‘Data, data and more data’ has been the demand of mobile-phone users for many years, whether they articulated it or not. This, along with a demand for high quality of service, has prompted considerable innovation in radio technology – GPRS, EDGE, UMTS, WiMAX and many more – with much of the latest effort going into deployment of LTE (long term evolution) systems.
LTE aims to solve the data challenge by providing higher data rates – theoretically up to 75 Mbps in the uplink and up to 300 Mbps in the downlink. The LTE specifications also bring in a number of other changes to the ways that radio networks are rolled out and managed.
LTE and its evolution, LTE-A, rely on direct communication between base stations – or eNodeBs – to optimise traffic allocation. This communication needs to occur with very low latency; according to standards body 3GPP, ‘the reason for distributing the intelligence amongst the base-stations in LTE is to speed up the connection set-up and reduce the time required for a handover. The transmission time interval is set to only 1 ms’.
In addition, LTE brings with it new network architectures and an increased role of so-called pico or femto cells, where a transceiver mounted on a lamppost or other street furniture serves a small but busy urban area – with perhaps a radius of 200 to 300 metres.
All of these things involve significant development on the radio side, but they also put pressure on the core network, the infrastructure that backhauls the mobile traffic. As Virginia Teixeira, consulting systems engineer at Cisco noted: ‘The radio is the expensive part so that has to be the bottleneck. You can’t have the transport layer as the bottleneck.’
According to a report published in October by telecoms market research firm Infonetics Research, global macrocell backhaul equipment revenue will be $8.1 billion in 2013, although this is starting to plateau; the figure is an increase of just one per cent from the 2012 revenue figure.
‘Though it’ll be slow growth ahead for the macrocell mobile backhaul equipment market, this is still a huge market, one for which we expect operators to spend a cumulative $43 billion over the five years from 2013 to 2017,’ said Michael Howard, principal analyst for carrier networks and co-founder of Infonetics Research.
The report also revealed that IP/Ethernet is the clear driver of the macrocell backhaul market, and that ‘94 per cent of macrocell backhaul equipment spending in 2013 will be on IP/Ethernet gear’.
Most interesting perhaps, for readers of Fibre Systems, is the increasing role that fibre specifically plays in mobile backhaul. Infonetics found that fibre is making up an increasing share of mobile backhaul installed connections, and will grow to 43 per cent of the mobile backhaul market in 2017.
‘Fibre is an integral part of backhaul and all Cisco equipment has optical interface support. We fully believe that fibre is a key transport mode,’ said Teixeira.
She noted that mobile networks have evolved quickly in the last few years from the low data rates of 2G when transport was based on E1 or ATM networks. ‘Most of the traffic was voice and this fitted well with this,’ she said.
However, for the data services of 3G, TDM and ATM did not scale well. Data uses greater bandwidth and the price per bit was too high. ‘It got to a point where bundles of E1 and ATM were not operationally possible to meet user demands and we needed to deploy IP networks to cell sites,’ she continued.
However, there is a need to maintain E1 and ATM support because there are older base stations that still have these interfaces. ‘The IP infrastructure we offer can handle those interfaces and emulate on IP,’ she said. The second challenge, she said, is to keep expanding the network in terms of the number of sites reached and also capacity. ‘We have seen a huge evolution in a very short time with 3G alone. We have to guarantee that transport will provide the necessary bandwidth, quality and latency.’
LTE brings with it architecture changes. As Teixeira explained: ‘2G and 3G are very hierarchical – with node Bs communicating with a radio network controller (RNC). LTE is different; cells communicate directly with the core. Now we have a fully meshed architecture, rather than hub and spoke.
‘With a fully meshed network, the requirements in terms of latency have changed and eNode Bs communicate in real time,’ she said. The eNode Bs are normally inter-connected via X2-interfaces.
‘Cell sites coordinate closely with regard to traffic and power requirements and there are tight controls between towers,’ explained John Hawkins, senior advisor, product and technical marketing at Ciena. ‘X2 traffic is very modest in terms of bandwidth but latency is crucial.’
When it comes to the core network, there are many pressures of the fibre too. ‘Many operators are integrated wired and wireless and it is important to have demarcation and aggregation,’ said Hawkins. ‘We’ve got installations at approaching 400,000 sites out in the field, and our largest customer is rolling out 1,200 cell sites per week.’
With many operators having both wireless and fixed networks, Yves Petit, senior manager for strategy and business development, broadband network solutions, for TE Connectivity said that physical network management is key.
‘Most operators are trying to connect mobile network to fixed network as quickly as possible. With LTE the number of cells is increasing and the size of cells is decreasing so the main challenge for operators is growing the existing network,’ he said.
He explained that, from a tower to the base station, an operator typically needs to manage 12 to 24 fibres if they are not shared with another operator. In the USA, he said that the trend is towards 24 to 36, and that shared backhaul typically requires 36 to 48 fibres between tower and base station. And this is just from one tower. ‘There are typically around 2,000 fibres going into one rack and we have customers that line up rack after rack,’ he continued. ‘Operators need good physical management to see quickly where they need to make an intervention.’
The density of number of connections per floor space also needs to be as high as possible, he continued. ‘Operators are often renting expensive city centre space.’
Using what’s there
Petit went on to say that the tower to fixed network connection is normally already there and once in place it’s usually over capacity.
There is, however, more frequently a need to add fibre at the tower. ‘Historically, the cable that is in place wasn’t fibre so you can’t exclude the possibility that operators will need to add fibre,’ he said. He added that, at that point, there are benefits to ensuring that the tower is over capacity. ‘The cost of adding more fibre at this time is marginal, while adding fibre in a later stage will be highly costly,’ he said.
Barry Zipp, director of industry marketing at Ciena, observed: ‘The major growth pain point is the raw capacity of the network. There is no lack of bandwidth capability provided you have fibre to the tower,’ he said. ‘In the USA, fibre is often available to the tower or it’s a case of a very short, opportunistic fibre run to an existing ring. The towers that require the most bandwidth tend to be near existing fibres.’
But there are challenges with legacy systems. ‘Cable companies are consistently struggling with footprint and power. Cable modules take up huge rack space and they have to constantly catch up,’ he continued. ‘The presence of legacy technology in the core network is what’s been driving much of our R&D, although it is becoming less and less of a case that fibre is not available.’
Teixeira, of Cisco, agreed on this challenge: ‘Depending on operators there will be different quantities and availability of fibre in the network. On the access side, fibre is not so common and we have to work with this,’ she said. ‘If there is already fibre to the site then that fibre is usually enough but with LTE we need more sites.’
And then there is the additional challenge that LTE is co-located with 2G and 3G equipment. ‘Most of the time we can reuse fibre backhaul as long as fibre still has capacity but then as we start to densify we need to add more fibre in new locations. In addition, existing sites 2G and 3G might have older backhaul solutions, for example copper,’ she said.
There is a particular issue with the low latency requirements of LTE. ‘Fibre is very good at low latency but the latencies provided by microwave and copper are a challenge. Throughput and latency will always be on the list of key characteristics to check,’ she said.
Managing costs
Cost is, of course, always a big issue for operators too. One of the drivers to develop LTE on the radio side was to bring down the cost per bit and this needs to happen on the core as well as with the radio. ‘On the one hand we need to have new networks and meet new requirements but on the other prices have been slashed,’ observed Teixeira.
Speed of installation is also an important factor, according to Petit, of TE Connectivity. ‘On the tower, speed of fibre installation is key. Our latest solutions are designed to increase the speed of installation and reduce labour. Not so long ago it took 12 man-days but now we can install a whole fibre tower system in three man-days – or three people working for one day. This means we can have the tower up and running as quick as possible so the operator can generate revenue more quickly.’
Hawkins of Ciena also noted the importance of a unified network management system. ‘It allows us to set up hundreds of cell sites in a matter of minutes,’ he said, adding that it is also important to have good tools to provide comprehensive visualisation and diagnostics.
Small cells
One key feature of LTE networks is the increasing role that small, low-power cells will play in providing additional capacity in built-up areas. In-building small cells are easier to install and maintain because they have less regulation and easier access. However, the vision is to include cell sites on street furniture such as lampposts and traffic lights too.
In a recent report, Infonetics predicted a rise in outdoor small cells. The analysis firm reported that small cell mobile backhaul equipment revenue totalled $39 million worldwide in 2012 and it projects that outdoor small cell backhaul connections will grow from fewer than 7,000 in 2012 to more than 850,000 in 2017.
Richard Webb, directing analyst for microwave and carrier WiFi at Infonetics and co-author of the report, observed: ‘We look for outdoor small cells to really kick into high gear beginning in 2014, and predict a cumulative $6 billion will be spent globally on outdoor small cell backhaul equipment between 2013 and 2017.’
The challenge with backhauling small cells is that, although there will be many more of these small cells, each operating at low power and serving a small area, their backhaul requirements are the same as those for a macrosite. And they come with more challenges too.
One of these challenges is the availability of fibre. ‘Fibre is a preferred transport medium. It has the best throughput, best delays and is very reliable. It is always the first option. But at small cell sites, especially outdoor, it is not so common that fibre is available where a small cell is deployed,’ said Teixeira.
Installation of fibre brings challenges. In particular, there are many regulatory – and cost – issues involved in digging up a busy street. And even where there is fibre, there can be cost issues too, as she explained.
‘One requirement with small cells is that they are very dense and cheap. It really kills the business case if the backhaul is very expensive,’ she said. ‘When operators need to sub-contract from third parties, price comes into place. Sometimes renting fibre is very expensive and operators think about going to other options.’
The option that many opt for in these situations is microwave backhaul. Microwave enables new greenfield capacity to be provisioned within days and when the link breaks down because of physical disruption the recovery takes place in minutes. In addition, maintenance of microwave backhaul does not require closing and digging up a busy street. Infonetics expects ethernet microwave radio to grow at a 36 per cent compound annual growth rate (CAGR) from 2012 to 2017, the highest of any equipment segment.
Powering small cells is also a challenge that has implications for backhaul. ‘Whatever goes into small cells needs to be low power,’ observed Teixeira. ‘It is not only the issue of the cost of power but also the availability of power.’
There are other things to take into account in designing small cells such as aesthetics. For example, in New York City only two types of box are permitted on the city’s streets, irrespective of whether the contents are the controls for traffic lights or part of a small cell for a mobile network.
Installation and maintenance costs are also going to be key considerations for operators when it comes to small cells. Operators cannot make too many visits to a lamppost, especially when it might involve things such as stopping traffic, so maintenance poses more logistical challenges.
Network sharing
Such factors are drivers towards network sharing. Teixeira noted that with small cells in busy urban environments it is likely that network sharing will be mandated. ‘Everybody trying to put equipment in the same place doesn’t work. Network sharing will be key,’ she said.
It’s not just with small cells either. Hawkins of Ciena noted that very few towers are single operator anymore.
‘Most wholesale fibre providers serve three to four operators,’ he said, adding that it is important to support complete separation between operators. He said that there is also a need to monitor the traffic and provide
periodic reports on bandwidth use, hot spots and failures.
Backhaul innovation
Of course, innovation doesn’t stop here. Users’ data demands continue to grow and radio technology will continue to evolve. Fibre backhaul providers are looking to be ready for whatever happens next.
‘We try to future proof as much as possible,’ said Petit of TE Connectivity. He noted that the quality of the fibre and fibre termination are very important.
‘Previously, grades C and D fibres were acceptable. Now the number of customers requiring grade B is increasing. We see operators that have their capacity limited because they have grade C fibre in the field.’
Better traffic management is a priority for Cisco. ‘Our vision is for self-organising backhaul networks. The target is really to make networks autonomous so that they can be optimised without human intervention,’ said Teixeira.
She pointed out that backhaul networks serve radio access and that requirements on the radio side change throughout the day and time of week. ‘What would be optimal would be to move backhaul resources to where the radio is being used to use all backhaul resources most efficiently,’ she said. ‘It would also work the other way too, for example, if the backhaul in a certain area fails the system would act on the radio part to redistribute traffic to neighbouring cells.’
‘If you have an intelligent network that knows where traffic is that improves OPEX costs and maintenance costs,’ she concluded. ‘Radio networks are alive and backhaul must follow.’
