1. Chapter 11 Data Link Control and Protocols

2. 11장 Data Link Control and Protocols
11.1 Flow and Error Control
11.2 Stop-and-Wait ARQ
11.3 Go-Back-N-ARQ
11.4 Selective Repeat ARQ
11.5 HDLC

3. Flow Control
Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.
Flow control is the regulation of the sender’s data rate so that the receiver buffer does not become overwhelmed.

4. Error Control
Error control in the data link layer is based on automatic repeat request, which is the retransmission of data.
Error control is both error detection and error correction.

5. Stop and Wait ARQ
In Stop-and-Wait ARQ, the sender sends a frame and waits for acknowledgment from the receiver before sending the next frame.

6. Stop-and-Wait ARQ
Normal Operation

7. Stop-and-Wait ARQ
Stop-and-Wait ARQ Lost Frame

8. Stop-and-Wait ARQ
Stop-and- Wait ARQ Lost ACK Frame

9. Stop-and-ARQ
In Stop-and-Wait ARQ, numbering frames prevents the retaining of duplicate frames.

10. Stop-and-Wait ARQ, delayed ACK

11. Stop-and-Wait ARQ, delayed ACK
Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost.

12. Piggybacking meaning combining data to be sent and acknowledgment
of the frame received in one single frame

13. Go-Back-N ARQ
Sender sliding window

14. Go-Back-N ARQ
Receiver sliding window

15. Go-Back-N ARQ
Control variables

16. Go-Back-N ARQ
Go-Back-N ARQ, normal operation

17. Go-Back-N ARQ
Go-Back-N ARQ, lost frame

18. Go-Back-N ARQ
Go-Back-N ARQ: sender window size

19. Go-Back-N ARQ
In Go-Back-N ARQ, the size of the sender window must be less than 2m; the size of the receiver window is always 1.

20. Selective-Repeat ARQ
Selective Repeat ARQ, sender and receiver windows

21. Selective-Repeat ARQ
Selective Repeat ARQ,
lost frame

22. Selective-Repeat ARQ
In Selective Repeat ARQ, the size of the sender and receiver window must be at most one-half of 2m.

23. Selective-Repeat ARQ
Selective Repeat ARQ, sender window size

24. Example 1 Solution Example
In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What is the bandwidth-delay product? If the system data frames are 1000 bits in length, what is the utilization percentage of the link?
The bandwidth-delay product is a measure of the number of bits
a system can have in transit.
Solution
The bandwidth-delay product is
1  106  20  10-3 = 20,000 bits
The system can send 20,000 bits during the time it takes for the data to go from the sender to the receiver and then back again. However, the system sends only 1000 bits. We can say that the link utilization is only 1000/20,000, or 5%. For this reason, for a link with high bandwidth or long delay, use of Stop-and-Wait ARQ wastes the capacity of the link.

25. Example 2 Solution Example
What is the utilization percentage of the link in Example 1 if the link uses Go-Back-N ARQ with a 15-frame sequence?
Solution
The bandwidth-delay product is still 20,000. The system can send up to 15 frames or 15,000 bits during a round trip. This means the utilization is 15,000/20,000, or 75 percent. Of course, if there are damaged frames, the utilization percentage is much less because frames have to be resent.

26. HDLC
A mode in HDLC is the relationship between two devices involved in an exchange; The mode of communication describes who controls the link
HDLC supports three modes of communication between stations
NRM(Normal Response Mode)
ABM(Asynchronous Balanced Mode)
HDLC is a protocol that implements ARQ mechanisms.
It supports communication over point-to-point or multipoint links.

27. HDLC (cont’d) NRM(Normal Response Mode)
refers to the standard primary-secondary relationship
secondary device must have permission from the primary device before transmitting

28. HDLC (cont’d) ABM(Asynchronous Balanced Mode)
all stations are equal and therefore only combined stations connected in point-to-point are used
Either combined station may initiate transmission with the other combined station without permission

29. HDLC (cont’d) HDLC Frame I (Information) Frame S (Supervisory) Frame
used to transport user data and control information relating to user data
S (Supervisory) Frame
used to only to transport control information, primarily data link layer flow and error controls
U (Unnumbered) Frame
is reserved for system management
Information carried by U-frame is intended for managing the link itself

30. HDLC (cont’d)
HDLC frame

31. HDLC (cont’d)
HDLC Frame types

32. HDLC (cont’d) Frame may contain up to six fields beginning flag
address
control
information
FCS(Frame Check Sequence)
Ending flag

33. HDLC (cont’d)
Flag Field serves as a synchronization pattern for the receiver

34. HDLC (cont’d) Address Field
~ contains the address of the secondary station that is either the originator or destination of the frame

35. HDLC(cont’d)
Control field

36. HDLC(cont’d)
Control field (extended mode)

37. HDLC(cont’d)
Poll/Final field in HDLC

38. HDLC(cont’d)
Information field

39. HDLC(cont’d)
FCS field

40. HDLC(cont’d) More about Frames s-frame
~ is used for acknowledgment, flow control, and error control

41. HDLC(cont’d)
U-Frame is used to exchange session management and control information between connected devices

42. HDLC(cont’d) U-Frame control command and response Command/ response
Meaning
SNRM
SNRME
SARM
SARME
SABM
SABME UP UI UA RD
DISC DM
RIM
SIM
RSET
XID
FRMR
Set normal response mode
Set normal response mode(extended)
Set asynchronous response mode
Set asynchronous response mode(extended)
Set asynchronous balanced mode
Set asynchronous balanced mode(extended)
Unnumbered poll
Unnumbered information
Unnumbered acknowledgement
Request disconnect
Disconnect
Disconnect mode
Request information mode
Set initialization mode
Reset
Exchange ID
Frame reject

43. HDLC (cont’d) Mode setting commands Unnumbered-Exchange
U-Frame
~ can be divided into five basic functional category
Mode setting commands
are sent by the primary station, or by a combined station wishing to control an exchange, to establish the mode of the session(see table)
SNRM, SNRME, SARM, SARME, SABM, SABME
Unnumbered-Exchange
are used to send or solicit specific pieces of data link information between devices (see table)
UP, UI, UA
Disconnection : RD, DISC, DM
Initialization Mode : RIM, SIM
Miscellaneous : RSET, XID, FRMR

44. HDLC (cont’d)
Example 3
Figure shows an exchange using piggybacking where is no error. Station A begins the exchange of information with an I-frame numbered 0 followed by another I-frame numbered 1. Station B piggybacks its acknowledgment of both frames onto an I-frame of its own. Station B’s first I-frame is also numbered 0 [N(S) field] and contains a 2 in its N(R) field, acknowledging the receipt of A’s frames 1 and 0 and indicating that it expects frame 2 to arrive next. Station B transmits its second and third I-frames (numbered 1 and 2) before accepting further frames from station A. Its N(R) information, therefore, has not changed: B frames 1 and 2 indicate that station B is still expecting A frame 2 to arrive next.

45. HDLC (cont’d)

46. HDLC (cont’d)
Example 4
In Example 3, suppose frame 1 sent from station B to station A has an error. Station A informs station B to resend frames 1 and 2 (the system is using the Go-Back-N mechanism). Station A sends a reject supervisory frame to announce the error in frame 1. Figure shows the exchange.

47. HDLC (cont’d) – Example 4

48. HDLC (cont’d) Bit stuffing
Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag.

49. HDLC (cont’d)
Bit stuffing and removal

50. HDLC (cont’d)
Bit stuffing in HDLC
< 15
≥ 15

51. HDLC (cont’d) LAP(Link Access Procedure)
LAPB(Link Access Procedure Balanced)
~ provides those basic control function required for communication between a DTE and a DCE
~ is used only in balanced configuration of two devices
~ is used in ISDN on B channels
LAPD(Link Access Procedure for D channel)
~ used in ISDN
~ use ABM(Asynchronous Balanced Mode)
LAPM(Link Access Procedure for Modem)
~ is designed to do asynchronous-synchronous conversation, error detection, and retransmission
~ has become developed to apply HDLC feature to modem