Logical Link Control LLC

The LLC provides services to the upper layers at a LAN station. It provides two forms of services for its users:

1. Unacknowledged connectionless service

2. Connection mode services.

Some brief comments are required to clarify the functions and limitations of each service. With unacknowledged connectionless service a single service access initiates the transmission of a data unit to the LLC, the service provider. From the viewpoint of the LLC, previous and subsequent data units are unrelated to the present unit. There is no guarantee by the service provider of the delivery of the data unit to its intended user, nor is the sender informed if the delivery attempt fails. Furthermore, there is no guarantee of ordered delivery. This type of service supports point-to-point, multipoint, and broadcast modes of operation.

As we might imagine with connection mode service, a logical connection is established between two LLC users. During the data-transfer phase of the connection, the service provider at each end of the connection keeps track of the data units transmitted and received. The LLC guarantees that all data will be delivered and that the delivery to the intended user will be ordered (e.g., in the sequence as presented to the source LLC for transmission). When there is a failure to deliver, it is reported to the sender.

IEEE (Ref. 2) defines the LLC as that part of a data station that supports the LLC functions of one or more logical links. The LLC generates command PDUs and response PDUs for transmission and interprets received command PDUs and response PDUs. Specific responsibilities assigned to the LLC include:

• Initiation of control signal interchange

• Interpretation of received command PDUs and generation of appropriate response PDUs

• 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.

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 include sequence numbering and 8 bits long for formats that do not. The three formats described for the control field are used to perform numbered information transfer (I-frame), unnumbered control (S-frames), and unnumbered information transfer (U-frames) functions. These functions are described in Section 10.10.3.

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