It's no secret that the future of mobile networks is small cells. As cellular networks evolve to LTE, traditional macro cellular architecture isn't going to cut it - not with mobile data growing at exponential rates and with more and more devices connecting to the network.
The pitch is simple: to get the most out of their spectrum capacity, cellcos will have to deploy base stations closer to users in a variety of coverage options - from metro cells and micro cells to pico cells and femto cells. And cellcos are already buying into that pitch. According to In-Stat, global shipments of small-cell gear will reach $14 billion in the next five years, with outdoor metro cells alone achieving a CAGR of 248% in that timeframe.
Interestingly, that small-cell future includes Wi-Fi as well. Once the scourge of mobile broadband, Wi-Fi has become an increasingly common offload strategy for cellcos looking to take advantage of both its unlicensed spectrum (i.e. free access capacity) and its relative ubiquity in mobile devices in order to lighten the load on their RANs. But the emergence of small cells doesn't mean Wi-Fi offload is a stopgap, says Selina Lo, president and CEO of Ruckus Wireless.
"It's not about whether operators should deploy Wi-Fi or cellular within smaller cells - they need to do both," she says. "By putting small cells closer to the subscriber, operators can lower the cost of transporting each bit while boosting wireless capacity. Users will access the network at much higher data rates and be able to get on and off the network more quickly - utilizing less network capacity from a spectrum and time perspective."
The result: the next-gen mobile broadband access network is going to be largely heterogeneous. And that's going to complicate things tremendously on a number of levels, from access and network management to backhaul and class-of-service (CoS) management.
By no coincidence, this year's Mobile World Congress served as a platform for mobile network suppliers to showcase various solutions to the heterogeneous-network (or "hetnet") problem.
To name three marquee examples:
Alcatel-Lucent showcased its lightRadio Wi-Fi solution, which builds on the compan's next-gen lightRadio architecture and promises cellcos a way to enable customers to "switch automatically from a cellular service to residential or public Wi-Fi networks and hotspots without having to login, worry about payments schemes, or even be aware of the shift."
Nokia Siemens Networks unveiled Flexi Zone - based on its next-gen Liquid Radio architecture - in which multiple, inter-connected low-power small cells (which can be HSPA, LTE and Wi-Fi) use a common pool of resources managed flexibly by a "zone controller". Result: local offloading of internet traffic based on coverage and capacity needs, "saving up to 80% of transport and mobile packet core costs".
Cisco announced two new enhancements to its mobile infrastructure portfolio: small-cell (i.e. Wi-Fi/femtocell) gateway functionality on its ASR5000 platform, and Wi-Fi access points for service providers compliant with the new Hotspot 2.0 specification established by the Wi-Fi Alliance (more on that later).
The common thread is Wi-Fi integration, which is arguably the top challenge cellcos face in the hetnet paradigm.
"The need for small cells as a capacity enhancement and data offload tool is well understood," said Peter Jarich, service director with Current Analysis, in a research note. "What is not as frequently discussed is the need for holistic small-cell solutions including licensed spectrum and Wi-Fi assets along with common transport and control backed by deployment and business consulting insights."
The reason for a holistic approach is because up to now, Wi-Fi services may be bundled in with fixed and mobile broadband packages as a supplemental connectivity option, but from a management point of view, Wi-Fi and cellular are two different networks.
Nelson Chu, manager and systems engineer for Cisco, points to the experience of Hong Kong's PCCW, which has been offering Wi-Fi as part of its broadband portfolio for years, but there's little question which service is easier to use.
"The challenge they see is making it easier for users to log on," Chu says. "Right now users have to select the SSID, which is troublesome because the average user may not know what SSID is, or which SSID to select, and then they have to enter a user name and password, which they have to remember. That reduces the likelihood of them using the service."
In broad terms, Cisco proposes to integrate Wi-Fi into the cellular RAN equation by way of a single centralized controller that manages multiple access points in the network - cellular and Wi-Fi - as well as an "intelligent service cloud" that knows what services a subscriber is entitled to use.
"Regardless of whether you use wireless or fixed - once you enter the service cloud, we can provide the control to allow or disallow different services for the users," says Chu. "So we can provide uniform access to users who are using different access methods."
In terms of simplifying Wi-Fi access, several industry groups have been promoting new standards-based solutions in the last few months. On the vendor/device side, for example, the Wi-Fi Alliance's HotSpot 2.0 framework leverages 802.11u, which enables fast, automatic and secure connections for Wi-Fi users (to include, incidentally, roaming on partner networks).
From the operator side, the Wireless Broadband Alliance (WBA) says its Next Generation Hotspot (NGH) initiative has a similar aim, making it easier for users to automatically connect to Wi-Fi hotspots. In February, the WBA said a number of its members had wrapped up NGH trials and that the first NGH deployments will take place over the next 12 months. Last month, the GSM Association officially announced it would work with the WBA on NGH, while also making use of the Wi-Fi Alliance's Passpoint certification technology, to develop technical and commercial frameworks for Wi-Fi roaming.
The CoS factor
But even as operators look to deal with hetnets in terms of managing connectivity on the access side, hetnets create new challenges in the backhaul side of the network as well.
Much of the backhaul challenge stems from the trend of operators deploying Carrier Ethernet as a backhaul solution to deal with mobile data growth, which is cheaper and more efficient compared to adding old-school E1 links to each base station, but becomes expensive for operators shifting from a macro cell to a small-cell architecture.
And it gets even less impractical once operators start factoring in CoS, which has become a crucial metric in the mobile broadband game. The changing nature of mobile data itself and the increasing importance being placed on the customer experience means cellcos need the ability to prioritize traffic in their backhaul links to ensure that low-latency apps like video and games run at optimum performance without resorting to throwing more bandwidth at the backhaul.
"Traditionally, when a service provider offered metro Ethernet, there was only one CoS for each circuit," says Aaron Tong, senior consulting specialist for mobility, Asia Pacific, at Juniper Networks. "So if a customer required multiple-CoS, they needed to lease multiple circuits, then map different types of traffic onto those circuits, which is more costly because the customer has to buy multiple circuits, and then they have to manage those circuits."
Udi Gordon, executive VP of marketing and business development for Ceragon Networks, adds that this becomes even more unmanageable under a hetnet scenario because service providers have to maintain CoS not only for different access networks, but also for the different services and applications running on each of them.
"If you have to maintain tens or hundreds of different services on your network and make sure every one of them has a unique service profile, that makes it much more complex and difficult to manage," Gordon explains. "If you just had a mobile access network, in many cases the base station was aggregating different users, and at the output end of the base station you can see your web users, your video users, and so on. But with a hetnet you have one base station that's 2G, one base station that's 4G, one Wi-Fi, one business access - each of which has different classes of service and you have to support them all."
In February the Metro Ethernet Forum addressed the problem with the release of its MEF 23.1 Multi-CoS Implementation Agreement (IA) for mobile backhaul - which itself is part of a broader release dubbed Carrier Ethernet 2.0 - that essentially standardizes ways for cellcos to implement multiple-CoS within a single Carrier Ethernet backhaul link. The MEF says this makes backhaul links lower-cost and easier to manage.
MEF 23.1 doesn't come without creating new challenges, of course. For a start, says Tong of Juniper, one element not laid out specifically under the new specs is how to define service attributes, especially SLAs, for things like latency and jitter, for instance.
Meanwhile, says Tong, even without the CoS element, hetnets create another backhaul challenge for cellcos - getting all those cell sites to communicate with each other.
"When we talk about hetnets, we're talking about coordinating multiple cell sites to offer higher bandwidth to individual users. That coordination requires some communication between cell sites," Tong explains. "In traditional cell sites, the backhaul is only in two directions - from the cell site to the controller and back, with almost no communication between the sites themselves. But in LTE cell sites are expected to talk to each other via the X2 [application protocol] interface so they decide which one of them can deliver the best connection to a mobile device."
That becomes problematic if the cell sites are linked by traditional point-to-point circuits in a hub-and-spoke architecture, because a base station's traffic has to go through the hub before it can reach the other cell sites it wants to communicate with.
Consequently, says Tong, some operators are looking at using Layer-3 VPN for mobile backhaul rather than a Layer 2 point-to-point circuit.
"If you have two cell sites coordinating together to deliver bandwidth to a particular device, they need to exchange X2 interface traffic frequently, so latency is a key here," he says. "With Layer 3 VPN, cell sites can communicate directly with each other, so in terms of latency it's much better, which is a key point."
SIDEBAR 1: Hetnet: the cost of getting it wrong
Operators planning to deploy heterogeneous networks (hetnets) of small cells to cope with data growth risk investing hundreds of millions of dollars for nothing if they get the business case wrong, according to a new study from Arieso.
The problem, says Arieso CTO Michael Flanagan, is that the hetnet approach creates new problems as operators find themselves faced with multiple offload options, complex subscriber behaviors, myriad charging models and a small minority of extreme users generating the majority of traffic.
"The result is a fiercely complicated business case with some brand new variables," he said in a statement. "The risk of getting it wrong, wasting money and still not solving the problem is significant".
Arieso says its research shows that half of the data traffic generated in a typical mobile network is carried by the busiest 10% of cells, and that one in five of those busy cells have fewer than 200 unique data users, while 2% have fewer than 50 data users.
What that means, says Dr Flanagan, is that cell splitting to provide capacity relief can be unjustified when there are too few users. He adds that even allowing for some high ARPU subscribers, an operator would struggle to justify a macro cell split for less than 200 unique users. Below 50 unique users per cell, Wi-Fi may well be the only cost effective solution.
"In a worst case scenario, where operators try and satisfy the demands of extreme users solely with macro sites, they will waste millions of dollars - $400 million across the industry this year alone," he says.
The $400 million figure is based on data from ABI Research estimating that $2.5 billion in capex is invested by the industry in capacity relief every year, as well as Arieso data that 80% of that investment will go into the areas covered by the 10% of busy cells. (If the remaining 20% is invested in macro sites with less than 200 users, that's $400 million invested in macros with a questionable business case, Arieso says.)
Arieso says that operators need to understand five new variables in their network planning that hetnets bring into play: unique users per cell, how much data they're consuming, device capabilities, spatial distribution of users within the cell, and temporal variations in demand.
SIDEBAR 2: Unlicensed backhaul
One notable challenge for small cells and hetnets is, of course, getting them all connected in the first place. LTE-level throughput speeds demand fiber-level backhaul capacity, but assuming the fiber is available at every site location (let alone bonded copper), with small cells potentially number in the tens of thousands in a metro service area, connecting every site with fiber is costly.
Wireless broadband is the next obvious solution, but technologies like microwave and even Wimax (which has been touted and utilized as a backhaul option for Wi-Fi) require spectrum licenses that, generally, aren't cheap.
Consequently, some vendors are pushing unlicensed spectrum as a viable backhaul option for hetnets.
Ruckus Wireless, for example, recently announced its SmartCell solution that not only integrates cellular and Wi-Fi connectivity, but also uses Wi-Fi mesh technology to provide low-cost backhaul links on the unlicensed ISM bands.
Meanwhile, NEC uses the less-populated unlicensed 60-GHz band for small-cell backhaul. According to Keita Ito, NEC's program director of small-cell solutions, the high channel re-use characteristics of that spectrum band are "ideally suited" to deliver high capacity and low latency connections to hundreds of cell sites.
He says, "60-GHz is capable of delivering 100 Mbps and more to the small cell, using virtually zero-cost radio spectrum." Ito adds that 60-GHz spectrum allows the design of compact equipment that can be easily installed and "aesthetically concealed within a wide variety of urban environments."
One tough selling point for using unlicensed spectrum for backhaul is, of course, the fact that it's unlicensed - i.e. anyone can use it - which raises potential QoS issues.
Ruckus says that SmartCell's use of adaptive directional antennas, predictive channel management and Wi-Fi mesh technology deliver "reliable backhaul for licensed cellular and unlicensed Wi-Fi traffic in both line of sight and non-line of site environments."
Ito says NEC's 60-GHz solution includes features for "intelligent provisioning of backhaul resources and protection against performance degradations, resulting in improved capacity efficiency and elimination of costly manual maintenance and troubleshooting."