Suppose that a network layer entity requests its data link layer to set up a connection to another network layer entity. In order to setup a connection in a data link, the initiating data link entity sends a SETUP frame, such as SABM in Figure 5.37. Upon receiving such a frame, the receiving data link entity sends an acknowledgment frame confirming receipt of the SETUP frame. Upon receiving this acknowledgment the initiating entity can inform its network layer that the connection has been setup and is ready to transfer information. This situation provides an example of how unnumbered acknowledgments can arise for confirmed services.