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Chapter 11 Data Link Control

Chapter 11 Data Link Control. Framing Flow and Error Control Protocols Noiseless Channels Noisy Channels HDLC Point-to-Point Protocol. Framing. Data link layer needs to pack bits into frames, so that each frame is distinguishable from another

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Chapter 11 Data Link Control

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  1. Chapter 11 Data Link Control Framing Flow and Error Control Protocols Noiseless Channels Noisy Channels HDLC Point-to-Point Protocol Data Communications, Kwangwoon University

  2. Framing • Data link layer needs to pack bits into frames, so that each frame is distinguishable from another • Separate a message from one source to a destination, or from other messages to other destinations, by adding a sender address and a destination address • Fixed-size framing: ATM (Chapter 18) • Variable-size framing • Need a way to define the end of the frame and the beginning of the next • Character-oriented approach and bit-oriented approach Data Communications, Kwangwoon University

  3. Character-Oriented Protocols • Frame structure • Byte stuffing: process of adding 1 extra byte whenever there is a flag or escape character in the text Data Communications, Kwangwoon University

  4. Bit-Oriented Protocols • Frame structure • Bit stuffing: process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data Data Communications, Kwangwoon University

  5. Flow and Error Control • Data link control = flow control + error control • Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgement • Error control in the data link layer is based on automatic repeat request (ARQ), which is the retransmission of data • ACK, NAK(Negative ACK), Piggybacking (ACKs and NAKs in data frames) Data Communications, Kwangwoon University

  6. Noiseless Channels: Simplest Protocol • Simplest protocol with no flow or error control Data Communications, Kwangwoon University

  7. Simplest Protocol • Sender-site algorithm • Receiver-site algorithm Data Communications, Kwangwoon University

  8. Stop-and-Wait Protocol • Simple tokens of ACK and flow control added Data Communications, Kwangwoon University

  9. Stop-and-Wait Protocol • Sender-site algorithm • Receiver-site algorithm Data Communications, Kwangwoon University

  10. Stop-and-Wait Protocol: Example Data Communications, Kwangwoon University

  11. Noisy Channels: Stop-and-Wait ARQ • Stop-and-wait Automatic Repeat Request (ARQ) • Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expires • In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic • Acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected. Data Communications, Kwangwoon University

  12. Stop-and-Wait ARQ Data Communications, Kwangwoon University

  13. Stop-and-Wait ARQ • Sender-site algorithm Data Communications, Kwangwoon University

  14. Stop-and-Wait ARQ • Receiver-site algorithm Data Communications, Kwangwoon University

  15. Stop-and-Wait ARQ: Example Data Communications, Kwangwoon University

  16. Go-Back-N ARQ • Pipelining improves the efficiency of the transmission • In the Go-Back-N Protocol, the sequence numbers are modulo 2m, where m is the size of the sequence number field in bits • The send window is an abstract concept defining an imaginary box of size 2m − 1 with three variables: Sf, Sn, and Ssize • The send window can slide one or more slots when a valid acknowledgment arrives. Data Communications, Kwangwoon University

  17. Go-Back-N ARQ • Receive window for Go-Back-N ARQ • The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn. The window slides when a correct frame has arrived; sliding occurs one slot at a time. Data Communications, Kwangwoon University

  18. Go-Back-N ARQ • Sliding windows, Timers, ACK, Resending a frame Data Communications, Kwangwoon University

  19. Go-Back-N ARQ: Send Window Size • In Go-Back-N ARQ, the size of the send window must be less than 2m; the size of the receiver window is always 1 • Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of the send window is 1 Data Communications, Kwangwoon University

  20. Go-Back-N ARQ: Sender Algorithm Data Communications, Kwangwoon University

  21. Go-Back-N ARQ: Receiver Algorithm Data Communications, Kwangwoon University

  22. Go-Back-N ARQ: Example 1 Data Communications, Kwangwoon University

  23. Go-Back-N ARQ: Example 2 Data Communications, Kwangwoon University

  24. Selective Repeat ARQ • Sender window size • Receive window size Data Communications, Kwangwoon University

  25. Selective Repeat ARQ Data Communications, Kwangwoon University

  26. Selective Repeat ARQ: Window Size • The size of the sender and receiver window must be at most one-half of 2m Data Communications, Kwangwoon University

  27. Selective Repeat ARQ: Sender-Site Algorithm Data Communications, Kwangwoon University

  28. Selective Repeat ARQ: Receiver-Site Algorithm Data Communications, Kwangwoon University

  29. Selective Repeat ARQ: Example Data Communications, Kwangwoon University

  30. Piggybacking • To improve the efficiency of the bidirectional protocols • Piggybacking in Go-Back-N ARQ Data Communications, Kwangwoon University

  31. HDLC • High-level Data Link Control • Two common transfer mode: normal response mode (NRM) and asynchronous balanced mode (ABM) Data Communications, Kwangwoon University

  32. HDLC: Frames • I(information)-frames, S(supervisory)-frames, U(unnumbered frame)-frames • Flag field: 01111110 to identify both the beginning and the end of a frame and serve as synchronization pattern for receiver • FCS field: 2- or 4-byte ITU-T CRC for error detection Data Communications, Kwangwoon University

  33. HDLC: Frames • Control Field: 1- or 2-byte segment of the frame used for flow and error control • Determine the type of frame and define its functionality • Control field for I-frame: P/F (poll/final bit for primary/secondary) Data Communications, Kwangwoon University

  34. HDLC: Frames • Control field for S-frame • Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective reject (SREJ) Data Communications, Kwangwoon University

  35. HDLC: Frames • Control field for U-frame Data Communications, Kwangwoon University

  36. HDLC: Example 1 • Connection and disconnection Data Communications, Kwangwoon University

  37. HDLC: Example 2 • Piggybacking without error Data Communications, Kwangwoon University

  38. HDLC: Example 3 • Piggybacking with error Data Communications, Kwangwoon University

  39. HDLC: Bit Stuffing and Unstuffing Data Communications, Kwangwoon University

  40. Point-to-Point Protocol: PPP • One of the most common protocols for point-to-point access • Many Internet users who need to connect their home computer to the server of an Internet service provider use PPP • A point-to-point link protocol is required to control and manage the transfer of data • PPP defines/provides • the format of the frame to be exchanged between devices • how two devices negotiate the establishment of the link and the exchange of data • how network layer data are encapsulated in the data link frame • how two devices can authenticate each other • multiple network layer services • connection over multiple links • Network address configuration • But, several services are missing for simplicity • no flow control, simple error control (detection and discard), no sophisticate addressing for multipoint configuration Data Communications, Kwangwoon University

  41. PPP Frame • Flag: 01111110 the same as HDLC, but it treated as a byte because of PPP is a byte-oriented protocol • Address: 11111111 (broadcast address) • Control: No need because PPP has no flow control and limited error control • PPP is a byte-oriented protocol using byte stuffing with the escape byte 01111101 Data Communications, Kwangwoon University

  42. PPP: Transition States Data Communications, Kwangwoon University

  43. PPP: Multiplexing • PPP uses another set of other protocols to establish the link, authenticate the parties, and carry the network layer data • Three sets of protocols defined for powerful PPP: LCP, two APs, several NCPs Data Communications, Kwangwoon University

  44. LCP: Encapsulated in a Frame Data Communications, Kwangwoon University

  45. LCP: Common Options • Options are inserted in the information field of the configuration packets Data Communications, Kwangwoon University

  46. Authentication • Authentication means validating the identity of a user who needs to access • PPP is designed for use over dial-up links  User authentication is necessary • PPP has two protocols for authentication • Password Authentication Protocol (PAP) • Challenge Handshake Authentication Protocol (CHAP) Data Communications, Kwangwoon University

  47. Password Authentication Protocol (PAP) Data Communications, Kwangwoon University

  48. Challenge Handshake Authentication Protocol (CHAP) • Three-way hand-shaking authentication protocol with greater security than PAP Data Communications, Kwangwoon University

  49. Network Control Protocol: NCP • PPP is a multiple-network layer protocol. • It can carry a network data packet from protocols defined by the Internet, OSI, Xerox, DECnet, AppleTalk, Novel • IPCP (IP Control Protocol) • Configures the link used to carry IP packets in the Internet Data Communications, Kwangwoon University

  50. IPCP Packet IP Datagram in a PPP frame Data Communications, Kwangwoon University

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