Guide to 5G Transport

What is 5G Transport?

5G Cell Site

We have often used the phrase the network is the glue that holds the 5G RAN together. At its most basic, 5G transport refers to the network that connects the 5G RAN to the core network.

5G is a complex technology and places new demands on the network. For those who are responsible for the network, there are key aspects of the technology, standards, and best practices that are most important. We have summarized these from the foundations to the applications with links to the best resources on these topics.

5G Architecture

5G Next Generation RAN (NG-RAN) takes the cell site functions and breaks them down into multiple functions. 5G gNodeB functions are the Radio Unit (RU), the Distributed Unit (DU) and the Centralized Unit (CU).  These interconnect via the transport networks and then connect to the core network. The DU and CU can be implemented as VNFs and centrally pooled for saving by not overprovisioning the cell site and keeping costs low. It is important to note that the 5G core is also virtualized and the location of some elements, like the User Plane Function (UPF), is application-specific. One DU can support multiple RUs and one CU can support multiple DUs. The CU also enables the separation of the control plane (CP) and user plane (UP) which is known as CUPS.

The new radio access technology for 5G is called “NR” and replaces “LTE”, and the new base station is called gNB (or gNodeB) and replaces the eNB (or eNodeB or Evolved Node B) in 4G.

Resource: For a comprehensive resource on 5G architecture see the 5GPPP’s “View on 5G Architecture” Feb 2020.

Standard: You can also go directly to the source, 3GPP, for the standard.

RAN Disaggregation and Functional Splits

The RAN has three basic building blocks:

  • Radio Unit (RU)
  • Distributed Unit (DU)
  • Centralized Unit (CU)

While the RU is at the cell tower, there is a great deal of flexibility in placing the DU and the CU which can operate as VNFs (virtualized software running on COTS hardware). The flexibility in how these components can be deployed and the different ways that they can be connected are known as functional splits. Functional splits in 5G enable a range of approaches to the transport network. The choice of functional splits is a major factor in the requirements for latency and bandwidth in the network.

The consensus is that the combination of split 7.2 and 2 will be the dominant implementation. As a result, transport networks allowing fronthaul, midhaul, and backhaul to be supported based on the specific needs for a given area. However, the DU and CU can be implemented as VNFs which optimizes flexibility.  The following diagram shows how the building blocks can be arranged and the terms for the network:

5G Transport Options

Resource: A good discussion of the various architectural approaches and options for High Level Splits (HLS) specifically was produced by NGMN “Overview on 5G RAN Functional Decomposition.”

Resource: IEEE 1914.1 discusses many different models with the key being the ability of the network to service all different requirements.

What Does the Transport Network need to support?

Some of the new 5G services will take advantage of the higher bandwidths available in 5G such as Enhanced Mobile Broadband (eMBB). High-speed connections in the gigabit range get a lot of attention because the existing transport networks were not designed to handle the cumulative impact of this much bandwidth from the user equipment. At the other extreme is Massive Machine to Machine Communications (mMTC) which, as the name implies will provide connections for a massive number of low-bandwidth IoT devices such as sensors.  The third class of services will be supported by Ultra-reliable Low Latency Communications (URLLC) which will provide new communications services for industrial automation, Smart City intelligent transportation systems, connected and autonomous vehicles, and telehealthcare.

Edge computing

Edge computing places high-performance compute, storage, and network resources as close as possible to end-users and devices, at the edge of the network, to fulfill the real-time requirements of 5G-enabled business applications.

Given the tight integration between edge computing and 5G, and the need for interoperability, standards are important. The edge computing standard from ETSI is Multi-Access Edge Computing (MEC). It is an open framework for applications and services that are tightly coupled with the Radio Access Network (RAN) via open interfaces to integrate software services into wireless networks.

Resource: ETSI white paper 32 is on “Network Transformation” that covers MEC. The white paper and standards can be access from ETSI.

Network slicing

Network slicing allows operators to deploy different “slices” of the network, where these virtual networks run on a common infrastructure. Network slices are defined as a set of virtual resources that lets operators provide portions of their networks for specific customer uses cases. 5G uses software-defined networking and network functions virtualization for the partitioning of networks into virtual elements.

Virtualization of network functions is a key enabler of network slicing. The dynamic provisioning and management of network slices must go all the way to the cell site (and its router) where having separate virtual routers each with its own administrative domain is essential to make this service practical.

Each network slice is isolated and tailored to the specific requirements required by very different applications like machine-type communication, ultra-reliable low latency communication, and enhanced mobile broadband content delivery.

Resource: GSMA published “An Introduction to Network Slicing.”

Resource: Volta’s Sales CTO, Jose-Miguel Pulido, has written a series of blog posts on network slicing.

A 5G Transport Ecosystem

The Telecom Infra Project is “a global community of companies and organizations working together to accelerate the development and deployment of open, disaggregated, and standards-based technology solutions.” TIP’s Open Optical & Packet Transport project group has defined a set of specifications that are relevant to 5G transport such as the Disaggregated Cell Site Gateway. The following diagram show TIP’s vision:

TIP does extensive testing and is a valuable resource.

Resource: TIP has published a specification for “Disaggregated Cell Site Gateways.”

Resource: TIP conducted an interesting set of practical tests with the results in “Learnings from Virtualized RAN Technology Trials over Non-ideal Fronthaul.”