Data and Computer Communications

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Data and Computer Communications

• Updated 2/9/2009

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Data Link Control Protocols

• They are required to provide a layer of logic to
  manage exchange of data over a link
• frame synchronization
• flow control
• error control
• addressing
• control and data
• link management

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Flow Control

• Objectives
• Sender does not flood the receiver - prevents
  buffer overflow
• Maximize throughput
• Basic Idea
• Sender throttled until receiver grants permission
• Impacted by
• Transmission time
• time taken to emit all bits into medium
• Propagation time
• time for a bit to traverse the link
• Two basic types are discussed
• Stop-and-Wait FC
• Sliding Window FC

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Space Time Diagrams

• A virtual time sequence diagram
• Illustrating the sender-receiver relationship

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Stop and Wait Flow Control

• Basic Idea
• Source transmits frame
• Destination receives frame and replies with
  acknowledgement (ACK)
• Source waits for ACK before sending next
• Destination can stop flow by not send ACK

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Performance

• Works well for a few large frames
• Stop and wait becomes inadequate if large block
  of data is split into small frames

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Efficiency of StopWait
We define parameter a
Utilization (or efficiency) can be calculated
as
What happens if a changes?
Examples of signal speed Light in vacuum 300
m/ms Light in fiber 200 m/ms Electricity 250
m/ms
The textbook uses B for tprop and L for tframe
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Performance Examples
How high is the satellite?
Assume data rate 56 Kbps Find tfrme, a, and
Utilization
Assume frame size is 500 bytesFind tfrme, a,
and Utilization
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Performance Examples
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Maximum Utilization
Utilization
Ideal
Plot the efficiency plot For the problems using
Matlab
Actual
a
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Sliding Windows Flow Control

• Allows multiple numbered frames to be transmitted
• Receiver has buffer W long
• Transmitter sends up to W frames without ACK
• Receiver sends an ACK
• ACK includes number of next frame expected
• Sequence number is bounded by size of field (k)
• frames are numbered modulo 2k
• giving max window size of up to 2k - 1
• Receiver can ack frames without permitting
  further transmission (Receive Not Ready)
• Must send a normal acknowledge to resume
• if have full-duplex link, can piggyback ACks

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Sliding Window Diagram
W5, 3 bits are required, ACK 0,1,2,3,4,0,1,
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Sliding Window Example
Why W is not 8?
Window shrinking until ACK is received
Window expands as soon as ACK is sent
Window expands as soon as ACK is received
W7, 3 bits are required, ACK 0,1,2,3,4,6,7,0,1,
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Efficiency of Sliding Window Protocol
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Error Control

• Detection and correction of errors such as
• Lost frames
• Damaged frames
• Common techniques use
• Error detection
• Positive acknowledgment
• Retransmission after timeout
• Negative acknowledgement retransmission

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Automatic Repeat Request (ARQ)

• Error control mechanisms used in
• stop and wait
• go back N
• selective reject (selective retransmission)

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ARQ in Stop and Wait
ACK0 / ACK1

• 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 alternate numbering and ACK0 / ACK1
• pros and cons
• simple
• inefficient

Remember Special case where W1
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ARQ in Go Back N

• Based on sliding window
• If no error, ACK (RR) as usual
• Use window to control number of outstanding
  frames
• If error, reply with rejection (REJ)
• Discard that frame and all future frames until
  error frame received correctly
• Transmitter must go back and retransmit that
  frame and all subsequent frames

Frames 5 6 are resent
Lost ACK, P1
Nothing sent until ACK is received
Lost ACK, P1
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Go Back N Scenarios which are handled

• Damaged Frame
• Frame received with error
• Frame lost
• Last frame lost
• Damaged Ack
• One ack lost, next one makes it
• All acks lost
• Damaged Nack
• Maximum Window 2n -1 with n-bit sequence numbers

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Selective Reject

• Also called selective retransmission
• Only rejected frames are retransmitted
• Subsequent frames are accepted by the receiver
  and buffered
• Cons and pros
• Minimizes retransmission
• Receiver must maintain large enough buffer
• More complex logic in transmitter
• Applications
• Less widely used
• Useful for satellite links with long propagation
  delays

Resend Only Corrupted one
Only frame 4 is resent
P1 Resent ACK
Nothing sent until ACK is received
Applet http//media.pearsoncmg.com/aw/aw_kurose_n
etwork_3/applets/SelectRepeat/SR.html
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Performance Comparison
P is the probability that a single frame is in
error Assume that ACK and NAK are never in
error.
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High Level Data Link Control (HDLC)

• An important data link control protocol
• station types
• Primary issue commands
• Secondary - issue Responses
• Combined - issues commands and responses
• link configurations
• Unbalanced - 1 primary, multiple secondary
• Balanced - 2 combined stations

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HDLC Transfer Modes

• Normal Response Mode (NRM)
• Response from secondary
• Asynchronous Balanced Mode (ABM)
• Combined Station, either station initiates
  transmission, has no polling overhead, widely
  used
• Asynchronous Response Mode (ARM)
• unbalanced config, secondary may initiate
  transmit without permission from primary, rarely
  used

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HDLC Frame Structure

• synchronous transmission of frames
• single frame format used for all data types

Data
Header
Trailer
Frame Check Sequence FCS
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Flag Fields and Bit Stuffing
From Jain
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Address Field

• Identifies secondary station that sends or
  receives frame
• Usually 8 bits long
• May be extended to multiples of 7 bits
• LSB indicates if is the last octet (1) or not (0)
• all ones address 11111111 is broadcast

Frames arte multiples of 8-bit (octets)
Last octet
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Control Field

• Different for different frame type (I, S, U)
• Information - data transmitted to user (next
  layer up)
• Flow and error control piggybacked on information
  frames (includes N(s) and N(R))
• Supervisory - ARQ when piggyback not used
• Unnumbered - supplementary link control
• First 1-2 bits of control field identify frame
  type
• I0, S10, U11

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Control Field

• Use of Poll/Final bit depends on context
• In command frame is P bit set to 1 to solicit
  (poll) response from peer
• In response frame is F bit set to 1 to indicate
  response to soliciting command
• Sequence number usually 3 bits
• can extend to 7 bits as shown below

SSupervisory function bit
http//www.erg.abdn.ac.uk/users/gorry/course/dl-pa
ges/hdlc-control.html
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Information FCS Fields

• Information Field
• Exists in information and some unnumbered frames
• It must contain integral number of octets
• Variable length
• Frame Check Sequence Field (FCS)
• Used for error detection
• Either 16 bit CRC or 32 bit CRC

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HDLC Operation

• consists of exchange of information, supervisory
  and unnumbered frames
• Consists of three phases
• Initialization
• by either side, set mode sequence
• data transfer
• with flow and error control
• using both I S-frames (RR, RNR, REJ, SREJ)
• disconnect
• when ready or fault noted

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HDLC Operation Example
Set normal response/responses
Set mode accepted
Full Duplex
Refer to HDLC Commands HDLC Commands
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HDLC Operation Example