LTE backhaul: Think twice

Posted by GenD on Tuesday, August 04, 2009 with No comments
When you start digging into LTE, you find it’s a pretty amazing technology – not just the speeds and feeds, but the way it was thought out from the bottom up. With technology migration always a painful problem for operators, LTE was designed to simplify deployment, maintenance and reduce operating costs with the concept of Self Organizing Networks (SONs) running over a flat IP infrastructure. Base stations are much more sophisticated than in 3G and other wireless models: they are responsible for managing their radios, optimizing service quality, discovering neighboring cells, and connecting themselves to the backhaul network.

But perhaps the most important change in LTE base stations (or “evolved Node Bs”as they are known), is their responsibility for managing the service itself. Where 2G and 3G networks rely on centralized radio network and base station controllers (RNC/BNC), LTE goes without: each tower communicates with its nearest peers to hand-off users as they roam from cell to cell. Both control plane (roaming and call control) and user data traffic pass directly between towers, connected in a mesh-style backhaul network. This distributed networking and intelligence can reduce latency and free core capacity by sending data directly to its destination without passing through a centralized aggregation point.
Sounds wonderful – a clear advance in mobile mechanics that takes full advantage of the advanced routing capabilities of today’s MPLS infrastructure – but like so many things, great ideas quickly run into roadblocks where the rubber hits the road. Here’s the tricky part: with LTE’s rates planned to ramp to 150Mbps per-user (!), the backhaul network has to be future-proofed day one. This means a lot of fiber to towers that are mainly fed today by a bundle of T1s over copper. In 3G this problem opened a whole new market: alternative access vendors (AAVs) such as cable MSOs, fiber-rich CLECs, pure-play backhaul providers and even utilities stepped up to fill the gap. Wholesaling backhaul is the name of the game in 3G, where the fastest deployments are ramping on the networks of others.
But this scheme doesn’t fit so well into LTE’s full-mesh architecture. Backhaul is traditionally provided over point-to-point links: AAVs deliver Ethernet in, Ethernet out, logically connecting each tower to a centralized switching center. The concept of tower-to-tower communication is beyond their domain, and their control. I’ve never heard of wholesale MPLS backhaul. Imagine the complexity getting everything talking? If it sounds like a major headache, it is, and no amount of “self-organization” will help.
So operators rolling out LTE have a difficult choice: go it alone with their own MPLS network (if they have one), or lease backhaul service based on point-to-point Ethernet. Towers can still talk to each another, but all the traffic that would just hop to the next cell now has to loop through the switching center just like in the good old days.
Twice the Trouble
Problem is, this has a serious impact on latency as the data path stretches out over a much longer distance. This added delay, combined with decentralized roaming control managed by the base stations spells out dropped and choppy calls… unless the AAVs deliver super-low latency. It’s hard enough delivering Ethernet backhaul with the tight performance demands of 3G, where tower to switching center latency needs to be in the single-digits of milliseconds. Bad news for AAVs: SLAs for LTE will cut this spec in half. Since control plane traffic has to pass from one cell to another, it effectively doubles the path length of what used to be centralized commands sent directly to the towers. So packets have to get there twice as fast. This isn’t a bandwidth issue – increasing capacity won’t do much for latency. This is more like a speed of light, switching performance and network optimization issue.
So what’s a cellco to do? Early LTE deployments precisely echo the backhaul dilemma; only the largest operators with significant MPLS footprint are in the game, and outsourced backhaul will only come into play where their own network can’t reach… and when it does, it’s a sure bet it’ll come with some of the tightest SLAs telecom has ever seen. The AAVs that can rise to the challenge are sure to win big, because there won’t be too many stepping up to the plate.