A bit-oriented, synchronous data communications protocol. SDLC supports high-speed transmission (56 kbps or better) over dedicated circuits in point-to-point or point-to-multipoint network configurations and operates in either half-duplex (HDX) or full duplex (FDX) mode. SDLC uses a cyclic redundancy check (CRC) error detection mechanism, specifically known as Frame Check Sequence (FCS) in this case. SDLC allows as many as 128 frames to be sent in a string, with each frame containing up to 7 blocks, each up to 512 characters. Each block within each frame is checked individually for errors. Errored blocks must be identified as such to the transmitting device within a given time limit, or they are assumed to have been received error-free.The SDLC frame consists of synchronizing bits, data bits, and control characters sent in a continuous data stream, frame-by-frame. The SDLC protocol was developed by IBM in the mid-1970s as a key element of its System Network Architecture (SNA). SDLC corresponds the Layer 2, the Data Link Layer of the OSI Reference Model. Figure S-1 illustrates the SDLC frame and its component fields.The same frame format applies to Highlevel Data Link Control (HDLC) frames and X.25 packets. The fields in the SDLC frame are as follows: