A Discussion on 5G Transport Network Planning

Driven by a joint effort from industry players, the 5G industry chain, from technical standards, network researches, to prototypes, has been ready for commercial use. To deliver a variety of services, it is necessary to develop transport networks. As a critical cornerstone of 5G commercialization, 5G transport network planning has become the industry’s focus.

5G Transport Network Deployment in SA and NSA Scenarios

There are two ideas for the evolution from 4G to 5G: standalone (SA) mode and non-standalone (NSA) mode. In the SA scenario, existing 4G networks are independent of future 5G networks. The two transport networks can be built separately, depending on service requirements and development. The existing 4G transport networks can be built into high-quality transport networks for delivering 4G, fleet users, and NB-IoT services, while 5G transport networks are specially used for  5G services. The NSA scenario is applicable to the initial stage of 5G networks, where LTE and NR terminals coexist for a long time. Therefore, when 5G networks are deployed initially in some areas, the convergent 4G/5G networks can best meet user experience needs. In this case, UEs can be connected to both 4G and 5G networks, and 4G LTE base stations serve as anchors. Operators can either upgrade existing transport networks to support 4G/5G shared transport, or build new standalone 5G transport networks for 4G/5G dual-plane transport.

Keys to 5G Transport Network Planning

5G transport networks contain MAN, core aggregation, and access layers. Whether existing 4G transport networks are upgraded or new 5G transport networks are built, the focus of network planning varies from layer to layer. The deployment of some new functions also needs to be considered in network planning.

Focusing on Fronthaul Scenario, High Bandwidth, L3 Function, and Ultra-Low Latency at the Access Layer

The biggest impact of 5G on transport networks lies in the access layer. Some 5G-related requirements must be considered at the access layer, including higher bandwidth, fronthaul transport needs brought by DU/CU integration, and other key functions such as L3 scheduling and ultra-low latency for traffic forwarding. The existing 4G transport and access devices can hardly meet these requirements, and thus need to be reconfigured or newly built.
Access devices need to provide 10GE and 25GE interfaces for access at the UNI side. 10GE is an interface for 5G NR base stations, and 25GE is an interface between AAU and DU . The line side needs to provide 40/50/100GE interfaces for networking at the NNI side.
Some factors make traffic flows more complicated, such as virtual functions of 5G mobile backhaul RAN and CN, distributed deployment of service anchors, and migration of L3 functions to base stations. In addition to the north-to-south traffic (from base stations to CN), the demand for east-to-west traffic also increases. It is therefore necessary for 5G transport and access devices to move L3 functions to the edge.
In the 5G era, vertical industries such as internet of vehicles and industrial control impose strict requirements on latency. As defined by 3GPP, the air interface latency in uRLLC scenarios is as low as 0.5 ms, and the one-way end-to-end latency is not more than 1 ms. For a backhaul network, the latency is 100 to 150 µs, about one third of the CN latency. Although the media plane of CN can move down in a distributed manner, the requirement for low latency is necessary for transport and access devices. Therefore, 5G transport and access devices need to support FlexE and time-sensitive network (TSN) to achieve ultra-low latency for traffic forwarding.

Focusing on High Bandwidth and Network Flattening at the Core Convergence Layer 

In the 4G phase, the bandwidth required on the line side at the core convergence layer is N×10GE and 100GE, while that in the 5G phase will exceed 400G. To meet 5G massive bandwidth needs, transport devices need to provide larger switching capacity and higher bandwidth on the line side.
The existing 4G transport networks use packet OTN overlay networking at the MAN and core convergence layers. Network architecture at the two layers face the challenges of high construction costs, insufficient equipment room space, high power consumption, slow service provisioning, and difficult network protection and O&M. As services evolve to be diverse and differentiated, 5G imposes higher requirements on network functions such as ultra low latency and network slicing. Network architecture has to be optimized to carry multiple services and reduce costs. Therefore, in the 5G phase, legacy packet OTN overlay networking and new integrated equipment networking are likely to become two network setup options for operators.

Introducing SDN for Traffic Engineering and Network Slicing

5G mobile backhaul network will use the architecture of centralized control and separate forwarding and control to make effective and on-demand routing calculation through SDN controllers. Only the routings with connection relationships are calculated and forwarded. This can implement L3 functions of the forwarding equipment at the access layer. While considerably reducing equipment performance pressure, services are effectively forwarded. Moreover, intelligent adjustment capability based on SDN controllers and traffic monitors can optimize networks in real time to meet network dynamic changes and address the issues in the 5G era involving more complicated east-to-west traffic flow between base stations, more complex interactive connection models between the clouds, and more connections.
With deployed SDN controllers and the separation feature based on FlexE, 5G transport networks can provide network slicing to carry services over the same physical network in different scenarios and to virtualize multiple service networks that are managed independently without any interference. SDN-based open networks can achieve rapid cross-domain service provisioning and adjustment and simplify network O&M.

Commercial Progress of 5G Transport Networks

The progress of 5G transport networks depends on the growth of the 5G industry chain and the maturity of the transport industry chain including standards and equipment maturity.
At the plenary meeting of ITU-T SG15 held in Geneva in June 2017, technical report Transport Network Support of IMT-2020/5G (TRGS-TN5G) , and research plan for 5G transport standards were formally approved, marking a key step towards 5G transport research in ITU-T.
Mainstream vendors worldwide have released 5G transport solutions and can provide 5G transport equipment for functional verification in small field tests. It is expected that 5G transport products will be qualified for pre-commercial use in 2018 when operators will be able to build small-scale 5G transport network trials. In 2019, 5G transport products will have the commercial capability to satisfy the requirements for large-scale commercial deployment in 2020.