 held the B-side licenses in other parts of the country and was interested in expanding its footprint. Small cellular operators nationwide were cashing in, selling their systems to larger operators that wanted to expand their footprints. European countries had, as a general rule, awarded countrywide licenses and were working to harmonize their systems into a seamless mesh, whereas the U.S. model of small local systems, perhaps efficient for taxicabs and plumbers, was being revamped by the market as systems consolidated. Only it was the original cellular licensees, not the gov- ernment, that were realizing the value of the licenses. By the 1990s it was clear that cellular mobile telephony was going to be very popular. Even though systems were cash-flow negative for many years, the potential future profits became obvious. Mobile telephone systems had heavy upfront expenses. Base stations had to be built. Towers were constructed all over-this was good revenue for tower-rental companies like American Tower, which grew rapidly. Central office switching systems, known as mobile tele- phone serving offices (MTSOs), had to be bought-this was a good revenue source for Lucent, Nortel, Ericsson, and Siemens, among others. And the cellu- lar phone companies generally subsidized the purchase of handsets, typically to the tune of $150 to $200 per unit, as each new customer signed up or renewed a contract. But revenues continued to rise, even as prices fell in a competitive mar- ket. The largest providers became profitable, though smaller ones, with fewer customers per cell site, never did break even. Networks Go Digital By the early 1990s, digital cellular systems were beginning to replace analog ones. In Europe, a multinational committee formed in 1987, originally known by its French name Group Speciale Mobile, created a standard that was adopted in most countries around the world. The GSM standard was later renamed "Global System for Mobile." It was a digital system, based on time division mul- tiple access (TDMA) technology