An offshoot of the Distributed Queue Dual Bus (DQDB) technology defined by the IEEE 802.6 standard for metropolitan area networks (MANs), as a means of extending the reach of a local area network (LAN) across a metropolitan area. SMDS was developed by Bellcore (now Telcordia Technologies), at the request of the Regional Bell Operating Companies (RBOCs), which at the time were confined to operating within the boundaries of relatively small Local Access and Transport Areas (LATAs). SMDS was originally described by Bellcore as a high-speed, connectionless, public, packet-switching service that extends LAN-like performance beyond the subscriber's premises, across a metropolitan or wide area. SMDS is a MAN network service based on cell-switching technology. Generally delivered over an SDH/SONET ring, SMDS has a maximum effective serving radius of approximately 30 miles (50 Km). SMDS enjoyed limited, short-lived success in the United States through deployment by most of the RBOCs and GTE. Under the name Connectionless Broadband Data Service (CBDS), SMDS also enjoyed moderate success in Western Europe, where the nations tend to be small in geographic terms and where the population density of large businesses is high in the major metropolitan areas. In the mid-1990s, SMDS was overwhelmed by frame relay and various metropolitan Ethernet offerings, and is now considered obsolete. IEEE 802.6 has been withdrawn. See also 802.6, Bellcore, cell switch, connectionless, DQDB, Ethernet, frame relay, IEEE, LATA, MAN, packet switch, RBOC, SDH, and SONET.
(Switched Multimegabit Data Service) A high-speed, switched data communications service offered by the local telephone companies for interconnecting LANs in different locations. It was introduced in 1992 and became generally available nationwide by 1995. Connection to an SMDS service can be made from a variety of devices, including bridges, routers, CSU/DSUs as well as via frame relay and ATM networks. SMDS can employ various networking technologies. Early implementations use the IEEE 802.6 DQDB MAN technology at rates up to 45 Mbps. Data are framed for transmission using the SMDS Interface Protocol (SIP), which packages data as Level 3 Protocol Data Units (L3_PDU). The L3_PDU contains source and destination addresses and a data field that holds up to 9188 bytes.