Ethernet in the mobile fronthaul v2 - Capgemini Engineering

ETHERNET IN THE MOBILE FRONTHAUL

THE WIRES THAT WILL ENABLE MORE WIRELESS CONNECTIVITY

03... Mobile and RAN Growth Forecast

05... Need for BBU and RH Separation

07... Fronthaul Network

10... Options for Fronthaul – OTN/WDM or Ethernet

18... Challenges Posed by Ethernet

20... Hardware Options

21... Altran’s Software Framework for Fronthaul Switches and Gateways

TABLE OF CONTENTS

Altran

The number of global unique mobile subscribers is expected to grow by 160 million from 5.7 billion subscribers in 2020 to 5.86 billion in 2025 [1]

Fueled by social media, user-generated content, mobile internet and apps, mobile data traffic will grow from 40.8 exabytes in 2020 to 77.5 exabytes in 2022[2] with a compound annual growth rate of 46%. Today, user expectations have become insatiable, demanding comprehensive services and co-operations from the mobility network, with governments, public utilities, local communities as well as private networks expected to deliver on these insistent demands.

MOBILE AND RANGROWTH FORECAST

03 - Mobile and RAN Growth Forecast Altran

[1] https://www.statista.com/statistics/740154/worldwide-unique-mobile-subscribers-by-region/[2] https://www.statista.com/statistics/271405/global-mobile-data-traffic-forecast/

While this presents a wealth of opportunities through basic and value-added services, operators and service providers will need to continuously add capacity to improve coverage and user experience while maintaining network quality. In turn, equipment designers and manufacturers will improve and innovate technology to enable the operators to function better. This increase in market potential has led to the introduction of new entrants bringing in differentiation, increasing the competition, putting negative pressure on prices, and consolidation of existing service providers.

Altran 04 - Mobile and RAN Growth Forecast

NEED FOR BBU AND RH SEPARATION

4G technology, powered by LTE (Long Term Evolution), provides the network architecture to transmit huge amounts of data over mobile radio interfaces. A predictive breakup of mobile connections by technology, in 2025 is below (source – GSMA).

85% or more of mobile connections are expected to be 4G or 5G. This will require a massive increase in the number of antennas to accommodate all of these new subscribers and other devices. This also will increase the amount of signal processing to be handled, thus requiring increasing amounts of space for cell towers, posing a signifi cant problem, especially in urban areas. The solution to this problem is a change in the network and system architecture by the introduction of Remote Radio Head (RRH) systems. The earlier cell site architecture consisted of a large radio component located along with the antenna.

Altran05 - Need for BBU and RH Separation

This new evolved architecture separates the radio component into an analog RF (Radio Frequency) component and a digital radio BBU (Baseband Unit) component. The analog RF component, co-located with the antenna, is also called the RRH (Remote Radio Head) since it is remote from the BBU. In the case of a conventional base station, a coaxial cable carries the high-frequency signal between the mast antenna and the base station. This limits the distance to about 30 meters. In the case of RRH design, optical fiber connects the BBU to the RRH, allowing the BBU to be located much further away from the RRH. This design reduces the equipment footprint at the cell site, thus enabling more antennas within the same area, and therefore increasing the network coverage and capacity accommodating a much larger number of subscribers with better throughput while reducing the operational costs for real estate requirement and energy demand at the cell site.

Altran 06 - Need for BBU and RH Separation

FRONTHAUL NETWORK

Though the concept of the RRH addresses the problem of space at the cell site, a different problem now arises – a lot more wires are required to be laid, monitored and maintained. This is leading to a signifi cant increase in operational costs. This can be reduced by introducing switches that aggregate the connections from the RRHs and carry them over a much higher bandwidth connection between the switch and the BBUs.

As the number of mobile connections and, therefore, the number of RRHs grows, it becomes imperative to replace the simplistic single fi ber between the switches by a network to transport the signals between the RRHs and BBUs. This network is the mobile fronthaul network. Considering the purpose of the fronthaul network, we can identify some key advantages (characteristics).

Altran07 - Fronthaul Network

• Multi-service capability – RRHs fall into three broad categories – 4G CPRI (Common Public Radio Interface), 5G CPRI, 5G eCPRI. The network must be capable of efficiently handling all these three types

• High reliability - the number of subscribers/services impacted, and the amount of data lost for every second of an outage is huge. So, the network must be built to minimize failures and to avoid any single point of failure

• Low latency - the one-way maximum tolerable latency between the RRH and BBU is 75 µs, including transmission over fibers up to 10 km. This typically means a maximum of 25 µs latency through any switch/gateway/router

• High capacity – the network may have to transport 5-10 Tbps depending upon the number of sites to be aggregated. This requires an appropriate design to minimize overheads in transporting the traffic

• Ability to distribute timing and synchronization - the RRH must be synchronized with the BBU to an accuracy of 5 ns. This places significant demands on the components chosen in the network elements and the wires/fibers constituting the network

• Optimal cost of maintenance and operations - fault isolation, recovery from failures and upgrades constitute a significant part of operational expenses. Optimizing these is critical to keep the network and services sustainable and profitable

Altran 08 - Fronthaul Network

• Adaptability - operators are increasingly embracing network virtualization and seeking new ways to optimize network costs, including sharing networks, sharing spectrum, selecting the best-suited components from multiple vendors and incorporating analytics to manage traffic better. The technology and components chosen must be adaptable to evolve as the industry and market evolves

The term CPRI (Common Public Radio Interface) used above in a generic manner refers to the signals between the RRH and BBU. Actually, there are multiple formats in which the information is exchanged and CPRI is one of them, which is widely deployed. Another is OBSAI (Open Base Station Standard Initiative). The OBSAI initiative is led by a consortium of vendors, primarily Nokia, NEC, LG and Samsung. The CPRI initiative was initially led by a consortium of vendors, primarily, Ericsson, Huawei, NEC, Nortel Networks and Siemens. In the rest of this article, CPRI is used to simplify the descriptions.

Altran09 - Fronthaul Network

OPTIONS FOR FRONTHAUL – OTN/WDM OR ETHERNET

PON (Passive Optical Network)

WDM (Wavelength Division Multiplexing)/OTN (Optical Transport Network)

There are three main contenders for the fronthaul network technology:

PON provides the required bandwidth, with TDM or WDM based variants. PON is very cost-effective, as well. However, transport latency is a big problem and there is no PON solution currently that can satisfy the fronthaul latency constraint.

This was the first choice and was widely used in previous generation products. Technically, WDM and/or OTN provide the best technical solution to connect the RRHs to the BBUs. With plain WDM, different wavelengths can be used to aggregate multiple RRH-BBU flows over the same fiber. With OTN, multiple ODUs can be used within the same wavelength. Different CPRI speeds can be accommodated with different ODU rates or with ODUflex. Depending upon the size of the fronthaul network, a pure passive solution, or a solution with active elements can be adopted. Transport latency is low and completely deterministic. The technology is transparent for both data and synchronization. OTN and WDM can carry asynchronous and synchronous traffic as well as TDM or packet payloads. The problem with OTN and WDM has been and continues to be, the high cost of components which have limited the variety of vendors and associated tools. At the time of this writing, the cost per bit for OTN is approximately double the cost per bit of Ethernet.

Altran 10 - Options for Fronthaul – OTN/WDM or Ethernet

Ethernet is a packet communication technology and does not natively support carrying TDM. TDM frames must first be converted into packets to transport them through the Ethernet network. Technologies for TDM transport over Ethernet using MPLS have been around for more than a decade but have been used to carry relatively lower speed TDM frames in the past. Carrying TDM over native Ethernet, though addressed in specifications, has seen relatively limited adoption. In addition, Ethernet is natively non-reliable and latency for traffic is unpredictable, non-deterministic and can vary significantly depending upon network load and traffic patterns. However, Ethernet provides enormous bandwidth, the capacity to scale, a huge number of tools, a large vendor ecosystem, and the economy of scale to build the fronthaul network at viable cost points rather than OTN/WDM.

Ethernet

Altran11 - Options for Fronthaul – OTN/WDM or Ethernet

Below is a brief comparison of Ethernet vs. OTN for suitability in the fronthaul network. The table uses some new terms which are explained later in this paper.


In the remaining part of this paper, we focus on the use of Ethernet to transport the signals between RRHs and BBUs.

Two options exist to transport the signals between RRHs and BBUs, through an Ethernet network. These are also shown in the pictures alongside.

• RRHs and BBUs transmit/receive CPRI frames. So, an Ethernet switch at each end maps CPRI frames into Ethernet frames (using some circuit emulation protocol) and transports the Ethernet frames through the fronthaul network. This is called RoE (Radio Over Ethernet)

• RRHs and BBUs transmit/receive Ethernet frames containing the same information as the CPRI frames so that the Ethernet switches have to do regular Ethernet switching. This is called eCPRI (evolved CPRI)

Ethernet transport between RRH and BBU

013 - Options for Fronthaul – OTN / WDM or Ethernet

Over Ethernet)


Besides the obvious advantage of simplifying the switches/gateways at the border between the RAN and the transport network, eCPRI offers another hidden benefit. The traffic load between an RRH and a BBU usually varies depending upon the number of subscribers connected at the time, the activity per subscriber, etc.

Since CPRI is a constant bit rate TDM stream, the switch converting CPRI into Ethernet passes a steady stream of packets into the network, offering a flat load. Considering that peak demand has to be accommodated, this flat load is often the peak load. Thus, CPRI-to-Ethernet conversion at the switch/gateway ends up using much more bandwidth in the transport network than is actually necessary. When the RRH and BBU use eCPRI, the packet stream from the RRH/BBU will vary based on the actual traffic load, so the load through the fronthaul transport network will vary with time. The use of eCPRI, therefore, reduces the average load through the transport network. However, this is beneficial only if the operator uses good planning tools to layout the network and engineer the traffic to utilize the best-deployed network resources. With good tools, the cost of the transport network can, therefore, be further reduced.

014 - Options for Fronthaul – OTN / WDM or EthernetAltran 14 - Options for Fronthaul – OTN/WDM or Ethernet

Though eCPRI offers several advantages, practically, 4G cell sites will continue for a few years and therefore, operators need to support both CPRI and eCPRI transport for the next few years.

The technology to transport CPRI frames over the Ethernet network, called RoE, Radio over Ethernet was first defined in IEEE 1904.3 and further updated and improved in IEEE 1914.3. Broadly, the original base station (a combination of RRH + BBU) comprises RRC, PDCP, RLC, MAC and PHY functions. The RLC, MAC and PHY can further be decomposed into High-RLC and Low-RLC, High-MAC and Low-MAC, and, High-PHY and Low-PHY functions.

So, this offers eight different ways to split the functionality between the RRH and BBU. In general,• Lower numbered options (Option 1 being the lowest numbered)

place higher processing load at the RRU and therefore increase the size and cost of cell sites

• Lower numbered options (Option 1 being the lowest numbered) allow greater latency through and demand lesser synchronization accuracy from, the transport network (since there is lesser coordination between RRH and BBU)

• Higher numbered options (Option 8 being the highest) demand much more information exchange between the RRU and BBU, and therefore increase the load and capacity demand on the fronthaul network

CPRI – Ethernet translation at the fronthaul gateway/switch


Currently, the options actively pursued in the industry are Option 8 and Option 7. We will look at these in a bit more detail.

Option 8 Split Between RRH and BBU

A CPRI frame (shown alongside) can carry different types of wireless traffi c – GSM, LTE, WiMax, UMTS, etc. The generic CPRI specifi cation that defi nes the TDM structure, the bit rates to be used, and the structure as shown in picture below. Each individual specifi cation then defi nes the format of the data as appropriate to the specifi c wireless technology. Further, there could be vendor-specifi c data within the frames that defi ne how a frame is to be processed, what actions are to be taken based on the content, etc.


This allows two different ways of mapping the CPRI frames into Ethernet frames.

Structure agnostic mapping• The mapping function understands the basic CPRI frame

boundaries and the framing rule• The mapper removes the TDM line coding when a frame is

received from the RRU at the switch• The mapper adds the TDM line coding when a frame is

transmitted from the switch to the RRU• The mapping function packs N (usually a configured number)

CPRI frames received from the RRU into an RoE frame which it transmits into the transport network

• The mapping function unpacks the RoE frame received from the transport network, into N (usually a configured number) CPRI frames which it transmits to the BBU

• The mapper or switch does not look any further into the CPRI frame and does not process the CPRI frame any more than above

Structure aware mapping• The mapping function looks beyond just the frame boundaries • The mapping function could have separate rules to map the data

and control signals in the IQ data field

• By looking further into the payload, the mapping function could optimize the number of bytes required in the RoE frame (for example, during the idle period, there is no user data in the IQ data portion of the CPRI payload)

Tunneling mode• The mapping function does not interpret the line coding bits• The mapping function includes the line coding bits also into the

Line code aware mode• The mapping function is aware of, and handles the line coding

bits• The mapping function removes the line coding bits when placing

the CPRI frame into the RoE payload• The mapping function inserts the line coding bits when

extracting the CPRI frame from the RoE payload

Tunneling mode is the simplest to implement but places maximum capacity demand on the transport network. Line-code aware mode, depending upon the line coding used can provide a 20% reduction in the capacity demand on the transport network. This comes with a higher implementation complexity in the switch. Structure aware mapping can yield a significant reduction in the capacity demand on the transport network. The estimated reduction depends on the actual traffic pattern, but a general thumb rule is a reduction of 30% to 40%. It also requires better network planning to realize the gains fully.

CHALLENGES POSED BY ETHERNET

Ethernet and IP networks are inherently unpredictable in terms of delays experienced by packets traversing the network. Time-sensitive radio traffic cannot tolerate this. Ethernet does not have an inbuilt way of synchronizing time (including frequency and phase) across network elements connected through Ethernet cables. These are addressed by some additions to the Ethernet standards.

These are mechanisms for all participating network elements (including switches, RRHs and BBUs) to have a common understanding of time. Installing a GPS clock in every device is cost-prohibitive and, in many installations, infeasible. The Precision Time Protocol (PTP), defined in IEEE 1588, addresses this. In our fronthaul network, typically, BBUs obtain their clocks from a Grand Master, which obtains its clock from a high precision source such as GPS. BBUs, in turn, serve as Master clocks to distribute the timing to the RRHs, which are the clients/slaves. Network devices in between, such as switches, serve as boundary clocks or transparent clocks. The IEEE 802.1CM specification defines a specific choice of options for use in fronthaul networks.

To address the issue of non-determinism and unpredictable latency, the IEEE has formulated a Time Sensitive Networking (TSN) task group within itself. This task group is defining standards for transmission of time sensitive data over Ethernet networks to achieve high predictability, very low transmission delay and high reliability for traffic. Broadly, the TSN standards address the following.

Timing and Synchronization requirements

Time Sensitive Networking (TSN)

Altran 18 - Challenges posed by Ethernet

Altran19 - Challenges posed by Ethernet

• Frame pre-emption: These are rules for switches to break up a large frame at one end of a link and reassemble the fragments at the other end, in order to transmit other high priority frames between the fragments. This is addressed by the IEEE 802.1Qbu standard. Switches advertise their capability for frame pre-emption through LLDP messages

• Forwarding and queuing enhancements for time sensitive streams are addressed by IEEE 802.1Qav

• Path selection and reservation: These are rules for network elements to select paths and reserve bandwidth as well as time slots for frames transiting the network

• Time aware traffic scheduling and shaping: These are rules, that the network elements must implement uniformly, for processing and forwarding Ethernet frames. This is addressed by the IEEE 802.1Qbv standard

• The IEEE 802.1CB recommendation for frame replication and elimination for reliability provides for duplicate copies of messages to be transmitted in parallel over disjoint paths with the receiver selecting the best to ensure high reliability for data

• The IEEE 802.1Qat Stream Reservation Protocol (SRP) proposes for the source of a stream/flow to send messages to request resource reservation at every node in the intended traffic path. The IEEE 802.1Qca recommendation proposes the same through an extension to the IS-IS routing protocol

Given the current scenario of the relatively fixed nature of the connectivity between the RRH and BBU, out of the above, the following are identified requirements for the fronthaul network.• Frame pre-emption as defined in IEEE 802.1Qbu• Time aware traffic scheduling and shaping as defined in IEEE

802.1Qbv

Standardization

The key industry groups working on standardization of the fronthaul are• IEEE 1914.3 • ORAN WG4

Altran 20 - Hardware options

Since traditional Ethernet switches were never intended to carry radio traffic, several new options are emerging to build the switches/gateways for the fronthaul transport network. Leading ethernet switch ASIC vendor, Broadcom, has been the first to release a “5G radio switch”[3]. This encompasses complete Ethernet switching, IP routing and MPLS functionality along with the RoE (Radio Over Ethernet) mapping as well as the extension for Frame Pre-emption, into a single compact ASIC. Others have taken or may take different approaches. Technology leader Intel has taken the FPGA route to address the unique requirements of fronthaul transport[4]. Xilinx has also taken an FPGA approach[5]. There are also industry initiatives towards standardizing the hardware design. One such is AT&T’s initiative in the OpenCompute project[6].The approach here is to distribute the RoE and switching functions into two separate devices, called FHG (FrontHaul Gateway) and CAS (Converged Access Switch), respectively.

HARDWARE OPTIONS

[3] https://www.broadcom.com/company/news/product-releases/12316[4] https://www.intel.in/content/www/in/en/wireline/products/programmable/applications/fpgas-enable-5g-fronthaul-video.html[5] https://www.xilinx.com/products/intellectual-property/ef-di-roe-framer.html[6] https://www.opennetworking.org/wp-content/uploads/2019/09/3.30pm-Sumithra-Bhojan-Emerging-Wireless-White-Boxes-RAN-Fronthaul.pdf

Altran21 - Altran’s Software Framework for Fronthaul Switches and Gateways

ALTRAN’S SOFTWARE FRAMEWORK FOR FRONTHAUL

SWITCHES AND GATEWAYS

The technology is complex and there is a bewildering array of material to digest and hardware choices to select from. And the market is moving! Altran offers a licensable software framework for fronthaul switches and gateways. This framework is based on software that has been widely deployed in carrier networks around the world for more than a decade. The Altran software framework includes support for CPRI and eCPRI transport over Ethernet networks, covering all the different modes of transport, together with the necessary functions for reliability, timing, synchronization, time sensitive networking and management. Comprehensive functionality and full support for Netconf with Yang models for the management of all functions enables the shortest time to market with the best chance to succeed in this fast-paced, evolving market. This is further complemented by Altran engineering services with extensive experience in both wired transport and wireless communication domains. This combination significantly reduces the uncertainty and time required to launch products. Read more about Altran’s fronthaul switch software framework at - https://northamerica.altran.com/software/mobile-fronthaul-switch-framework.

About the Author

Rajesh Kumar Sundararajan, [email protected]

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Ethernet in the mobile fronthaul v2 - Capgemini Engineering

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