1 Kyung Hee University Data Link Protocols

2 Kyung Hee University 11 장 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 Kyung Hee University 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 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.Flow control is the regulation of the sender’s data rate so that the receiver buffer does not become overwhelmed.

4 Kyung Hee University 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 Kyung Hee University 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 Kyung Hee University Stop-and-Wait ARQ  Normal Operation

7 Kyung Hee University Stop-and-Wait ARQ  Stop-and-Wait ARQ Lost Frame

8 Kyung Hee University Stop-and-Wait ARQ  Stop-and- Wait ARQ Lost ACK Frame

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

10 Kyung Hee University Stop-and-Wait ARQ, delayed ACK

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

12 Kyung Hee University Piggybacking  meaning combining data to be sent and acknowledgment of the frame received in one single frame

13 Kyung Hee University 11.3 Go-Back-N ARQ  Sender sliding window

14 Kyung Hee University Go-Back-N ARQ  Receiver sliding window

15 Kyung Hee University Go-Back-N ARQ Go-Back-N ARQ  Control variables

16 Kyung Hee University Go-Back-N ARQ  Go-Back-N ARQ, normal operation

17 Kyung Hee University Go-Back-N ARQ  Go-Back-N ARQ, lost frame

18 Kyung Hee University Go-Back-N ARQ Go-Back-N ARQ  Go-Back-N ARQ: sender window size

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

20 Kyung Hee University 11.4 Selective-Repeat ARQ  Selective Repeat ARQ, sender and receiver windows

21 Kyung Hee University Selective-Repeat ARQ  Selective Repeat ARQ, lost frame

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

23 Kyung Hee University Selective-Repeat ARQ  Selective Repeat ARQ, sender window size

24 Kyung Hee University Example Example 1 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? Solution The bandwidth-delay product is 1  10 6  20  = 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. The bandwidth-delay product is a measure of the number of bits a system can have in transit.

25 Kyung Hee University Example Example 2 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 Kyung Hee University 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 Kyung Hee University 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 Kyung Hee University 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 Kyung Hee University HDLC (cont’d)  HDLC Frame  I (Information) Frame l used to transport user data and control information relating to user data  S (Supervisory) Frame l used to only to transport control information, primarily data link layer flow and error controls  U (Unnumbered) Frame l is reserved for system management l Information carried by U-frame is intended for managing the link itself

30 Kyung Hee University HDLC (cont’d)  HDLC frame

31 Kyung Hee University HDLC (cont’d)  HDLC Frame types

32 Kyung Hee University HDLC (cont’d)  Frame may contain up to six fields  beginning flag  address  control  information  FCS(Frame Check Sequence)  Ending flag

33 Kyung Hee University HDLC (cont’d)  Flag Field serves as a synchronization pattern for the receiver

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

35 Kyung Hee University HDLC(cont’d)  Control field

36 Kyung Hee University HDLC(cont’d)  Control field (extended mode)

37 Kyung Hee University HDLC(cont’d)  Poll/Final field in HDLC

38 Kyung Hee University HDLC(cont’d)  Information field

39 Kyung Hee University HDLC(cont’d)  FCS field

40 Kyung Hee University HDLC(cont’d)  More about Frames  s-frame ~ is used for acknowledgment, flow control, and error control

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

42 Kyung Hee University 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 Kyung Hee University HDLC (cont’d)  U-Frame ~ can be divided into five basic functional category  Mode setting commands l 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) l SNRM, SNRME, SARM, SARME, SABM, SABME  Unnumbered-Exchange l are used to send or solicit specific pieces of data link information between devices (see table) l UP, UI, UA  Disconnection : RD, DISC, DM  Initialization Mode : RIM, SIM  Miscellaneous : RSET, XID, FRMR

44 Kyung Hee University 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 Kyung Hee University HDLC (cont’d)

46 Kyung Hee University 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 Kyung Hee University HDLC (cont’d) – Example 4

48 Kyung Hee University 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 Kyung Hee University HDLC (cont’d)  Bit stuffing and removal

50 Kyung Hee University HDLC (cont’d)  Bit stuffing in HDLC ≥ 15 < 15

51 Kyung Hee University 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