LONDON -- Backhaul Strategies & Core Convergence for Mobile Operators -- To say that backhaul for Long Term Evolution (LTE) networks is complex could be a mighty understatement going by the talk from operators and vendors at Light Reading's backhaul and mobile core networks conference here in London yesterday.

But as the first next-generation mobile broadband networks are only just being launched now, the best way to sum up the situation with LTE backhaul seems to be, "it's complicated." (See Two Views on LTE Backhaul and Heavy Reading Reports on LTE Backhaul.)

In the opening keynote address, Andy Sutton, principal architect for access transport at EE -- the UK's largest mobile operator following the joint venture between Orange UK and T-Mobile (UK) -- said that LTE backhaul doesn’t even fit the definition of backhaul in today's 2G and 3G networks, which typically refers to the connection between a cell site and its network controller.

But there are no radio network controllers in LTE's flat network architecture, so the backhaul network will have to connect to the Evolved Packet Core (EPC) in a way that is different to 2G and 3G networks. That means there needs to be a new definition of backhaul for LTE networks. For Sutton, the definition of LTE backhaul is from the cell site to an aggregation router.

In addition, Sutton stressed the importance of incorporating all the legacy 2G and 3G backhaul networks into any LTE backhaul planning. But those legacy backhaul networks already comprise complex technology cocktails even before LTE is introduced into the mix.

Sutton explained that in the transport layer, there could be TDM, frame relay, ATM, and IP. On top of that, there are "network enablers" PDH/SDH, Ethernet, MPLS, and pseudowires. Then, of course, all of that has to be mapped onto the physical network layer, where the choices are copper, microwave radio, or optical fiber. Sutton said that he came up with more than 158 different backhaul combinations in terms of the way all the technologies can be "mixed and matched."

So, when it comes to LTE, "don't forget you have to add that to a GSM backhaul network, which may be in various states of evolution," said Sutton.

In other words, he explained, LTE will be added to converged backhaul network as GSM backhaul migrates from TDM to IP-over-Ethernet, UMTS backhaul networks evolve from ATM to IP-over-Ethernet, and LTE introduces IP from day one. (See Packet Backhaul Busts the 'Pain Barrier'.)Operator uncertainty
Given the complexity involved in LTE backhaul and the myriad networking decisions required, it's not surprising that many operators do not yet know how they will implement backhaul in their next-generation mobile broadband networks. Indeed, according to a survey of about 70 operators that Heavy Reading conducted in late 2009, just one in four mobile operators said that they understood the requirements for LTE backhaul. And three out of four operators said "variations of 'we're not sure yet' or 'we have to look at it'," according to Heavy Reading analyst Patrick Donegan.

100 Mbit/s is not enough
But one requirement is clear: LTE cell sites will need backhaul capacity of more than 100 Mbit/s. In fact, the consensus at yesterday's conference is that each eNodeB (LTE base station) will need between 200 Mbit/s and 300 Mbit/s.

"100 Mbit/s is not enough," said Robert Synnestvedt, RAN backhaul lead, global mobile Internet marketing, at Cisco. "It will be 200-300 Mbit/s. It's going to be much more about architecting your network to scale."

And fiber has emerged as the physical network of choice for LTE backhaul among early deployments. In Sweden, for example, both Telia Company and Net4Mobility HB have rolled out fiber to their LTE cell sites. (See Net4Mobility Chooses Transmode for NGN, Mobile Backhaul – a Booming Opportunity for Wholesaler Carriers? , LTE Showdown in Sweden , and Swedish LTE Challengers Wield Unlimited Offers .)

— Michelle Donegan, European Editor, Light Reading Mobile