Data Link Protocols

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Data Link Protocols

• Asynchronous Protocols
• Synchronous Protocols
• Character-Oriented Protocols
• Bit-Oriented Protocols

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DATA LINK PROTOCOLS

• DLP are sets of specification used to implement
  the data link layer.
• To this end, they contain rules for line
  discipline, flow control, and error handling,
  among others.

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Figure 11-1
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Figure 11-2
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Asynchronous protocols

• A number of AP have been developed. See fig 11.2
• These protocol are employed mainly in modems.
• Has been replaced by synchronous protocol due to
  its slowness (start and stop bit)
• Adv. Not complex and inexpensive to implement.

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XMODEM

• Develop in 1979 for telephone-line communication
  between PCs. (FTP)
• The first field is a one-byte SOH. The second
  field is a two-byte header.
• The first header byte, the sequence number,
  carries the frame number. The second header byte
  is used to check the validity of the sequence
  number.
• The fixed data field holds 128 bytes of data. The
  last field, CRC, checks for errors in the data
  field only. See fig. 11.3

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Figure 11-3
XMODEM
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YMODEM

• Protocol similar to XMODEM, with the following
  major differences
• The data unit is 1024 bytes
• Two CANs are sent to abort a transmission
• ITU-CRC-16 is used for error checking.
• Multiple files can be sent simultaneouly.

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Other Asynchronous Protocol

• Zmodem-newer protocol combination of xmodem and
  ymodem
• BLAST-blocked asynchronous transmisstion is more
  powerful than XMODEM. Full-duplex with sliding
  window flow control.
• Allows the transfer of data and binary files
• Kermit is currently the most widely used
  asynchronous protocol.
• The file transfer protocol is similar to XMODEM
  but much more powerful. Designed at columbia
  university.
• See for details http//www.columbia.edu/kermit/

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Synchronous protocols

• Better choice due to its speed. Used for LAN, MAN
  and WAN technology. Can be divided into 2 classes
  fig. 11.4
• Bit-oriented protocols
• The frame or packet is interpreted as a series of
  bits.
• Character-oriented protocols(byte oriented
  protocols)
• The frame or packet is interpreted as a series of
  characters
• All control info is in the form of an existing
  character encoding system. E.g. ASCII.

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Character-oriented protocols (COP)

• Not as efficient as bit-oriented protocols.
• Not widely used now. Consider obsolete
• But easy to comprehend.
• As foundation to understand bit-oriented
  protocols
• In all DLP, control information is inserted into
  the data stream as separate control frames or as
  additions to existing data frames.
• In COP, this info is in the form of code words
  taken from existing character sets such as ASCII.
  These multibit characters carry info about line
  discipline, fc, and error control.
• Best known COP is binary synchronous
  communication (BSC) developed by IBM.

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Figure 11-4
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Binary synchronous communication (BSC)

• BSC was developed in 1964.
• Usable in PPP and multipoint configurations.
• It support half-duplex transmission using
  stop-and-wait ARQ flow control and error
  correction.
• BSC doesnt support full-duplex transmission or
  sliding window protocol.
• Table 11.1 is a list of standard control
  characters used in BSC frame.

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Table 11.1

• Need more ASCII table for code used in
  information interchange?
• refer. http//www.neurophys.wisc.edu/www/comp/docs
  /ascii.html

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BSC Frames

• The BSC protocol divides a transmission into
  frames.
• If a frame is used strictly for control purposes,
  it is called control frame.
• Control frames are used to exchange info between
  comm devices. E.g. connection oriented.
• If a frame contains part or all of the message
  data itself, it is called a data frame.
• Data frames are used to transmit info, but may
  also contain ctrl info applicable to that
  information. See fig 11.5 for brief.

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Figure 11-5
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Simple data frame

• Refer fig 11.6. The arrow shows the direction of
  transm.
• Begins with 2 or more SYN char which alert the
  receiver to the arrival of a new frame and
  provide a bit pattern used by the receiving
  device to synchronize its timing with that of the
  sending device.
• STX start of text character. End of the control
  info. And the next byte will be data.
• Data or text can consist of varying number of
  characters
• An end of text (ETX) indicates the transition
  between text and more control characters
• Finally one or two characters called the block
  check count BCC are included for error detection.

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Figure 11-6
Simple Frame
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Frame with header

• Usually header will be added to include the
  address of the receiving device, the address of
  the sending device and the identifying number of
  the frame (0 or 1) for stop and wait ARQ.
• This info is included in a special field called a
  header, which begins with a SOH character.
• The header comes after the SYNs and b4 the STX
  character.
• Refer fig. 11.7

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Figure 11-7
A Frame with Header
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Multiblock frames

• To minimize error, text in a messgae is often
  divided between several blocks.
• Each block, except the last one, starts with an
  STX char and ends with an intermediate text block
  (ITB).
• The last block start with an STX but ends with an
  ETX.
• Immediately after each ITB or ETX is a BCC field.
• In that way, the receiver can check each block
  separately for errors, thereby increasing the
  likelihood of detection.
• If any block contains an error, however, the
  entire frame must be retransmitted.

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Figure 11-8
Multiblock Frame
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Multiframe transmission

• Some messages, may be too long to fit into the
  format of a single frame.
• In such cases, the sender can split the message
  not only among blocks but among frames.
• Several frames can carry continuation of a single
  message.
• To let the receiver know that the end of the
  frame is not the end of transmission, the ETB
  character in all frames but the last one is
  replaced by end of transmission (ETX).
• The receiver must acknowledge each frame
  separately but cannot take over control of the
  link until it sees the ETX in the last frame.
  (See fig. 11.9)

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Multiframe Transmission
Figure 11-9
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Control frame

• A control frame is used by one device to send
  commands to, or solicit info from, another
  device.
• A control frame contains control char but no
  data it carries info specific to the functioning
  of the data link layer itself. See fig. 11.10.
• Control frame serve 3 purposes establishing
  connection, maintaining flow and error control
  during data transmission and terminating
  connections see fig 11.11

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Figure 11-10
Control Frame
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Figure 11-11
Control Frames
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Figure 11-11-continued
Control Frames
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Figure 11-11-continued
Control Frames
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Data transparency problem in COP

• E.g. a receiver seeing the bit pattern 0000011 in
  data and reads it as an ETX char (ctrl char).
• Data transparency in BSC is achieved by a process
  called byte stuffing.
• It involves two activities
• Defining the transparent text region with the
  data link escape (DLE) characters and preceding
  any DLE character within the transparent region
  by an extra DLE.
• To define the transparent region, we insert one
  DLE char b4 the STX char at the beginning of the
  txt field and another just b4 the ETX.

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• Cont
• The first DLE tells the receiver that the text
  may contain control characters and to ignore
  them.
• The last DLE tells the receiver that the
  transparent region has ended.
• Problems may still arise if the transparent
  region contains a DLE char as text. In that case,
  we insert an additional DLE just b4 each DLE
  within the text.
• See fig 11.12.

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Figure 11-12
Byte Stuffing