- BISYNC uses the
ASCII code:
- If a node that is
idle
(= has no frame to send), will transmit:
000101100001011000010110... SYNC SYNC SYNC .... (00010110 = 16 Hex = Synchronous Idle --- See: ASCII table)
- Note:
- There is always some
signal (= voltage) on
a communication medium
(e.g., copper wire)
- 0 volts is still
a
value
- In fact, 0 volt is low and will convey some meaning
- 0 volts is still
a
value
- Therefore:
- it is impossible not to transmit anything at all !!!
- To perform the "transmit nothing"
function,
the sender
transmits a serie of
null frames:
000101100001011000010110... Interpretation: 00010110"NULL frame"00010110"NULL frame"00010110...
- There is always some
signal (= voltage) on
a communication medium
(e.g., copper wire)
- A node will
transmit
a frame
as follows (= protocol):
- Transmits the
Start-of-Header (SOH)
code
- Then transmit the
header data
The header data contains:
- The sender ID
- The receiver ID
- Other meta data (such as priority, etc)
- Then transmit the
Start-of Text (SOT) code
- Then transmit the
text (= "data")
The text portion contains:
- The user's data
- The BISYNC frame is
ended by the
End-of Text (EOT) code
- A CRC check sum is appended after the BISYNC frame
- Transmits the
Start-of-Header (SOH)
code
- The behavior of
a sender using
BISYNC is
as follows:
- Problematic case:
- What if the
header/message
itself contains a
special character, like
a End-of-Text (00000011)
character:
This situation will cause this error:
- The special character EOT in the message will terminate the frame prematurely !!!
- What if the
header/message
itself contains a
special character, like
a End-of-Text (00000011)
character:
- Solution: use
character stuffing
- A special (= control) character
SPEC is
transmitted as
follows:
DLE SPEC
- The DLE character is the
Data Link Escape character and
it's character code
is 00010000
- The DLE character
is also a
special character !!!
- To transmit the
DLE character
as
normal data, we must
transmit the following:
DLE DLE
- The DLE character
is also a
special character !!!
The BISYNC frame using character stuffing has the following format:
- The BISYNC frame
is as follows:
SYN SYN DLE SOH 00010110 00010110 00010000 00000001 Header bytes .... DLE STX .... 00010000 00000010 Message bytes .... DLE ETX CRC bytes .... 00010000 00000011 cccccccc cccccccc
- A special (= control) character
SPEC is
transmitted as
follows:
- Example:
- Suppose a
BISYNC frame contains
the following
information:
Header bytes: 00000001 00001111 Message bytes: 00010000 00000011 CRC code: 10101010 10101010
- The sender will
transmit the following
BISYNC frame:
SYN SYN DLE SOH Header DLE STX 00010110 00010110 00010000 00000001 00000001 00001111 00010000 00000010 Message DLE ETX CRC bytes 00010000 00010000 00000011 00010000 00000011 10101010 10101010Note:
- We must use 00010000 (DLE) 00010000 (DLE) to transmit the data byte 00010000 (DLE) !!!
- Suppose a
BISYNC frame contains
the following
information:
- NOTE:
- BISYNC Wikipedia page:
click here
- The BISYNC protocol is
no longer used in
today's networks... (defined in 1967 !)
- BISYNC Wikipedia page:
click here