In ccna wendell odom pdf free download, IBM developed the first bit-oriented protocol, SDLC, from work done for IBM in the early 1970s. The 8044 controller is still in production by third party vendors.
Frames not a multiple of 8 bits long are illegal in SDLC, but optionally legal in HDLC. HDLC optionally allows addresses more than 1 byte long. HDLC has an option for a 32-bit frame check sequence. Some features were added in HDLC, and subsequently added back to later versions of SDLC.
SNRME U frame, were added to SDLC after the publication of the HDLC standard. This is particularly useful in SDLC loop mode. The CFGR command contains a 1-byte payload which identifies some special diagnostic operation to be performed by the secondary. A payload byte of 0 stops all diagnostic modes. The secondary echoes the byte in its response.
Disable all frame generation, becoming silent, but do not stop carrier or loop mode operation. Enter local loopback, connecting the secondary’s input to its own output for the duration of the test. Rather than echoing TEST commands verbatim, generate a TEST response consisting of a number of copies of the first byte of the TEST command. Initialization mode, and the associated RIM and SIM U frames, are so vaguely defined in HDLC as to be useless, but are used by some peripherals in SDLC.
HDLC, its function having been superseded by asynchronous response mode. UP frame with the poll bit clear if it has data to transmit. The TEST U frame was not included in early HDLC standards, but was added later. A special mode of SDLC operation which is supported by e.
Zilog SCC but was not incorporated into HDLC is SDLC loop mode. In this mode, a primary and a number of secondaries are connected in a unidirectional ring network, with each one’s transmit output connected to the next’s receive input. Each secondary is responsible for copying all frames which arrive at its input so that they reach the rest of the ring and eventually return to the primary. When a secondary is powered off, a relay connects its input directly to its output. When powering on, a secondary waits for an opportune moment and then goes “on-loop” inserting itself into the data stream with a one-bit delay. A similar opportunity is used to go “off-loop” as part of a clean shutdown.