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A BRIEF DESCRIPTION OF HDLC/SDLC
PROTOCOLS
General
The High Level Oata Link
Control
(HOLC)
is
a standard
communication
link protocol established by International
Standards Organization (ISOL HOLC is the discipline
used
to
implement ISO X.25 packet
switching
systems.
The Synchronous
Oata Link Control (SOLC)
is
an·
IBM
communication link protocol
used
to
implement the
System Network Architecture
(SNAL 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), ttlereoy reducing interconnect hardware
cosis.
Since both the protocols are speed independent, reducing
interconnect hardware
could become
an
important
application.
Network
In both the HOLC and SOLC line protocols, according to a
pre-ass.igned hierarchy, a
PRIMARY (Control) STATION
controls
the overall network (data link) and issues
commands
to
the SECONOARY (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
IOLE state.
Frames
A single communication element
is
called 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 AOORESS FIELO (A),
an
eight bit CONTROL
FIELO
(C),
a variable (N-bit) INFORMATION FIELO (I), a
sixteen bit FRAME CHECK SEQUENCE (FCS), and
an
eight
bit end FLAG
(F),
having the same bit pattern
as
the
beginning
flag. In HOLC the Address (A) and Control (C)
bytes are
extendable. The HOLC and the SOLC use three
types
of
frames;
an
I nformation 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
cUlnrnunication,
enhancing the
tiue
fu:l-
duplex
nature
of
the HOLC/SOLC protocols.
I
n contrast, BISYNC is basically half-duplex (two way
alternate) because
of
necessity to transmit immediate
acknowledgement frames.
HOLC/SOLC therefore saves
propagation
delay times and have a potential
of
twice
the
throughput
rate
of
BISYNC.
It
is
possible
to
use HOLC
or
SOLC 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
HOLC/SOLC 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
Genera/Information,
IBM,
GA27-
3093-1.
Standard
Network
Access
Protocol
Specification,
DATAPAC,
Trans-
Canada
Telephone
System
CCGlll
Recommendation
X.25.
ISO/CCITT
March
2.
1976.
IBM
3650
Retail
Store
SysternLoop
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
III
FRAME
CHECK
SEQUENCE
(FCS)
CLOSING
F1.AG
(F)
01111110
8 BITS 8 BITS
VARIABLE LENGTH
16BITS
01111110
(ONLY IN I FRAMESI
Figure
1,
Frame Format
9-68