ly, backhaul networks have consisted of between two and four T1 cir- cuits carried over wireless microwave links. T1 technology is suitable for the pre- dominantly voice traffic carried on 2G cellular networks. However, T1 service has inadequate capacity and is inefficient for transmitting the large amounts of data traffic generated by smartphones and tablet computers on 3G and 4G networks. Worldwide, microwave is still the primary technology used in backhaul networks. New microwave services are available that transmit signals in IP packets using Carrier Ethernet. For more information on Carrier Ethernet, see Chapter 5, "Broadband and Wide Area Network Services." In addition, microwave has increased capacity and can now reach 1Gbps. Carriers, particularly those in high-traffic metropolitan areas, are upgrading to these new microwave technologies or to fiber-optic networks. In the United States, AT&T and Verizon Wireless have upgraded much of their backhaul network to fiber. In Europe and in developing countries, most backhaul net- works operate on microwave services. This is particularly true in the access portion of mobile networks because there is less fiber already in the ground in these areas. Some of these networks use microwave in the access portion and fiber-optic cable in aggregation links. The higher costs for fiber are viable because more traffic is car- ried on these links. Fiber is likely to be more readily available in aggregation networks where carriers have previously laid fiber that often has spare capacity. In many instances, the landline divisions of mobile carriers build these networks. When Verizon Wireless upgraded its backhaul network to fiber, it used the wholesale division of Verizon Communications to build it out. Verizon's wholesale division offers backhaul services on a wholesale basis to other providers in addition to Verizon Wire- less. Cable Multiple System Operators (MSOs), AT&T, and other long-haul network carriers also provide backhaul links to mobile carriers. The traffic is tran