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In article <1992Dec31.023213.21653@magnus.acs.ohio-state.edu> sgururaj@magnus.acs.ohio-state.edu (Sanjay R Gururaj) writes:
>Hi!
>
>Does anyone know what the difference is between SDLC (synchronous data
>link protocol) and HDLC (????)
>Any pointers to documentation on the above protocol would be appreciated.
>Thanks in advance.

Well, for a rough first cut, SDLC is mostly a subset of HDLC.  ISO
HDLC specifications include 3 general operating modes for HDLC,
NRM (Normal Response Mode), ARM (Asynchronous Response Mode) and
ABM (Asynchronous Balanced Mode), together with numerous options
within those modes.  Ignoring some of the bizarre varients of SDLC
(like ring mode operation), SDLC is a NRM subset of HDLC. HDLC/LAPB
which is one of the most common flavors of HDLC (used by X.25) is
an ABM version of HDLC.  In NRM HDLC (or SDLC) there is one "Primary"
Station and one or more "Secondary" stations.  The HDLC address is
used to identify the secondary involved in a transaction.  Commands
are always to a secondary and Responses are always from a secondary.
A secondary is only allowed to send responses when "polled" by the primary.
This means a command frame with the "Poll" bit set.  When the secondary
is finished talking to the primary, it sets the "Final" bit in the
last response frame sent to the primary.  The primary can now move on
to some other secondary.  In contrast, in an ABM version (like LAPB)
either end can issue a command or response frame at any time.  But
ABM HDLC is usually limited to point-to-point operation.

As for documents, there is an IBM document describing SDLC.  There
are a series of ISO documents for HDLC (sorry I'm at home and don't have
the references).

>sanjay gururaj

Art Berggreen