Specifications

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Communication Overview
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it then establishes communication parameters that define
instructions for processing given bit sequences. Finally, the actual
data will be transmitted, and then followed by several frames
that validate the transmission. BiSync transmission is also
governed by a strict set of rules for data transmission. These
rules require frequent handshaking and validation--speaking in
sentences rather than paragraphs between pauses. As a result of
the extensive handshaking, and because communication can take
place in only one direction, BiSync communication is best suited
for low-speed applications.
Bit-Oriented Synchronous Protocols
In bit-oriented protocols, data is accepted as a long string of bits
whose order does not impart specific instructions to the receiver.
Data is flagged by a set bit pattern at either end, and is validated
by a single frame check sequence at the end of the message that
either accepts it or demands retransmission. Any bits received
outside of valid flag sequences are ignored as placeholders.
Clearly, bit-oriented communication requires considerably less
overhead than byte-oriented communication, because it is not
constantly attempting to match bit sequences to numerous
predetermined arrangements. The only sequence the bit-oriented
protocol is concerned with identifying is the flag sequence. Bit
protocols have other advantages over byte protocols as well. In
a byte-oriented system, because of the constant handshaking,
communication can take place in only one direction at a time.
In bit-oriented systems, both ends can talk to each other at once,
enabling effective use of full-duplex networks. Further, a single
master device using bit protocols can communicate with multiple
slave devices by using an address field following the start of
message flag. This address field is tailored to each individual
slave, and slaves only process data that is specifically addressed
to them. Likewise, when the master receives from the slave, it
knows from precisely where the transmission originated.
The two main bit-oriented protocols used today are Synchronous
Data Link Control (SDLC) and High-Level Data Link Control (HDLC).
SDLC, which was developed by IBM in the 1970s, is based around
a network of primary and secondary network nodes. The primary
controls the network and continually polls the secondaries to
determine whether they have data to transmit. Four
configurations are available for SDLC networks:
Point to Point: one primary connected to one secondary
Multipoint: one primary connected to multiple secondaries
Loop: one primary connects to the first and last secondary,
and secondaries in the middle pass messages through
each other to the primary.
Hub go-ahead: Uses one inbound and one outbound channel.
The primary sends on the outbound channel and the
secondaries send on the inbound channel. Secondaries pass
messages through each other back to the primary.
Key to SDLC communication is the control characters it uses to
maintain data integrity. The figure below shows the six fields
that comprise a single SDLC data frame.
The Flag field starts and ends error checking. The Address field
is used to indicate the intended data destination, and can
be a single address, a group of addresses, or a broadcast
to the entire network. The Data field is the information
being transmitted, and the Frame Check Sequence (FCS)
is generally a Cyclic Redundancy Check (CRC) calculation.
A calculation on the transmitted data is done by the
transmitter and the result is sent in the FCS. This calculation is
then performed by the receiver after data transmission is complete.
If the results don't match, an error is assumed.
The Control field uses three different formats depending on the
type of SDLC frame. The diagram below breaks out the different
data bits in the control field. Explanations for the three control
formats follow on the next page.
Flag Address Control Data
Frame
Check
Sequence
Flag
(SDLC Data Frame)
Control Field (1 or 2 bytes)
Information Frame Format
Supervisory Frame Format
Unnumbered Frame Format
Receive
Sequence
Number
Receive
Sequence
Number
Function
Code
Poll
Final
Poll
Final
Poll
Final
Send
Sequence
Number
Function
Code
Function
Code
11
01
0
(Control Field Formats for SDLC Frames)