1 11 장 Data Link Protocol 11.1 Asynchronous protocol 11.2 Synchronous protocol 11.3 Character-Oriented protocol 11.4 Bit-Oriented protocol 11.5 요약

2 zData Link Protocol ~ is a set of specifications used to implementation the data link layer

3 zData Link Protocol yAsynchronous protocol ~ treat each character in a bit stream independently ySynchronous protocol ~ take the whole bit stream and chop it into characters of equal size

Asynchronous protocol ~ used primarily in modems, feature start and stop bits and variable length gabs between characters

5 Asynchronous protocol(cont’d) zXmodem yWard Christiansen designed a file transfer protocol for telephone-line communication between PCs(1979) yhalf-duplex stop-and-wait ARQ protocol

6 Asynchronous protocol(cont’d) zXMODEM frame

7 Asynchronous protocol(cont’d) zXMODEM frame ySOH(Start of Header) : 1 byte yHeader : 2 byte(Sequence number, check the validity of sequence number) yData(Binary, ASCII, Boolean, Text, etc.) : 128 byte yCRC : check for error in the data field

8 Asynchronous protocol(cont’d) zYMODEM ~ is a protocol similar to XMODEM ydata unit is 1024 bytes yTwo CANs are sent to abort a transmission yITU-T CRC-16 is used for error checking yMultiple files can be sent simultaneously

9 Asynchronous protocol(cont’d) zZMODEM ~ is a newer protocol combining features of both XMODEM and YMODEM zBLAST(Blocked Asynchronous Transmission) ~ is full-duplex with sliding window flow control zKermit ~ designed at Columbia University ~ is the most widely used asynchronous protocol

Synchronous protocol ~ for LAN, MAN, WAN

11 Synchronous protocol(cont’d) zcharacter-oriented protocol ~ frame or packet is interpreted as a series of characters zbit-oriented protocol ~ frame of packet is interpreted as a series of bits

Character-Oriented protocol ~ are not as efficient as bit-oriented protocols and therefore one now seldom used zpopular protocol : BSC(Binary synchronous communication)

13 Character-Oriented protocol(cont’d) zBSC(Binary Synchronous Communication) ydeveloped by ZBM in 1964 yusable in both point-to-point and multipoint configuration ysupport half-duplex transmission using stop-and- wait ARQ flow control and error correction ydo not support full-duplex transmission or sliding window protocol

14 Character-Oriented protocol(cont’d) zControl character for BSC Character ASCII Code ACK 0 ACK 1 DLE ENQ EOT ETB ETX ITB NAK NUL RVI SOH STX SYN TTD WACK DLE and 0 DLE and 1 DLE ENQ EOT ETB ETX US NAK NULL DLE and < SOH STX SYN STX and ENQ DLE and ; Function Good even frame received or ready to receive Good odd frame received Data transparency maker Request for a response Sender terminating End of transmission block; ACK required End of text in a message End of intermediate block in a multiblock transmission Bad frame received nothing to send Filler character Urgent message from receiver Header information beings Text beings Alerts receiver to incoming frame Sender is pausing but not relinquishing the line Good frame received but not ready to receive more

15 Character-Oriented protocol(cont’d) zASCII code ~ whatever the system, not all control characters can be represented by a single character. Often they must be represented by two or three characters(table 11.1)

16 Character-Oriented protocol(cont’d) zBSC frame

17 Character-Oriented protocol(cont’d) zData frame

18 Character-Oriented protocol(cont’d) zHeader field One or more bytes defining address and/or other information(nonstandardized)

19 Character-Oriented protocol(cont’d) zMultiblock Frame ytext in a message is often divide between several blocks

20 Character-Oriented protocol(cont’d) zMultiframe Transmission

21 Character-Oriented protocol(cont’d) zControl Frame ~ is used by one device to send commands to, or solicit information from, another device One or more characters

22 Character-Oriented protocol(cont’d) zControl Frame ~ serve three purpose yestablishing connections ymaintaining flow and error control during data transmission yterminating connection

23 Character-Oriented protocol(cont’d) zControl Frame(1)

24 Character-Oriented protocol(cont’d) zControl Frame(2)

25 Character-Oriented protocol(cont’d) zControl Frame(3)

26 Character-Oriented protocol(cont’d) zData Transparency ~ means we should be able to send any combination of bits as data(bit stuffing)

Bit-Oriented protocol ~ can pack more information into shorter frames and avoid the transparency problem of character-oriented protocol

28 Bit-Oriented protocol(cont’d) zSDLC(Synchronous Data Link Control) ~ developed by ZBM in 1975 zHDLC(High-Level Data Link Control) ~ developed by ISO in 1979 zLAPs(LAPS, LAPD, LAPM, LAPX, etc) ~ developed by ITU-T since 1981 zPPP, frame relay ~ developed by ITU-T and ANSI

29 Bit-Oriented protocol(cont’d) zHDLC yAll bit-oriented protocols are related to high-level data link control(HDLC), which published by ISO. yHDLC support both half-duplex and full-duplex modes in point-to-point and multipoint configurations yHDLC can be characterized by their station types, their their configurations, and their response modes

30 Bit-Oriented protocol(cont’d) zStation Types yprimary : send commands ysecondary : send response ycombined : send command and response

31 Bit-Oriented protocol(cont’d) zConfiguration ~ refers to the relationship of hardware devices on a link

32 Bit-Oriented protocol(cont’d) z

33 Bit-Oriented protocol(cont’d) z

34 Bit-Oriented protocol(cont’d) zMode of communication ~ describes who controls the link yNRM(Normal Response Mode) yARM(Asynchronous Response Mode) yABM(Asynchronous Balanced Mode)

35 Bit-Oriented protocol(cont’d) zNRM(Normal Response Mode) yrefers to the standard primary-secondary relationship ysecondary device must have permission from the primary device before transmitting

36 Bit-Oriented protocol(cont’d) zARM(Asynchronous Response Mode) ysecondary may initiate a transmission without permission from the primary whenever the channel is idle ydo not alter the primary-secondary relationship in any other way

37 Bit-Oriented protocol(cont’d) zABM(Asynchronous Balanced Mode) yall stations are equal and therefore only combined stations connected in point-to-point are used yEither combined station many initiate transmission with the other combined station without permission

38 Bit-Oriented protocol(cont’d) zHDLC modes

39 Bit-Oriented protocol(cont’d) zFrame yI( Information ) Frame ~ used to transport user data and control information relating to user data yS( Supervisory ) Frame ~ used to only to transport control information, primarily data link layer flow and error controls yU( Unnumbered) Frame ~ are reserved for system management

40 Bit-Oriented protocol(cont’d) zHDLC Frame types

41 Bit-Oriented protocol(cont’d) zFrame ~ many contain up to six field ybeginning flag yaddress ycontrol yinformation yFCS(Frame Check Sequence)

42 Bit-Oriented protocol(cont’d) zFlag Field ~ serve as a synchronization pattern for the receiver

43 Bit-Oriented protocol(cont’d) zBit 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 flag

44 Bit-Oriented protocol(cont’d) zBit Stuffing in HDLC

45 Bit-Oriented protocol(cont’d) zAddress Field ~ contain the address of the secondary station that is either the originator or destination of the frame

46 Bit-Oriented protocol(cont’d) zControl field

47 Bit-Oriented protocol(cont’d) zPoll/Final field in HDLC

48 Bit-Oriented protocol(cont’d) zInformation field

49 Bit-Oriented protocol(cont’d) zPiggybacking ~means combining data to be sent and acknowledgment of the frame received in one single frame

50 Bit-Oriented protocol(cont’d) zFCS field

51 Bit-Oriented protocol(cont’d) zMore about Frames ys-frame ~ are used for acknowledgment, flow control, and error control

52 Bit-Oriented protocol(cont’d) zRR(Receive Ready) yACK yPoll yNegative response to poll yPositive response to select

53 Bit-Oriented protocol(cont’d) zRNR(Receive Net Ready) yACK ySelect yNegative response to select zREJ(Reject) zSREJ(Selective-reject)

54 Bit-Oriented protocol(cont’d) zUse of P/F bit in polling and selection

55 Bit-Oriented protocol(cont’d) zU-Frame ~are used to exchange session management and control information between connected devices

56 Bit-Oriented protocol(cont’d) z

57 Bit-Oriented protocol(cont’d) zU-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

58 Bit-Oriented protocol(cont’d) zU-Frame ~ can be divided into five basic functional category yMode setting yUnnumbered-Exchange yDisconnection yInitialization Mode yMiscellaneous

59 Bit-Oriented protocol(cont’d) zMode Setting Command ~ are sent by the primary station, or by a combined station wishing to control an exchange, to establish the mode of the session(table 11.2) zUnumbered-Exchange ~ are used to send or solicit specific pieces of data link information between device(table 11.2) zDisconnection zInitialization Mode zMiscellaneous

60 Bit-Oriented protocol(cont’d) zExample 1 : Poll/Response

61 Bit-Oriented protocol(cont’d) zExample 2 : Select/Response

62 Bit-Oriented protocol(cont’d) zExample 3 : Peer Devices(1)

63 Bit-Oriented protocol(cont’d) zExample 3 : Peer Devices(2)

64 Bit-Oriented protocol(cont’d) zLAP(Link Access Procedure) yLAPB(Link Access Procedure Balanced) ~ provide those basic control function required for communication between a DTE and a DLE ~ used only in balanced configuration of two device ~ is used in ISDN on B channel yLAPD(Link Access Procedure for D channel) ~ used in ISDN ~ use ABM(Asynchronous Balanced Mode) yLAPM(Link Access Procedure for Modem) ~ is designed to do asynchronous-synchronous conversation, error detection, and retransmission ~ has become developed to apply HDLC feature to modems