Backhaul: mixing wireless and wired
Written by: James Hayes | Published:
Wireless backhaul is relatively quick and inexpensive to deploy compared with fibre; credit: CBNL

James Hayes explores whether wireless or traditional wired backhaul is the best option for mobile network operators to meet growing data demands

Backhaul is bearing the brunt of the massive surge in mobile traffic. But while mounting pressure on backhaul may seem good news for mobile network operators (MNOs), it also poses challenges for intermediate connectivity within their infrastructures.

Understanding backhaul, and the part it plays in mobile networking hierarchies, is easier when picturing how the mobile ‘skeleton’ joins together. With core networks representing the spine, backhaul has been characterised as the ‘arms and legs’ of that skeleton; those limbs have had to rapidly acquire extra reach to enable them to cope with the escalating demands placed on them.

These backhaul limbs empower UK network operators to carry more and move faster. Building out backhaul capability to meet escalating demand – let alone plan for longer- term requirements – is an ongoing balancing act between trend-driven market dynamics and MNOs’ ambitions and resources.

The shift in how smartphone users now consume various content, from streamed movies to corporate videoconferencing – has had repercussions for mobile backhaul in general, and is expected to drive wireless backhaul ‘densification’ in particular. The trend is obvious to video-hungry subscribers in the UK, but as voice calls and texts become a marginalised part of the traffic make-up MNOs are having to rethink their backhaul provisioning strategies.

Wireless backhaul infrastructures are primarily based on wired (mainly fibre) or wireless (mostly microwave, though satellites can be used) technologies, and transmit data from endpoint devices to a node in a backbone network. In general, the function of the mobile backhaul network is to aggregate the bandwidth of thousands of cell towers back to centralised Mobile Telephone Switching Offices (MTSO). ‘Backhaul’ can also refer to the transmission of network data over an alternative wireless route when the usual route becomes overburdened or unavailable.

Technology vendors selling in this sector are focused on finding ways to boost equipment performance while continuing to meet MNOs’ needs for flexibility and cost-effectiveness.

“Significantly higher capacity is available now with wider RF channels of 56 MHz, higher order modulation of 512 quadrature amplitude modulation (QAM), and co-channel cross polarisation operation with Cross Polarisation Interference Cancellation (XPIC). These help with general backhaul capacity management throughout the network,” explains Andy Sutton, principal network architect at BT’s chief architect’s office. “Techniques such as microwave radio repeaters and space diversity links have more recently been added to the solutions toolkit to address rural coverage.”

Wired or wireless?
Backhauling wirelessly is also a popular option with MNOs because of the capital expenditure flexibility it promises. One of the joys of wireless backhaul for MNOs is that the core technology can be physically redeployed as consolidated markets justify follow-up fibre, explains Steve Greaves, founder and CEO at CCS, a specialist in backhaul systems for small cells.

“Wireless backhaul provisioning brings the capability to deploy quickly, flexibly, and relatively inexpensively,” Greaves says. “If operators could ‘fibre-up’ every small cell to potentially deliver unlimited capacity this would be a technically preferable solution, [but] costs are prohibitively expensive. Wireless backhaul represents, in that sense, the bow wave of innovative service delivery – once demand and capacity reach a critical mass then fibre can be deployed.”

MNOs are starting to understand that wired and wireless options are not mutually exclusive, says John Naylon, CTO and co-founder of Cambridge Broadband Networks (CBNL). “The UK now has a fairly even split between wired and wireless backhaul, which we see continuing in a balanced strategy,” Naylon reports. “With modern wireless technology holding the capacity to backhaul all generations of mobile networks envisaged... the balance tips towards wireless. [This happens] when operators need to quickly scale their networks, or increase return on investment.”

Even when operators have fibre strategies, adds Naylon, they are pragmatic about its use: “It’s commonplace for operators to use wireless backhaul to quickly expand their network footprint. Once the fibre catches up the operator can either use the wireless as a backup, or redeploy to the next phase of expansion.”

So while the operational benefits of wired and wireless options are not mutually exclusive, and a short-term wireless strategy can often prove the first stage of an eventual upgrade toward wired build-out, both wired and wireless have a role in any backhaul provisioning plan, argues Stuart Broome, chief sales and marketing officer at Fastback Networks: “One does not exclude the other. We have customers that deploy wirelessly to gain quick access, and customers to provision at a later stage with fibre as and when demand justifies it.”

Fibre and wireless options “can now be used as appropriate,” agrees BT’s Sutton. “[BT- owned] EE has a good mix of fibre and point- to-point microwave radio solutions. The key is to get the balance right. Chains of up to four or five links were quite common [with] GSM. The evolution through UMTS (Universal Mobile Telecommunications System) towards HSDPA (High Speed Downlink Packet Access), however, resulted in smaller chains of microwave radio, and more fibre-based points of presence. Chains of microwave radio links tend to be longer in more rural locations.”

Strategy going forward
From this perspective, and with next-generation standards being prepared, MNOs’ backhaul infrastructure strategy is destined to be fluid and difficult to formulate too prescriptively, Sutton predicts. “Moving from 3G to 4G was a big learning curve in terms of anticipating future needs,” he recalls. “It went a long way to helping network architects understand techniques for minimising latency and packet loss to gain optimal performance.”

Understanding the impact of 4G on mobile backhaul transport is critical to deploying efficient, cost-effective systems that meet MNOs’ expectations for performance, reliability and cost, says Paul Gowans, marketing manager for location intelligence and RAN solutions at Viavi Solutions: “Everyone now expects a 4G signal wherever they are. Coverage is a key topic in the UK, particularly for rural areas,” he explains. “Given this, and assuming the bandwidth is available, many forms of backhaul can be considered – wired, wireless, line-of-sight, non-line-of-sight, and satellite.”

Savvy operators now look at a combination of these to meet customers’ needs, Gowans says: “Strategically positioned small cells can provide a valuable lifeline for consumers to deliver targeted data services. So any backhaul, regardless of medium, would be welcome.”

“Capacity demands inform the choice of wireless technology solutions deployed, but technology is also governing the evolution of network topologies,” says Sutton. “The capacity demands per 4G cell site result in greater deployment of fibre points and shorter chains of microwave radios.”

He adds: “You need to understand how to combine technologies to get optimal performance – and that means minimising latency and packet loss. The evolution of network topology is leading the way to technological changes. The need to extend 4G availability into the UK’s rural subscriber base is being led by microwave.”

The UK “still has a long way to go to provide sufficient 4G coverage for rural areas,” says Carsten Brinkschulte, CEO at Core Network Dynamics (CND). “An interesting approach could be to retire 3G and re-farm the spectrum for 4G usage, as is the strategy in the US.”

With LTE and LTE Advanced standards, meanwhile, wireless backhaul again meets the capacity needs because it is much lower cost

and faster to deploy when compared to fibre, according to Gary Croke, senior director of marketing and strategy at Aviat Networks. But wireless backhaul is also proving an attractive option, he says, for replacing legacy fibre infrastructures whose limitations have been highlighted by the newer, more demanding mobile transmission standards.

“Most legacy backhaul infrastructure is not optimised to support the capacity needs of 4G, and now needs to be upgraded,” Croke points out. “In addition, as MNOs are moving into other applications like IoT many are moving to new IP/multiprotocol label switching (MPLS)-capable backhaul infrastructure for advanced quality of service, flexibility in service delivery, and management over a converged mobile infrastructure.”

Naylon explains: “Whereas point-to- point has historically been the customary wireless backhaul choice, appetite is growing for more efficient and versatile technologies, such as point-to-multipoint microwave and millimetre wave.”

CCS’ Greaves agrees: “4G delivers more capacity and enables better services, [but it] also creates a network densification requirement because higher capacity services are delivered to a larger number of customers. [And it is also] a technology that leads ultimately to 5G, which requires a very dense network.”

Demand grows, but revenue slows
Mobile backhaul strategies are influenced by one of MNOs’ greatest dilemmas: how to derive revenue from network demand being driven by OTT services that make no direct revenue contribution to the build and upkeep of their enabling technology. This is compounded by the rising popularity of accessing mobile OTT services via untariffed public Wi-Fi.

More than 50 per cent of the traffic on EE networks is now video, adds BT’s Sutton, and the move from SD to HD further complicates that. “Part of good network capacity planning for MNOs is the ability to study those kinds of usage trends, understand the dynamics driving that demand, make educated decisions on what shape it will take going forward, and plan accordingly,” Sutton says.

This bugbear is set to worsen. According to CND’s Brinkschulte, over the next 24 months further bandwidth demand will certainly be “predominantly driven by OTT services. Expect a decline in voice communication and an increase in video and music streaming.”

“While consumer data demand rises rapidly, the average revenue per user remains level,” explains CBNL’s Naylon, so operators must deliver more data at a flat cost. “This is driving operators to optimise their backhaul and augment the use of more efficient technology to build a more attractive business case. With backhaul accounting for approximately 30 per cent of an operator’s total operational cost it offers potential to drive efficiency. More cost-effective and spectrally efficient wireless technologies, such as PMP microwave, have a significant role to play in achieving this.”

Vendors are responding to urban densification by designing small-form factor backhaul radios, that can be mounted on any tower, building or street asset; credit: Fastback Networks

LTE and LTE Advanced
The LTE standard itself is evolving to a newer version, LTE-Advanced (LTE-A), which increases spectrum utilisation up to 100 MHz. Also looming is the impact of wide-scale deployment of networks based on small cells.

Sometimes referred to as ‘densification’, macro- cell deployment could gradually give way to small cell expansion to fill in coverage gaps, increase available capacity, and improve service quality in crowded urban areas.

“LTE-Advanced is focused on providing a faster, better mobile broadband experience on smartphones,” says CND’s Brinkschulte. “With regard to backhaul, LTE Advanced Pro [does mean] increased pressure on bandwidth... LTE Advanced Pro continues to stretch the throughput boundaries for smartphone use cases. [These standards] are also a stepping stone towards 5G, with support for new use-cases; IoT applications in particular.”

LTE as a protocol “can be run on big macro cells or on small cells, but to deliver maximum performance requires the inherent densification delivered by small cell deployments,” argues CCS’ Greaves. “What’s interesting is that the technology advances and architecture of LTE have enabled a new class of mobile network operator, meaning many more players are now entering the market.” These new players may have their own ideas about how their mobile backhaul is provisioned – ones that differ from traditional models.

“With the new spectrum-sharing protocol being delivered by the Federal Communications Commission, anyone [in the US] can now become a MNO and build a mobile network... The core and access network technology is becoming commoditised. However, what cannot be commoditised as yet is the carrier class backhaul – that will always require a high quality, specialised solution,” he concludes.

“As with every iteration of 3GPP, LTE Advanced and LTE Advanced Pro will lead to backhaul upgrades that can manage their higher data rates,” predicts CBNL’s Naylon.

However, this is unlikely to take the form of new technologies as the data requirement of these standards is “well within the capacity of existing wireless solutions”. Instead, backhaul methods will be developed to better support the densification strategies needed to manage the shorter ranges of these access technologies. “Wireless will become an essential tool in providing the agility and business case to achieve this – a key reason the forecasts for wireless backhaul remain so positive,” he adds.

Provided that the 4G rollout and coverage “continues at a fast pace, and that the tariffs further promote the use of mobile broadband applications”, and that consumer smartphone penetration continues to rise, “we should expect continuous pressure on the backhaul to provide adequate throughput”, believes CND’s Brinkschulte.

If the commercial relationship between MNOs and OTT companies changes backhaul networks “may need to participate in the rapid setup and delivery of OTT- based services over the converged mobile network infrastructure,” thinks Aviat Networks’ Croke. “This will mean support for carrier Ethernet and IP/MPLS services as well as advanced quality of service and reporting capabilities.”

Backhaul planning
To meet future network architectures, where sites are in close proximity, backhaul needs to go from being more than a medium that connects two locations, says Broome at Fastback Networks. It needs to be ‘intelligent’ to manage the environment it operates in, and flexible enough to evolve with the network.

“The main issue may not be about whether the mobile backhaul is wired or wireless,” suggests Brinkschulte, “but rather how best to avoid backhaul congestion and avoid central core network overload, given the projected influx of massed IoT devices in coming years.”

Meanwhile, adds Viavi Solutions’ Gowans, MNOs should bear in mind that planning for backhaul “is often about access... You want to place cell sites as much as you can where fibre can maximise backhaul bandwidth... You may choose, for example, as part of your overall network optimisation plan to place a site in a potentially sub-optimal location [for RF propagation]. It is all about coverage and capacity. There may not be a great gain from subscribers to have good coverage but very poor capacity. Five bars’ signal strength does not always, in practice, mean good service.”

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