SONET (Synchronous Optical Network):

First introduced in 1994, SONET is a North American standard for multiplexing slower streams of traffic onto fiber-optic cabling and transporting it at optical carrier (OC) speeds. The international standard for the same functions is synchronous digital hierarchy (SDH). SONET/SDH was a major innovation in enabling carriers to carry enormous amounts of voice and data traffic reliably on fiber networks. As SONET equipment prices dropped, large enterprises adopted it as well.

SONET equipment transports high-speed traffic on fiber-optic network between the following:
- Central offices in metropolitan areas (the metropolitan core)
- Remote terminals (digital loop carriers) in metropolitan networks (metropolitan access networks) and central offices
- Long-haul backbone networks and metropolitan areas
- Points of presence (POPs) in long-haul, core networks
- Enterprises and data centers where backup data is stored
- Enterprises and points of presence (POPs) that carry their long distance traffic
- Enterprises to separate central offices for redundancy in case of a central office failure or a fiber cut

SONET also can carry ATM and IP traffic and television signals. However, as increasing amounts of traffic is data rather than voice and more of the data and a growing percentage of the traffic is IP based, lower-priced gear is becoming available to transport IP traffic more efficiently and at lower costs on redundant fiber rings. These rings found in MPLS (Mulitprotocol Label Switching) networks and some metro-area networks are based on dense wavelength division multiplexing.