Telecommunication Technologies
Week 8
Flow Control
Error Control
EIE325: Telecommunication Technologies
Maciej J. Ogorzałek, PolyU, EIE
Where are we?
 Up until now:
Physical Layer
 Now: Data Link
Layer
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Flow Control
Error Control
“sending
“sendingdata
signals
overover
a data
a
communications
transmission link”
link”
 Later…
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Synchronisation
Multiplexing
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
 Propagation time

Time for a bit to traverse the link
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Stop and Wait
 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 send ACK
 Works well for a few large frames
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Fragmentation
 Large block of data may be split into small
frames
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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
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Question
 If the data rate is 10 Mbps and transmission is
over a distance of 100 m, how long will it take
to send 1 Kb in eight 128 byte chunks with stop
and wait FC? [assume v = 2×108 ms-1].
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Stop and Wait Link Utilization
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Sliding Window Flow Control
 Allow multiple frames to be in transit
 Receiver has buffer W long
 Transmitter can send up to W frames without
ACK
 Each frame is numbered
 ACK includes number of next frame expected
 Sequence number bounded by size of field (k)
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Frames are numbered modulo 2k
W = 2k-1
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Sliding Window
Enhancements
 Receiver can acknowledge frames without
permitting further transmission (Receive Not
Ready)
 Must send a normal acknowledge to resume
 If duplex, use piggybacking
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If no data to send, use acknowledgement frame
If data but no acknowledgement to send, send last
acknowledgement number again, or have ACK
valid flag (TCP)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Error Detection
 Additional bits added by transmitter for error
detection code
 Parity
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Value of parity bit is such that character has even
(even parity) or odd (odd parity) number of ones
Even number of bit errors goes undetected
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Cyclic Redundancy Check
 For a block of k bits transmitter generates n bit
sequence
 Transmit k+n bits which is exactly divisible by
some number
 Receive divides frame by that number

If no remainder, assume no error
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Error Control
 Detection and correction of errors
 Lost frames
 Damaged frames
 Automatic repeat request
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Error detection
Positive acknowledgment
Retransmission after timeout
Negative acknowledgement and retransmission
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Automatic Repeat Request (ARQ)
 Stop and wait
 Go back N
 Selective reject (selective retransmission)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Stop and Wait
 Source transmits single frame
 Wait for ACK
 If received frame damaged, discard it
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Transmitter has timeout
If no ACK within timeout, retransmit
 If ACK damaged,transmitter will not recognize
it
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Transmitter will retransmit
Receive gets two copies of frame
Use ACK0 and ACK1
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Stop and Wait ARQ
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Stop and Wait - Pros and Cons
Simple
Inefficient
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Go Back N
 Based on sliding window
 If no error, ACK as usual with next frame
expected
 Use window to control number of outstanding
frames
 If error, reply with rejection
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Discard that frame and all future frames until error
frame received correctly
Transmitter must go back and retransmit that frame
and all subsequent frames
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Go Back N - Damaged Frame
 Receiver detects error in frame i
 Receiver sends rejection-i (REJ-i)
 Transmitter gets rejection-i
 Transmitter retransmits frame i and all
subsequent
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Go Back N - Lost Frame (1)
 Frame i lost and transmitter sends i+1
 Receiver gets frame i+1 out of sequence
 Receiver sends reject i
 Transmitter goes back to frame i and
retransmits
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Go Back N - Lost Frame (2)
 Frame i lost and no additional frame sent
 Receiver gets nothing and returns neither
acknowledgement nor rejection
 Transmitter times out and sends
acknowledgement frame with P bit set to 1
 Receiver interprets this as command which it
acknowledges with the number of the next frame
it expects (frame i )
 Transmitter then retransmits frame i
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Go Back N - Damaged
Acknowledgement
 Receiver gets frame i and send
acknowledgement (i+1) which is lost
 Acknowledgements are cumulative, so next
acknowledgement (i+n) may arrive before
transmitter times out on frame i
 If transmitter times out, it sends
acknowledgement with P bit set as before
 This can be repeated a number of times before
a reset procedure is initiated
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Go Back N - Damaged Rejection
 Receiver sends rejection which is lost
 As for lost frame (2)
 (Transmitter times out and sends
acknowledgement frame with P bit set to 1)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Selective Reject
 Also called selective retransmission
 Only rejected frames are retransmitted
 Subsequent frames are accepted by the
receiver and buffered
 Minimises retransmission
 Receiver must maintain large enough buffer
 More complex log at transmitter
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Performance Issues
 What is the line utilisation for various flow
control methods?
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Stop and Wait
Sliding Window
 What is the line utilisation for various ARQ
schemes?
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Stop and Wait
Selective Reject
Go Back N
EIE325: Telecommunication Technologies
Which is best ???
Maciej Ogorzałek, PolyU, EIE
Stop and Wait Flow Control
 Assume stations A and B are communicating
frames F1,F2,F3,…,Fn
 Total transmission time T = nTF where TF is the
time to transmit one frame and receive an
acknowledgement
 TF = tprop+tframe+tproc+tprop+tack+tproc
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tprop: propagation delay
tframe: time spent transmitting a frame
tproc: processing time
tack:time spent transmitting an acknowledgement
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Stop and Wait Flow Control
 TF = tprop+tframe+tproc+tprop+tack+tproc
= 2tprop+tframe
T = n(2tprop+tframe)
 T is the total time to transmit, the actual time
spent transmitting is only ntframe
 Line utilisation U = ntframe/T
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Line utilisation
 Define line utilisation U as the ratio of
transmission time to the time taken to transmit
data
 For Stop and Wait
define a = tprop/tframe, then U = 1/(1+2a)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
The parameter a
 We have defined a = tprop/tframe
 Alternatively, define
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V: propagation speed (ms-1)
d: transmission distance (m)
R: data rate (bps)
L: frame size (bits)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Spread Spectrum
 Analog or digital data
 Analog signal
 Objective: Spread data over wide bandwidth
 Makes jamming and interception harder
 Frequency hoping
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Signal broadcast over seemingly random series of frequencies
 Direct Sequence
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Each bit is represented by multiple bits in transmitted signal
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Chipping code
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
General model for SS
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Spread Spectrum
 Frequency hoping
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Signal broadcast over seemingly random series
of frequencies
 Direct Sequence
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Each bit is represented by multiple bits in
transmitted signal
Chipping code
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Direct Sequence
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Question
 What is the bandwidth of a digital data
stream encoded with direct sequence
spreading?
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Generating noise
 Need to generate same “noise” at source
and destination
 Computers are deterministic – generating
true noise is not possible (i.e. no truly
random numbers)
 May use
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pseudo-random algorithm
predetermined sequences (e.g. Gold
sequences)
chaos
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE