data and supervise error recovery. A most important function of this layer is recovery from abnormal conditions. The data-link layer services the network layer or logical link control (LLC; in the case of LANs) and inserts a data unit into the INFO portion of the data frame or block. A generic data frame generated by the link layer is illustrated in Figure 10.7. Several of the more common data-link layer protocols are: CCITT LAPB, LAPD; IBM SDLC; and ANSI ADCCP (also the U.S. government standard). Network layer. The network layer moves data through the network. At relay and switching nodes along the traffic route, layering concatenates. In other words, the higher layers (above layer 3) are not required and are utilized only at user end-points. The concept of relay open system is shown in Figure 10.23. At the relay switching point, only the first three layers of OSI are required. The network layer carries out the functions of switching and routing, sequencing, logical channel control, flow control, and error-recovery functions. We note the duplication of error recovery in the data-link layer. However, in the network layer error recovery is network-wide, whereas on the data-link layer error recovery is concerned only with the data link involved. The network layer also provides and manages logical channel connections between points in a network such as virtual circuits across the public switched network (PSN). It will be appreciated that the network layer concerns itself with the network switching and routing function. On simpler data connectivities, where a large network is not involved, the network layer is not required and can be eliminated. Typical of such connectivities are point-to-point circuits, multipoint circuits, and LANs. A packet-switched network is a typical example where the network layer is required. The best-known layer 3 standard is CCITT Rec. X.25 (Ref. 22). Transport layer. The transport layer (layer 4) is the highest layer of the services asso- ciated with the provider of communication services.