Connectionless BroadbandDataService (CBDS) is a connectionlessdatagram service for interconnecting local area networks (LAN).
CBDS is the European counterpart to the American Switched Multimegabit Data Service (SMDS) developed by Bellcore. It is a cell-oriented transport protocol for city networks (MAN), based on IEEE 802.6.
The layer model of CBDS is defined in ETSI-300211 and, like the SMDS layer model, is structured in three layers. On the physical layer, Layer 1, is Distributed Queue Dual Bus (DQDB), a dual bus concept for MANs. Layer 2 is supported by Service Management Protocol Data Units (SMPDU) and Layer 3 by Initial MAC PDUs (IMPDU). The E-transmission interfaces for the physical layer are defined by ETSI with 2.048 Mbit/s for the E1 line and with 34.368 Mbit/s for E3.