[Asterisk-Users] HDLC for Dummies?

[Michael Welter]

Eric Bishop wrote:
> Can any give me or point me to a short and simple explanation of what HDLC is?
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SDLC (Synchronous Data Link Control) was invented by IBM way, way back 
when.  It is a full-duplex, synchronous, point-to-point protocol.  It is 
the lowest layer in the SNA protocol suite.

A packet in SDLC begins with a flag (01111110), data bytes, a CRC byte, 
and a closing flag byte.  Between frames you'll see all marking bits. 
Back in the 70s one could purchase a USART (Universal Synchronous 
Receiver Transmitter) in a 40 pin package.  Don't know about today.

I don't want to start a flame war with the Europeans, but I recall that 
  a European standards committee (CCITT? ISO?) decided that SDLC was 
pretty cool, so they changed one bit, changed the name to HDLC 
(High-level Data Link Control), and called it their own.