dB =10log P1 P2

! Control frames are not control characters.
! They are used to send commands to or solicit
information from another device.
! Such a frame contains no data but it carries information
specific to the function of the DLL itself.
! Control frames serves in establishing connections,
maintaining flow, and error control during data
transmission and terminating connections.
! BSC was originally designed to transport textual
messages
! a user is just as likely to send binary sequences that
contain non textual information and commands.
! In 1975 IBM pioneered development of SDLC and lobbied ISO
to make SDLC the standard.
! In 1979, ISO answered with HDLC, which was based on SDLC.
! Adoption of HDLC by ISO led to its adoption and extension by
other organizations.
! ITU-T was one the first organizations that embraced HDLC.
! LAPs (LAPB, LAPD, LAPX, etc), all based on HDLC.
! Other protocols such as Frame relay and PPP developed both by
ITU-T and ANSI also derive from HDLC, as do most LAN access
control protocols.
! In short all bit oriented protocols in use today either derive from
or are sources for HDLC.
! Thus, through HDLC we can obtain a basic understanding for
others.
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! Byte oriented protocols
! bits are grouped into predefined patterns (characters)
! Bit-oriented protocols
! more information into shorter frames
! Avoid the transparency problems of character oriented
protocols.
! Broadly categorized into
! SDLC (Synchronous Data Link Control)
! HDLC (High-level Data Link Control)
! LAPs (Link Access Protocols)
! HDLC (ISO 3009, ISO 4335) is a bit oriented DLL
protocol designed to support both half duplex and
full duplex communication over point to point and
multi-point links.
! Systems using HDLC can be characterized by their
station types, configurations and their response
modes.
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