This part of IEC 62056 specifies the data link layer for connection-oriented, HDLC-based, asynchronous communication profile. In order to ensure a coherent data link layer service specification for both connection-oriented and connectionless operation modes, the data link layer is divided into two sub-layers: the Logical Link Control (LLC) sub-layer and the Medium Access Control (MAC) sub-layer. This specification supports the following communication environments: • point-to-point and point-to-multipoint configurations; • dedicated and switched data transmission facilities; • half-duplex and full-duplex connections; • asynchronous start/stop transmission, with 1 start bit, 8 data bits, no parity, 1 stop bit. Two special procedures are also defined: • transferring of separately received Service User layer PDU parts from the server to the client in a transparent manner. The server side Service user layer can give its PDU to the data link layer in fragments and the data link layer can hide this fragmentation from the client; • event reporting, by sending UI frames from the secondary station to the primary station. Annex B gives an explanation of the role of data models and protocols in electricity meter data exchange.