Transcript ppt

ECEN5553 Telecom Systems
Dr. George Scheets
Week #2
Read [3] "A Roadmap to 100G Ethernet at the Enterprise
Data Center"
Exam #1 Lecture 15, 21 September (Live)
No later than 28 September (Remote DL)
Outline 7 October 2015, Lecture 22 (Live)
No later than 14 October (Remote DL)
Outlines
Received
due 7 October (local)
14 October (remote)
0%
Fiber Optic Cable
1 1/4 inch
SC
Physical Layer Ailments...
Attenuation
Signal power weakens with distance
 Distortion
Pulse shapes change with distance

Copper cabling
High frequencies attenuate faster
Pulses smear
 Fiber cabling
Frequencies propagate at different speeds
Dispersion (Pulses change shape)

Generating a Square Wave...
5 Hz
+
15 Hz
+
25 Hz
+
35 Hz
1.5
0
-1.5
0
1.0
cos2*pi*5t - (1/3)cos2*pi*15t
+ (1/5)cos2*pi*25t - (1/7)cos2*pi*35t)
Effects of Dispersion...
5 Hz
+
15 Hz
+
25 Hz
+
35 Hz
1.5
0
-1.5
0
1.0
cos2*pi*5t + (1/3)cos2*pi*15t
+ (1/5)cos2*pi*25t + (1/7)cos2*pi*35t)
In this example the 15 and 35 Hz signals have suffered a
phase shift (which can be caused as a result of different
propagation speeds) with respect to the 5 and 25 Hz
signals. The pulse shape changes significantly.
Smearing (a.k.a. Inter-symbol Interference)
4.5
input
output
z
k
z2
k
0
4.5
0
0
20
40
60
80
k
100
120
140
127
Pulse energy is no longer confined to a T second time interval.
Makes receiver symbol detector's life more difficult.
Examples of Amplified Noise
Radio Static (Thermal Noise)
 Analog TV "snow"
2 seconds

of White Noise
SNR = Average Signal Power = Infinity
Average Noise Power
4.5
z2 x 0
k k
4.5
0
0
20
60
40
k
Binary Signal
Sequence = 0011010111
80
100
99
SNR = 100
4.5
z2 x 0
k k
4.5
0
0
20
40
60
80
k
Signal a sequence +1 and -1 volt pulses
For your info, SSD BER ≈ 0.0
100
99
SNR = 10
4.5
z2 x 0
k k
4.5
0
0
20
40
60
80
k
Signal a sequence +1 and -1 volt pulses
For your info, SSD P(BE) = 0.000783 = 1/1277
100
99
SNR = 1
4.5
z2 x 0
k k
4.5
0
0
20
60
40
80
k
Signal a sequence +1 and -1 volt pulses
For your info, SSD P(BE) = 0.1587 = 1/6.3
100
99
SNR = .1
8.5
z2 x 0
k k
8.5
0
0
20
60
40
80
k
Signal a sequence +1 and -1 volt pulses
For your info, SSD P(BE) = 0.3759
100
99
Single Sample Detector: SNR = 1
Threshold is placed midway between nominal Logic 1 and 0 values.
4.5
0
4.5
0
0
20
40
60
k
80
100
99
Detected sequence = 0011010111 at the receiver,
but there were some near misses.
Fall 2002 Final

'Average' based on 1 test chosen at random
126.00 out of 150


Analogous with "Single Sample" Detector
'Average' based on 10 tests chosen randomly
109.44 out of 150
Analogous with "Multiple Sample" Detector
 Average based on 10 samples tends to be more
accurate than "Average" based on 1 sample


Actual Midterm Average
106.85 out of 150
Matched Filter Detector: SNR = 1
Orange Bars are average voltage over that symbol interval.
4.5
0
4.5
0
0
20
40
60
k
80
100
99
Averages are less likely to be way off the mark.
SSD P(BE) = 0.3759, MFD P(BE) = 0.000783 (10 samples/bit)
Receiver Detection
SNR tends to worsen with distance
 Digital Receiver Symbol Detectors

Examine received symbol intervals (T sec.)
 Decide which of M symbols was transmitted
 Single Sample Detectors
Sample each symbol once
Compare sampled value to a threshold
 Matched Filter Detectors (Optimal)
Sample each symbol multiple times
& generate an average
Compare the average value to a threshold

Channel Capacity

Bandwidth affects usable symbol rate
Rapidly changing symbols need hi frequencies
 Baud rate too high? Distortion!!


M-Ary allows increased bit rate


SNR


Each symbol can represent multiple bits
Affects RCVR ability to tell symbols apart
Bandwidth & SNR affect usable bit rate
Channel Capacity (C)
Bandwidth, Bit Rate, SNR, and BER related
 Channel Capacity defines relationship
C = Maximum reliable bit rate
C = W*Log2(1 + SNR) bps

Bandwidth sets the maximum Baud rate
If move too many Baud, symbols will smear.
SNR sets the maximum number of
different symbols (the "M" in M-ary)
you can reliably tell apart.
Channel Capacity
(a.k.a. Shannon-Hartley Theorem)
Claude Shannon
Ralph Hartley
Power Line Communications
Transformers don't pass high frequency signals.
Image Source
Wikipedia
Power Lines act as Antennas
Source: http://www.doobybrain.com/2008/02/03/electromagnetic-fields-cause-fluorescent-bulbs-to-glow/
Home Power Line Networks

Possible using home wiring
Outside Step Down Transformer blocks
further wired transmission
 Signal radiates off internal house wiring


Netgear claims 500 Mbps
Normalized Propagation Delay
NPD = End-to-End Propagation Delay
Average time to inject a Packet
 NPD > 1

1 or more packets can simultaneously be in transit


If NPD = 5, 5 average sized packets can be physically on the line
NPD < 1
Packet front end hits far side before back end transmitted
Transmitter
High Speed
Low Speed
Receiver
Types of Traffic...
Computer Data
Bursty
Highly sensitive to errors
Not as time sensitive as voice or video
 Interactive Voice/Video
Fixed Rate (if not compressed)
*Not very sensitive to errors
Fixed or Variable Rate (if compressed)
*Sensitive to errors
Time Sensitive

IEEE
Institute for Electrical & Electronics
Engineers
 Largest Technical Organization in the
World
 Promotes betterment of Electrical
Engineering

Journals & Magazines
 Conferences
 Standards

IEEE 802.3 Ethernet
Based on late 1970’s technology
 Covers OSI Layers 1 & 2
 10 Mbps Line Speed
 Logical Bus
 Designed to move Computer Data
 The various 802.3 flavors dominate the
wired LAN

Serial Bit Stream: NRZ Coding
Logic
One
Logic
Zero
0
0
T
volts
+1
Called ‘Non Return to Zero’
because voltage never dwells
on zero volts.
0
time
-1
T
Ethernet Uses Manchester Coding
Logic
One
Logic
Zero
0
0
volts
+1
All symbols
have a transition
in the middle.
0
time
-1
T
Ethernet Uses Manchester Coding
volts
+1
0
time
-1
T
High Pass Filters Emphasize Change
High Pass Filter Output
+1
0
time
-1
Rectify (Absolute Value)
+1
0
T
time
-1
Result always has pulses T seconds apart.
Useful for receiver synchronization.
What is CSMA/CD?
 Polite
One
Conversation
node active at a time
No deliberate interruptions
Collisions sometimes occur
after a pause
802.3 Back-Off Algorithm

choose random number
1st Collision
0, 1
2nd Collision
0, 1, 2, 3
3rd Collision
0, 1, ..., 6, 7
4th Collision
0, 1, ..., 14, 15
10th Collision
0, 1, ..., 1022, 1023
15th Collision
0, 1, ..., 1022, 1023
16th Collision
Punt
 Wait (Random Number*.0000512) seconds