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11 September 2019

ORAN Alliance helps drive consensus on the disaggregated network

The past few years, AT&T has spearheaded many open source initiatives related to key enablers of next generation networks and services. These include Open Network Automation Protocol (ONAP – see below), Acumos for AI, Akraino for edge computing and others. One that looks set to be very influential in the emerging, and challenging, area of disaggregated virtualized RANs is Open RAN (ORAN), which has attracted support from a wide range of operators, including AT&T’s arch-rival Verizon.

The ORAN Alliance is looking to specify open interfaces between the elements of a disaggregated RAN – the central unit (CU), distributed unit (DU) and remote radio unit (RRU). These are designed to avoid these key interfaces being defined by individual vendors in proprietary or semi-proprietary ways (as happened with the CPRI fronthaul interface), so preventing operators from mixing equipment from different suppliers in the same RAN.

One of the complications of disaggregated vRAN is that there are so many ways to split the network functions between virtual and physical, and between centralized, distributed and cell site locations. The 3GPP standardizes eight options (13 including sub-options), with varying degrees of centralization. Only a few are likely to be supported on a large scale by operators and therefore vendors, but the role of industry alliances will be important to drive consensus and scale behind the options which will best support common 5G architectures.

The 3GPP is mainly focused on one high layer split, Option 2 (also called the F1 interface), for highly centralized RANs (with most functions virtualized in a central controller, often in the cloud). It is also working on two lower layer splits, with more functions pushed out to distributed controllers or the radio site. These two are

Option 6, with a MAC/PHY split, and Option 7, with an intra-PHY split, and offering three different variants 7-1, 7-2, 7-3.

The main 3GPP split supported by ORAN is Split 7.2, while another industry alliance, Small Cell Forum, has aligned its own nFAPI (network function API) interface with Split 6. The two organizations are increasingly cooperating to ensure they work on platforms that can support both these options, to drive more scale into chips and equipment, and to support MNOs such as Rakuten Mobile, which want to deploy networks with multiple split options for different scenarios.

These issues are becoming urgent on the eve of dense 5G roll-outs. Many studies show that the economics of dense, disaggregated 5G networks will not work without open interfaces. These will support the ability to mix and match different network elements, from different suppliers, allowing operators to achieve the best price points and the highest levels of innovation without technology dead ends.

Many operators are taking the view that Option 6 is the strongest choice for affordable deployments in lower capacity scenarios, especially indoors; while Option 7.2 will be superior for high capacity and ultra-reliable requirements, because it has strong support for multi-point transmission.

A common concern was that there are still too many options and that MNOs either do not know which to prioritize, or each will want a different combination – leading to fragmentation and lack of scale, with consequent risks for pricing and time to market. Some operators, like BT, are already driving the market towards a common hardware platform that could support all split options in software, to avoid the need to choose just one and to future-proof early deployments. In all cases, cost and availability of transport will be a key consideration.

Ericsson recently announced a vRAN platform which provides operators with full flexibility about where they site physical computing resources to support virtualized RAN; and how they split the RAN baseband functions between central and site-based, and between virtual network functions (VNFs) in the cloud and physical network functions in the radio/antenna unit.

As well as the functional splits, operators are also trying to drive uniformity and openness into the fronthaul connections which will support physical connectivity between the different disaggregated elements. The aim is to move away from semi-proprietary CPRI towards its successor, eCPRI, or several Ethernet-based alternatives. There are concerns that eCPRI may be implemented differently by each vendor, as CPRI was, and so kill the dream of multivendor interoperability. But major operators, as well as open alliances like ORAN and Telecom Infra Project, are pushing back against this risk.

Last week, AT&T said it had successfully tested eCPRI on 5G millimeter wave spectrum, using equipment from Nokia and Samsung. This holds out the hope that large MNOs will drive the industry towards connectivity which is truly standardized, and can be deployed on wireless as well as fiber links, reducing cost and enabling plentiful mmWave spectrum to be used for both access and backhaul, especially in small cell environments.

“The CPRI interface today has proprietary aspects which can result in a slower or more costly network build as we increase the bandwidth served requiring more fibers per radio,” wrote AT&T’s Gordon Mansfield in a blog post. “eCPRI is an enhancement of that technology. It’ll increase the efficiency to support higher bandwidth across fewer fibers. It is also what open interfaces are being built on, making it easier to use multiple vendors in a build. The benefits of this expanded flexibility and capacity will make it easier to deploy mmWave in markets where laying fiber is difficult or impractical, and promote cost effective operations by giving carriers the flexibility to use a variety of vendors to help bring fastest deployments at the best cost.”