Data Link Protocols
• Specifications to implement data link layer
• Asynchronous Protocols:
– Primarily used in modems
– Feature start and stop bits and variable length
gaps between characters
• Synchronous Protocols:
– Developed for higher speed networks
– No start and stop bits, much lower overhead,
faster transmission
Winter 2006
ECE
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Computer Interfacing and Protocols
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Synchronous Protocols
• Character-oriented protocols:
– Frames are interpreted as a sequence of characters
– Example: Binary Synchronous Communication (BSC)
• Character count-oriented protocols:
– Frame length specified within the header
– Example: Digital Data Communication Message
Protocol, Count Type (DDCMP)
• Bit-oriented protocols:
– Frames are interpreted as a sequence of bits
– Example: High-Level Data Link Control (HDLC)
Winter 2006
ECE
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Computer Interfacing and Protocols
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BSC
• Half duplex, can be used with ASCII,
EBCDIC, and Six Bit Transcode
• Control information is in the form of code
words taken from the character set
• Control information is carried in separate
frames as well as within data frames
– Line control, flow control, error control
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ECE
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BSC
• Data Frames:
S
Y
N
S
Y
N
S
O
H
Header
S
T
X
…Data…
E
T
X
B
C
C
B
C
C
– SYN = (0)0010110 (ASCII 268)
– Header:
• Address, sequence number for stop-and-wait ARQ
• Non-standardized
– BCC: Block Check Character
• 1-character LRC
• 2-character CRC
Winter 2006
ECE
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BSC
• Multiblock frame:
Block
Block
S S S
Y Y O
N N H
Header
S
T
X
I
T
B
B
C
C
B
C
C
S
T
X
…Data…
I
T
B
B
C
C
B
C
C
S
T
X
…Data…
I
T
B
B
C
C
B
C
C
S
T
X
…Data…
…Data…
E
T
X
B
C
C
B
C
C
…Data…
E
T
B
B
C
C
B
C
C
…Data…
E
T
X
B
C
C
B
C
C
• Multiframe transmission:
S
Y
N
S
Y
N
S
O
H
B
C
C
B
C
C
A S
C1 Y
K N
S
Y
N
S
Y
N
S
O
H
B
C
C
B
C
C
A S
C0 Y
K N
Winter 2006
ECE
Header
S
T
X
S
Y
N
Header
S
T
X
S
Y
N
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Computer Interfacing and Protocols
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BSC
• Control Frames:
– Frames used only for signaling
• SYN SYN {Control Characters} BCC
– Connection establishment
(bid, poll, select, positive and negative
responses)
– Connection termination (end of transmission)
– Flow and error control
(ACK0, ACK1, NAK, WACK (wait and ACK),
reverse interrupt, temporary delay)
Winter 2006
ECE
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Computer Interfacing and Protocols
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BSC
• Data Transparency
– When sending binary data, certain sequences
may be identical to control characters
– Solution: Define transparent blocks and use
character stuffing
– Start transparent text block with DLE STX
– End transparent text block with DLE ETX,
DLE ITB, DLE ETB
– To send DLE character, send DLE DLE
Winter 2006
ECE
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Computer Interfacing and Protocols
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DDCMP
• Character-count-oriented protocol
• Instead of using control characters to mark
the end of the frame, send the length of
the frame
• Can also be used asynchronously
• What happens when header is damaged?
S
Y
N
Winter 2006
ECE
S
Source
Y HEADER
Dest.
N
MSG
Byte
Count
C
R
C
…Data…
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Computer Interfacing and Protocols
C
R
C
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HDLC
• Bit-oriented protocol
• Special bit pattern to signal beginning and end
of frame (01111110)
• This pattern cannot be repeated in the frame
• Solution: Bit stuffing
– Transmitter automatically stuffs dummy 0 after five 1s
– If after five 1s the sequence 10 is received, it is the
reserved pattern
– If less than 15 1s are received, it signals abort,
otherwise idle channel
Flag Address Control
Winter 2006
ECE
Variable length data
ECE 766
Computer Interfacing and Protocols
Check Flag
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