f frequencies on DSL and in cable TV networks. Hertz refers to cycles per second. DSL service that supports 30 megahertz (or as commonly expressed, 30MHz) has greater capacity than DSL at 12 megahertz (12MHz). High-Speed Access via Carrier Gigabit Ethernet HIGH-SPEED ACCESS VIA CARRIER GIGABIT ETH- ERNET Carrier Gigabit Ethernet (GigE) is a site-to-site and Internet access high-speed service. When used by enterprises, Gigabit Ethernet is intended to increase network access speeds SO that they more closely match the speeds in local area networks (LANs). While Gigabit Ethernet operates mainly over fiber-optic cabling, new Ethernet services are becoming available that operate over copper cabling. GigE transmits data, video, images, and packetized voice (VoIP). Employees at carriers use the term "Gigabit Ethernet" or "GigE" when they refer to Gigabit Ethernet broadband service. However, to distinguish carrier broadband service from Gigabit Ethernet on LANs, IT personnel might use the term "Carrier Gigabit Ethernet." Some Gigabit Ethernet switch manu- facturers use the term "Carrier Gigabit Ethernet" and others use "Gigabit Ethernet." Both terms are correct and are used in this chapter for the broadband service. A key advantage is that Carrier Gigabit Ethernet uses the same protocol used within buildings in LANs, making it less complex to connect to a customer's net- work and simpler to upgrade to higher speeds. Ethernet does not require a CSU/DSU device (used for T1-type service) as an interface between the LAN and the carrier's network. A CSU/DSU is used for network diagnostics and clocking. Clocking keeps T1-type services synchronized with a carrier's network. See the section "Channel Ser- vice Units and Digital Service Units," later in this chapter, for information on CSU/ DSU equipment. Carrier Gigabit Ethernet was first introduced in the late 1990s. At that time, it was not widely available both because it required fiber-optic cabling to the customer and because the central offices of providers were not widely e