1. Data Link Protocols Asynchronous Protocols Synchronous Protocols
Character-Oriented Protocols
Bit-Oriented Protocols

2. Data Link Protocol Categories
A protocol in data communications is the set of rules or specifications used to implement one or more layers of the OSI model.
Data link protocol is a set of specifications used to implement the data link layer

3. Data Link Protocol Categories
Asynchronous protocols
treat each character in a bit stream independently
use start and stop bits to frame the data units
inexpensive
used primarily in modems
Synchronous protocols
take the whole bit stream and chop it into characters of equal size
faster than asynchronous transmission

4. Asynchronous Protocols

5. XMODEM Frame - half-duplex stop-and-wait ARQ protocol
Signal from receiver side
- ACK
- NAK
- CAN (cancel)
SOH –Start of Header (1 byte)
CRC - Cyclic Redundancy Check

6. BLAST (Blocked asynchronous transmission)
YMODEM
similar to XMODEM
Major differences
Data unit = 1024 bytes
CANs = 2
Multiple files can be sent simultaneously
ZMODEM
Combining features of both XMODEM and YMODEM
BLAST (Blocked asynchronous transmission)
More powerful than XMODEM.
Full-duplex
KERMIT
Most widely used asynchronous protocol

7. Synchronous Protocols
Character-oriented protocols
(or byte-oriented protocols)
- the frame or packet is interpreted as a series of characters
Bit-oriented protocols
- the frame or packet is interpreted as a series of bits

8. Character-Oriented Protocols

9. Binary Synchronous Communication (BSC)
- the best-known character-oriented protocol
Supports half-duplex transmission
Uses stop-and-wait ARQ flow control and error correction

10. Data Frame Simple Data Frame BSC Frame with Header SOH – 0000001
SYN –
STX –
ETX –
BSC Frame with Header
SOH –
ITB –
Multiblock Frame

11. Multiframe Transmission
ETB –
ETB – End of Transmission Block
ETX – End of Transmission

12. BSC Control Frames

13. BSC Control Frames cont…

14. Bit-Oriented Protocols
IBM ISO ITU-T IEEE
SDLC – Synchronous Data Link Control
HDLC – High-Level Data Link Control
LAPs – Link Access Protocols
LAN – Local Area Nelwork

15. HDLC
Supports half/full duplex communications over point to point or multipoint links
Characterized by: station types, configuration, response modes
Station Types:
Primary – issues commands
Secondary – issues responses
Combined – issues both command & response

16. HDLC Configuration
HDLC does not support balanced multipoint

17. HDLC Modes Who controls the link
Figure 11-15
HDLC Modes
Who controls the link
NRM – Normal Response Mode ( unbalanced configuration)
ARM – Asynchronous Response Mode
ABM – Asynchronous Balanced Mode
* Mode for symmetrical & balanced can be set using U-frames

18. HDLC Frame Types
Information frame
Supervisory frame
Unnumbered frame

19. HDLC Flag F& FCS field

20. HDLC Address Field

21. HDLC Control Field
Code (S-frame) – carry coded flow and error control information
Code (U-frame) – identify the type of U-frame and its function

23. Use of P/F Field

24. U-Frame Control Field

25. U-Frame Control Field Set NRM Set NRM (extended) Set ARM
Set ARM (extended)
Unnumbered info
Disconnect
Request disconnect
Set initialization mode
Unnumbered poll
Frame reject
Exchange ID
Reset
Request information mode