1. Data Link Protocols ผศ.ดร. อนันต์ ผลเพิ่ม
Asst.Prof. Anan Phonphoem, Ph.D.
Computer Engineering Department
Kasetsart University, Bangkok, Thailand
Protocol: set of rules or specifications used to implement one or more layers of the OSI model
Data Link Protocol: a set of specifications used to implement data link layer
rules –> line discipline, flow control, and error handling

2. Data Link Protocols Asynchronous Protocols Synchronous Protocols
Xmodem
Ymodem
Zmodem
BLAST
Kermit
Asynchronous: treat each character in a bit stream independently
Synchronous: take whole bit stream and chop it into characters of equal size
Character-oriented
Bit-oriented

3. Asynchronous Protocols
Long, long…time ago
Not complex and easy to implement
Slow
Required start/stop bit and space
Now mainly used in modem
 Replaced by high speed synchronous

4. XMODEM frame Half-duplex, stop-and-wait ARQ protocol
1979: designed for FTP via telephone-line between PCs
SOH: Start of Header
TX  start by sending NAK (from receiver)
Sender must wait for ACK before sending next frame
Half-duplex, stop-and-wait ARQ protocol

5. Data Link Protocols Asynchronous Protocols Synchronous Protocols
Xmodem
Ymodem
Zmodem
BLAST
Kermit
Ymodem  data unit changes to 1024 bytes (Xmodem=128)
use CRC16
multiple files accepted
Zmodem  combination of X and Ymodem
BLAST (Blocked Asynchronous Transmission)  better than Xmodem (full-duplex, sliding window flow conrol)
Kermit (Columbia U)  most widely used asyn. Protocol (operation same as Xmodem)
Character-oriented
(Byte-oriented)
Bit-oriented
BSC

6. Synchronous Protocols
Character-oriented protocol
Based on one byte (8-bit)
Use ASCII for control character
Not efficient  seldom used
Bit-oriented protocol
Based on individual bits
One or multiple bits for control
More efficient
Speed  better than asynchronous

7. IBM’s Binary Synchronous Communication (BSC)
Character-oriented protocol
Half-duplex, stop-and-wait ARQ
2 frame types
Data frame
(data transmission)
Control frame
(connect/disconnect and flow/error control)

8. A simple BSC data frame SYN = Synchronous idle = 0010110
SYN : Alert the receiver for the incoming frame
BCC : can be LRC (longitudinal redundancy check) or CRC (cyclic redundancy check)
This simple frame is seldom used
SYN = Synchronous idle =
STX = Start of text =
ETX = End of text =

9. A BSC frame with a header
Header Fields:
address (sender/receiver)
#frame identifier (0/1 for stop-and-wait ARQ)
SOH: start of Header

10. A multiblock frame ITB = Intermediate text block
Probability of error: Frame size increases, error increases  multiple faults occurs  Difficult to detect errors (error cancel each others)
 Message is divided in several blocks
 Each block has STX, ITB and BCC
 Ending with ETX (end of text)
 Error detected, whole frame is discarded (needs retransmission)
 ACK for entire frame
 one frame is entire message
ITB = Intermediate text block

11. Multiframe transmission
“Large Message” is broken down to multiple frame
 need ETB (End of transmission Block)
 need ETX (End of text)
 Half-duplex so ACK 0 and ACK 1 alternately
ETB = End of transmission Block

12. Control frame Note: Control Frame is used to send command
Control frame  not control character
Note: Control Frame is used to send command
* Establish connection
* Maintaining flow & error control
* terminating connection

13. Control frames

14. Control frames

15. Control frames

16. Data Transparency BSC is designed for text message
Now, non-text message (graphics,…)
Problem?
BSC control character problem
Data transparency: should be able to send any data

17. Byte stuffing DLE = data link escape Byte Stuffing 2 activities:
- Defining the transparent text region with DLE
- Preceding any DLE character within the transparent region (extra DLE)
Problem still exist if text = DLE ?
 Insert an addition DLE next to the character (DLE DLE)
DLE = data link escape

18. Data Link Protocols
Asynchronous
Protocols
Synchronous
Protocols
Xmodem
Ymodem
Zmodem
BLAST
Kermit
Character-oriented
(Byte-oriented)
Bit-oriented
BSC

19. Bit-oriented protocol
Represent more information into shorter frame
Avoid the transparency problems

20. Bit-oriented Protocols
SDLC
HDLC
LAPs
LANs
SDLC: Synchronous data link control – IBM
HDLC: High-level data link control – ISO
LAPs : Link access procedure
Most of the bit-oriented protocols are proprietary
Most famous  HDLC

21. HDLC
Support half/full – duplex over point-to-point and multipoint links
HDLC system characterization
Station types
Configurations
Communication modes
Frames

22. HDLC station types Primary station Secondary station Combined station
The station that controls the medium by sending “command”
Secondary station
The station that “response” to the primary station
Combined station
The station that can both command and response

23. HDLC configurations The relationship of hardware devices on a link
3 configurations of all stations (primary/secondary/combined)
Unbalanced
Symmetrical
Balanced

24. HDLC Configurations: Unbalanced (master/slave)
Can be point-to-point (for 2 devices) or multipoint

25. HDLC Configurations: Symmetrical

26. HDLC Configurations: Balanced
Only point-to-point

27. HDLC communication modes
Mode : describe “Who controls the link”
NRM: Normal response mode (master/slave)
ARM: Asynchronous response mode
(secondary can initiate if idle, all transmissions are made to primary station)
ABM: Asynchronous balanced mode (point-to-point equal)
ARM: need to communicate through the Primary (even for secondary to secondary primary)

28. HDLC frame 3 frame types Information frame (I-frame)
Supervisory frame (S-frame)
For ACK, Flow/Error controls
Unnumbered frame (U-frame)
For Mode setting, Initialize, Disconnect

29. HDLC Frame

30. HDLC Frame

31. HDLC Frame: Flag field Flag:  beginning and ending of a frame
 Last flag can be the start of the next flag
Flag  similar to “Control Character”
 problem for transparency !!!  Bit Stuffing

32. Bit Stuffing How to differentiate data and flag?
Adding one extra 0 whenever there are five consecutive 1s in the data

33. HDLC: Bit stuffing
TX more than 5 consecutives “1”  insert (stuffs) one redundant bit “0” after the fifth “1”
Example: 
No Matter that the sixth bit is one or not !
3-Exceptional:
1. it’s really a Flag (6 consecutive “1”)
2. Tx is being aborted (7-14 consecutive “1”)
3. Channel is in Idle state (>= 15 consecutive “1)

34. HDLC frame: Address field
Primary station creates a frame
 destination address
Secondary station creates a frame
 source address
Can be one byte or more

35. HDLC Frame: Address field
One byte = 128 stations (one bit is used for another purpose)
Large network needs multiple byte address

36. HDLC Frame: Control field
N(R)  can be think as “ACK”
if correct  N(R) = next frame seq
else  N(R) = number of damaged frame (need reTx)
In S-Frame  not transmit data, so do not need N(S)
 S-Frame for response (return N(R) )
Code  flow and error control information

37. HDLC frame: Poll / Final
P/F: dual purposes
1) P/F = 0 no meaning (regular data)
2) P/F = 1 means “poll” when send by primary
P/F = 1 means “final” when send by secondary

38. HDLC Frame: Information field

39. HDLC Frame: FCS field
FCS: Frame check sequence

40. HDLC: S-Frame

41. HDLC: Use of P/F field

42. HDLC: Use of P/F field
Piggybacking:
data + ack

43. HDLC: Use of P/F field

44. HDLC: Use of P/F field

45. HDLC: S-Frame Acknowledgement

46. HDLC: S-Frame Positive AcknowledgementRR
Receiver sends “Positive Ack” (no data to send)
N(R) = seq of next frame
RNR
Receiver sends “Positive Ack”
Receiver tells sender that sender cannot send any frame until ‘RR’ frame is received

47. HDLC: S-Frame Negative Acknowledgement
Reject (REJ)
Go-back-n ARQ
N(R) = # of damage frame (and follow)
Selective-Reject (SREJ)
N(R) = # of damage frame

48. HDLC: U-Frame control field
For session management and control information

49. HDLC: U-Frame control field
SNRM : set normal response mode
DISC: Disconnect

50. HDLC: Polling example

51. HDLC: Selecting example

52. HDLC: Peer-to-peer example
SABM: Set asynchronous balanced mode
UA: Unnumbered ack

53. HDLC: Peer-to-peer example