1. A physical organization of communications facilities, each higher level covering a wider or more general area of operation than the next lower level. As an example, a large bank might have a local area network within each branch, connecting the teller stations. Several branches might be linked to a data concentrator, connected to regional concentrators, which in turn link to the bank’s main data processing centre.

2. A logical organization of communication facilities, in which the lowest levels deal with the physical network while higher levels deal with the communication between specific applications. See also protocol.