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History of HDLC and LAP


I did some research into the history of HDLC just to verify my 
memory.  HDLC is the ISO variant of Synchronous Data Link Control (SDLC) 
protocol from IBM.  SDLC is the data link protocol under Systems Network 
Architecture (SNA).  SNA replaced Binary Synchronous (BSC) protocol in the 
1970s at the primary mainframe to mainframe communications protocol.  SDLC 
was and has remained the data link protocol for remote terminal and data 
entry equipment for mainframe systems. The Normal Response Mode (NRM) of 
HDLC is equivalent to SDLC.

There are five primary variants of HDLC, Normal Response Mode (NRM), Link 
Access Procedure (LAP), Link Access Procedure Balanced (LAPB), Link Access 
Procedure for ISDN D Channel (LAPD), and Link Access Procedure for Modems 
(LAPM).  LAP was the original data link protocol for X.25.  It was replaced 
in X.25 by LAPB.  Point to Point Protocol (PPP) from IETF is a variant of 
LAPB.  Link Access Procedure for SDH (LAPS) is a variant of the original 
LAP protocol.

HDLC uses a complex link initialization procedure.  I believe that it is 
this procedure which is also in PPP that causes a major delay in traffic 
restoration of Internet Protocol (IP) links over optical/SONET/SDH 
protected networks.  I did some experimentation with this in 1999.  The 
best that I could achieve was a traffic restoration of ~800ms.   The use of 
LAPS to transport Ethernet will in all likely hood have the same traffic 
restoration delay that is seen in PPP.  Compared to a 2ms traffic 
restoration delay that is seen with optical GbE,  the Ethernet over LAPS 
PHY will not have the resilience of IEEE 802.3 PHYs.

Thank you,
Roy Bynum