r a more detailed discussion of TDM operation.) In addition, speeds are inflexible. The only way to increase speeds is to upgrade from T1 to T3, or add new T1 or T3 circuits. In both cases, new equipment is required and customers must wait from two to four weeks for carriers to implement these changes. Introduction In contrast to TDM's fixed capacities, Gigabit Ethernet over fiber is scalable. It supports speeds ranging from 10Mbps to 10Gbps in some cases. At higher speeds, Gigabit Ethernet is less costly than equivalent T3 services. Moreover, prices for Giga- bit Ethernet have recently decreased enough to be affordable for midsize customers. Another advantage is that upgrades to higher capacity can be achieved by program- ming alone, without the time-consuming and costly changes for T1 and T3 upgrades. This is also the case for Ethernet over copper, which supports from 2Mbps up to 100Mbps. Additionally, both Gigabit Ethernet and Ethernet over copper operate on the same Ethernet technology used in both core carrier networks and internal enterprise networks. This makes them simpler to install because there are fewer requirements to convert between Ethernet on Local Area Networks (LANs) and TDM, and vice versa. Moreover, Ethernet interfaces between the carrier and the customer cost considerably less than those for TDM. Like T1/E1 and T3/E3, Digital Subscriber Line (DSL) is an older technology, but it is used extensively worldwide. DSL's key advantage is that it operates over the existing outside copper cabling located in the vicinity of most customers. Thus, it is a relatively low-cost option for carriers to deploy and for subscribers to lease for Internet access. Businesses also use it in areas where T1/E1 services are cost prohibitive. The number of DSL connections is decreasing as more fiber is installed, and as next-generation, high-speed mobile services become available and consumers substi- tute mobile broadband for DSL. Both of these changes offer the possibilities for higher speeds and cost savings. In addi