JOHN WILLIAMSON C RAN NETWORKS
C-RAN NETWORKS o er solution to data logjam
C apacity and connectivity demands placed on next-generation mobile networks are reaching crisis levels. ABI Research predicts that the two billion global 4G (LTE) subscribers in early 2017 could swell to four billion during 2022, and average user consumption could rise from 1.2 Gbytes to 5.7 Gbytes per month in that timeframe. RAN ING ON EMPTY This presents service providers with mounting operational challenges in the Radio Access Network (RAN) in terms of its capacity, manageability, responsiveness, service quality, energy consumption, upgradeability and running costs. One potential solution is the centralised- or cloud-RAN (C-RAN) model. Here the processing functions of the Baseband Unit (BBU) and the radio functions of the Radio Unit (RU) are disaggregated, with BBUs clustered together in remote locations, and the RUs mounted on cell towers to operate as Remote Radio Heads (RRHs). C-RANs could o er substantial benefits. Speaking in March in a fronthaul futures webcast organised by IHS Markit, Nokia Principal Marketing Manager, IP/Optical Networks, Hector Menendez, listed some as: performance improvement via resource pooling; faster load balancing; better interference management between cells; energy and real estate savings; greater flexibility in network programmability; and simplified site acquisition. “The move towards C-RAN also aids in the move to 5G, with operators being able to virtualise network resources and support mobile edge computing,” he observes. “This allows mobile operators to move network resources closer to end users, improving the performance of the network – for example, in the form of reduced latency,” Menendez commented. FIBRE TO THE FORE Key to C-RAN delivering on these promises is the use of optimised technology in fronthaul networks – the links between the RRHs and the BBU farms using the Common Public Radio Interface – and in the backhaul network – the links between the BBU farms and the core network. Although there are di erent technology options for implementing backhaul and fronthaul systems – including microwave radio, G.fast and copper as trialled by BT, and coax – optical connectivity solutions are popular. “Where fibre exists it is the logical means of connecting the BBU
A bottleneck at the optical network- mobile interface can be uncorked with smart use of front and backhaul technologies. By John Williamson.
JOHN WILLIAMSON
typically group their fronthaul o erings into passive, semi-passive - passive transmission with additional management capabilities - and active options, with varying cost points and functionality. BACK TO FRONT Baldry describes how today’s fronthaul and backhaul networks actually di er quite considerably. The former support the CPRI protocol, which is a Layer 1 arrangement over a point-to-point connection over a maximum of 20 km. The latter is typically Layer 2 or above, and is a complete network with aggregation, protection and longer distance transmission. As the move to 5G gathers pace, however, the two networks will begin to overlap, and often be required to run over the same fibre infrastructure. Baldry believes that therefore it will be an advantage if one optical platform can support both Layer 1-based fronthaul and Layer 2 packet-optical based backhaul flexibly in the same chassis. The coming together of fronthaul and backhaul is expected to have an impact on CPRI. In 5G the merging infrastructure will need to have di ering characteristics at various parts of the network. “There will still be di erent parts of this hybrid network that will be CPRI/fronthaul-like and parts that will be Ethernet-based,” concludes Baldry. “The exact nature of this is currently being discussed in the standards bodies. Overall, however, they will merge into a single 5G mobile transport network.”
to the core. Optical communications provides higher bandwidth to a cell site and enables tra c from multiple cell sites to be aggregated together for more e cient transport, “ points out Infinera’s Metro Marketing Director Jon Baldry, adding that where fibre doesn’t exist cell sites are typically backhauled over point- to-point microwave links to the nearest fibre-based aggregation point. Baldry says fronthaul is even more inclined towards fibre: “As the BBU creates the RF signal for transmission, the CPRI protocol carries digitised RF and therefore requires more bandwidth than backhaul. For example, a BBU with a 1G backhaul link could generate up to 3 x 10G of fronthaul, 10G per 120 degree sector.” GETTING REAL In the real world, though, not all service providers are equally invested in fibre. “So what’s needed is a solution set that ultimately leverages technologies that make good economic sense for the operators,” said Menendez. Practicalities and economics will also determine which of the di erent optical technologies, or combinations, will make the C-RAN cut. According to Baldry, Optical Transport Network (OTN) technology is great in many parts of the network but su ers from too high latency for most fronthaul requirements. Meanwhile, Wavelength Division Multiplexing (WDM) can use OTN framing or other techniques that can give the better sync performance required for fronthaul. Baldry notes that vendors
LATENCY AND SYNCHRONISATION KEY TO FRONTHAUL Fronthaul networks have very strict latency and synchronisation requirements in order to carry digitised RF signals. Infinera’s Metro Marketing Director Jon Baldry says the overall latency budget is largely consumed by the processing within the BBU, and the available budget for optical transport equates to about 20 km of fibre. “Any optical infrastructure used in the fronthaul network therefore has to be ultra-low latency to avoid significantly reducing the reach of the fronthaul network,” he remarks. “Equally high quality Layer 1 sync transfer is also critical for any fronthaul network to ensure no impact on the digitised RF signal being carried.”
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| ISSUE 9 | Q2 2017
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