Data communication protocols can be asynchronous or synchronous handling respective data formats. Asynchronous protocols are character oriented while synchronous protocols can be either character oriented or bit oriented protocols. Most commonly used Asynchronous character oriented protocol is IBM's asynchronous data link protocol 83B. Most commonly used Synchronous character oriented protocol is IBM's BISYNC which is addressed in this presentation while most common bit oriented portals are SDLC and HDLC which are addressed in next presentation.

1. DATA COMMUNICATION
PROTOCOLS in Centralised
Networks (in Master/slave
environment)

2. DATA COMMUNICATION PROTOCOLS in Centralised Networks
(in Master/slave environment)
✓Set of rules governing orderly exchange of information.
✓LCU controls the flow of data between application programs and
remote terminal.
✓Set of rules LCU follows are called Data Link Protocols.
✓Polling: An invitation from primary to secondary to transmit
message.
✓ A secondary can not poll a primary.
✓Selection: When primary has a data to send to a secondary, it
identifies the secondary by ‘selecting’ it.
✓ It is interrogation of secondary to determine its status
✓ - whether ready to receive a message or not.
✓ Secondary can not select a primary.
✓Secondary transmits to primary only, while primary transmits to
all.

3. Asynchronous Protocols
✓Use asynchronous data format and asynchronous modems.
✓Character oriented protocols.
✓Like ‘end of transmission (EOT)’ and ‘start of text (STX).
✓Character occurring anywhere can not be anything but mean the
action designated to them.
✓ e.g. 04H (EOT) if received by secondary , it clears all operations
and comes into line-monitor mode.
✓Data link characters must never occur in data.
✓Secondary station limited to single terminal/printer pair.
✓Commonly used protocols are:
✓ Bell System’s SELECTIVE CALLING SYSTEM(8A1/8B1).
✓ IBM’s ASYNCHRONOUS DATA LINK PROTOCOLS(83B).

4. Asynchronous Protocols
✓Polling Sequence:
!
!
!
✓3 character sequence.
✓EOT-
✓ a clearing character, clears all actions of secondary.
✓ Precedes all polling sequences.
✓ Brings all secondaries in line monitor mode.
✓DC3 –
✓ Indicates that next character is the polling address of station.
!
✓A , the polled secondary, must respond now.
E
O
T
D
C
3
A ASCII Codes
EOT – End of transmission – 00000100 – 04H
DC3 – Device control 3 -- 00010011 - 13H

5. Acknowledgement sequences to Polling
✓ 1.
!
✓ No message to transmit but ready to receive.
!
!
✓2.
!
✓ No message to transmit, not ready to receive either.
!
!
✓3. Formatted data sent
A
C
K

6. Asynchronous Protocols
✓Selection Sequence:
!
!
!
✓2 character sequence.
✓EOT-
✓ A clearing character, clears all actions of secondary.
✓ Precedes all selection sequences.
✓ Brings all secondaries in line monitor mode.
✓X , the selected secondary, must respond now.
E
O
T
X

7. Acknowledgement sequences to Selection
✓ 1.Ready to receive.
!
!
!
✓ 2.Not ready to receive. Terminal in local or printer mode
!
!
!
✓ 3. Not ready to receive but has a formatted data to transmit.
A
C
K
* *

8. Message format
!
!
!
✓SOH is start of header.
✓Header can include special data e.g. date, time, message number,
message priority till STX is received.
✓STX is start of text.
✓ It terminates header and starts data.
✓STX blinds other stations to the data. Only EOT can bring them to line
monitor mode.
S
O
H
Heading
S
T
X
Message
E
O
T
Message format-

9. Acknowledgement to message
A
C
K
1. Acknowledged.
No error--
N
A
K
2. Not Acknowledged.
Message has error
C
A
N
3. Cancel or abort
transmission

10. Asynchronous Protocols – TYPES
XMODEM
✓By Ward Christiansen in 1979.
✓FTP for telephone line communication between PC’s.
✓Half Duplex.
✓Stop/wait ARQ.
✓Message divided into characters.
✓Each character contains start bit and stop bits.
✓CRC for data only.
✓CAN sent to abort transmission.
SOH CRC
Header 128 bytes data
Start bit Stop bit

11. Asynchronous Protocols – TYPES
YMODEM
✓Same as XMODEM except…
✓Data 1024 bytes.
✓2 CAN sent to abort transmission.
✓2 Bytes CRC-16.
✓Multiple frames can be sent simultaneously.
!
!
!
✓Exercise: Find details for other asynchronous modems and
submit.

12. Synchronous Protocols
!
!
✓Use synchronous data format and synchronous modems.
✓Can be Character oriented protocol or Bit oriented protocol.
✓ Secondary station LCU can serve up to 50 terminal/printer pair.
✓Commonly used protocols are:
✓Character oriented protocol –
✓ e.g. IBM’s 3270 Binary Synchronous Communication
Protocol(BSC or Bisync).
✓Bit oriented protocol –
✓ e.g. IBM’s Synchronous Data Link Communication
Protocol(SDLC).

13. IBM’s Bisync Protocol (BSC)
!
!
✓Point to point or multipoint.
✓Half duplex mode using stop and wait ARQ and flow
control.
✓Does not support full duplex mode or sliding window
protocol.
✓Bisync because sync characters are sent in pair.

14. IBM’s Bisync Protocol (BSC)
!
!
!
!
!
✓PAD in beginning is called leading pad.
✓PAD can be 55H(01010101) or AAH(10101010).
✓Used for CLK recovery at receiver for bit synchronization.
!
✓2 SYNC characters required for CLK recovery at receiver
for character synchronization.
✓SYNC can be 16H in ASCII ( 00010110)
OR 32H in EBCDIC repeated twice.
P
A
D
S
Y
N
S
P
A
S
P
A
S
Y
N
E
O
T
P
A
D
S
Y
N
S
Y
N
P
A
D
E
N
Q
“ “
General
Polling
Sequence

15. IBM’s Bisync Protocol (BSC)
!
!
!
!
!
✓EOT, a clearing character, brings all secondaries to line
monitor mode.
!
✓PAD is all 1’s string, gives enough time to secondaries to
clear their assignments and come to line monitor mode.
!
✓2 SYNC characters available further to synchronize if PAD
disturbs synchronization.
P
A
D
S
Y
N
S
P
A
S
P
A
S
Y
N
E
O
T
P
A
D
S
Y
N
S
Y
N
P
A
D
E
N
Q
“ “
General
Polling
Sequence

16. IBM’s Bisync Protocol (BSC)
!
!
!
!
!
✓SPA will hold secondary station’s polling address in ASCII.
✓Transmitted twice to avoid any spurious sequence
initiating a poll.
✓Each secondary has unique SPA.
✓ “ is used as it is twice to indicate that it is a general poll.
✓Any device connected to secondary can transmit the data.
(keyboard, Scanner etc.)
✓If more than one ready, secondary’s LCU decides the
transmitter.
P
A
D
S
Y
N
S
P
A
S
P
A
S
Y
N
E
O
T
P
A
D
S
Y
N
S
Y
N
P
A
D
E
N
Q
“ “
General
Polling
Sequence

17. IBM’s Bisync Protocol (BSC)
!
!
!
!
!
✓ENQ – request for response – format line turn around
character is an enquiry character.
✓Ending polling sequence, primary now wants a response.
✓H/D transmission.
✓ PAD at end is trailing pad.
✓7FH or DEL character.
✓Ensures that RLSD is kept active long enough for the DTE
to receive all character correctly from DCE.
✓Low RLSD will eliminate carrier and RD pin will be inactive.
P
A
D
S
Y
N
S
P
A
S
P
A
S
Y
N
E
O
T
P
A
D
S
Y
N
S
Y
N
P
A
D
E
N
Q
“ “
General
Polling
Sequence

18. IBM’s Bisync Protocol (BSC)
!
!
!
!
!
✓It is not general poll, hence “ is not used.
✓A specific device with address mentioned in field DA ,
under station with address mentioned under SPA, is
polled.
P
A
D
S
Y
N
S
P
A
S
P
A
S
Y
N
E
O
T
P
A
D
S
Y
N
S
Y
N
P
A
D
E
N
Q
D
A
D
A
Specific
Polling
Sequence

19. IBM’s Bisync Protocol (BSC)
!
!
!
!
✓Station polling address is replaced by station selection
address.
✓Selection is always specific.
✓It is for the station to divert the incoming data to
appropriate device.
P
A
D
S
Y
N
S
S
A
S
S
A
S
Y
N
E
O
T
P
A
D
S
Y
N
S
Y
N
P
A
D
E
N
Q
D
A
!
D
A
Selection
Sequence

20. IBM’s Bisync Protocol (BSC) Acknowledgement to
polling!
!
!
!
!
✓Handshake is negative acknowledgement to polling.
✓It has no formatted data to transmit.
P
A
D
S
Y
N
S
Y
N
E
O
T
P
A
D
1. Handshake

21. IBM’s Bisync Protocol (BSC) Acknowledgement to
polling
!
!
!
!
!
✓Data 128 bytes minimum.
✓BCC is block check sequence.
✓1 byte LRC for ASCII or 2 bytes CRC for EBCDIC.
✓BCC is done between SOH and ETX, excluding the two.
P
A
D
S
Y
N
S
Y
N
S
O
H
header
2. Formatted Message
S
T
X
Data
P
A
D
B
C
C
E
T
X

22. IBM’s Bisync Protocol (BSC) Acknowledgement to
polling
!
!
!
!
!
!
!
✓ Message can be broken into Multiple blocks and accommodated in
one frame.
✓ ITB - intermediate text block.
✓ BCC done on smaller blocks is always preferred.
✓ Finally frame will be terminated by ETX.
Multi-block frame
P
A
D
S
Y
N
S
Y
N
S
O
H
header
S
T
X
Data
S
T
X
B
C
C
I
T
B
Data
P
A
D
B
C
C
E
T
X

23. IBM’s Bisync Protocol (BSC) Acknowledgement to
polling
!
!
!
!
!
!
!
!
✓If message too long to accommodate in one frame.
✓Multiple frames transmitted.
✓All frames except last, will be terminated in ETB.( More to come)
✓Last frame terminated in ETX.(End of message)
Multi-frame block
P
A
D
S
Y
N
S
Y
N
S
O
H
header
S
T
X
Data
S
T
X
B
C
C
I
T
B
Data
P
A
D
B
C
C
E
T
B
P
A
D
S
Y
N
S
Y
N
S
O
H
header
S
T
X
Data
S
T
X
B
C
C
I
T
B
Data
P
A
D
B
C
C
E
T
X
ACK
More frames and ACK…

24. IBM’s Bisync Protocol (BSC) Acknowledgement to
selection
!
!
!
!
!
!
✓ Ready to receive first frame using ACK0.
✓ DLE is data link escape character.
✓ DLE 0 will be taken as number 0.
✓ ASCII for ACK0 is DLE and 0
P
A
D
S
Y
N
S
Y
N
D
L
E
0
1. Positive
acknowledgement
P
A
D

25. IBM’s Bisync Protocol (BSC) Acknowledgement to
selection
✓
2.Negative acknowledgement- not ready to
receive
P
A
D
S
Y
N
S
Y
N
N
A
K
P
A
D
P
A
D
S
Y
N
S
Y
N
D
L
E
3.Reverse Interrupt (RVI) – Not ready to receive,
but has urgent message to transmit.
<
P
A
D

26. IBM’s Bisync Protocol (BSC) Acknowledgement to
DATA
!
!
!
!
!
!
!
!
!
!
✓Stop and wait error control.
✓DLE0 for all even frame correct reception.
✓DLE 1 for all odd frames correct reception.
1.Positive acknowledgement- No error
P
A
D
S
Y
N
S
Y
N
D
L
E
0
P
A
D
P
A
D
S
Y
N
S
Y
N
D
L
E
1
P
A
D
or
2. Negative acknowledgement- Error
P
A
D
S
Y
N
S
Y
N
N
A
K
P
A
D

27. IBM’s Bisync Protocol (BSC) Some more on BISYNC
1.Point to point
connection asked
S
Y
N
S
Y
N
E
N
Q
2.Wait and ACK- ACK but not
ready to receive more.
S
Y
N
S
Y
N
W
A
C
K
3.Temporary delay S
Y
N
S
Y
N
T
T
D
4.Connection
termination
S
Y
N
S
Y
N
E
O
T

28. TRANSPARENCY
✓LCU look-alike characters appearing in data force the LCU to go
into control mode.
✓Data 03H appearing in data block will be taken as ETX and block will
be terminated at receiver.
✓LCU is made transparent to data by DLE(Data link escape).
✓LCU will not search for data link control characters in the block of
data encapsulated by 2 DLE’s.
✓DLE STX places LCU in transparent mode.
✓DLE ETX, DLE ITB, and DLE ETB brings LCU out of transparent
mode.
✓DLE DLE if ASCII of DLE appears in data.
✓DLE SYN used only with transparent messages more than 1second
long. Two SYN characters to ensure character synchronization at
receiver.