CPEG 419 Introduction to Networks

1
CPEG 419Introduction to Networks

• Week 2

2
Administrative Issues

• Homework 1 assigned.
• Due in 2 weeks.

3
Transmission Impairments

• Types of impairments
• Attenuation.
• Delay distortion.
• Noise.
• Multi-path Fading (wireless only).

4
Attenuation

• Weakening of the signals power as it propagates
  through medium.
• Function of medium type
• Guided medium (wired) logarithmic with distance.
  
• Unguided medium (wireless) more complex
  (function of distance and atmospheric conditions).

5
Attenuation

• Problems and solutions
• Insufficient signal strength for receiver to
  distinguish between the signal and noise use
  amplifiers/repeaters to boost/regenerate signal.
• Attenuation increases with frequency special
  amplifiers to amplify high-frequencies
  (equalization).

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Attenuation
Let Rf be the received signal power at frequency
f Let Tf be the transmitted signal power at
frequency f
The attenuation in dB is
7
Delay Distortion

• Speed of propagation in guided media varies with
  frequency.
• Different frequency components arrive at receiver
  at different times (more about this later).
• Solution
• equalization techniques to equalize distortion
  for different frequencies.
• Use fewer frequencies.

8
Noise

• Noise undesired signals inserted anywhere in the
  source/destination path.
• Different categories thermal (white), crosstalk,
  impulse, etc.

noise
received signal is an attenuated version of the
transmitted signal plus noise.
attenuation
transmitter

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Thermal Noise

• Any conductor and electronic device has noise due
  to thermal agitation of electrons
• The thermal noise found in 1Hz is
• N k ?T (W/Hz)
• k 1.3 e 23 (Boltzmanns constant)
• T is the temperature in Kelvin
• N is noise power in watts per 1Hz of bandwidth
  (dBW)
• Total noise is
• N k ?T ? B
• B is total bandwidth.

10
Crosstalk

• Wires act as antennas. They broadcast energy when
  the signal switches and receive energy for any
  other source (e.g., other wires, radios,
  microwave ovens, the big bang, etc.).
• Crosstalk can be reduced by careful shielding and
  using twisted pairs.
• The longer the wires, the more significant the
  crosstalk.

power found on the wire of interest
Crosstalk gain is
power at other wires
Suppose that 10 dBW is transmitted on other
wires. And the crosstalk gain is 3. Then the
noise received had power is 7 dBW.
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Other noises

• Coupling through common impedance (power supply
  noise). This is a major source at the transmitter
  and receiver.
• Galvanic Action. Dissimilar metals and moisture
  produce a chemical wet cell (battery).
• Triboelectric effect from bends in cable.
• Shot Noise. Present in semiconductors.
• Contact noise. Due to imperfect contacts.
• Popcorn noise. Minor defects in junction in a
  semiconductor, often due to metallic impurities.

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Decibel and Signal-to-Noise Ratio

• Decibel (dB) measures relative strength of 2
  signals.
• Example S1 and S2 with powers P1 and P2.
• NdB 10 log10 (P1/P2)
• Signal-to-noise ratio (S/N)
• Measures signal quality.
• S/NdB 10 log10 (signal power/noise power)

13
SNR
Suppose that we transmit at a very high power, so
thermal and other noises are small compared to
crosstalk.
This depends on the cable. Furthermore, it may
not be possible to transmit at such a high power
that other noises can be neglected.
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SNR13
2 times the bit-rate
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Multi-path Fading (wireless)
Because of reflections, a signal may take many
paths from transmitter to receiver.
transmitter
Objects such as buildings, people, etc.
receiver
Signals that take alternative paths will arrive
later.
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Multi-path reflection or delay spread (wireless)
late arriving signals
line of sight signal
getting small
received signal
At 10Mbs, if the difference in paths is 30
meters, then the alternative signals arrive at
exactly the next slot. (Use the fact that light
travels a 300000000 m/s.
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Channel Capacity 1

• Channel Capacity is the rate at which data can be
  transmitted over communication channel.
• We saw earlier that to send a binary data at a
  rate R, the channel bandwidth must be greater
  than ½ R.
• So, if the bandwidth of the channel is B, it
  might be possible to transmit at a rate of 2B.

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Channel Capacity 2

• For a fixed bandwidth, the data rate can be
  increased by, increasing number of signal levels.
  However, the signal recognition at receiver is
  more complex and more noise-prone.
• The data rate becomes
• C 2B log2V, where V is number voltage levels.
• Is it possible to continually increase V to make
  C arbitrarily large?

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Channel Capacity 3

• Noisy channel Shannons Theorem
• Given channel with B (Hz) bandwidth and S/N (dB)
  signal-to-noise ratio, C (bps) is
• C B log2 (1S/N)
• Theoretical upper bound since assumes white noise
  (e.g., thermal noise, not impulse noise, etc).

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Transmission Media Chapter 4

• Physically connect transmitter and receiver
  carrying signals in the form electromagnetic
  waves.
• Types of media
• Guided waves guided along solid medium such as
  copper twisted pair, coaxial cable, optical
  fiber.
• Unguided wireless transmission (atmosphere,
  outer space).

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Guided Media Examples 1

• Twisted Pair
• 2 insulated copper wires arranged in regular
  spiral. Typically, several of these pairs are
  bundled into a cable. (What happens if the twist
  is not regular? Reflection?)
• Cheapest and most widely used limited in
  distance, bandwidth, and data rate.
• Applications telephone system (home-local
  exchange connection).
• Unshielded and shielded twisted pair.
• What is a differential amplifier?

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Guided Media Examples 1

• Twisted pair continued
• Category 3 Unshielded twisted pair (UTP) up to
  16MHz.
• Cat 5 UTP to 100 MHz.
• Table 4.2. Suppose Cat 5 at 200m (the limit of
  100Mbps ethernet is 300m).
• The dB attenuation at 100m is 22.0. So at 200m,
  the attenuation is 44. Suppose we transmit at
  80dBW. Then the received signal has energy of
  124dBW.
• The near-end crosstalk is 32dB per 100m. So the
  crosstalk energy is at 144dBW.
• The SNR is 20dB (neglecting thermal noise).

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Examples 2

• Coaxial Cable
• Hollow outer cylinder conductor surrounding inner
  wire conductor dielectric (non-conducting)
  material in the middle.
• Less capacitance than twisted pair, so less loss
  at high frequencies. Also, Coaxial has more
  uniform impedance.
• Applications cable TV, long-distance telephone
  system, LANs.
• Repeaters are required every few kilometers at
  500MHz.
• s Higher data rates and frequencies, better
  interference and crosstalk immunity.
• -s Attenuation at high frequency (up to 2 GHz
  is OK) and thermal noise.

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Examples 3

• Optical Fiber
• Thin, flexible cable that conducts optical waves.
• Applications long-distance telecommunications,
  LANs (repeaters every 40km at 370THz!).
• s greater capacity, smaller and lighter, lower
  attenuation, better isolation,
• -s Not currently installed in subscriber loop.
  Easier to make use to current cables than install
  fiber.

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Examples 3 types of fiber

• Step-index multimode

lower index of refraction
shorter path
longer path
absorbed
higher index of refraction
total internal reflection
Since the signal can take many different paths,
the arrival the received signal is smeared.
Input Signal
Output Signal
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Examples 3 types of fiber

• Single mode

If the fiber core is on the order of a
wavelength, then only one mode can
pass. Wavelengths are 850nm, 1300nm and 1550nm
(visible spectrum is 400-700nm). 1550nm is the
best for highest and long distances.
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Wavelength-division multiplexing (WDM)

• Wavelength-division multiplexing
• Multiple colors are transmitted.
• Each color corresponds to a different channel.
• In 1997, Bell Labs had 100 colors each at 10Gbps
  (1Tbps).
• Commercial products have 80 colors at 10Gbps.

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Wireless Transmission

• Omni-directional the signal is transmitted
  uniformly in all directions.
• Directional the signal is transmitted only in
  one direction. This is only possible for high
  frequency signals.

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Terrestrial Microwave

• Parabolic dish on a tower or top of a building.
• Directional.
• Line of sight.
• With antennas 100m high, they can be 82 km (50
  miles).
• Use 2 40 GHz.
• 2 GHz bandwidth 7MHz, data rate 12 Mbps
• 11 GHz bandwidth 220MHz, data rate 274 Mbps

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Satellite Microwave

• 1 10 GHz (Above 10 GHz, the atmosphere
  attenuates the signal, and below 1 GHz there is
  too much noise).
• Typically, 5.925 to 6.425 GHz for earth to
  satellite and 4.2 to 4.7 GHz for satellite to
  earth. (Why different frequencies?)
• A stationary satellite must be 35,784 km (22000
  miles) above the earth.
• The round-trip delay is about ½ a second.

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Other

• Cell phones Omni-directional. GSM-900 uses
  900MHz, GSM-1800 and GSM-1900 (PCS). Typical data
  rate seems to be around 40kbps. But the protocol
  is specified to 171kbps.
• 802.11 wireless LANs
• Omni-directional
• 802.11b 2.4 GHz up to 11Mbps
• 802.11a 5 GHz up to 54Mbps
• Infrared Line of sight, short distances.

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Types of Connections

• Long-haul about 1500km (1000 miles) undersea,
  between major cites, etc. High capacity
  20000-60000 voice channels. Twisted pair,
  coaxial, fiber and microwave are used here.
  Microwave and fiber are still being installed.
• Metropolitan trunks 12km (7.5 miles) 100,000
  voice channels. Link long-haul to city and within
  a city. Large area of growth. Mostly twisted pair
  and fiber are used here.
• Rural exchange trunks 40-160km link towns.
  Twisted pair, fiber and microwave are used here.
• Subscriber loop run from a central exchange to
  a subscriber. This connection uses twisted pair,
  and will likely stay that way for a long time.
  Cable uses coaxial and is a type of subscriber
  loop (it goes from central office to homes). But
  a large number of people share the same cable.
• Local area networks (LAN) typically under 300m.
  Sizes range from a single floor, a whole
  building, or an entire campus. While some use
  fiber, most use twisted pair as twisted pair is
  already installed in most buildings. Wireless
  (802.11) is also being used for LAN.

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Data Encoding

• Transforming original signal just before
  transmission.
• Both analog and digital data can be encoded into
  either analog or digital signals.

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Digital/Analog Encoding
Encoding
g(t)
g(t)
(D/A)
Encoder
Digital Medium
Decoder
Source
Destination
Source System
Destination System
Modulation
g(t)
g(t)
(D/A)
Modulator
Analog Medium
Demodulator
Source
Destination
Source System
Destination System
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Encoding Considerations

• Digital signaling can use modern digital
  transmission infrastructure.
• Some media like fiber and unguided media only
  carry analog signals.
• Analog-to-analog conversion used to shift signal
  to use another portion of spectrum for better
  channel utilization (frequency division muxing).

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Digital Transmission Terminology

• Data element bit.
• Signaling element encoding of data element for
  transmission.
• Unipolar signaling signaling elements have same
  polarization (all or all -).
• Polar signaling different polarization for
  different elements.

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More Terminology

• Data rate rate in bps at which data is
  transmitted for data rate of R, bit duration
  (time to emit 1 bit) is 1/R sec.
• Modulation rate baud rate (rate at which signal
  levels change).

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Digital Transmission Receiver-Side Issues

• Clocking determining the beginning and end of
  each bit.
• Transmitting long sequences of 0s or 1s can
  cause synchronization problems.
• Signal level determining whether the signal
  represents the high (logic 1) or low (logic 0)
  levels.
• S/N ratio is a factor.

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Comparing Digital Encoding Techniques

• Signal spectrum high frequency means high
  bandwidth required for transmission.
• Clocking transmitted signal should be
  self-clocking.
• Error detection built in the encoding scheme.
• Noise immunity low bit error rate.

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Digital-to-Digital Encoding Techniques

• Nonreturn to Zero (NRZ)
• Multilevel Binary
• Biphase
• Scrambling

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NRZ Techniques

• Use of 2 different voltage levels.
• NRZ-L positive voltage represents one binary
  value negative voltage, the other.
• NRZI (Nonreturn to zero, invert on ones)
  transition (low-to-high or high-to-low)
  represents 1 no transition, 0.
• NRZI is an example of differential encoding
  decoding based on comparing polarity of adjacent
  signal elements.

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Multilevel Binary

• Use more than 2 signal levels.
• Bipolar-AMI 0 no signal 1 positive and
  negative pulse consecutive 1s alternate in
  polarity avoid synchronization loss.
• Pseudoternary opposite representation.
• Long sequence of 0s or 1s still a problem for
  bipolar-AMI and pseudoternary respectively.

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Biphase

• Manchester transition in the middle of bit
  period.
• Carries data and provides clocking.
• Low-to-high 1.
• High-to-low 0.
• Differential Manchester
• Mid-bit transition only provides clocking.
• 0 transition in the beginning of bit interval.
• 1 no transition.

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Scrambling

• Avoid long sequences of 0s or 1s.
• Bipolar with 8-zeros substitution (B8ZS)
• Inserts transitions when transmitting 8
  consecutive 0s.
• High-density bipolar-3 zeros (HDB3)
• Inserts pulses when transmitting 4 consecutive
  0s.
• Receiver must recognize insertions and
  re-generate original signal.

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Digital-to-Analog Encoding

• Transmission of digital data using analog
  signaling.
• Example data transmission of a PTN.
• PTN voice signals ranging from 300Hz to 3400 Hz.
• Modems convert digital data to analog signals
  and back.
• Techniques ASK, FSK, and PSK.

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Amplitude-Shift Keying

• 2 binary values represented by 2 amplitudes.
• Typically, 0 represented by absence of carrier
  and 1 by presence of carrier.
• Prone to errors caused by amplitude changes.

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Frequency-Shift Keying

• 2 binary values represented by 2 frequencies.
• Frequencies f1 and f2 are offset from carrier
  frequency by same amount in opposite directions.
• Less error prone than ASK.

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Phase-Shift Keying

• Phase of carrier is shifted to represent data.
• Example 2-phase system.
• Phase shift of 90o can represent more bits aka,
  quadrature PSK.

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Analog-to-Digital Encoding

• Analog data transmitted as digital signal, or
  digitization.
• Codec device used to encode and decode analog
  data into digital signal, and back.
• 2 main techniques
• Pulse code modulation (PCM).
• Delta modulation (DM).

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Pulse Code Modulation 1

• Based on Nyquist (or sampling) theorem if f(t)
  sampled at rate gt 2signals highest frequency,
  then samples contain all the original signals
  information.
• Example if voice data is limited to 4000Hz, 8000
  samples/sec are sufficient to reconstruct
  original signal.

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PCM 2

• Analog signal -gt PAM -gt PCM.
• PAM pulse amplitude modulation samples of
  original analog signal.
• PCM quantization of PAM pulses amplitude of PAM
  pulses approximated by n-bit integer each pulse
  carries n bits.

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Delta Modulation (DM)

• Analog signal approximated by staircase function
  moving up or down by 1 quantization level every
  sampling interval.
• Bit stream produced based on derivative of analog
  signal (and not its amplitude) 1 if staircase
  goes up, 0 otherwise.
• Parameters sampling rate and step size.

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Analog-to-Analog Encoding

• Combines input signal m(t) and carrier at fc
  producing s(t) centered at fc.
• Why modulate analog data?
• Shift signals frequency for effective
  transmission.
• Allows channel multiplexing frequency-division
  multiplexing.
• Modulation techniques AM, FM, and PM.

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Amplitude Modulation (AM)

• Carrier serves as envelope to signal being
  modulated.
• Signal m(t) is being modulated by carrier cos(2p
  fct).
• Modulation index ratio between amplitude of
  input signal to carrier.

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Angle Modulation

• FM and PM are special cases of angle modulation.
• FM carriers amplitude kept constant while its
  frequency is varied according to message signal.
• PM carriers phase varies linearly with
  modulating signal m(t).

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Spread Spectrum 1

• Used to transmit analog or digital data using
  analog signaling.
• Spread information signal over wider spectrum to
  make jamming and eavesdropping more difficult.
• Popular in wireless communications

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Spread Spectrum 2

• 2 schemes
• Frequency hopping signal broadcast over random
  sequence of frequencies, hoping from one
  frequency to the next rapidly receiver must do
  the same.
• Direct Sequence each bit in original signal
  represented by series of bits in the transmitted
  signal.

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Transmission Modes

• Assuming serial transmission, ie, one signaling
  element sent at a time.
• Also assuming that 1 signaling element represents
  1 bit.
• Source and receiver must be in sync.
• 2 schemes
• asynchronous and
• synchronous transmission.

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Asynchronous Xmission 1

• Avoid synchronization problem by including sync
  information explicitly.
• Character consists of a fixed number of bits,
  depending on the code used.
• Synchronization happens for every character
  start (0) and stop (1) bits.
• Line is idle transmits 1.

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Asynchronous Xmission 2

• Example sending ABC in ASCII
• 0 10000010 1 0 01000010 1 0 110000 1 1111
• Timing requirements are not strict.
• But problems may occur.
• Significant clock drifts high data rate
  reception errors.
• Also, 2 or more bits for synchronization
  overhead!

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Synchronous Xmission 1

• No start or stop bits.
• Synchronization via
• Separate clock signal provided by transmitter or
  receiver doesnt work well over long distances.
• Embed clocking information in data signal using
  appropriate encoding technique such as Manchester
  or Differential Manchester.

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Synchronous Xmission 2

• Need to identify start/end of data block.
• Block starts with preamble (8-bit flag) and may
  end with postamble.
• Other control information may be added for data
  link layer.

8 -bit flag
8 -bit flag
Control
Control
Data
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Data Link Layer

• So far, sending signals over transmission medium.
• Data link layer responsible for error-free
  (reliable) communication between adjacent nodes.
• Functions framing, error control, flow control,
  addressing (in multipoint medium).

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Flow Control

• What is it?
• Ensures that transmitter does not overrun
  receiver limited receiver buffer space.
• Receiver buffers data to process before passing
  it up.
• If no flow control, receiver buffers may fill up
  and data may get dropped.

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Stop-and-Wait

• Simplest form of flow control.
• Transmitter sends frame and waits.
• Receiver receives frame and sends ACK.
• Transmitter gets ACK, sends other frame, and
  waits, until no more frames to send.
• Good when few frames.
• Problem inefficient link utilization.
• In the case of high data rates or long
  propagation delays.

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Sliding Window 1

• Allows multiple frames to be in transit at the
  same time.
• Receiver allocates buffer space for n frames.
• Transmitter is allowed to send n (window size)
  frames without receiving ACK.
• Frame sequence number labels frames.

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Sliding Window 2

• Receiver acks frame by including sequence number
  of next expected frame.
• Cumulative ACK acks multiple frames.
• Example if receiver receives frames 2,3, and 4,
  it sends an ACK with sequence number 5, which
  acks receipt of 2, 3, and 4.

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Sliding Window 3

• Sender maintains sequence numbers its allowed to
  send receiver maintains sequence number it can
  receive. These lists are sender and receiver
  windows.
• Sequence numbers are bounded if frame reserves
  k-bit field for sequence numbers, then they can
  range from 0 2k -1 and are modulo 2k.

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Sliding Window 4

• Transmission window shrinks each time frame is
  sent, and grows each time an ACK is received.

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Example 3-bit sequence number and window size 7

• A B
• 0 1 2 3 4 5 6 7 0 1 2 3 4... 0 1 2 3 4 5
  6 7 0 1 2 3 4

0
1
2
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
RR3
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
3
0 1 2 3 4 5 6 7 0 1 2 3 4
4
5
0 1 2 3 4 5 6 7 0 1 2 3 4
RR4
6
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4