erage revenues per user (ARPU) but increasingly now should be measured either in terms of AMPU (average margin per user) or more accurately AMPD (average margin per device). The scale shift is, however, quite remarkable. An exabyte by the way is a million terabytes. Telecommunications Economies of Scale 30.00 25.00 20.00 15.00 10.00 Laptop Devices iPAD & Other Handheld Smartphones iPhones Ordinary Phones Figure 4.5 Mobile data volume per month (Exabytes) 2010 to 2016. Reproduced with permission of The Mobile World. The impact on the relative revenue mix between voice and data is shown in Figure 4.7 with projections forward to 2016. However, revenue is not the same as profitability. If a network is lightly loaded, incremen- tal loading, as long as it has some revenue attached to it, will improve network investment return rates. If the network is heavily loaded and additional load requires additional capac- ity to be added, then return on investment is dependent on the investment per subscriber Laptop Devices iPAD & Other Handheld Smartphones iPhones Ordinary Phones Figure 4.6 Annual mobile data traffic (Exabytes) 2010 to 2016. Reproduced with permission of The Mobile World. Making Telecoms Work Voice Nonvoice Figure 4.7 Mobile service revenue - 2010 to 2016. Reproduced with permission of The Mobile World. per megabyte. This is not a metric that operators have previously needed to be concerned about. Adding capacity may also imply an investment in new spectrum. The economics of this are complex as a return on investment is dependent on ensuring an adequate supply of performance competitive market competitive user devices are available. For reasons that we will analyse this is often much harder to achieve than one might expect and is a function of RF cost economics. RF cost economics are a function of the band plan but also the technology deployed in each supported band. Figure 4.8 shows the absolute dominance of GSM on a global basis. This is effectively a technopoly - a market dominated by a legacy technology. Thi