EE3900 Computer Networks Data Link Control Slide 1

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Data Link Control Chapter 7
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Placement of the Data Link Protocol
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Functions of Data Link Control

• Frame synchronization
• Flow control
• Error control
• Addressing
• Control and data on same link
• Link management
• By performing all of the above functions, the
  datalink layer aims at providing a reliable
  point-to-point communication link for used by the
  upper layers

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Frame Synchronization

• Why framing?
• Easier to detect errors by breaking the bit
  stream up into discrete frames and compute the
  checksum for each frame
• Data in upper layer (e.g. IP layer in TCP/IP) is
  organized in units of packets

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Frame Synchronization

• Common methods
• Character count
• Starting and ending characters, with character
  stuffing
• Starting and ending flags, with bit stuffing
  (will be discussed in HDLC)
• Others

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Character Count
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Starting/ending Characters
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Flow Control

• to assure that transmitting entity does not
  overwhelm receiving entity with data
• size of receiver's buffer is limited

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Common Flow Control Methods

• Stop and Wait Protocol
• Sliding Window Protocols

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We first consider flow control for for error-free
transmission. For transmission with errors,
techniques such as Automatic Repeat reQuest (ARQ,
will be discussed later) are used.
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Stop and Wait

• Source transmit a frame, stop and wait for
  acknowledgement
• Destination send back an acknowledgement after
  reception
• Source send the next frame when ACK is received
• Destination can stop flow by not send ACK
• Works well for a few large frames

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Fragmentation

• Large block of data may be split into small
  frames
• Limited buffer size
• Errors detected sooner (when whole frame
  received)
• On error, retransmission of smaller frames is
  needed
• Prevents one station occupying medium for long
  periods
• Stop and wait becomes inadequate

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Stop and Wait - Utilization

• a (Propagation Delay)/(Frame transmission
  time)
• agt1 under-utilized
• alt1 inefficiently utilized (since time is still
  wasted in waiting the ACK)
• therefore, not suitable for very high data rates
  or very long distance transmission (why?)

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Sliding-Window

• Efficiency improved if multiple frames can be
  transmitted at the same time
• Consider transmission from A to
• B
• - B can buffers n frames
• - A can send up to n frames without ACK
• - or window size n
• - frame sequence number 0 to m-1
• - nltm, and m is a power of 2

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Sliding Window Enhacements

• Receiver can acknowledge frames without
  permitting further transmission (Receive Not
  Ready)
• Must send a normal acknowledge to resume
• If duplex, use piggybacking
• If no data to send, use acknowledgement frame
• If data but no acknowledgement to send, send last
  acknowledgement number again

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Why Error Detection?

• Consider a transmission system with BER1E-6
• Frame size 1000 bits
• Prob that a frame received with no error
  0.999, or 1 error frame per 1000 transmitted
  frames, too large!
• Frame error rate increases when frame size
  increases

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Cyclic Redundancy Check (CRC)

• K-bit message
• n-bit frame check sequence (FCS)
• use Modulo 2 Arithmetic, just the same as
  exclusive-or operation
• 1111 11001
• 1010 11
• --------- -----------
• 0101 11001
• 11001
• -----------
• 101011

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Generation of FCS

• Define
• T(kn)-bit transmitted frame, with nltk
• Mk-bit message
• Fn-bit FCS
• Pa predetermined (n1)-bit divisor
• T2nM F, where FRemainder of (2nM)/P

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Example of FCS Generation

• M1010001101 (10 bits)
• P110101 (6 bits)
• F to be calculated, should be 5 bits
• 1101010110
• P? 110101? 101000110100000 ? 2nM
• 110101
• 111011
• 110101
• 111010
• 110101
• 111110
• 110101
• 101100
• 110101
• 110010
• 110101
• 01110 ? F

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Example of Error Checking

• M1010001101 (10 bits)
• P110101 (6 bits)
• F01110
• 1101010110
• P? 110101? 101000110101110 ? T
• 110101
• 111011
• 110101
• 111010
• 110101
• 111110
• 110101
• 101111
• 110101
• 110101
• 110101
• 00000 ? No Error!

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Widely Used Polynomials

• CRC-16 X16 X15 X2 1
• CRC-CCITT X16 X12 X5 1
• CRC-32 X32 X26 X23 X16 X12 X11
• X10 X8 X7 X5 X4 X2 X 1

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Automatic Repeat Request (ARQ)

• Why ARQ? Error-free transmission is not possible
  in real life
• ARQ involves
• Error detection
• Positive acknowledgement
• Retransmission after timeout
• Negative acknowledgement
• and retransmission

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Stop-and-Wait ARQ

• based on Stop-and-Wait flow control, plus timeout
  mechanism
• simple
• inefficient

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Stop and Wait ARQ

• Source transmits single frame
• Wait for ACK
• If received frame damaged, discard it
• Transmitter has timeout
• If no ACK within timeout, retransmit
• If ACK damaged,transmitter will not recognize it
• Transmitter will retransmit
• Receive gets two copies of frame
• Use ACK0 and ACK1

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Go-back-N ARQ

• allow multiple frames to be
• transmitted at the same time
• to improve performance
• ve ack (RRReceive Ready)
• for sliding-window flow
• control
• -ve ack (REJReject) for
• frame retransmission request

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RR2 Receive Ready 2, or the receiver is now
ready to receive frame 2, or the receiver is now
looking for frame 2 P bit indicates that P-bit
timer expires
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Selective-reject ARQ

• Only frames with -ve ack (SREJ) are
  retransmitted
• more efficient
• larger buffer than Go-back-N

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High-level Data Link Control (HDLC)

• The most important data
• link control protocol
• 3 station types
• 2 link configurations
• 3 data transfer models

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Station Types

• Primary station
• Controls operation of link
• Frames issued are called commands
• Maintains separate logical link to each secondary
  station
• Secondary station
• Under control of primary station
• Frames issued called responses
• Combined station
• May issue commands and responses

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HDLC Link Configurations

• Unbalanced
• One primary and one or more secondary stations
• Supports full duplex and half duplex
• Balanced
• Two combined stations
• Supports full duplex and half duplex

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Data Transfer Modes

• Normal response mode
• (NRM)
• used with unbalanced configuration
• used on multidrop lines
• Asynchronous response
• mode (ARM)
• rarely used
• Asynchronous balanced
• mode (ABM)

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Asynchronous Balanced mode (ABM)

• most widely used of the 3 modes
• used with a balanced configuration
• each combined station may initiate
• transmission without receiving
• permission from the other
• used as the data link layer protocol
• of the widely used packet-switched
• X.25 networks

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Frame Structure

• Synchronous transmission
• All transmissions in frames
• Single frame format for all data and control
  exchanges

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• Flag Field
• delimit the frame at both ends with 01111110
• a single flag may be used as the closing flag of
• one frame and the opening flag of the next
• receiver continuously hunting for the flag, if
• found, it continues to hunt for ending flag
• if the pattern 01111110 appears inside the
  frame,
• then ...

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Bit stuffing
Figure 6. 11 Bit Stuffing
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• Address Field
• for identifying the secondary station
• not needed for point-to-point link (e.g. PPP)
• 11111111 means all stations, for broadcast use
• Control Field
• HDLC defines 3 types of frames, each with a
  different
• control field format
• Information frames (I-frames) carry the data
• Supervisory frames (S-frames) provide the ARQ
• mechanism when piggybacking is not used (e.g.
• when there is acknowledgement to be sent, but
  no
• data to be sent back)
• Unnumbered frames (U-frames) provide
  supplemental
• link control functions

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• Notes
• 3-bit sequence numbers are used
• N(S) is the sequence number of the frame
• N(R) which number I-frame expected to be
  received
• S indicate the flow control and error control
  functions
• Receive Ready (RR)
• Receive Not Ready (RNR)
• Reject (REJ) initiate the go-back-N ARQ
• Selective Reject (SREJ) request retransmission
  of
• just a single frame

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• Poll/Final Bit
• Use depends on context
• Command frame
• P bit
• 1 to solicit (poll) response from peer
• Response frame
• F bit
• 1 indicates response to soliciting command
• Information Field
• present only in I-frames and some U-frames
• in I-frames it contains upper layer data (e.g.
  IP packets)
• Frame Check Sequence Field
• normal code is the 16-bit CRC-CCITT

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HDLC Operations

• Consists of the exchange of I-frames, S-frames,
  and U-frames
• Involves 3 phases
• - Initialization
• - Data Transfer
• - Disconnect

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• Initialization
• signals the other side that initialization is
  requested
• specifies the mode (NRM, ABM, or ARM)
• specifies whether 3- or 7-bit sequence numbers
  are used
• example

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Examples of Operation (1)
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Examples of Operation (2)
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Other Data Link Protocols

• LAPB
• used in X.25
• subset of HDLC which provides only the
• Asynchronous Balanced Mode (ABM)
• LAPD
• used in ISDN
• also similar to HDLC
• LAPDm used in GSM
• Point-to-Point Protocol (PPP)
• use subset of HDLC
• widely used in dialup access to Internet
• also widely used in connecting WAN routers
• LLC
• IEEE 802, used in LAN