Intel MCS48 User Manual page 350

8273
A
BRIEF DESCRIPTION
OF
HDLC/SDLC
PROTOCOLS
General
The High
Level Data Link Control
(HDLC)
is
a standard
communication
link
protocol established
by
International
Standards Organization
(ISO).
HDLC
is
the discipline
used
to
implement ISO
X.25 packet switching systems.
The Synchronous
Data Link Control
(SDLC)
is
an
IBM
communication
link
protocol used
to
implement
the
System Network
Architecture (SNA).
Both the protocols
are
bit
oriented,
code
independent,
and
ideal for
full
duplex communication.
Some common
applications
include terminal
to terminal,
terminal
to
CPU,
CPU
to
CPU,
satellite
communication, packet switching and
other
high
speed
data
links. In
systems which
require
expensive
cabling
and interconnect hardware, any
of the
two
protocols could be used
to simplify interfacing (by
going
serial),
thereby reducing interconnect
hardware
costs.
Since both the protocols are speed independent, reducing
interconnect hardware could
become
an important
application.
Network
In
both
the
HDLC
and
SDLC
line
protocols,
according
to a
pre-assigned hierarchy, a
PRIMARY
(Control)
STATION
controls the
overall
network
(data
link)
and
issues
commands
to
the
SECONDARY
(Slave)
STATIONS. The
latter
comply
with instructions
and respond by sending
appropriate
RESPONSES.
Whenever
a transmitting
station
must end
transmission prematurely
it
sends an
ABORT
character.
Upon
detecting
an abort character, a
receiving station ignores the transmission block called a
FRAME. Time
fill
between frames can be accomplished by
transmitting either
continuous frame preambles
called
FLAGS
or
an
abort character.
A
time
fill
within a
frame
is
not permitted.
Whenever
a station
receives a
string of
more
that
fifteen
consecutive ones,
the station
goes
into
an
IDLE
state.
Frames
A
single
communication element
ist;alled
a
FRAME
which
can be used
for
both Link Control
and
data transfer
purposes.
The
elements
of
a
frame
are the
beginning
eight
bit
FLAG
(F)
consisting of
one
zero, six
ones,
and
a zero,
an
eight
bit
ADDRESS
FIELD
(A),
an
eight
bit
CONTROL
FIELD
(C),
a variable
(N-bit)
INFORMATION
FIELD
(l),
a
sixteen
bit
FRAME CHECK SEQUENCE
(FCS),
and an
eight
bit
end
FLAG
(F),
having the
same
bit
pattern as the
beginning
flag. In
HDLC
the
Address
(A)
and
Control
(C)
bytes are extendable.
The
HDLC
and
the
SDLC
use
three
types
of
frames;
an
Information
Frame
is
used
to transfer
data, a
Supervisory
Frame
is
used
for control
purposes,
and
a
Non-sequenced Frame
is
used
for initialization
and
control of the
secondary
stations.
Frame
Characteristics
An
important characteristic of a
frame
is
that
its
con-
tents are
made
code transparent
by use
of
a zero
bit
insertion
and
deletion
technique. Thus, the user can adopt
any
format
or
code
suitable for his
system
—
it
may
even
be a
computer word
length or a
"memory
dump".
The
frame
is bit
oriented that
is,
bits,
not characters
in
each
field,
have
specific
meanings.
The Frame Check
Sequence
(FCS)
is
an error detection
scheme
similar to
the Cyclic
Redundancy Checkword (CRC)
widely used
in
magnetic
disk
storage
devices.
The
Command
and
Response
information
frames contain
sequence numbers
in
the control
fields identifying
the sent
and
received
frames.
The sequence numbers
are
used
in
Error
Recovery Procedures (ERP) and
as
implicit
acknowledge-
ment
of
frame communication, enhancing
the
true
full-
duplex nature
of
the
HDLC/SDLC
protocols.
In
contrast,
BISYNC
is
basically
half-duplex (two
way
alternate)
because
of
necessity
to
transmit
immediate
acknowledgement
frames.
HDLC/SDLC
therefore
saves
propagation delay times
and have
a
potential of
twice the
throughput
rate of
BISYNC.
It
is
possible to
use
HDLC
or
SDLC
over
half
duplex
lines
but there
is
a
corresponding
loss
in
throughput because
both are primarily
designed
for full-duplex
communi-
cation.
As
in
any synchronous
system, the
bit
rate
is
determined by
the clock
bits
supplied by the
modem,
protocols themselves
are
speed independent.
A
byproduct
of
the use of zero-bit insertion-deletion
technique
is
the non-return-to-zero invert
(NRZI) data
transmission/reception
compatibility.
The
latter
allows
HDLC/SDLC
protocols
to
be used with
asynchronous
data
communication hardware
in
which the clocks are
derived
from
the
NRZI encoded
data.
References
IBM
Synchronous Data Link Control General
Information,
IBM,
GA27-
3093-1.
Standard Network Access Protocol
Specification,
DATAPAC,
Trans-
Canada
Telephone System
CCG111
Recommendation
X.25,
ISO/CCITT March
2,
1976.
IBM
3650
Retail
Store
System Loop
Interface
OEM
Information,
IBM,
GA
27-3098-0
Guidebook
to
Data Communications, Training Manual, Hewlett-Packard
5955-1715
IBM
Introduction
to
Teleprocessing, IBM,
GC
20-8095-02
System Network
Architecture,
Technical Overview, IBM,
GA
27-3102
System Network
Architecture
Format and
Protocol,
IBM
GA
27-3112
OPENING
FLAG(F)
ADDRESS
FIELD
(A)
CONTROL
FIELD
(C)
INFORMATION
FIELD
(I)
FRAME CHECK
SEQUENCE
(FCS)
CLOSING
FLAG
(F)
01111110
8 BITS
8 BITS
VARIABLE LENGTH
(ONLY
IN
I
FRAMES)
16
BITS
01111110
Figure
1.
Frame Format
8-60
00743A