work traffic is now based on IP, there are still many organizations and residential consumers whose voice service is based Carrier Networks on circuit switching. Thus, core and metropolitan networks need equipment that han- dles both types of traffic. SONET equipment is used on fiber-optic networks between central offices in metropolitan areas, and between high-speed switches in the core. central office is the building in which telephone companies install a switch that con- nects calls to residences, and businesses. In addition, there are SONET rings between enterprises and Points of Presence (POPs) that carry their long-distance traffic. Ring technology provides an active circuit to carry traffic, and a backup path and electron- ics, in case the active circuit or equipment fails. Carrier Gigabit Ethernet Gigabit Ethernet is used in both Local Area Networks (LANs) and Wide Area Net- works (WANs). It operates mainly over fiber-optic cabling. When it is used in a pro- vider's network, it is sometimes referred to as Carrier Gigabit Ethernet. There are four capacities of Gigabit Ethernet (also referred to as GigE): 1, 2.5, 10, and 100. These are used in core Metropolitan Area Networks (MANs) and access networks. Carrier Gigabit Ethernet is used for VoIP and all IP-packetized traffic. This includes video, data, and graphics traffic that is bundled in packets. GigE is not, however, suitable for circuit-switched voice, because it does cur- rently not have the timing sources required for voice. A timing source provides fixed, guaranteed capacity that circuit-switched voice requires. Without it, circuit-switched voice sounds choppy because of brief, intermittent delays, which are otherwise imper- ceptible in IP data. An International Telecommunications Union (ITU) standard called Synchronous Ethernet is being developed for chips with timing sources. Synchronous Ethernet will require costly hardware upgrades in Carrier Gigabit Ethernet switches. In the interim, GPS-based local clocking is being introduced as a stand-in tim