Baseband Signaling and
Modulation
Part 1:
Baseband Signaling
Part 1 of a 2-part presentation
Eric L. Michelsen
Inductive Logic
If You Could Tell Your Audience
Only One Sentence...
Transmitting data requires not only the
signaling of bit values, but also bit timing.
1
or
0
time
sample here & here & here ...
If I could tell them a second sentence, it would be:
DC is bad.
1/8/2003
Inductive Logic
2
Topics: Baseband Signaling (day 1)
•
•
•
•
•
•
On-Off signaling
Antipodal signaling
Timing recovery
NRZI
Multilevel: 2B1Q
DS1 & DS3
•
•
•
•
•
Manchester encoding
4B/5B encoding
8B/10B encoding
Multi-Level Transition
A modern line code
Topics: Modulation (day 2)
•
•
•
•
•
•
•
Cosine review
Sums of cosines
Spectra
Fourier transforms
Baseband signaling
Why cosine waves?
Transfer functions
1/8/2003
•
•
•
•
•
Communication channels as
filters
Amplitude modulation
Amplitude demodulation
Quadrature multiplexing
DMT ADSL
Inductive Logic
3
Where in the Stack?
•
•
•
Signaling and modulation are ways of transmitting data
They are the lowest sublayers in Layer 1 (physical layer)
In this context, “signaling” means “transmitting data”
(not call setup/teardown)
7. Application
OSI stack
6. Presentation
5. Session
4. Transport
bit serial
(payload)
3. Network
2. Link
1. Physical
V.35, framing framing framing framing framing
HSSI,
SDSL
DS1
DS3
ADSL IDSL SONET
electrical
1/8/2003
Inductive Logic
optical
bit serial
(line)
signaling &
modulation
4
A Matter of Values
•
On-Off binary signaling




amplitude

simple
indicates 1 (on) or 0 (off)
by itself, does not explicitly convey timing
works for electrical and optical signals
Used by Ethernet 10Base5 and 10Base2 (w/ additional line coding)
1
1
0
1
1
0
0
time
bit period
1/8/2003
0
Inductive Logic
5
Time Is of the Essence
•
•
•
•
With separate clock and data, the transmitter gives the receiver timing
on one signal, and data on another
Requires two signals (clock and data): can be expensive
Data values are arbitrary (no restrictions)
Used by local interfaces: V.35, (synchronous) EIA-232, HSSI, etc.
As distance and/or speed increase, clock/data skew destroys timing
sample on
rising edge
of clock
clock
•
data
sample times
centered in data bits
1/8/2003
time
Inductive Logic
6
•
•
•
•
No Clock:
Do You Know Where Your Data Is?
Most long-distance or high speed signaling is self timed: it has no
separate clock; the receiver recovers timing from the data itself
Receiver knows the nominal data rate, but requires transitions in the
signal to locate the bits, and interpolate the sample points
Receiver tracks the timing continuously, to stay in synch

Tracking requires sufficient transition density throughout the data stream
Used in all DSLs, DS1, DS3, SONET, all Ethernets, etc.
data
transitions locate data
interpolated sample times
(bit centers)
1/8/2003
Inductive Logic
time
7
Timing Recovery
•
All self-timed line codes provide sufficient signal
transitions for timing recovery. Some methods used:







1/8/2003
Scrambling
Return to zero (RTZ)
Zero substitution
Manchester encoding
4B/5B
8B/10B
Multi-level transition
Inductive Logic
8
All For One ... or Zero
•
amplitude
•
•
•
•
•
On-Off binary signaling: simple, but not energy efficient
(SNR)
At unit distance (A = 1), average energy = A2/2 = 0.5
For balanced data, DC (Direct Current) ~= 0.5 (bad)
Also known as Non-Return to Zero (NRZ)
Requires sufficient data transition density, or scrambling
Works for electrical and optical signals
1
distance
1
0
1 1
0
0
time
bit period
1/8/2003
0
Inductive Logic
9
Pluses and Minuses
•
•
•
•
•
Antipodal binary signaling: energy efficient (SNR)
At unit distance (A = 0.5), average energy = A2 = 0.25
(3 dB better than on-off signaling)
Requires sufficient data transition density, or scrambling
For balanced (or scrambled) data, DC ~= 0 (good)
For electrical signaling only (negative light?)
Ethernet 10BaseT, EIA-232, V.35, V.36, HSSI
amplitude

+0.5
0
1
0
1
1
0
0
distance
time
-0.5
Can you say “tip-ring reversal?”
1/8/2003
Inductive Logic
10
NRZI (Non-Return to Zero Inverted)
•
•
•
•
•
Data value coded as transition = 1, no transition = 0
Used in combination with antipodal or on/off binary signaling
With scrambling, DC ~= 0
Why NRZI? Can you say “tip-ring reversal?”
Requires sufficient data 1s (signal transition) density, or scrambling
+
0
?
1
1 0
1
0
time
Equivalent
NRZI signals
+
0
?
1
1 0
0
time
1/8/2003
1
Inductive Logic
11
Multilevel Signaling: 2B1Q
•

A pair of bits in a single symbol is a dibit
AKA 4-PAM (4-level Pulse Amplitude Modulation)
Requires data transitions, or scrambling
With scrambling, DC ~= 0
Used in SDSL, IDSL, ISDN BRI
Other PAMs exist: 16-PAM (G.shdsl), 256-PAM, etc.
+3
amplitude
•
•
•
•
•
4 is better than 2:
Encodes 2 Binary bits into 1 Quatenary (4-level) symbol
+1
-1
-3
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11
10
01
00
time
Usually described as
“distance 2”: -3, -1, +1, +3
Inductive Logic
12
AMI: Alternate Mark Inversion
•
•
•
•
mark = 1
Bipolar, tri-state (+, 0, and -)
space = 0
50% duty cycle RTZ (Return To Zero)
Pulses alternate polarity (DC = 0)
Used by DS1 (Digital Service 1, ref. T1.107, T1.403): 2 pair (4 wire)




- amplitude +

1/8/2003
Line rate = 1.544 Mbps, including 8 kbps framing/OAM
Payload rate = 1.536 Mbps
Generic digital service, can carry T1, PRI, GR-303, Frame Relay, etc.
Timing recovery requires at least 2 pulses (ones) every 16 bits
B8ZS (Binary 8-Zero Substitution) provides transparency
1
0
1
1
0
idealized
pulse
0
25%
50%
25%
time
alternate
polarity
UI = Unit Interval (bit period)
Inductive Logic
13
•
•
•
•
•
•
•
AMI: DS3
Digital Service 3 (ref. T1.107, T1.404): 2 coax, 75 
RTZ (Return To Zero) pulse, very similar to DS1
AMI (Alternate Mark Inversion), (DC = 0)
Line rate = 44.736 Mbps, including ~530 kbps framing/OAM
Payload rate = 44.736 x (84 / 85)  44.210 Mbps
Generic digital service: can carry T3, Frame Relay, ATM, etc.
Timing recovery requires at least one pulse every 3 bits
- amplitude +

1/8/2003
B3ZS (Binary 3-Zero Substitution) provides transparency
Deliberate bipolar violation,
substitutes for 3 zeros
1
0
1
1
0
0
0
0
X
alternate
polarity
0
0
time
X bits inserted as needed to make BPVs
alternate polarity, to maintain DC = 0
Inductive Logic
14
Double Time: Manchester Encoding
•
•
“Coding” in this sense is applicable to any binary (2-state) signal
(on-off, antipodal, FSK, etc.)
Provides a transition in the center of every bit

DC = 0, exactly (with antipodal signaling)
Data bit is value in last half of bit (or could be first half)
Used in Ethernet 10Base5, 10Base2, 10BaseT
Equivalent to 1B/2B encoding
Not spectrally efficient: requires transmitting 2 signal events for each
bit (100% bandwidth expansion)
signal state
B
A
•
•
•
•
•

no density requirement
High information content: allows rapid timing recovery
1/8/2003
1
0
1
1
0
0
time
Inductive Logic
15
Enough is Enough: 4B/5B Encoding
•
•
•
•
•
•
•
Encodes 4 payload bits into 5 line bits
Guarantees transitions; no user data restrictions or scrambling needed
Extra codewords available for control (Idle, SSD, ESD, ...)
More BW efficient than Manchester: 25% expansion
Data
DC >> 0 (bad), but used with NRZI or MLT, DC ~= 0
0
1 1 1 1 0
1
0 1 0 0 1
Checks line integrity by counting invalid codes
2
1 0 1 0 0
Used in Ethernet 100BaseTX, FDDI
3
1 0 1 0 1
Control
1/8/2003
1 1 1 1 1
IDLE
used as inter-stream fill code
1 1 0 0 0
J
1 0 0 0 1
K
0 1 1 0 1
T
0 0 1 1 1
R
Start-of-Stream Delimiter, Part 1 of 2;
always used in pairs with K
Start-of-Stream Delimiter, Part 2 of 2;
always used in pairs with J
End-of-Stream Delimiter, Part 1 of 2;
always used in pairs with R
End-of-Stream Delimiter, Part 2 of 2;
always used in pairs with T
Inductive Logic
4
5
6
7
8
9
A
B
C
D
E
F
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
1
0
1
0
1
0
1
0
1
0
1
16
•
•
•
•
•
•
Twice as Good: 8B/10B Encoding
Encodes 8 payload bits into 10 line bits
Guarantees 3 to 8 transitions per 10-bit codeword
Maximum run-length of 5
Code Group
Octet
Current RD – Current RD 
25% BW expansion (same as
Na me
Value
abcdei fghj abcdei fghj
D0.0
00
100111 0100 011000 1011
4B/5B)
D1.0
01
011101 0100 100010 1011
D2.0
02
101101 0100 010010 1011
12 control codes (start of packet,
D3.0
03
110001 1011 110001 0100
D4.0
04
110101 0100 001010 1011
end of packet, error, etc.)
D5.0
05
101001 1011 101001 0100
:
:
:
:
Alternately inverts non-zero-DC
codewords to achieve zero DC
Code Group
Octet
Current RD –
Current RD 
Na
me
Value
abcdei
fghj
abcdei fghj
(similar to AMI)
K28.0
1C
001111 0100
110000 1011

•
•
Worst case codeword imbalance is
6/4
Checks line integrity by counting
invalid codes
Used in Gigabit Ethernet, Fiber
Channel (FC), some backplanes
1/8/2003
K28.1
K28.2
K28.3
K28.4
K28.5
K28.6
K28.7
:
3C
5C
7C
9C
BC
DC
FC
:
001111 1001
001111 0101
001111 0011
001111 0010
001111 1010
001111 0110
001111 1000
:
110000 0110
110000 1010
110000 1100
110000 1101
110000 0101
110000 1001
110000 0111
:
Notes
1
1,2
1
1
1
2
1
1,2
NOTE 1 — Reserved .
NOTE 2 — Conta ins a c omma .
Inductive Logic
17
Saving Bandwidth:
MLT-3 (Multi-Level Transition)
Bipolar, tri-state signal (+, 0, and -)
Like a combination of NRZI and AMI
Transition = data 1, no transition = 0
Non-zero signals alternate polarity
Cuts bandwidth in half (and SNR as well)
Used by Ethernet 100BaseTX (with 4B/5B and
scrambling)
- amplitude +
•
•
•
•
•
•
1/8/2003
1
0
1 1
0
0
distance
Inductive Logic
1
1
time
18
A Modern Line Code
A
B
C
D
E
F
G
H
I
J
K
L
M
1/8/2003
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
• Binary signaling (on and off, not
dits and dahs)
• Pulse Width Modulated (PWM)
• Return to zero coded (RTZ, vs.
NRZ or NRZI)
• Variable rate
• Self timed
• Asynchronous at word level
• Variable length encoding
• Data compressed
• Forward error corrected (English)
Interesting history of
pen and paper
Inductive Logic
19
Just Do It
D
3 1 1 3
dah
dit
size size
inter- intersymbol letter
space space
•
O
I T
7
minimum
inter-word
space
Receiver recovers unit time interval from dits and intersymbol spaces; extrapolates other intervals
1/8/2003
Inductive Logic
20
Data Compression: English
size
A 8
B 12
C 14
D 10
E 4
F 12
G 12
H 10
I 6
J 16
K 12
L 12
M 10
frequency
.082
.014
.028
.038
.131
.029
.020
.053
.063
.001
.004
.034
.025
avg.
.65
.17
.39
.38
.52
.35
.24
.53
.38
.02
.05
.41
.25
size
N 8
O 14
P 14
Q 16
R 10
S 8
T 6
U 10
V 12
W 12
X 14
Y 16
Z 14
frequency
.071
.080
.020
.001
.068
.061
.105
.025
.009
.015
.002
.020
.001
avg.
.57
1.12
.28
.02
.68
.49
.63
.25
.11
.18
.02
.32
.01
Avg letter size: 11.2
units
English weighted avg letter size: 9.0 (~20% savings)
Opt. Eng. weighted avg letter size: 8.6 (within 5%)
1/8/2003
Inductive Logic
21
Signaling States
Transition Coding
on-off
Ethernet 10Base5, 10Base2
on-off + DC bias
Manchester
Morse Code
on-off
RTZ
Ethernet 10BaseT
antipodal
EIA-232, V.35, HSSI
antipodal
Manchester
none (NRZ,
separate clock
and data)
optical
multi-level
Interface
antipodal
Baseband Summary
1/8/2003
Ethernet 100BaseTX, FDDI
(electrical)
SDSL, IDSL, ISDN BRI
MLT (3-level)
4B/5B, scrambled
2B1Q (= 4-PAM)
scrambled
G.shdsl
16-PAM
scrambled
DS1, DS3
Gigabit Ethernet (optical), Fiber
Channel
SONET
AMI (3-level)
RTZ, BxZS
on-off (optical)
8B/10B
on-off (optical)
scrambled
FDDI (optical)
on-off (optical)
4B/5B, NRZI
Inductive Logic
22
Baseband Signaling and
Modulation
Part 2:
Modulation
Eric L. Michelsen
1/8/2003
Inductive Logic
23
Another Day, Another Sentence
Modulation avoids baseband problems of
signal overlap and DC error.
If I could tell them a second sentence, it would be:
Bandwidth is not capacity!
If I could tell them a third sentence, it would be:
Bandwidth is not capacity!
But first, a review of Fourier analysis...
1/8/2003
Inductive Logic
24
Topics: Modulation
•
•
•
•
•
•
•
Cosine review
Sums of cosines
Spectra
Fourier transforms
Baseband signaling
Why cosine waves?
Transfer functions
1/8/2003
•
•
•
•
•
Communication channels as
filters
Amplitude modulation
Amplitude demodulation
Quadrature multiplexing
DMT ADSL
Inductive Logic
25
Definitions
•
Baseband signaling


•
Communicating a signal in its original form for a given medium
(e.g., audio)
or
Communicating a signal with components down to DC (or almost
DC)
Carrier modulation


1/8/2003
Communication based on modifying (modulating) a cosine wave
signal
Other forms of modulation exist (non-carrier modulation, e.g.,
PAM, PWM, PCM(?), but that’s another story)
Inductive Logic
26
Cosine: A Function of Angle
Basis function for frequency analysis and for modulation
0o
- amplitude +
•
30o o
70
90
450 540
270 360
120o
630 720
angle
(degrees)
one cycle
y
y
0o
1 unit
1/8/2003
180
x
30o
x
Inductive Logic
y
y
120o
70o
x
x
27
•
Cosine Wave: A Function of Time
Fully characterized by 3 parameters:
A
f

Amplitude (e.g., 10 V)
Frequency (e.g., 2 Hz)
Phase
(e.g., 60)
cosine wave = A cos(f*360t + )
= A cos(360ft + )
A = 10 V
10cos(360(2)t + 60o)
60o
0.25
0.5
time
(sec)
1
f = 2 Hz
60o
t=0
1/8/2003
132o
204o
t = 0.1
Inductive Logic
240o
t = 0.2
t = 0.25
28
Sums of Cosines
s(t) = A1cos(360f1t) + A2cos(360f2t) + A3cos(360f3t) + ...
1
0.8
0.6
0.4
0.2
0
-0.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.5
0.6
0.7
0.8
0.9
0.8
0.9
1
-0.4
-0.6
-0.8
-1
1
0.8
0.6
0.4
0.2
0
0.0
0.1
0.2
0.3
0.4
1.0
-0.2
-0.4
-0.6
-0.8
-1
1/8/2003
Inductive Logic
29
Spectrum: A Bar Chart of Cosines
A
Progressively denser bar charts give way to a simple graph
A
•
f
A
A
f
f
1/8/2003
f
Inductive Logic
30
Why Cosine Waves?
•
Cosines are the only basis functions (aka eigenfunctions) of
Time Invariant Linear Systems


•
•

System: produces output from input
Linear: if Ia  Oa, then kIa  kOa
and if Ib  Ob, then (Ia + Ib)  Oa + Ob
Time invariant: it does the same thing all the time
If input is a cosine, then output is a cosine of same frequency,
but different amplitude and phase
Linear  Cosine components of input don’t interact
input is
any cosine
TILS
output is cosine of exactly
the same frequency...
...but different
amplitude and
phase
time
1/8/2003
Inductive Logic
time
31
Triangles Are Not Cosines
•
•
•
If input is not a cosine, output is not a multiple of the input
Single triangle wave input produces complex output
What a mess!
input is a
triangle wave
TILS
time
1/8/2003
Inductive Logic
output is NOT a
triangle wave
time
32
Transfer Functions
•
•
A TILS multiplies each input frequency amplitude (& shifts its phase)
The multiplier (and phase-shift) are functions of frequency
TILS
Ain
time
H(f ) = Aout / Ain
or
Aout = H(f )Ain
Aout
at same frequency, f
at frequency, f
We can graph the amplitude multiplier as a function of frequency,
the amplitude transfer function, H(f ):
H(f )
•
time
f
1/8/2003
Inductive Logic
We can graph the phaseshift as a function of
frequency: the phase
transfer function, (f )
(but we won’t)
33
Transfer Functions at Work
•
•
•
Since cosine components of the input signal do not interact, each
cosine is multiplied by the transfer function at its frequency
Thus, the output spectrum is the input spectrum multiplied by the
transfer function, at each frequency
Every TILS has a transfer function, and
a transfer function defines a TILS.
TILS
Input signal
spectrum
1/8/2003
Transfer function
of linear system
Inductive Logic
Output signal
spectrum
34
Any communication channel is imperfect
A time invariant linear channel is described by its transfer function
A filter is a TILS that passes some frequencies, and blocks others
Transfer function
for a copper loop
H(f )
•
•
•
The Communication Channel as Filter
Transfer function
for a copper loop
with a splitter
H(f )
Transfer function for
a transistor amplifier
H(f )
f
1/8/2003
f
This is why
DC is bad.
Inductive Logic
f
35
•
•
90+% of energy is in the first lobe
Part of the first, and all of the other lobes can be discarded without
much degradation
This is also the spectrum of 2B1Q, and all PAMs
A
•
The Spectrum of Square Wave
Antipodal Signaling
time
fsym
2fsym
3fsym
fsym
2fsym
3fsym
A
square wave
time
filtered square wave
1/8/2003
Inductive Logic
36
Amplitude Modulation
•
•
•
Given a signal, i(t)
And a carrier, cos(360ct)
We modulate the signal onto the carrier by multiplying the two at each
instant in time: i(t)cos(360ct)
cos(360ct)
i(t)
x
modulator
i(t)
cos(360ct)
=
i(t)cos(360ct)
1/8/2003
Inductive Logic
37
Know Your Identity
1. cos(-a) = cos(a)
2. cos(90-a) = -cos(90+a)
3. cos(a+b) = cos(a)cos(b) - cos(90-a)cos(90-b)
cos(a - b) + cos(a + b)
4. cos(a)cos(b) =
cos(90-a)cos(90-b)
2
Demonstration
of identity #3
90-b
90-a
cos(90-a)
b
Recall that
for any right
triangle:
a
a
H•cos(a)
1/8/2003
b
cos(a+b)
cos(a)cos(b)
Inductive Logic
cos(90-a)cos(90-b)
38
Spectral View of Amplitude Modulation
Modulating a baseband cosine onto a carrier
i(t) = cos(360wt)
A
•
(simple) baseband spectrum:
a cosine of frequency ‘w’
f
w
cos(360ct)
A
carrier spectrum:
a cosine of frequency ‘c’
f
c
A
Modulated signal spectrum:
Using identity #4:
cos(360wt)cos(360ct) =
cos[360(c-w)t] + cos[360(c+w)t]
c-w c c+w
1/8/2003
f
Pop Quiz: Is a modulator a TILS?
Inductive Logic
39
Deja View of Amplitude Modulation
Modulating a complicated baseband signal onto a carrier
complicated
baseband spectrum
i(t)
A
•
(AM radio BW = 5 kHz)
f
bandwidth
cos(360ct)
A
carrier spectrum
(AM radio carrier = 540 - 1600 kHz)
c
f
A
Modulated signal spectrum;
using identity #4 for each
frequency component
c
bandwidth
1/8/2003
f
Notice that the modulated bandwidth is
twice the baseband signal bandwidth
(AM radio BW = 10 kHz)
Inductive Logic
40
Demodulation: Getting It Back
A
•
•
Given a modulated signal:
Multiply by the carrier again:
modulated
spectrum
i(t)cos(360ct)
i(t)cos(360ct)cos(360ct)
= i(t)[cos(0) + cos(360(2ct))]
= i(t) + i(t)cos[360(2ct)]
f
c
A
i(t)cos[360(2ct)]
i(t)
A
c
f
filtered and fully
demodulated
spectrum
filter transfer
function
c
1/8/2003
2c
almost
demodulated
spectrum
2c
Inductive Logic
f
41
A
All Together Now
time
2fsym
3fsym
2fsym
3fsym
A
Energy Efficient Signaling fsym
time
A
Filtered Baseband Signal fsym
Modulated Carrier
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42
-
+
Comparison of Modulated and
Unmodulated Carrier
unmodulated
carrier
modulated
carrier
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A
•
Quadrature Multiplexing:
Two for the Bandwidth of One
Consider a signal modulated with the wrong carrier phase, off by 90.
We attempt to demodulate (recall identity #4):
i(t)cos(360ct + 90)cos(360ct)
modulated
= i(t)[cos(90) + cos(360(2ct) + 90)]
spectrum
= i(t)cos[360(2ct) + 90]
f
c
A
i(t)cos[360(2ct) + 90]
A
c
2c
filter transfer
function
attempted
demodulated
spectrum
f
filtered signal
spectrum
f
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Quadrature Multiplexing: Part Deux
Consider two signals, i(t) and q(t), modulated with two carriers of the
same frequency, but different by 90:
i(t)cos(360ct) + q(t)cos(360ct + 90)
q(t)
A
i(t)
A
•
baseband spectra
f
f
A
modulated signal spectrum:
generally not symmetric
c
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Quadrature Demodulation
•
•
Given a quadrature multiplexed modulated signal:
i(t)cos(360ct) + q(t)cos(360ct + 90)
Demodulate each channel separately, each with its own carrier:
carrier for i(t)
[ i(t)cos(360ct) + q(t)cos(360ct + 90) ]cos(360ct)
= i(t)cos(360ct)cos(360ct) + q(t)cos(360ct+90)cos(360ct)
A
i(t)
demodulated
spectrum
c
2c
f
carrier for q(t)
A
[ i(t)cos(360ct) + q(t)cos(360ct + 90) ]cos(360ct + 90)
= i(t)cos(360ct)cos(360ct+90) + q(t)cos(360ct+90)cos(360ct+90)
c
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q(t)
demodulated
spectrum
f
46
•
•
DMT ADSL
Discrete Multi-Tone
Up to 255 “separate” carriers,




•

Each carrier is quadrature multiplexed multi-level PAM
Two to 15 bits per symbol per carrier (2 - 256 PAM per I/Q axis)
Optimum filling of data into the carriers for maximum total SNR
All share the same time, frequency, and phase references
Lower carriers omitted for baseband voice
Carrier spacing is 4312.5 Hz
A
upstream
downstream
baseband
voice
300 Hz
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3600 Hz
N x 4312.5 Hz
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47
DMT ADSL
•
Two kinds of FEC:

•
•
•

“Fast” path (low latency): Trellis Coded Modulation (TCM)
“Interleaved” path (higher latency): Reed-Solomon block interleaved
Framing structure built into the modulation
Integral number of bytes per frame, 4000 user data frames per second
= N x 32 kbps data rates
G992.1 defines two services: STM and ATM


The industry standard is ATM over STM (HEC delineation)
No one uses G992.1’s ATM mode
Superframe: 17 ms
frame frame frame frame
frame frame synch
...
0
1
2
3
66
67 symbol
over
head
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Fast bytes
over
head
FEC
over
head
Interleaved bytes
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over
head
48