Chapter 11 Data Link Control

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Chapter 11 Data Link Control

1. Framing
2. Flow and Error Control
3. Protocols
4. Noiseless Channels
5. Noisy Channels
6. HDLC
7. Point-to-Point Protocol

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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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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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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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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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Noiseless Channels Simplest Protocol

• Simplest protocol with no flow or error control

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Simplest Protocol

• Sender-site algorithm

• Receiver-site algorithm

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Stop-and-Wait Protocol

• Simple tokens of ACK and flow control added

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Stop-and-Wait Protocol

• Sender-site algorithm

• Receiver-site algorithm

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Stop-and-Wait Protocol Example
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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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Stop-and-Wait ARQ
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Stop-and-Wait ARQ

• Sender-site algorithm

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Stop-and-Wait ARQ

• Receiver-site algorithm

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Stop-and-Wait ARQ Example
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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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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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Go-Back-N ARQ

• Sliding windows, Timers, ACK, Resending a frame

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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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Go-Back-N ARQ Sender Algorithm
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Go-Back-N ARQ Receiver Algorithm
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Go-Back-N ARQ Example 1
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Go-Back-N ARQ Example 2
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Selective Repeat ARQ

• Sender window size

• Receive window size

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Selective Repeat ARQ
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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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Selective Repeat ARQ Sender-Site Algorithm
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Selective Repeat ARQ Receiver-Site Algorithm
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Selective Repeat ARQ Example
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Piggybacking

• To improve the efficiency of the bidirectional
  protocols
• Piggybacking in Go-Back-N ARQ

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HDLC

• High-level Data Link Control
• Two common transfer mode normal response mode
  (NRM) and asynchronous balanced mode (ABM)

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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

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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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HDLC Frames

• Control field for S-frame
• Receive ready (RR), Receive not ready (RNR),
  Reject (REJ) Selective reject (SREJ)

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HDLC Frames

• Control field for U-frame

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HDLC Example 1

• Connection and disconnection

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HDLC Example 2

• Piggybacking without error

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HDLC Example 3

• Piggybacking with error

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HDLC Bit Stuffing and Unstuffing
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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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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

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PPP Transition States
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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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LCP Encapsulated in a Frame
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LCP Common Options

• Options are inserted in the information field of
  the configuration packets

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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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Password Authentication Protocol (PAP)
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Challenge Handshake Authentication Protocol (CHAP)

• Three-way hand-shaking authentication protocol
  with greater security than PAP

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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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IPCP Packet
IP Datagram in a PPP frame
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Multiple PPP
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Example (1)
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Example (2)