Nonbroadcast Multiaccess

In computer networks, a nonbroadcast multiaccess (NBMA) medium or transmission system is one to which more than two users can connect, but that does not support broadcasts to all participants, or multicast to some or all participants. NBMA is typically a logical, not necessarily physical, implementation of star or hub-and-spoke topology.

Some of the most common transmission systems, operating at the medium access control layer (i.e., Layer 2 or the Data Link Layer of the Open Systems Interconnection Reference Model are frame relay and Asynchronous Transfer Mode, which create bidirectional paths between a hub point and a group of spoke points. In a typical NBMA system, the spokes cannot communicate with one another. Yet another technology, sometimes called "layer 2.5", which can support NBMA in its unidirectional point-to-multipoint mode, is Multi-Protocol Label Switching.

NBMA, when used with the Internet Protocol, breaks a basic Internet architectural principle called the "local vs. remote assumption". This assumption means that if two devices are on the same subnet, they should be able to communicate directly, via a data link layer protocol. If they are on different subnets, they must communicate via routers on their indvidual subnets.

It may be completely impossible for spoke devices to communicate via layer 2, although in many applications for NBMA, this is a feature rather than a bug; the spokes may be end users that have no need to see one anothers' data. One proprietary technique from Cisco Systems, called "private virtual local area network (VLAN)", imposes a NBMA topology on a normally broadcast-capable Ethernet-style transmission system. The typical application for private VLANs are cable television networks, where subscribers are on spokes and the hubs is a service provider concentration point.