critical for the application’s processing. Furthermore, PDU Set information also allows the network to determine whether it is acceptable to discard the remaining PDUs within a Set when one of its PDUs is lost. These handling mechanisms for PDU Sets can result in more efficient resource allocation, power savings, and improved congestion response in the network.
3GPP also defined a Data Burst as a set of multiple PDUs generated and sent by the application in a short period of time. A Data Burst can be composed of one or multiple PDU Sets. Many XR applications generate traffic, such as video frames, in periodic data bursts. Knowledge of traffic periodicity, as well as starting and ending of data bursts, can be utilized by the network especially for power saving.
How is application awareness achieved?
In Release 18, the focus of PDU Set handling is on applications that transmit their data using the Real-time Transport Protocol (RTP). RTP senders can derive the PDU Set information from the application data, such as video, by using Network Abstraction Layer (NAL) unit headers. This information includes sequence numbers to indicate the ordering of different PDU Sets and the PDUs within a PDU Set, as well as the size and relative importance of a PDU Set for the application.
3GPP introduced an RTP header extension to transmit the PDU Set information from applications to the 5G network. In addition to the PDU Set information, the application may also signal the end of a data burst (EoDB). For downlink direction, when an RTP packet with the PDU Set header extension is received, the User Plane Function (UPF) parses the PDU Set information and inserts it into the GPRS Tunnelling Protocol User Plane (GTP-U) header, which is then sent to the RAN.
Furthermore, 3GPP expanded the QoS framework to include new parameters that define the requirements imposed on a PDU Set in a QoS flow. These new QoS parameters are provided to the RAN from the CN via Control Plane, similar to traditional QoS parameters. The CN may receive them from the application via an Exposure API. The RAN applies the PDU Set QoS parameters to handle the PDU Sets identified by the GTP-U headers. It can utilize the PDU Set Importance to influence its decisions on which PDU Sets to discard in congestion situations prioritizing the least important ones.
The RAN has the capability to optimize the power consumption of User Equipment (UE) for periodic traffic by acquiring knowledge or by learning from the periodicity of the traffic. By providing an explicit EoDB indication, the transition of the UE from active to inactive state can be made more accurate and faster, thereby improving the efficiency of mechanisms such as Discontinuous Reception (DRX).
The flow of packets marked with PDU Set information from an XR Application Server (AS) to a user device. The example shows how the network elements like UPF and RAN use the gained application awareness for optimizing delivery.
Who will benefit from the PDU Set framework?
The PDU Set framework emerges as a game-changer in the realm of XR application awareness, fostering enhanced collaboration between application developers and network operators.
Thanks to this framework, mobile consumers worldwide can enjoy a superior user experience, including improved video quality and reduced freezing even in situations of limited network bandwidth. XR applications can now provide the network with information on the boundaries and sizes of their application data units. Moreover, applications can assign different levels of importance to PDU Sets, enabling them to indicate to the network which PDU Sets are critical for seamless operation. This allows application developers to differentiate their applications by incorporating network awareness, ultimately providing a superior user experience. Network operators can optimize their networks for XR traffic, resulting in improved efficiency and better resource utilization. Furthermore, device manufacturers can make use of these optimizations to improve power efficiency.
How Nokia is contributing
Nokia has been a key contributor in 3GPP to both PDU Set handling aspects in the 5G core network and RAN as well as development of the RTP header extension for PDU Set marking.
Nokia’s Real-time Extended Reality Multimedia (RXRM) is a Proof-of-Concept demonstration of the benefits of the PDU Set framework. The solution is enhanced to apply PDU Set marking by the sender application. In the 5G RAN, the PDU Set information was used to react in real-time to the radio cell load by prioritizing the PDU Sets. This allowed most of the content stream to be transmitted in high quality, both for low and very high cell loads.
Future perspectives
As we continue to embrace the era of metaverse, the collaboration between XR application developers and network operators, facilitated by the PDU Set framework, will be crucial in shaping the future of mobile connectivity. 3GPP has recently started its Release 19 work which aims to enhance the PDU Set framework in terms of application marking and network handling. One of the key study topics is how individual media streams and PDU Sets can be identified when they are transported using the QUIC protocol that lacks unencrypted header extensions for such purposes. A proposed solution is to create a new framework that allows applications and the network to securely exchange in-band per-session, per-flow or per-packet metadata. This is expected to happen through an application to UPF tunnelling mechanism, where the application traffic is embedded. If successful, this framework will unlock new opportunities for how applications can interact with the network beyond XR.
As the research and technology evolves, Nokia continues to be among the key drivers of innovation related to the PDU Set framework. We firmly believe that the true value of an innovation lies in the benefits it brings to the end user. With this in mind, we are dedicated to creating solutions that meet the needs of the industry and are easy to implement. Therefore, we are committed to developing XR application awareness to enhance user experience even in challenging network conditions.
