2. Computer Networking 网络课件 双语教学 模拟实验 计算机网络教研室

3. Department of Computer Networking Application CHAPTER 7 DATA LINK CONTROL PROTOCOL 1 The first section 2 Exercises 3 Online lecture

4. Chapter 7 Data Link Control Protocol 7.1 INTRODUCTION A set of rules that define the exact format of messages exchanged between computers or between computers and people is called a protocol.exact format exchanged protocol Data link protocols are necessary so that all of the devices on the network are operating by the same set of rules and are able to communicate with each other.Data link protocols www.gxmu.edu.cn

5. 7.2 DATA LINK PROTOCOLS FUNCTIONS  Communication startupCommunication startup  Character identification and framingCharacter identification and framing  Message identificationMessage identification The DTE must separate the characters on the communications line into messages.separatemessages Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

6. 7.2 DATA LINK PROTOCOLS FUNCTIONS  Line ControlLine Control The part of a protocol consisting of the rules that specify the way the receiving terminal signals the sending terminal if it has received data correctly, how and under what circumstances the line will be turned around, and whether the receiving terminal can accept more data is called line control.circumstances Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

7. 7.2 DATA LINK PROTOCOLS FUNCTIONS  Error ControlError Control The part of the protocol consisting of rules specifying what happens when an error is detected, what to do if communications suddenly and unexplainably cease, and the way communications are reestablished after they are broken is called error control.unexplainably ceasereestablished  TerminationTermination The part of a protocol consisting of rules for ending the communications under normal and abnormal circumstances is called termination. circumstances Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

8. 7.3 DESIRABLE ATTRIBUTES OF DATA LINK PROTOCOLSDESIRABLE ATTRIBUTES OF DATA LINK PROTOCOLS  TransparencyTransparency The ability of data terminal equipment (DTE) to be able to transmit and receive any bit pattern as data is called transparency. bit pattern Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

9. 7.3 DESIRABLE ATTRIBUTES OF DATA LINK PROTOCOLSDESIRABLE ATTRIBUTES OF DATA LINK PROTOCOLS  Code independenceCode independence The protocol allow the transmission of data from any data coding system.  EfficiencyEfficiency The protocol should use as few characters as possible to control the data transmission so that most of the line capacity can be used for actual data transmission.capacity Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

10. 7.4 PROTOCOL IMPLEMENTIONPROTOCOL IMPLEMENTION A data link protocol is implemented by transmitting certain bit patterns or characters on the communications circuit.implemented certain bit patterns or characters 7.5 PREDETERMINED COMMUNICATIONS PARAMETERSPREDETERMINED COMMUNICATIONS PARAMETERS Predetermined communication parameters are those that are set manually by switches or specified as parameters in software.set manually by switches or specified as parameters in software. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

11. 7.7 LINE ACCESSLINE ACCESS When two stations on a circuit transmit at the same time, a(n) collision occurs.collision There are three primary ways that circuit access can be obtained:contention,polling,and token passing.obtainedcontention,polling,and token passing 7.7.1 ContentionContention When any station that has traffic looks to see if the circuit is free, and if it is, begins sending its traffic the line control technique is called contention.station Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

12. Contention systems work best _____. a. on circuits that have only two DTEs b. when the speed of the circuit is relatively fast c. on circuits that have not too heavy message trafficnot too heavy message traffic 7.7.2 PollingPolling When a central or control station asks each station on the circuit or network if it has traffic to send it is using a line control technique called polling.central or control station Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

13. 7.7.2 PollingPolling  Roll call pollingRoll call polling Roll call polling is the most common implementation of a polling system.the most common implementation In order to know what stations to poll and in what sequence, the master station uses a polling list. polling list Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

14. 7.7.2 PollingPolling  Fast select pollingFast select polling Fast select polling speeds up the polling process because a slave station that doesn’t have traffic isn’t required to return a character to the master station.slave station master station  Hub polling P128Hub polling 7.7.3 Token passingToken passing Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

15. 7.8 MESSAGE FORMATMESSAGE FORMAT Data messages normally consist of three parts: the header, text, and trailer.the header, text, and trailer The header of a message contains and conveys information about the message, such as the destination node’s address, a sequence number, and perhaps a date and time.destination node’s addresssequence number Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

16. Figure 7-1 Message format showing examples of the kinds of control characters the protocol might employ. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

17. 7.9 FLOW CONTROLFLOW CONTROL Techniques to ensure that a fast transmitting node does not send data faster than the receiving node can receive and process it are called flow control. 7.9.1 Stop-and-wait Flow ControlStop-and-wait Flow Control Stop-and-wait Flow Control is the simplest form control. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

18. Figure 7-2 The data flow of stop-and-wait flow control. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

19. 7.9.2 Sliding Window Flow ControlSliding Window Flow Control The flow control technique that allows multiple frames to be in transit on the line at one time is called sliding window. multiple frames There are two common implementations of sliding window flow control:go-back-n and selective repeat.implementationsgo-back-n and selective repeat Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

20. Figure 7-3 The data flow in a simple sliding window flow control. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

21. 7.10 ASYNCHRONOUS DATA LINK PROTOCOLSASYNCHRONOUS DATA LINK PROTOCOLS Asynchronous data link protocols deal with individual characters rather than blocks of data.individual characters rather than blocks of data 7.10.1 Point-to-Point Protocol(PPP)Point-to-Point Protocol(PPP) A protocol that is primarily used by PC workstations to send IP over dial-up lines, typically when dialing in to an ISP for connection to the Internet, is called PPP.dial-up lines PPP has largely replaced the SLIP protocol because it has better error detection.SLIPerror detection Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

22. Figure 7-4 PPP’s frame format. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

23. 7.10.2 XMODEM ProtocolXMODEM Protocol The XMODEM protocol was developed for use between PCs, especially for transfers of data files between them. 7.10.3 Other Asynchronous ProtocolsOther Asynchronous Protocols A sophisticated check character at the end of each block of data is called a cyclic redundancy check(CRC).A sophisticated check character at the end of each block of data cyclic redundancy check(CRC) Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

24. 7.11 SYNCHRONOUS DATA LINK PROTOCOLSSYNCHRONOUS DATA LINK PROTOCOLS Synchronous data link protocols typically deal with blocks of data,not individual characters. The three types are character-oriented protocol,byte-count-oriented protocol,and bit-oriented protocols.character-oriented protocolbyte-count-oriented protocol bit-oriented protocols Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

25. 7.11.1 Character-Oriented Protocols A character-oriented protocol uses special characters to indicate the beginnings and ends of messages.indicate The best known character-oriented protocol is the Binary Synchronous Communications Protocol(BSC),also known as BISYNC.Binary Synchronous Communications Protocol(BSC) BISYNC is defined to only support the 6-bit transcode (SBT), EBCDIC, and ASCII.6-bit transcode (SBT) Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

26. 7.11.2 Byte-Count-Oriented Protocols Byte-count-oriented protocols have a special characters to mark the beginning of the header,followed by a count field that indicates how many characters are in the data portion of the messages.indicatedata portion of the messages The best known byte-count-oriented protocol is the Digital Data Communications Message Protocol(DDCMP).Digital Data Communications Message Protocol(DDCMP) Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

27. 7.11.3 Bit-Oriented Protocols A bit-oriented protocol uses only one special character,called the flag character,which marks the beginning and end of parts of the message.flag character The best known bit-oriented protocols are the ISO’s High-Level Data Link Control (HDLC) protocol.ISO’s High-Level Data Link Control SDLC is a subset of HDLC. 7.12 OTHER DATA LINK PROTOCOLS P143 Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

28. 7.13 PROTOCOL CONVERSIONPROTOCOL CONVERSION A protocol converter changes one protocol to another, when, for example, a message is sent between two networks that use different protocols. Chapter 7 Data Link Control Protocol www.gxmu.edu.cn

29. 课后习题 C 、 a frame is beginning or ending 1.When an HDLC node receives a flag character, it knows that （ ）. C A 、 an error has occurred B 、 it should signal the sender to stop sending traffic D 、 it should switch to transparent mode Answer Redo Next × × × √ Exercises Chapter 7 Data Link Control Protocol

30. 课后习题 D 、 collision 2.When two stations on a circuit transmit at the same time, a(n) （ ） occurs. D A 、 division B 、 polling C 、 bit stuffing Answer Redo Next × × × √ Exercises Chapter 7 Data Link Control Protocol

31. 课后习题 B 、 polling list 3.In order to know what stations to poll and in what sequence, the master station uses a （ ）. B A 、 message header D 、 cyclic redundancy check C 、 start of header character Answer Redo Next × × × √ Exercises Chapter 7 Data Link Control Protocol

32. 课后习题 D 、 sliding window flow control 4. Go-back-n and selective repeat are two common implementations of （ ）. D A 、 bit oriented protocols B 、 byte protocols C 、 byte-count oriented protocols Answer Redo Next × × × √ Exercises Chapter 7 Data Link Control Protocol

33. 课后习题 A 、 stop-and-wait 5. When a receiver must acknowledge every block of data before the next block is sent, the （ ） type of flow control is being used. A B 、 stop-and-check D 、 sliding window C 、 sliding frame Answer Redo Next × × × √ Exercises Chapter 7 Data Link Control Protocol

34. 课后习题 6.SDLC is a subset of ________. HDLC Answer Exercises 7.BISYNC is defined to only support the 6-bit transcode (SBT), EBCDIC, and _________. ASCII Answer 8.A set of rules that define the exact format of messages exchanged between computers or between computers and people is called a ______________. protocol Answer Chapter 7 Data Link Control Protocol

35. 课后习题 9. A technique used to ensure that a fast transmitting node does not send data faster than a receiving node can receive and process it is called ___________. flow control Answer Exercises 10.A protocol that is primarily used by PC workstations to send IP over dial-up lines, typically when dialing in to an ISP for connection to the Internet, is called _____. PPP Answer Chapter 7 Data Link Control Protocol

36. www.gxmu.edu.cn Chapter 7 Data Link Control Protocol