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```									CPEG 419
Introduction to Networks

[Week 2]

University of Delaware CPEG 419   1

Homework #1 assigned.
 Due in 2 weeks.

University of Delaware CPEG 419   2
Transmission Impairments

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

University of Delaware CPEG 419   3
Attenuation

Weakening of the signal’s 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).

University of Delaware CPEG 419   4
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).

University of Delaware CPEG 419   5
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:

 Rf                 
A f  10 log                     
T                   
 f                  

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Delay Distortion

Speed of propagation in guided media
varies with frequency.
Different frequency components arrive at
later).
Solution:
equalization techniques to equalize distortion
for different frequencies.
Use fewer frequencies.

University of Delaware CPEG 419     7
Noise

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

noise
received signal is an attenuated
attenuation
transmitter                    +          version of the transmitted signal
plus noise.
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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 (Boltzmann’s 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.

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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.

 Sf            power found on the wire of interest
Crosstalk gain is C f  10 log        
O              power at other wires
 f     

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
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)

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SNR

Suppose that we transmit at a very high power, so thermal
and other noises are small compared to crosstalk.

 Received Signal 
SNR  10 log                    10 log Received Signal  10 log Noise
     Noise       
 10 log transmitted power  Attenuatio n - 10log transmitted power  Crosstalk
 Attenuatio n - 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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SNR=13
0 1 1 0 1 0 1 1 0                                                0 1 1 0 1 0 1 1 0
2                                                                2

0                                                                0

5    4   3   2   1   0   1   2   3    4   5                      5    4   3   2   1   0   1   2   3   4   5

0.5 times the bit-rate                                           0.75 times the bit-rate

0 1 1 0 1 0 1 1 0                                                0 1 1 0 1 0 1 1 0
2
2

0
0

5    4   3   2   1   0   1   2   3   4   5
5    4   3   2   1   0   1   2   3    4   5

1 times the bit-rate                                             2 times the bit-rate
University of Delaware CPEG 419                                          14
Because of reflections, a signal may take
many paths from transmitter to receiver.
transmitter
Objects such as
buildings, people, etc.

Signals that take alternative paths will arrive later.

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Multi-path reflection or delay
line of sight signal                                  late arriving signals
1

f( t ) . .6   ( f( t   D ) . .3   f( t   1.5 . D ) . .1 )

f( t )                                                      0.5

f( t     D)                                                                                                 getting
f( t     1.5 . D )                                           0                                               small
.5

0.5
2   1     0         1     2      3
t

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: Shannon’s Theorem
Given channel with B (Hz) bandwidth and
S/N (dB) signal-to-noise ratio, C (bps) is
C = B log2 (1+S/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                        total internal                            higher index of refraction
reflection

Since the signal can take many different paths, the arrival the received signal is smeared.
Input Signal                                                               Output Signal
1.5                                                                      40

1
f( t )
g( t ) 20
0.5

0
0.6   0.4   0.2   0   0.2       0.4   0.6     0.8   1   1.2          0
t                                             0.6   0.4   0.2     0   0.2       0.4   0.6   0.8        1   1.2
t
University of Delaware CPEG 419                                                   25
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.
University of Delaware CPEG 419   31
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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University of Delaware CPEG 419   33
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 mux’ing).
University of Delaware CPEG 419   36
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:

Clocking: determining the beginning and
end of each bit.
Transmitting long sequences of 0’s or 1’s 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.

University of Delaware CPEG 419   40
Digital-to-Digital Encoding
Techniques

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 “1”s
alternate in polarity: avoid synchronization
loss.
Pseudoternary: opposite representation.
Long sequence of 0’s or 1’s 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 0’s or 1’s.
Bipolar with 8-zeros substitution (B8ZS)
Inserts transitions when transmitting 8
consecutive “0”s.
High-density bipolar-3 zeros (HDB3)
Inserts pulses when transmitting 4
consecutive “0”s.
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.

University of Delaware CPEG 419   47
Frequency-Shift Keying

2 binary values represented by 2
frequencies.
s(t )  A cos(2f1t )," "
1
s(t )  A cos(2f 2t ),"0"
Frequencies f1 and f2 are offset from
carrier frequency by same amount in
opposite directions.
Less error prone than ASK.
University of Delaware CPEG 419   48
Phase-Shift Keying

Phase of carrier is shifted to represent
data.
Example: 2-phase system.
s(t )  A cos(2f ct  ),"1"
s(t )  A cos(2f ct ),"0"
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 > 2*signal’s highest
frequency, then samples contain all the
original signal’s 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 -> PAM -> 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 signal’s 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.

S AM (t )  [1  m(t )] cos(2f ct )
Signal m(t) is being modulated by carrier
cos(2p fct).
Modulation index: ratio between
amplitude of input signal to carrier.
University of Delaware CPEG 419   55
Angle Modulation

FM and PM are special cases of angle
modulation.
FM: carrier’s amplitude kept constant
while its frequency is varied according to
message signal.
PM: carrier’s phase varies linearly with
modulating signal m(t).

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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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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:
University of Delaware CPEG 419   61
Synchronous Xmission 1

No start or stop bits.
Synchronization via:
Separate clock signal provided by transmitter
or receiver; doesn’t 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 Control                                             8 -bit
Data                 Control
flag                                                       flag

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So far, sending signals over transmission
medium.
Data link layer: responsible for error-free
(reliable) communication between
Functions: framing, error control, flow
control, addressing (in multipoint
medium).
University of Delaware CPEG 419   64
Flow Control

What is it?
Ensures that transmitter does not overrun
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.
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 ack’s frame by including
sequence number of next expected frame.
Cumulative ACK: ack’s multiple frames.
and 4, it sends an ACK with sequence
number 5, which ack’s receipt of 2, 3, and
4.

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Sliding Window 3

Sender maintains sequence numbers it’s
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

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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...               0123456701234
0
1
0123456701234                              2 0123456701234
RR3
0123456701234
0123456701234
3
4 0123456701234
0123456701234                               5
RR4               6
0123456701234                                 0123456701234
University of Delaware CPEG 419        71

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