How Does SDLC Work?
SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connections between IBM equipment. A variation of the ISO HDLC protocol called HDLC Normal Response Mode (NRM) is essentially the same as SDLC. SDLC is not a peer to peer protocol like HDLC, Frame Relay or X.25. An SDLC network is made up of a primary station that controls all communications, and one or more secondary stations. Where multiple secondaries are connected to a single primary, this is known as a multipoint or multidrop network. The primary is usually a mainframe or midrange central computer, and the secondaries are either terminals, or controllers which act as concentrators for numbers of local terminals. The different drops can be arranged by the transmission line supplier to be in different locations. Thus, for instance, a mainframe in Los Angeles may support a multidrop line with controllers connected to drops in offices in Boston, New York and Washington. SDLC uses the same frame format a
