Switched Multimegabit Data Service

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Switched Multimegabit Data Service (SMDS) was a connectionless packet service defined by Bellcore (now Telcordia) in 1989. Initially it was marketed to the RBOCs and other LECs as a MAN service.

Access rates to a network providing SMDS range from 64 kbps to 34 Mbps. High-speed access to the network uses the IEEE 802.6 DQDB access control over DS-1 or DS-3 transmission facilities, while low-speed access is frame based.

SMDS sets up calls using ISDN telephone-number-like addresses as defined in ITU-T Recommendation E.164, one of the addressing schemes used by ATM networks. SMDS provides a connectionless data service (á la ITU-T AAL Class D) and uses a cell format similar to AAL3/4.

The similarity between SMDS connectionless service and the ITU-T AAL Class D service deserves further comment. Bellcore originally defined SMDS based on the IEEE 802.6 DQDB standard for two reasons. First, DQDB was an available standard, and ATM was not. Second, and perhaps more important, DQDB was designed to be compatible with ATM. SMDS “packets” can also be transported in ATM cells or DXI frames.

SMDS was designed as a service, whereas ATM is a technology. As a service, SMDS defined such things as performance criteria, addressing, security, and network management. In the words of Steven Taylor, “SMDS is connectionless ATM” (Data Communications, November 1993).

IN today's networks SMDS is obsolete, having been replaced by Ethernet and MPLS.