CLASSIFICATONS OF DLCP
• CHARACTER ORIENTED PROTOCOLS
– Interpret a frame of data as a group of successive
bits combined into predefined patterns of fixed
length bits each representing a unique character
– Data frames for these protocols include control
characters that convey important information
pertaining to DLCP functions
– Also called BYTE ORIENTED Protocols
– Examples: X-Modem, Y-Modem, ZModem, BLAST, IBM 83B, IBM BSC

2

• BIT-ORIENTED PROTOCOLS
– A discipline for serial bit-by-bit information
transfer over a data communications channel
– Information is transferred as a series of successive
bits that may be interpreted individually on a bitby-bit basis or in groups of several bits as opposed
to a fixed length group of bits
– Examples: SDLC, HDLC


3

• ASYNCHRONOUS PROTOCOLS
– Simple, character oriented protocols generally
used on two-point networks using asynchronous
data and asynchronous modems
– Examples: X-MODEM, Y-MODEM, IBM 83B

• SYNCHRONOUS PROTOCOLS
– Protocols used on point to multi-point networks
using synchronous data and synchronous modems
– Examples: IBM BSC, SDLC, HDLC

4

ASYNCHRONOUS DLCP
• XMODEM
– Developed by Ward Christiansen in 1979
– The first file transfer protocol (FTP) designed as an
asynchronous, character-oriented protocol to
facilitate transferring data between two personal
computers
– Specifies a half-duplex stop-and-wait protocol
using a data frame comprised of four fields


5

ASYNCHRONOUS DLCP
• XMODEM PROCEDURAL SEQUENCE
– Destination station sends a negative acknowledgement (NAK)
to the source station
– The source sends the first data frame and waits for an
acknowledgement
– The destination receives and evaluates the data frame from
the source. It responds with an ACK if there is no error or a
NAK if an error is detected
– If the source receives an ACK it sends the next frame. If it
receives a NAK it retransmits the previous frame. If it does
not receive an ACK or a NAK after a predetermined length of
time, it times out and retransmits the previous frame
– A single cancel (CAN) character may be used to abort an
ongoing transmission

6

ASYNCHRONOUS DLCP
• XMODEM FRAME FORMAT
S
O
H

8
8
Bits Bits

8
Bits

8
Bits

Start of
Heading
Header Field
(Sequence Number with
Complementary Redundancy)

Data Field
128 bytes
Fixed length


C
R
C
Error
Detection
(8 bits)

7

ASYNCHRONOUS DLCP
• YMODEM
– Similar to XMODEM except for the following:
1. The information field has a capacity of 1024 bytes.
2. Two CAN characters are required to abort a
transmission.
3. ITU-T-CRC 16 is used to calculate the frame check
sequence.
4. Multiple frames can be sent in succession and then
acknowledged with a single ACK or NAK characters
(Sliding window flow control)


8

SYNCHRONOUS DLCP
• BINARY SYNCHRONOUS COMMUNICATION (BSC)
– A synchronous character-oriented data-link protocol
developed by IBM
– Also called bisync or bisynchronous communication
– Each data transmission is preceded by a unique
synchronization character sent twice


9

SYNCHRONOUS DLCP
• BSC FRAME FORMAT

S
Y
N

S
Y
N

Message

Bisync Field
Synchronization Characters
Transmitted in Pairs

• Poll
• Selection
• Message
• Acknowledgement

• ASCII
• EBCDIC


: 16H (00010110)
: 32H (00110010)

10

BSC POLLING SEQUENCES
• GENERAL POLLING SEQUENCE
P
A
D

S
Y
N

S
Y
N

E
O
T

P
A
D

S
Y
N

• PAD. Pad
• Leading Pad (55H or AAH)
• Trailing Pad (FFH)
• EOT. End of Transmission
• ENQ. Inquiry (Line Turnaround)

S
Y
N

S
P
A

S
P
A

“

“

E
N
Q

P
A
D

• SYN. Synchronization Character
• SPA. Station Polling Address
• “. General Poll


11

• SPECIFIC POLLING SEQUENCE
P
A
D

S
Y
N

S
Y
N

E
O
T

P
A
D

S
Y
N

• PAD. Pad

S
Y
N

S
P
A

S
P
A

D
A

D
A

E
N
Q

P
A
D

• SPA. Station Polling Address
• DA. Device Address


12

• POLLING SEQUENCE NEGATIVE ACKNOWLEDGEMENT
P
A
D

S
Y
N

S
Y
N

E
O
T

P
A
D

• PAD. Pad


13

BSC SELECTION SEQUENCES
• SELECTION SEQUENCE
P
A
D

S
Y
N

S
Y
N

E
O
T

P
A
D

S
Y
N

• PAD. Pad

S
Y
N

S
S
A

S
S
A

D
A

D
A

E
N
Q

P
A
D

• SSA. Station Selection Address
• DA. Device Address


14

• SELECTION SEQUENCE POSITIVE ACK
P
A
D

S
Y
N

S
Y
N

D
L
E

0

P
A
D

• PAD. Pad
• DLE. Data Link Escape

15

• SELECTION SEQUENCE NAK (RVI. Reverse Interrupt)

P
A
D

S
Y
N

S
Y
N

D
L
E

6

P
A
D

• PAD. Pad
• DLE. Data Link Escape

16

BSC MESSAGE SEQUENCES
• MESSAGE SEQUENCE
P
A
D

S
Y
N

S
Y
N

S
O
H

Heading

• PAD. Pad
• SOH. Start of Heading
• STX. Start of Text

S
T
X

Message

E
T
X

B
C
C

P
A
D

• ETX. End of Text
• ETB. End of Text Block
• May be used in place of ETX
• BCC. Block Check Character
• LRC for ASCII
• CRC-16 for EBCDIC


17

• MESSAGE SEQUENCE POSITIVE ACKNOWLEDGEMENT
P
A
D

S
Y
N

S
Y
N

D
L
E

P
A
D

S
Y
N

S
Y
N

D
L
E

0

P
A
D

FOR EVEN-NUMBERED
BLOCKS

1

P
A
D

FOR ODD-NUMBERED
BLOCKS


18

• MESSAGE SEQUENCE NEGATIVE ACKNOWLEDGEMENT
P
A
D

S
Y
N

S
Y
N

N
A
K

P
A
D

• PAD. Pad
• NAK. Negative Acknowledgement


19

BSC TRANSPARENCY
• A process that disables the BSC controller’s
capability to identify data link control characters
• Used for communication with devices that use
non-ASCII and non-EBCDIC codes such as those in
microprocessor controlled security alarm systems
or environmental monitoring systems
• Transparent mode is activated when the STX
character is preceded by a DLE


20

OTHER DLE APPLICATIONS
1. DLE ETB. Used to terminate all blocks of data
except the final block
2. DLE ITB. Used to terminate blocks of transparent
text other than the final block when ITB (end of
intermittent block) is used for block-terminating
character
3. DLE SYN. With bisync, two SYN characters are
inserted in the text in messages lasting more than
one second to ensure that the receive controller
maintains character synchronization

21

BSC CHARACTER CODES
Character
ACK0 / ACK1 (Alternating
positive
acknowledgements)

Hex
1070
1061

DISC (Disconnect)

1037

DLE (Data Link Escape)

10

ENQ (Enquiry)

2D

Use
Indicates that text was received without errors. Also is a positive response
(acknowledgement) to an ENQ (bid) on a point-to-point line. In 3780
Bisynchronous Communications, this positive acknowledgement alternates
between 1070 (even) and 1061(odd).
Indicates a mandatory disconnect on a switched line connection. The station
sending this message will disconnect without having to receive confirmation
from the receiving station.
Changes the meaning of the control character immediately following the DLE.
For example, the sequence DLE EOT indicates that both stations on a
switched connection should disconnect from the line.
Has the following uses:
-- Initiates (bids for) control of transmission on a point-to-point connection
-- Indicates the end of a polling or address sequence on a multi-point
connection
-- Indicates a request for a station to repeat its response (received from target
station)
-- Indicates that a block of text should be ignored (if the blocks ends with a
ENQ)
-- Indicates the end of an identification sequence on a switched connection

22

BSC CHARACTER CODES
EOT (End Of Transmission) 37

Indicates the end of a transmission for the sending system. This is also sent
when the sending system exhausts its retry limit of sending an ENQ to a
remote system without a response.

ETB (End Of Text Block)

26

Indicates the end of a block of data, but not the last block transmitted (more
data will be coming from the station that transmits this).

ETX (End Of Text)

03

Indicates the end of a message if ETBs are used (text is transmitted in
blocks). Also can indicate the end of the last block of text transmitted
(completed text transmission)

ITB (Intermediate Text
Block)

1F

Divides a block of text into smaller groups of text (Intermediate Text Blocks)
for error checking.

NAK

3D

Indicates a Not Ready condition (most common), or indicates that an error
occurred. Also is sent as a negative response to a line bid or switched line
security identification.

Pad (Leading Pad
Character)

55 or At least 2 leading pad characters are transmitted to synchronize the clocks at
AA
the transmitting and receiving stations.

Pad (Trailing Pad
Character)

FF

A trailing pad character ensures that all bits of the last character of a
message are transmitted.

23

BSC CHARACTER CODES
RVI (Reverse Interrupt)

107C

Transmitted by the receiving station rather than an ACK to indicate a positive
acknowledgement and to request that the transmitting station stop
transmitting so that the receiving station can transmit. (some people may
refer to this as a "turnaround request")

SOH (Start Of Header)

01

Indicates the start of heading information.

STX (Start Of Text)

02

Indicates the start of text.

SYN (Sync)

32

Establishes and maintains character synchronization.

TTD (Temporary Text
Delay)

022D

Indicates to the receiving station that the transmitting station is temporarily
not ready to transmit. (station that transmits this is having an issue)

WACK (Wait Before
Transmitting)

106B

Transmitted by the receiving station to indicate to the transmitting station a
positive acknowledgement and a temporary not-ready-to-receive condition
(this is usually recovered by the application at which point the transmission
continues).


24 (END)

Data link control protocol(2)

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