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Chapter 7 Data Link Control Protocols - ECE-UNH Home

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Chapter 7 Data Link Control Protocols - ECE-UNH Home Powered By Docstoc
					ECE 734 /834
Data Network and
Communications

Chapter 7
Data Link Control Protocols
Data Link Control Protocols
• need layer of logic above Physical
• to manage exchange of data over a link
  —flow control
  —error control
  —addressing
  —control and data
  —link management
Flow Control
• Ensuring the sending entity does not overwhelm
  the receiving entity
  —Preventing buffer overflow
• Transmission time
  —Time taken to emit all bits into medium
  —tframe = frame_size / data_rate
• Propagation time
  —Time for a bit to traverse the link
  —tprop = path_length / prop_velocity + switching delays
Model of Frame Transmission
Stop and Wait Flow Control
• Source transmits frame
• Destination receives frame and replies with
  acknowledgement (ACK)
• Source waits for ACK before sending next frame
• Destination can stop flow by not sending ACK
• Works well for large frames and short data
  paths
Stop and Wait Utilization
Stop and Wait Performance
•   Tf =total time to deliver 1 frame successfully
•   Tf = tframe + tprop + tproc + tack + tprop + tproc
•   Assume: tack = tproc = 0
•   Then, Tf = 2 * tprop + tframe
•   tframe= frame_size / data_rate = fbits / rdata
•   Effective data rate = frame size / time to deliver
•   reff = fbits / Tf = fbits / (2 * tprop + fbits / rdata)
•   Utilization = U = reff / rdata
•   U = (fbits / rdata) / (2 * tprop + fbits / rdata)
•   U = 1 / ((2 * tprop / tframe ) + 1)
Model of Frame Transmission
Error Control
•   Detection and correction of errors
•   Lost frames
•   Damaged frames
•   Automatic repeat request (ARQ)
    —Error detection
    —Positive acknowledgment
    —Retransmission after timeout
    —Negative acknowledgement and retransmission
Stop and Wait ARQ
• Source transmits single frame
• Source waits for ACK
• If received frame OK
  —Return ACK
• If received frame damaged, but header OK
  —Return NAK – (negative acknowledgement)
• If received frame header damaged, discard it
  —Transmitter has timeout
  —If no ACK within timeout, retransmit
Stop and Wait ARQ (cont)
• If ACK damaged or lost, transmitter will not
  recognize it
  —Transmitter will retransmit
  —Receive gets two copies of frame (need frame ID in
   header so receiver can ACK but then discard
   duplicate frame)
• Frame header needs:
  —To & from addresses
  —CRC (possibly separate header and data CRCs)
  —Frame ID (frame sequence number)
Stop and Wait -
Diagram
Stop and Wait - Pros and Cons
• Simple and reliable
• Inefficient
  — Except for slow data rates, large data frames and
   short data paths
  — Need: tprop << tframe
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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