 interchange Interpretation of received command PDUs and generation of appropriate response Organization of data flow Actions regarding error-control and error-recovery functions in the LLC sublayer As shown in Figure 11.3, the LLC accepts higher level user data and encapsulates it, forming an LLC PDU. The resulting LLC frame is embedded into the MAC user field for transmission. ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS The LLC is another derivative of HDLC, which was discussed in Section 10.10.3. It is based on the balanced mode of that link-layer protocol with similar formats and functions. This is particularly true when operating in the connection mode. 11.5.3.1 LLC PDU Structure. As shown in Figure 11.3, the LLC appends a header forming the LLC PDU. The PDU frame format is illustrated in Figure 11.4. The header consists of address and control information; the information field contains the user data. The control field is identical with HDLC control field, illustrated in Figure 10.29 and described in Section 10.10.3.1. However, the LLC control field is two octets long and there is no provision to extend it to 3 or 4 octets in length as there is in HDLC. As mentioned previously, the LLC destination address is the user address at an SAP inside the destination LAN station. It is called the destination service access point (DSAP). The SSAP is the source service address point and it indicates the message originator inside a particular LAN station. Each has fields of 8 bits, as shown in Figure 11.5. However, only the last seven of these bits are used for the actual address. The first bit in the DSAP indicates whether the address is an individual address or a group address (i.e., addressed to more than one SAP). The first bit in the SSAP is the C/R bit, which indicates whether a frame is a command frame or a response frame. The control field is briefly described in Section 11.5.3.2. 11.5.3.2 LLC Control Field and Its Operation. The LLC control field is illustrated in Figure 11.6. It is 16 bits long for formats that 