1. Computer Interfacing and Protocols
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
ECE 766
Computer Interfacing and Protocols

2. 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
ECE 766
Computer Interfacing and Protocols

3. Computer Interfacing and Protocols
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
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols

4. Computer Interfacing and Protocols
BSC
Data Frames:
SYN = (0) (ASCII 268)
Header:
Address, sequence number for stop-and-wait ARQ
Non-standardized
BCC: Block Check Character
1-character LRC
2-character CRC
S Y N
S O H
S T X
E T X
B C C
Header
…Data…
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols

5. Computer Interfacing and Protocols
BSC
Multiblock frame:
Multiframe transmission:
S Y N
S O H
S T X
I T B
B C C
Header
…Data…
E T X
Block
S Y N
S O H
S T X
I T B
B C C
Header
…Data…
E T B
A C1 K
S Y N
S O H
S T X
I T B
B C C
Header
…Data…
E T X
A C0 K
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols

6. Computer Interfacing and Protocols
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
ECE 766
Computer Interfacing and Protocols

7. Computer Interfacing and Protocols
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
ECE 766
Computer Interfacing and Protocols

8. Computer Interfacing and Protocols
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
HEADER
Source Dest.
MSG Byte Count
C R C
…Data…
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols

9. Computer Interfacing and Protocols
HDLC
Bit-oriented protocol
Special bit pattern to signal beginning and end of frame ( )
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
Variable length data
Check
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols