Sequential link is utilized to interconnect DTEs and DCEs. The DCEs are information correspondence hardware that created or got information, and the DCEs are information correspondence gear that main transferred information. There are two distinct sorts of links, one for interfacing a DTE to a DCE, and one more for associating two DTEs straightforwardly to one another.

The DTE/DCE interface for a specific standard characterizes the guidelines including the quantity of pins and connector types, voltage levels for 0 and 1, characterizes the capacity for every one of the flagging lines in the point of interaction and arrangement of occasions for communicating information.

Figure-1 delineates the first RS-232 standard that was characterized as the association among DTEs and DCEs (modems). Nonetheless, we have likewise the necessity to interface two DTEs, for instance, two switches or two PCs.

For this reason, an extraordinary link called an invalid modem can be utilized which wipes out the necessity of DCEs. With an invalid modem association, the Trans and get lines are cross-connected between the two closures of the link. Figure 2 outlines the invalid modem link.