5G Transport Network Requirements and Architecture Part II (continued from NANOG 76)
Several fundamental changes in the radio access network (RAN) architecture were introduced in the evolution from 4G to 5G. The radio capacity increase in 5G by utilizing new spectrum and beamforming radios with the desire for more deployment flexibility to account for the variety of use cases. This led into the introduction of new splits in the RAN protocol stack. The lower layer split, between radio unit and baseband unit, was in 4G based on the common public radio interface (CPRI) in the fronthaul transport segment, while for 5G to provide better rate efficiency and node scalability more functionality was moved into the radio and the new Ethernet based eCPRI interface was introduced.
Another notable change is the use of packet technology in eCPRI instead of time-division multiplexing (TDM) in CPRI. By utilizing an Ethernet/IP network in fronthaul instead of point-to-point TDM links, the mobile network can provide superior performance utilizing legacy and new spectrum while making optimal use of the underlying network infrastructure and RAN baseband resources. A packet network allows using the Ethernet/IP ecosystems which offers well-proven and standardized tools for easing OAM processes and to improve network resiliency, reliably, and availability.
The packet fronthaul transport network architecture defines how RAN applications (eCPRI), will use L2 Ethernet for its connectivity with tight characteristics requirements. These can be fulfilled with a L2 Ethernet fabric in small Edge deployments, but when the fabric introduces additional paths and more scale is needed then it becomes complex to manage and control using L2 control protocols. An overlay-underlay network architecture makes it possible to separate the physical network structure from the application service needs. An IP underlay can handle networks from simple to full-mesh, using protocols like IP etc. Such protocols have been used and are well-understood for years for sharing network topology and characteristic information. The eCPRI L2 Ethernet traffic is handled as a virtual overlay Ethernet service with strict service requirements operating over the underlay network fabric.
The overlay is the virtual network running on top of the underlay to create connectivity for the L2 EVPN and L3 IP-VPN services. The Multi-Protocol Boarder Gateway Protocol and automation as well as controller functions are used to simplify the creation and control of the virtual networks and services where following applies:
Scalability – small to large deployments using the same architectural principles
Resiliency – a robust network to minimize effects of failures
Simplicity – a network easy to dimension and deploy
An overlay-underlay networking structure further improves ease of deployment and connection flexibility by being able to establish software-defined overlays with optional traffic engineered paths on the underlaying transport switch fabric. A distributed IP based control protocol on the infrastructure makes the underlay self-contained and gives fast response to failures. Extensions in the routing protocol makes it possible to distribute the MPLS forwarding information including traffic engineering with Segment Routing (SR-MPLS). This also removes the need for multiple control planes, but it still uses IP and MPLS in the forwarding plane.
In addition to the above, the packet fronthaul synchronization architecture implies that when changing from CPRI to eCPRI in the fronthaul, time distribution between baseband and RU has moved from being carried by the CPRI frame itself to being carried by the packet-based precision time protocol (PTP). ITU-T has specified several PTP profiles with relevant properties for telecom. For the RAN to support full functionality, a performant synchronization solution must be available.
This presentation discuss the necessary evolution of the transport network interface to comply with the increased requirements in 5G networks, and explains the design rationales behind the functional splits and the packet switch fabric architecture.