Inside the 5G core: current status and research opportunities

3GPP is the organization responsible for the specification of the 5G architecture and its operation, which is currently arrived at Release 15. [1] The entire 5G architecture  can be divided into an access network (AN), composed of all the physical apparatus and logical elements needed for radio connectivity to user equipment (UE – e.g., Smartphones, IoT devices, vehicles, etc.), and the 5G core (5GC), which manages the user end-to-end connection to a requested service (i.e., Data Network – DN). The 5G core has been designed in order to offer boosted data rates and performances for traditional services and to support a wide range of new use cases for vertical industries (e.g., network slicing).

The 5G core follows clear control-/data plane separation principles, which enable the use of a variety of data plane solutions, data plane programmability and a more flexible deployment of associated functionalities. With similar targets, 3GPP has defined in the latest release the Service Based Architecture (SBA), which splits the control plane into different network functions logically connected by a bus. In the following figure, the SBA as defined by 3GPP is depicted, whereas different communication principles and protocols are considered in between some control plane functions (AMF, SMF) and the user plane.

The 5GC user plane is composed by User Plane Functions (UPFs) though which the data traffic is forwarded. Different UPFs may perform different operations, but for each user connection there must be one UPF acting as a data plane anchor, used to anchor a Packet Data Unit (PDU) session for the mobility device. According to Release 15, an overlay is assumed using the GRPS Tunneling Protocol for User Plane (GTP-U) between the data plane anchor and the mobile device’s current radio base station in the AN.

GTP-U is a tunneling protocol which uses particular identifiers (TEID – Tunnel Endpoint ID) to identify and forward encapsulated traffic in the core network on the N9 and N3 interfaces, while N6 interface assumes routing plain IP PDUs. GTP-U has also been used in past generations and it is now re-considered in the view of different 5G use cases and associated requirements, as well as protocol costs (overhead, complexity, limitations). For this reason, a hot topic discussion (e.g., in IETF DMM working group) is in place to find a more suitable data plane protocol: various candidates for such role are Segment Routing for IPv6 (SRv6), Locator-Identifier separation protocols (e.g., LISP), and locator re-write solutions (e.g., ILA or LEAPS) in support of forwarding plain IP PDUs in the transport network. [2]

With reference to the previous figure describing the 5GC, data plane alternatives to GTP-U are considered for interfaces N9 and N3. Currently, some emerging proposals are focusing on investigating optimizations on the N6 interface. The UPF connected on this interface is a centralized data plane anchor for the user traffic (depending on the specific deployment, it can be the only UPF on the data path, or another UPF as in figure may be present, acting as uplink classifier for traffic). A more decentralized anchor, deployed close to the device’s current radio base station, would be preferable for some services (e.g., edge computing). The works investigating on this (again, an ongoing topic is present in IETF DMM working group) aim at moving the data plane anchor to the access network, and as a result have to consider new requirements for traffic steering, QoS and metering support on the N6 interface.

Enabling such data plane deployment as well as control of data traffic treatment and forwarding policies on N3, N9 and N6 interfaces enables optimization opportunities in support of various industry verticals’ needs, such as URLLC, edge computing, route optimization or service relocation.

Data plane protocol options, their use and control, as well as means for optimization are researched in the SPOTLIGHT project.

References:

[1] 3GPP, “System Architecture for the 5G System (Release 15), TS 23.501; june-2018.

[2] F.Giust, A.Malik, M.Liebsch, “Simplification as Design Principle for 5G: A Native IP Data Plane for a Lean Packet System”, IEEE 28th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), Montreal, 2017