• Asynchronous vs. Synchronous
• Character-oriented vs. Bit-oriented

• normally character-oriented
• each character stands on its own, wrapped in start & stop bits.
• Examples : XMODEM, YMODEM, kermit, others

XMODEM (half-duplex, stop-and-wait) Frame format is:

1. SOH (start of header) byte
2. 2 header bytes
3. 128 data bytes
4. CRC byte

• Either character or bit-oriented
• Frame received as whole, find fields based on offset.
• Examples: BSC, HDLC

• Binary Synchronous Communication (bisynch)
• Character-oriented
• Data frames and control frames
• Half-duplex stop-and-wait
• Can be used to transmit binary data if "byte stuffing" is used
• For byte stuffing, use DLE (datalink escape) to mark start and end of transparent data.
  If data contains DLE bit pattern, stuff a DLE

BSC Data Frame format is:

1. Two SYN bytes
2. Optional SOH (start of header) followed by header
3. STX (start text)
4. Variable amount of data
5. ETX (end text)
6. CRC

• High-level Data Link Control
• Bit-oriented
• Developed by ISO
• Basis for all bit-oriented protocols (including LAN protocols)
• Supports half-duplex or full-duplex sliding window.
• Information (data) frames, supervisory (control) frames, unnumbered (link mgmt) frames
• I-frames, S-frames, and U-frames
• Bit-stuffing required because flag pattern (01111110) can occur
• Bit-stuffing: stuff 0 after 5th consecutive 1.

HDLC Frame format is:

1. Flag (01111110)
2. Address (station address)
3. Control (frame type: I or S or U, flow control info)
4. Data
5. Frame Check Sequence. The CRC.
6. Flag (01111110)

Related Home Pages: notes | CSC 465 | Peter Sanderson | Computer Science | SMSU

Peter Sanderson ( pete@csc.smsu.edu )