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Transcript Spread Spectrum
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
Flow Control
Error Control
“sending
“sendingdata
signals
overover
a data
a
communications
transmission link”
link”
Later…
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
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)
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
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
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
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
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
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
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?
Stop and Wait
Sliding Window
What is the line utilisation for various ARQ
schemes?
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
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
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
Signal broadcast over seemingly random series of frequencies
Direct Sequence
Each bit is represented by multiple bits in transmitted signal
Chipping code
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
General model for SS
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Spread Spectrum
Frequency hoping
Signal broadcast over seemingly random series
of frequencies
Direct Sequence
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
pseudo-random algorithm
predetermined sequences (e.g. Gold
sequences)
chaos
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE