Data Link

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Chapter 11
Data Link Protocols
2
Figure 11-1
Data Link Protocol Categories

• Asynchronous protocols treat each character in a
  bit stream independently.
• Synchronous protocols take the whole bit stream
  and chop it into characters of equal size.

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Figure 11-2
Asynchronous Protocols

• Asynchronous protocols, used primarily in modems,
  feature start and stop bits and variable length
  gaps between characters.

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Figure 11-3
XMODEM Frame
Start of header
2 bytes

• A half-duplex stop-and-wait ARQ protocol

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Other Asynchronous Protocols

• YMODEM
• Data unit 1024 bytes
• Two CANs (Cancel signal) are sent to abort a
  transmission
• ITU-T CRC-16
• ZMODEM
• Combining features of both XMODEM and YMODEM
• BLAST (Blocked asynchronous transmission)
• Full-duplex with sliding window flow control
• Kermit
• Currently the most widely used asynchronous
  protocol.

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Figure 11-4
Synchronous Protocols

• The speed of synchronous transmission makes it
  the better choice, over asynchronous
  transmission, for LAN, MAN, and WAN technology.
• In a character(byte)-oriented protocols, the
  frame or packet is interpreted as a series of
  characters.
• In bit-oriented protocols, the frame or packet is
  interpreted as a series of bits.

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Figure 11-5
Character-oriented Protocols Binary Synchronous
Communication (BSC) Frames

• Developed by IBM in 1964.
• Half-duplex transmission with stop-and-wait ARQ.
• Control characters (Table 11.1)

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Figure 11-6
A Simple BSC Data Frame
SYN Synchronization STX Start of text ETX End
of text BCC Block check counter LRC
Longitudinal redundancy check CRC Cyclic
redundancy check
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Figure 11-7
A BSC Frame with a Header
SOH Start of header
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Figure 11-8
A Multiblock Frame

• The more bits in a frame, the greater the
  likelihood that one of them will be corrupted in
  transmit.
• If any block contains an error, the entire frame
  must be retransmitted.
• The receiver sends a single acknowledgment for
  the entire frame.

ITB Intermediate text block
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Figure 11-9
Multiframe Transmission
ETB End of transmission block ACK required
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Figure 11-10
BSC Control Frame

• Control frames severe three purposes
• - establishing connections,
• - maintaining flow and error control during
  data transmission,
• - terminating connections.

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Figure 11-11
Control Frames
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Figure 11-12
Data Transparency

• Confusion between control information and data is
  called a lack of data transparency.
• ? In BSC, data transparency is achieved byte
  stuffing process.
• ? If the transparent region contains a DLE
  character as text.
• An additional DLE just before each DLE is
  inserted within the text.

Byte Stuffing
DLE data link escape
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Figure 11-13
Bit-Oriented Protocols

• Synchronous data link control (SDLC)
• - IBM (1975)
• High-level data link control (HDLC)
• - ISO standard (1979 based on SDLC)
• - Support half- and full-duplex modes over
  point-to-point and multipoint configurations.
• Link access procedure (LAP)
• - ITU-T (based on HDLC)
• Frame relay, PPP ITU-T and ANSI
• LANs access control protocols

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Figure 11-14
HDLC Configuration

• Station types
• - Primary station
• - Secondary station
• - Combined station
• Configuration
• - Unbalanced
• - Symmetrical
• - Balanced
• HDLC does not support balanced multipoint.

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Figure 11-15
HDLC Modes

• A mode in HDLC is the relationship between two
  devices involved in an exchange the mode
  describes who controls the link.
• Normal response mode (NRM)
• - A secondary device must have permission from
  the primary device before transmitting.
• Asynchronous response mode (ARM)
• - A secondary may initiate a transmission
  without permission from the primary whenever the
  channel is idle.
• Asynchronous balanced mode (ABM)
• - All stations are equal and therefore only
  combined stations connected in point-to-point are
  used.

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Figure 11-16
HDLC Frame Types

• Information frames (I-frames)
• Supervisory frames (S-frames)
• - data link layer flow and error control.
• Unnumbered frames (U-frames)
• - reserved for system management.

FCS (Frame check sequence)
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Figure 11-17
HDLC Frame Types
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Figure 11-18
Bit Stuffing and Removal

• Bit 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 a flag.

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Figure 11-19
Bit Stuffing in HDLC
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Figure 11-20
HDLC Addressing Field

• If a primary station creates a frame, address
  field contains a to address.
• If a secondary station creates the frame, address
  field contains a from address.

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Figure 11-21
HDLC Control Fields
Flow and error control sequences
S-Frame Return N(R) when the receiver does not
have data of its own to send.
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Figure 11-22
HDLC Control Fields in the Extended Mode
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Figure 11-23
Poll/Final Field in HDLC

• P/F bit
• It has meaning only when it is set (bit 1) and
  can mean poll or final.
• It means poll when the frame is sent by a primary
  station to a secondary station.
• It means final when the frame is sent by a
  secondary station to a primary station.

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Figure 11-24
Information Field in HDLC
Piggybacking means combing data to be sent and a
acknowledgment of the frame received in one
single frame.
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Figure 11-25
Frame Check Sequence Field
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Figure 11-26
S-Frame Control Field

• RR
• - ACK
• - Poll
• - Negative response to poll
• - Positive response to select
• RNR
• - ACK
• - Select
• - Negative response to select
• REJ
• - go-back-n ARQ
• SREJ
• - selective-reject ARQ

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Figure 11-27
Use of P/F Bit
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Figure 11-28
U-Frame Control Field in HDLC

• Five basic functional categories
• Mode Setting
• Unnumbered-Exchange
• Disconnect
• Initialization Mode
• Miscellaneous

??) Table 11.2 U-frame control command and
response
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Figure 11-29
To address
Example of Polling
From address
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Figure 11-30
Example of Selecting
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Figure 11-31
Example of Peer-to Peer Communication
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Figure 11-32
Example 11.4
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Link Access Protocols

• Link access procedures, balanced (LAPB)
• Basic control functions required for
  communication between a DTE and a DCE.
• Used in Integrated Services Digital Network
  (ISDN) on B channels.
• Link access procedure for D channel (LAPD)
• Integrated Services Digital Network (ISDN)
• Use asynchronous balanced mode (ABM)
• Link access procedure for modems (LAPM)

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Summary

• Asynchronous Protocols
• XMODEM, YMODEM, ZMODEM, BLAST, and Ketmit, etc.
• Synchronous Protocols
• Character-oriented protocols
• Binary synchronous communication (BSC)
• Bit-oriented protocols
• High-level data link control (HDLC)