Overhauling Backhaul

It is amazing how the connectivity systems within a mobile network (Backhaul/Transport) have evolved over the  last two decades – from 2G voice only services to our modern LTE world where even voice could be served through a packet switched network. Such evolution is always haphazard, driven by real-time traffic demands and ends up making the network messier and messier over time.

GSM systems started with TDM based connectivity between the BTS and BSC. In fact, I remember that during the greenfield launch of the network in my previous employer, we had a star-topology realized between all the BTS and BSC using P2P Microwave links becausewe started with about 26 BTS covering an entire metro city!

As we added capacity, we realized that the star-topology to be expensive and had some of the newer BTS added a PDH MUX based conectivity to create clusters of BTS connected. We used to have link failures (it was a coastal city and cyclonic storms would selecctively knock off one intermediate Microwave link) causing multiple BTS outages. Besides, if the chain had too many BTS, we needed higher order MUX at every site in PDH.

This made us go for SDH – with ADM capability at E1 level and support for Ring-topology that can overcome single link failures easily. But some sites remained on PDH because the benefit analysis didn’t prove it worth the cost!

Although I moved away from the operator around the advent of GPRS, I am sure the journey through EDGE, 3G, HSPA to LTE would have made many more changes – whilst some sites may not have changed at all through this journey.

For example – GPRS would have introduced a packet-switched core (originally suggested by standards to be a Frame Relay system) which would have run out of switching capability when EDGE technology was introduced necessitating the adoption of ATM, and, in a few blessed cases, directly to an IP MPLS system. With 3G, definitely this core had to be changed to IP MPLS – but there would have still been some GPRS/EDGE components left in the network somewhere that may have necessitated ATM switching, if not Frame Relay!

The base station capacities have also increased to an extent where fibre is required to connect up the SDH ring (SONET) and with Gigabit speeds, Ethernet over Fibre. We have seen the upgrades to bring in Ethernet card on NodeBs and Cell Site Routers being introduced with Pseudo-wire interfaces for colocated 2G equipment. There may still be some stages where Ethernet was introduced but the traffic was hauled over SDH (EoSDH)!

Yikes! The possible combinations that would have come in over time is mind boggling, to say the least!

With SDN/NFV coming in, perhaps is it time to overhaul backhaul and streamline the chaos? But, technology by itself will not guarantee success.

In my opinion, the success of this transformation is going to be how well this is planned and how the cost-benefit tradeoffs are understood for every combination of history that continues to remain in the  network.


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