1. Chapter 11 Data Link Control
Framing
Flow and Error Control
Protocols
Noiseless Channels
Noisy Channels
HDLC
Point-to-Point Protocol
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2. Data Communications, Kwangwoon University
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
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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
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4. Bit-Oriented Protocols
Frame structure
Bit stuffing: process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data
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5. Data Communications, Kwangwoon University
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)
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6. Noiseless Channels: Simplest Protocol
Simplest protocol with no flow or error control
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7. Data Communications, Kwangwoon University
Simplest Protocol
Sender-site algorithm
Receiver-site algorithm
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8. Stop-and-Wait Protocol
Simple tokens of ACK and flow control added
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9. Stop-and-Wait Protocol
Sender-site algorithm
Receiver-site algorithm
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10. Stop-and-Wait Protocol: Example
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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.
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12. Data Communications, Kwangwoon University
Stop-and-Wait ARQ
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13. Data Communications, Kwangwoon University
Stop-and-Wait ARQ
Sender-site algorithm
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14. Data Communications, Kwangwoon University
Stop-and-Wait ARQ
Receiver-site algorithm
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15. Stop-and-Wait ARQ: Example
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16. Data Communications, Kwangwoon University
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.
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17. Data Communications, Kwangwoon University
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.
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18. Data Communications, Kwangwoon University
Go-Back-N ARQ
Sliding windows, Timers, ACK, Resending a frame
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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
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20. Go-Back-N ARQ: Sender Algorithm
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21. Go-Back-N ARQ: Receiver Algorithm
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22. Go-Back-N ARQ: Example 1
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23. Go-Back-N ARQ: Example 2
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24. Data Communications, Kwangwoon University
Selective Repeat ARQ
Sender window size
Receive window size
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25. Data Communications, Kwangwoon University
Selective Repeat ARQ
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26. Selective Repeat ARQ: Window Size
The size of the sender and receiver window must be at most one-half of 2m
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27. Selective Repeat ARQ: Sender-Site Algorithm
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28. Selective Repeat ARQ: Receiver-Site Algorithm
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29. Selective Repeat ARQ: Example
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30. Data Communications, Kwangwoon University
Piggybacking
To improve the efficiency of the bidirectional protocols
Piggybacking in Go-Back-N ARQ
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31. Data Communications, Kwangwoon University
HDLC
High-level Data Link Control
Two common transfer mode: normal response mode (NRM) and asynchronous balanced mode (ABM)
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32. Data Communications, Kwangwoon University
HDLC: Frames
I(information)-frames, S(supervisory)-frames, U(unnumbered frame)-frames
Flag field: 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
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33. Data Communications, Kwangwoon University
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)
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34. Data Communications, Kwangwoon University
HDLC: Frames
Control field for S-frame
Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective reject (SREJ)
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35. Data Communications, Kwangwoon University
HDLC: Frames
Control field for U-frame
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36. Data Communications, Kwangwoon University
HDLC: Example 1
Connection and disconnection
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37. Data Communications, Kwangwoon University
HDLC: Example 2
Piggybacking without error
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38. Data Communications, Kwangwoon University
HDLC: Example 3
Piggybacking with error
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39. HDLC: Bit Stuffing and Unstuffing
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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
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41. Data Communications, Kwangwoon University
PPP Frame
Flag: the same as HDLC, but it treated as a byte because of PPP is a byte-oriented protocol
Address: (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
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42. PPP: Transition States
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43. Data Communications, Kwangwoon University
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
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44. LCP: Encapsulated in a Frame
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45. Data Communications, Kwangwoon University
LCP: Common Options
Options are inserted in the information field of the configuration packets
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46. Data Communications, Kwangwoon University
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)
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47. Password Authentication Protocol (PAP)
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48. Challenge Handshake Authentication Protocol (CHAP)
Three-way hand-shaking authentication protocol with greater security than PAP
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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
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50. IP Datagram in a PPP frame
IPCP Packet
IP Datagram in a PPP frame
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51. Data Communications, Kwangwoon University
Multiple PPP
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52. Data Communications, Kwangwoon University
Example (1)
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53. Data Communications, Kwangwoon University
Example (2)
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