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By Anthony Plewes

Backhauling mobile traffic used to be a straightforward affair for most operators: Cell sites linked to their core networks with E1/T1 leased lines, and when the operator needed more capacity it simply provisioned more lines.

Mobile broadband services, which demand substantial increases in capacity throughout the network, not simply in the access portion, have shattered this cozy world. Consequently, mobile operators and their providers must look beyond legacy technologies to boost their backhaul capacity substantially without dramatically increasing their costs.

The arrival of HSDPA, which promises users DSL-like speeds at DSL prices, is jolting mobile operators out of their voice-centric backhaul comfort zones. Mobile broadband finally is changing how people use their mobile phones, with rich content downloads, Internet browsing and E-mail becoming commonplace. This explosion in bandwidth is putting the operators' mobile backhaul networks under increasing strain, with some operators experiencing a three-fold increase in data traffic in less than a year.

If operators continue down this path, service quality for both voice and data applications ultimately will suffer. To prevent this happening, operators must overhaul their backhaul, since the incremental addition of more leased lines will prove economically unjustifiable. To make matters worse, much of the spending on mobile backhaul is ending up with wireline operators, which have for years seen retail revenues fall at the expense of rising mobile phone use. According to analysts at Infonetics Research, this spending amounted to USD19.5 billion in 2006 alone.


Regional variations

Choosing new backhaul architecture and cutting backhaul costs, however, is not entirely straightforward, as few operators have the same legacy backhaul infrastructure. Traditional mobile backhaul is a mixture of leased lines, private circuits and microwave circuits, so no single solution will meet the migration needs of all operators.

In the United States, leased-line T1s have been priced more competitively than in Europe. As a result, U.S. operators are more reliant on leased-line connections. In Europe, more than 60 percent of mobile operators have built their own microwave backhaul solutions, but a large base of leased-line users still exists, particularly among the mobile subsidiaries of former incumbents. Fiber is popular in the Asia Pacific, especially in countries with a rapidly developing infrastructure, such as China, and the technology is also starting to make inroads into both the U.S. and Europe.


Scalability

Besides increasing capacity requirements, this new data traffic is better suited to different technologies. "Operators need to look using different backhaul technologies for their data requirements," says Nadine Manjaro, an ABI Research analyst. "Voice traffic is perfectly matched to sharing a 2Mbps T1, but if you have multiple users all trying to stream over 1Mbps of data you are going to have a bottleneck in your backhaul."

The key is to use a more scalable backhaul technology. While it is possible to increase capacity by adding more T1/E1 lines, it is costly and can take too long to meet the operator's immediate needs. Ideally, operators would like to have quickly scalable bandwidth they can use on a pay-as-you-use basis without having to change the infrastructure radically.

Ethernet is emerging as one of the best technologies to do just that. "Ethernet is attractive for operators as it matches how they are using the network," Manjaro explains. "Because it is bursty, as opposed to fixed-like ATM, Ethernet gives you the ability to scale up and down as you need it while providing quality of service assurances."


All roads lead to IP

Decisions about mobile backhaul must be made in view of an operator's migration to an ALL IP infrastructure. IP is being introduced in the core, while IMS is being deployed in both fixed and mobile networks. To gain the most from IP in the core, however, a mobile operator must introduce IP throughout its entire network, so while the starting point for most operators may be different, the end result will eventually be the same.

Mobile operators can take three main routes to an eventual ALL IP infrastructure:

• Continue using traditional backhaul infrastructure, aggregate leased lines, share bandwidth between 2G and 3G networks, optimize the network and move over to a packet infrastructure when the technology is mature;
  
  
• Adopt a hybrid infrastructure that uses TDM for voice and offloads HSPA data traffic to a separate packet network that will eventually carry all of the operator's traffic; or
  
  
• Move directly to an ALL IP packet infrastructure, taking advantage of bandwidth savings immediately.

"Moving to ALL IP infrastructure is a gradual process for most operators," explains Gaby Junowicz, director of business development for cellular at RAD, an equipment supplier. "The most common RFQ we see is for hybrid solutions that keep the real-time traffic on the operator's TDM network and offload HSDPA traffic onto packet transport. But all of them have a migration plan to move eventually to the IP world, as it can give them massive savings." For one operator, according to RAD, this was calculated to lead to savings of EUR80 million (USD114 million) a year in backhaul alone.

The hybrid approach is popular in Europe as it solves the most pressing issue for mobile operators: how to handle the growth in data traffic while still preserving their investment in current backhaul technology. Backhaul solutions based on xDSL are a good match for many European operators, as connectivity is widespread and asymmetrical connections such as ADSL2+ mirror HSDPA, which offers much higher download than upload speeds.

A main concern about xDSL is the lack of robust QoS, so most operators plan to keep their voice on their existing backhaul infrastructures and just offload data traffic. Much of the data traffic they currently generate is from Internet browsing or e-mail downloads, which means operators can get away with a best-effort level of service.

Of course, operators with no legacy backhaul can skip TDM and move directly to a packet network. An example is Japan's EMOBILE, which launched last spring with a HSDPA-powered mobile broadband solution. It backhauls all traffic over Ethernet transport using ATM pseudowires.


Packet advantage

Although next-generation microwave and IP Ethernet take mobile operators out of their comfort zones, they will, as noted, allow them to realize significant savings. Infonetics estimates the cost of mobile backhaul over traditional leased lines is around two and half times that of next-generation technology, such as Ethernet.

Given the savings, there is little surprise operators are already testing and trialling Ethernet backhaul, with major deployments expected to begin in earnest in 2008. Infonetics says that while Ethernet made up just 1 percent of total mobile backhaul equipment sales in 2006, it will account for as much as 41 percent, or USD2.5 billion, of worldwide sales by 2010.

Microwave will continue to be a key transport mechanism in Europe, with the move away from point-to-point technology helping to reduce costs significantly.

European operators that have made significant investments in microwave likely will continue to develop the technology through to fully packet-based solutions. "Legacy microwave systems are TDM-based and few of them support IP streams," says Frank Chevalier, senior consultant at Mason Consulting. "If the equipment is less than two years old, however, operators are likely to be able to upgrade it fairly easily."

Point-to-multipoint technology allows operators to carry out radio planning more easily, using fewer antennas to cover the same area. It also allows them to aggregate traffic from multiple base stations within a single deployment, allowing them to share traffic and use the available bandwidth more efficiently.

Operators, including Vodafone, have already jumped on the microwave bandwagon to avoid high leased-line costs. "The majority of our global networks have already switched to microwave backhaul based on PDH and SDH technologies," says a Vodafone spokesperson. "We have begun to explore how we can upgrade this technology to something that offers even more capacity. Right now, we are exploring the potential of Ethernet microwave, which offers an intelligent form of modulation to boost capacity still further and makes more economical use of spectrum."

Although some operators reportedly are trialing it, fixed WiMAX is not yet particularly strong as a mobile backhaul option, even though it was initially promoted as such. This is partly because much of the development effort is focused on the technology's 802.16e mobile version, and also because of doubts surrounding WiMAX's ability to handle QoS in backhaul networks.


Outsourcing strategies

To deal with the complexities of mobile backhaul, a number of operators are looking at outsourcing. In July 2007, T-Mobile signed a 5-year deal with UK's BT wholesale to manage its backhaul network, migrating the backhaul from leased lines to Ethernet. According to T-Mobile, the flexibility of BT's 21CN Ethernet network was a key factor in the decision.

"By partnering with BT we are able to achieve cost savings and focus on our core business, while BT takes care of our access backhaul requirements," said Emin Gurdenli, T-Mobile's technology director. Previously, T-Mobile reportedly had been investigating the potential of DSL to backhaul its HSDPA traffic and was said to be planning to eventually backhaul its entire traffic over DSL to an Ethernet aggregation point.

Network sharing, which has been moot since 3G networks emerged, is another approach to cost savings in backhaul networks. The concept gained a significant boost early this year with the announcement by Vodafone and Orange in the UK that they planned to share their radio access networks. The exact details have not yet been released, but those operators will benefit more if they share the entire infrastructure, including the backhaul network. If this partnership is successful, the model will likely be replicated across Europe.

Whatever the technology migration path or strategic approach operators adopt to increase backhaul capacity, it's clear the old days are fading fast. The take-up of data services is forcing operators to move away from their voice-centric infrastructures: They need fully converged infrastructures in both backhaul and core networks. The technology to do this has been around for some time, but operators are only now taking action. (From telecommagazine.com)

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