Routing in 5G Transport Networks

5G Cell Site5G opens up a broad range of new services to Mobile Network Operators (MNOs).  Since these new services will focus more on enterprises connecting things than end users connecting devices, new network architectures and significant investment in the transport network will be needed.

The 5G transport network is the glue that holds together the disaggregated RAN components, connecting the cell site to the core network. In 5G, the RAN consists of the Radio Unit (RU), the Distributed Unit (DU), and the Centralized Unit (CU). While the RU is at the cell site, the DU and CU may be located remotely, virtualized, and operated as a pool of resources to reduce costs.  This flexible RAN disaggregation has specific performance requirements for fronthaul, mid-haul, and backhaul implementation which correspond to the various RAN deployment options as shown in the diagram. Each imposes requirements for latency and distance.

5G Transport Options

There are more requirement that the new 5G transport networks must support:

  • Much more bandwidth per cell site as speeds to user equipment scale to the multi-Gbps range.
  • Many more cell sites, especially small cells which are expected to double to 8.4 million in 2025.
  • New services with stringent latency and bandwidth requirements
  • Network slicing to support different applications on common network infrastructure

5G services cannot be delivered without a robust and modern transport network. Routing is integral to these networks so that traffic can be managed to meet service level requirements. As end users take advantage of the greater bandwidth in 5G, the transport network will also need to scale to handle the increased aggregate traffic. In addition, many operators plan to converge their various networks. With 5G, both the core and access network can be converged so the same fiber plant that connects cell towers can also handle business and residential wireline services.

Virtualization and Cloud

Virtualization and the cloud will be integral to deploy these 5G services economically and at scale.  Network Function Virtualization (NFV) provides the basis for network operators to move away from proprietary hardware to software implementations. There are a number of NFV initiatives such ETSI and OPNFV. Like enterprises, operators see the economic and operational benefits of virtualization using commodity hardware. Virtualization improves scalability and agility by allowing service providers to deliver new network services and applications on demand.

NFV will require 5G operators to invest in computing resources closer to cell towers to run their virtualized network equipment in order to meet latency requirements in 5G. Those facilities can also be used for a wider edge computing application like ETSI’s Multi-Access Edge Computing (MEC). For many new 5G applications like industrial, medical, drone, and transportation, the reliability and latency requirements surpass bandwidth needs. These can only be met by integrating edge computing close to the end user.

Cell Site Gateway Routers

Since the networks will be IP based and there are stringent requirements including latency, service providers expect to use routing extensively. The Telecom Infra Project’s (TIP) Disaggregated Cell Site Gateway (DCSG) project specifies the hardware and software requirements to connect a small number of cell sites per router. Moreover, they envision using the DCSG to connect and cover business and broadband services. The DCSG is expected to support MPLS, Segment Routing, QoS, and timing and synchronization. The ability to deliver different service levels is critical to services using network slicing. Fundamentally, using routing for the service intelligence will be critical to ensure the performance of the various applications running on the 5G transport network.

The Need for Virtual Routing

Routing must be flexible and very cost effective, which is driving a high interest in virtual routing. Legacy routing vendors have attempted to implement router virtualization with approaches like logical routers, virtual routing and forwarding (VRF) and partitions. However, none of these meets the full requirements, especially on cost.

Like server virtualization, a virtual router must be one of multiple, separate workloads within a given piece of hardware. This means that there should be multiple routing elements on a single piece of hardware. A routing element must be a separate management domain with its own control plane and dedicated forwarding resources (logical or physical ports).

RAN sharing is a way for multiple mobile network operators to share radio access network infrastructure to share capital costs and better serve customers. Virtual routing allows each service provider to have their own router that can operate and be managed with complete separation of both the control plane and the management plane. This will be best accomplished by having multiple virtual routers on the cell site gateways.

Increasingly MNOs are specifying the ASICs to be used in cell site gateway routers as is the case with DCSG. Volta’s cloud-based approach to router virtualization provides the only means of implementing multiple virtual routers on these ASIC-based platforms. This will be critical to support applications like RAN sharing, network slicing and MEC. VM or VNFs of router software running on servers may have a role within the compute infrastructure but are unlikely to meet the throughput or cost requirements of 5G transport infrastructure.

Given that the initial investments in 5G edge buildouts will include much more routing, MNOs will need to ensure that their choices can meet this set of requirements to futureproof their choices. Virtualization and disaggregation will help keep costs low. They will also be critical to service agility which will allow MNOs to maximize their time to revenue.