Deep Dive in 5G Transport – SRAN & O-RAN

In light of the ongoing discussions in the industry, the development, and the testing of multivendor O-RAN (Open Radio Access Network) as a possible successor to the SRAN (Single Radio Access Network) that is presently firmly established by the small number of well-known incumbent RAN vendors, the transport networks are about to undergo major changes in their design and capabilities.

One of the most important changes is indicated by the word ”open”: open interfaces, open fronthaul, or open midhaul. Open RAN means the disaggregation of the different functions from a monolithic base station approach to a distributed approach.

Where does this transport take place?

If an operator needs to interconnect the disaggregated components of the RAN provided by multivendors, the interfaces must necessarily be open, predefined, and compatible and not restricted by proprietary installations of single vendors.

What are the key challenges posed by disaggregation?

  • Cell sites can continue to use Remote Radio Unit only insofar as centralization gains are achieved that demand fronthaul connectivity.
  • 4G and 5G fronthaul have very high data rates and extremely low latencies (<100 usec).
  • Standard Commercially of the shelf servers have drawbacks compared to traditional custom-built Baseband Units with high-density enhanced Common Public Radio Interface ports.
  • The transport solution – located at the edge for encompassing all ports between Central Unit/ Distributed Unit (midhaul) and between Distributed Unit and Remote Radio Unit (fronthaul) – requires a substantial upgrade.
  • Transporting between sync source, Distributed Unit and Radio Unit is also a specific challenge for Open RAN; traditional SRAN handles it more efficiently.

Standardized protocols must be implemented by all players to guarantee interoperability.

Trends and conclusions in transport caused by introduction of 5G and O-RAN

Mobile access catches up with fixed access in data speed

“Gigabit Society” is one of the most frequently used buzzwords. The industry’s promise behind it: being able to deliver “up to” 1 Gbps to the end user at the access layer. The introduction of 5G enables mobile operators for the first time to provide data speeds comparable to those of fixed-line operators or, under certain circumstances (e.g., when copper-based technologies or older GPON versions are in use), even outperforming them. The drawback is the need for enormous investments. A much larger number of base stations and huge investments in transport networks, both for capacity and coverage, are required to serve the micro and pico base stations.

(R)evolution of interface bandwidth results in higher demand for number of interfaces at access level/aggregation level

5G and its data capability change the picture dramatically:

  • Owing to the aggregation of high bandwidth, more and more physical interfaces become necessary.
  • Owing to the disaggregation in O-RAN, more physical interfaces are available.
  • Owing to the increase in data speed, interfaces for access and aggregation become more and more expensive – these types of interfaces have previously been seen only in backbones and are really expensive.
  • Owing to the limitations in the number of interfaces at switches and routers, the required investments rise. Major investments in access and aggregation are necessary in the transport network.

(Provider) backbone routers in aggregation

The layers of transport are separated simply by speed classes and feature classes. Access is cheap, aggregation is even cheaper while backbone is expensive, redundant, secure, and feature-rich.

  • Backbone: N x 100Gbit/s up to Tbits/s
  • Aggregation: N x 1 or 10 Gbit/s
  • Access: some 100Mbit/s up to 1 Gbits/s

The choice of the right transport equipment and right boxes ultimately ends in a search for expensive backbone boxes even for aggregation and access layer because only they provide adequate physical interfaces and switching capacity. In a manner of speaking, the "misuse" of backbone boxes will continue until manufactures provide tailored solutions for access and aggregation. In short, one significant cost factor relates to the boxes at the right transport layer.

New data center trend: spine-leaf architecture

When 5G (virtual Distributed Unit, virtual Central Unit) runs on a cloud infrastructure as network function virtualization (NFV), there is a merger of “classic” telecommunications structures and design with well-established “classic” information technology (IT) structures and design familiar from data centers. The two are combined to provide the desired 5G RAN and core functionalities. But that also leads to a higher number of switches and routers and other network elements and requires investments encompassing “classic” Network Technology and IT design.

Increasing demand for high-speed optical fiber transport

Enabling 5G in most countries means allocating new frequency resources above the frequencies used today. It is simply a matter of radio wave physics: the lower the frequency, the farther the propagation, the better the coverage, and the better the interior penetration as well. The allocation of higher frequencies reduces the effectiveness of wave propagation and penetration, limiting them to “line of sight”; the corollary is that more base stations are needed to ensure a certain level of coverage. Examples include small inner-city base stations on each power pole, each light pole, at every bus stop, on every advertisement board, or on every traffic light. Since the required bandwidth makes it impossible to serve base stations with more than 10G using microwaves, only one conclusion is possible: optical fiber – and huge investments in excavation – will be required at inner-city power poles, light poles, bus stops, advertisement boards, and traffic lights.

Author of this article is Torsten Soltmann.

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