What is smartphone signalling?

01 January 2011 |

Worldwide mobile broadband subscriptions topped 600 million in 2009 according to the ITU, and a growing proportion of those subscriptions are for smartphones, often with all-you-can-eat data plans.

Already smartphone users, are complaining of spotty coverage. A single Youtube video uses the same amount of data as half a million SMS messages, and this has created problems for some operators. But there’s a hidden problem that needs to be solved – smartphone signalling. Some operators struggle with dropped calls and sluggish data across entire cities, not just in particular cells. This indicates that the problem is not simply a few heavy data users, and may be because signalling channels are being overloaded.

Why is there more signalling all of a sudden?

In short: smartphone apps and smartphone design. Applications like instant messaging (IM), Facebook or Skype can easily overload signalling channels. A single instant message can generate 30 signalling messages, which roughly equates to the network usage required by a voice call.

Smartphones are causing network congestion because of the chattiness of common applications which keep data channels open even if they are running in the background. These applications generate a great deal of underlying signalling traffic, even if they don’t generate that much data traffic, with the result that the 3G data network has spare capacity, but performance suffers. Adding more capacity for data traffic (for example increased backhaul or deploying another radio carrier) will not, by itself, solve the problem. With a device typically updating itself every one or two minutes, the background signalling traffic created per device is comparable to 1,000 voice calls each day.

You’ll have to be quick, my smartphone battery is almost flat

In fact, smartphone battery life is a large part of the problem. Many manufacturers choose to rapidly disconnect and reconnect from the data network in order to conserve smartphone battery life. This fast dormancy increases the amount of signalling traffic. Most of this congestion has been reported in markets with many iPhone users, but with the Android phones also on the market now, many more operators might well experience similar problems in the near future.

Constant signalling also drains the smartphone’s battery, and so manufacturers have an incentive to use the Cell_PCH feature from Nokia Siemens Networks (NSN) where it is implemented. It will use the network operator’s timings to put the handset into a connected low power state. Also major application developers may learn that what works on the internet does not necessarily work so well in 3G, and adapt accordingly.

What can we do?

Doing nothing is not an option. This background signalling happens whenever the smartphone is switched on, so messages are being generated 24 hours a day and generally without the knowledge, or activity, of the user. Any solution has to start with the network. Incorporating a NSN feature called Cell_PCH in the network design reduces overall signalling load by as much as 80% in NSN tests. In Cell_PCH state, the Radio Network Controller (RNC) and packet core will keep the packet bearer active. Signalling in the network is cut down because the RNC tracks device mobility during the Cell_PCH state.

Careful planning and optimisation will be necessary to optimise networks for the next generation of smartphone users, but NSN has made a study of mobile broadband with HSPA and LTE which shows that monthly network capex and opex can be kept below E3 per user over eight years, with at least 500 users per site using less than 2Gb of data per month. It’s more efficient to use macro sites, such as Flexi base stations, which offer 40% or 50% more capacity. Quality of service differentiation between users, and offloading traffic to femtocells will also help in the longer term.

Does it work outside the lab?

NSN has already worked with Telefonica O2’s central London network to upgrade performance for smartphone users. It identified the bottlenecks caused by smartphone data consumption and signalling.

The solution has combined an enhancement of signalling capacity with the introduction of new Flexi Multiradio base stations, doubling the number of mobile sectors from three to six in each base station’s area of coverage. Signals Research Group conducted a field test of smartphones in Montreal, comparing NSN’s Telus HSPA network to the Rogers Wireless HSPA network. “We conclude that the combination of Cell_ PCH and the selection of appropriate T1, T2 and T3 timer settings can significantly reduce the amount of signalling traffic while increasing the life of the battery… and we observed as much as a 65% reduction in the amount of signalling traffic,” says the research firm.