# ATM QOS The Myth and the Legend

Document Sample

```					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:

1/8/2003                           Inductive Logic                2
Topics: Baseband Signaling (day 1)
•   On-Off signaling                 •   Manchester encoding
•   Antipodal signaling              •   4B/5B encoding
•   Timing recovery                  •   8B/10B encoding
•   NRZI                             •   Multi-Level Transition
•   Multilevel: 2B1Q                 •   A modern line code
•   DS1 & DS3
Topics: Modulation (day 2)
•   Cosine review                    •   Communication channels as
•   Sums of cosines                      filters
•   Spectra                          •   Amplitude modulation
•   Fourier transforms               •   Amplitude demodulation
•   Baseband signaling               •   Quadrature multiplexing
•   Why cosine waves?                •   DMT ADSL
•   Transfer functions

1/8/2003                   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

2. Link                V.35, framing framing framing framing framing
bit serial
HSSI,
(line)
SDSL    DS1     DS3    ADSL IDSL SONET
1. Physical
signaling &
modulation
electrical               optical
1/8/2003                       Inductive Logic                                       4
A Matter of Values
•   On-Off binary signaling
     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)
amplitude

1

1        0     1    1    0     0
0
time
bit period

1/8/2003                                    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
clock

rising edge
of clock

sample times
centered in data bits
data

time

1/8/2003                      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.
transitions locate data
data

time
interpolated sample times
(bit centers)

1/8/2003                                Inductive Logic                                 7
Timing Recovery
•   All self-timed line codes provide sufficient signal
transitions for timing recovery. Some methods used:
     Scrambling
     Zero substitution
     Manchester encoding
     4B/5B
     8B/10B
     Multi-level transition

1/8/2003                            Inductive Logic        8
All For One ... or Zero
•   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)
•   Requires sufficient data transition density, or scrambling
•   Works for electrical and optical signals
amplitude

1                           distance
1       0     1    1    0     0
0
time
bit period

1/8/2003                                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

+0.5
amplitude

1   0     1    1    0       0
0
distance          time
-0.5
Can you say “tip-ring reversal?”

1/8/2003                                Inductive Logic                                10
•   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      1     0
time
-

1/8/2003                      Inductive Logic                                 11
Multilevel Signaling: 2B1Q
•   4 is better than 2:
Encodes 2 Binary bits into 1 Quatenary (4-level) symbol
     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

+1
11 10    01    00
-1                                           time
Usually described as
“distance 2”: -3, -1, +1, +3
-3

1/8/2003                               Inductive Logic                              12
AMI: Alternate Mark Inversion
•               Bipolar, tri-state (+, 0, and -)                           mark = 1

•               Pulses alternate polarity (DC = 0)
•               Used by DS1 (Digital Service 1, ref. T1.107, T1.403): 2 pair (4 wire)
   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
- amplitude +

idealized
pulse
1        0      1   1     0      0             25%          50%          25%
time
alternate
polarity                                    UI = Unit Interval (bit period)

1/8/2003                                     Inductive Logic                                           13
AMI: DS3
•           Digital Service 3 (ref. T1.107, T1.404): 2 coax, 75 
•           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
       B3ZS (Binary 3-Zero Substitution) provides transparency
Deliberate bipolar violation,
substitutes for 3 zeros
- amplitude +

1        0      1   1        0       0       0      0      0       0
X                   time
alternate                            X bits inserted as needed to make BPVs
polarity                              alternate polarity, to maintain DC = 0

1/8/2003                                      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
     no density requirement
     High information content: allows rapid timing recovery
•   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)
1   0    1    1      0     0
signal state
B

time
A

1/8/2003                             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
Checks line integrity by counting invalid codes             1   0 1 0 0 1

•   Used in Ethernet 100BaseTX, FDDI
2
3
1 0 1 0 0
1 0 1 0 1
Control                                 4   0   1   0   1   0
5   0   1   0   1   1
1 1 1 1 1   IDLE   used as inter-stream fill code            6   0   1   1   1   0
7   0   1   1   1   1
1 1 0 0 0   J      Start-of-Stream Delimiter, Part 1 of 2;   8   1   0   0   1   0
always used in pairs with K               9   1   0   0   1   1
1 0 0 0 1   K      Start-of-Stream Delimiter, Part 2 of 2;   A   1   0   1   1   0
always used in pairs with J               B   1   0   1   1   1
0 1 1 0 1   T      End-of-Stream Delimiter, Part 1 of 2;     C   1   1   0   1   0
always used in pairs with R               D   1   1   0   1   1
0 0 1 1 1   R      End-of-Stream Delimiter, Part 2 of 2;     E   1   1   1   0   0
always used in pairs with T               F   1   1   1   0   1

1/8/2003                        Inductive Logic                                             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
•   25% BW expansion (same as                  Code Group
Na me
Octet
Value
Current RD – Current RD 
abcdei fghj abcdei fghj
4B/5B)                                         D0.0
D1.0
00
01
100111 0100 011000 1011
011101 0100 100010 1011

•   12 control codes (start of packet,             D2.0
D3.0
D4.0
02
03
04
101101 0100 010010 1011
110001 1011 110001 0100
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              Notes
(similar to AMI)                         Na me
K28.0
Value
1C
abcdei fghj
001111 0100
abcdei fghj
110000 1011              1
     Worst case codeword imbalance is               K28.1
K28.2
3C
5C
001111 1001
001111 0101
110000 0110
110000 1010
1,2
1
6/4                                            K28.3        7C       001111 0011   110000 1100   1

•
K28.4        9C       001111 0010   110000 1101   1

Checks line integrity by counting                    K28.5
K28.6
BC
DC
001111 1010
001111 0110
110000 0101
110000 1001
2
1
invalid codes                                        K28.7
:
FC
:
001111 1000
:
110000 0111
:
1,2

•   Used in Gigabit Ethernet, Fiber                  NOTE 1 — Reserved .
NOTE 2 — Conta ins a c omma .
Channel (FC), some backplanes

1/8/2003                             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   0   1   1    0     0   1   1
distance           time

1/8/2003                           Inductive Logic               18
A Modern Line Code
A          N            • Binary signaling (on and off, not
B          O              dits and dahs)
C          P            • Pulse Width Modulated (PWM)
E          R              NRZ or NRZI)
F          S            • Variable rate
G          T            • Self timed
H          U            • Asynchronous at word level
I          V            • Variable length encoding
J          W            • Data compressed
K          X            • Forward error corrected (English)
L          Y
M          Z              Interesting history of
pen and paper

1/8/2003        Inductive Logic                           19
Just Do It
D                O                 I T
3 1 1 3                        7
dah      dit
minimum
size size
inter-word
inter- inter-                space
symbol letter
space space

•   Receiver recovers unit time interval from dits and inter-
symbol spaces; extrapolates other intervals

1/8/2003                      Inductive Logic                    20
Data Compression: English
size    frequency   avg.                 size   frequency   avg.
A 8         .082      .65                N 8        .071       .57
B 12        .014      .17                O 14       .080      1.12
C 14        .028      .39                P 14       .020       .28
D 10        .038      .38                Q 16       .001       .02
E 4         .131      .52                R 10       .068       .68
F 12        .029      .35                S 8        .061       .49
G 12        .020      .24                T 6        .105       .63
H 10        .053      .53                U 10       .025       .25
I 6         .063      .38                V 12       .009       .11
J 16        .001      .02                W 12       .015       .18
K 12        .004      .05                X 14       .002       .02
L 12        .034      .41                Y 16       .020       .32
M 10        .025      .25                Z 14       .001       .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
Baseband Summary
Interface                           Signaling States   Transition Coding
on-off

Ethernet 10Base5, 10Base2           on-off + DC bias   Manchester
Morse Code                          on-off             RTZ
Ethernet 10BaseT                    antipodal          Manchester
antipodal

none (NRZ,
EIA-232, V.35, HSSI                 antipodal          separate clock
and data)
Ethernet 100BaseTX, FDDI
MLT (3-level)      4B/5B, scrambled
(electrical)
multi-level

SDSL, IDSL, ISDN BRI                2B1Q (= 4-PAM)     scrambled
G.shdsl                             16-PAM             scrambled
DS1, DS3                            AMI (3-level)      RTZ, BxZS
Gigabit Ethernet (optical), Fiber
on-off (optical)   8B/10B
Channel
optical

SONET                               on-off (optical)   scrambled
FDDI (optical)                      on-off (optical)   4B/5B, NRZI

1/8/2003                                 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                   •   Communication channels as
•   Sums of cosines                     filters
•   Spectra                         •   Amplitude modulation
•   Fourier transforms              •   Amplitude demodulation
•   Baseband signaling              •   Quadrature multiplexing
•   Why cosine waves?               •   DMT ADSL
•   Transfer functions

1/8/2003                  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
     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)

1/8/2003                        Inductive Logic                             26
Cosine: A Function of Angle
•   Basis function for frequency analysis and for modulation

0o
- amplitude +

30o o
70
90    180                450 540                   angle
270 360                 630 720   (degrees)
120o

one cycle

y                           y                   y                   y
30o                                  120o
0o                                         70o
x                   x                       x                 x
1 unit

1/8/2003                                               Inductive Logic                                 27
Cosine Wave: A Function of Time
•   Fully characterized by 3 parameters:
A           Amplitude (e.g., 10 V)
f           Frequency (e.g., 2 Hz)      cosine wave = A cos(f*360t + )
           Phase    (e.g., 60)                    = A cos(360ft + )
A = 10 V                        10cos(360(2)t + 60o)

60o                                                               time
0.25           0.5                       1          (sec)

f = 2 Hz

132o         204o
60o                                                  240o

t=0                   t = 0.1              t = 0.2                 t = 0.25

1/8/2003                          Inductive Logic                                        28
Sums of Cosines
s(t) = A1cos(360f1t) + A2cos(360f2t) + A3cos(360f3t) + ...
1
0.8
0.6
0.4
0.2
0
0    0.1    0.2    0.3     0.4   0.5   0.6   0.7   0.8   0.9    1
-0.2
-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   0.5   0.6   0.7   0.8    0.9   1.0
-0.2

-0.4

-0.6

-0.8

-1

1/8/2003                                        Inductive Logic                               29
Spectrum: A Bar Chart of Cosines
•   Progressively denser bar charts give way to a simple graph
A

A
f                                 f
A

A

f                                 f

1/8/2003                 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                               output is cosine of exactly
any cosine                                the same frequency...
TILS                                      ...but different
amplitude and
phase

time                                                   time

1/8/2003                            Inductive Logic                                           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                         output is NOT a
triangle wave                          triangle wave
TILS

time                                        time

1/8/2003                      Inductive Logic                      32
Transfer Functions
•   A TILS multiplies each input frequency amplitude (& shifts its phase)
•   The multiplier (and phase-shift) are functions of frequency
TILS
H(f ) = Aout / Ain
or
Ain                  Aout = H(f )Ain                 Aout                time
time
at frequency, f                                       at same frequency, f

•   We can graph the amplitude multiplier as a function of frequency,
the amplitude transfer function, H(f ):

We can graph the phase-
shift as a function of
H(f )

frequency: the phase
transfer function, (f )
(but we won‟t)
f

1/8/2003                              Inductive Logic                                         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              Transfer function             Output signal
spectrum                 of linear system               spectrum

1/8/2003                     Inductive Logic                                 34
The Communication Channel as Filter
•   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

H(f )
for a copper loop
f

Transfer function
H(f )
for a copper loop
with a splitter
f

Transfer function for
H(f )

a transistor amplifier           This is why

1/8/2003                      Inductive Logic                                35
The Spectrum of Square Wave
Antipodal Signaling
•   90+% of energy is in the first lobe
•   Part of the first, and all of the other lobes can be discarded without
•   This is also the spectrum of 2B1Q, and all PAMs

A
time

square wave
A             fsym         2fsym           3fsym

time

filtered square wave       fsym         2fsym           3fsym

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
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) =             2                 cos(90-a)cos(90-b)
90-b
Demonstration
of identity #3                      cos(90-a)
90-a
b

Recall that
for any right
triangle:

a
a                               b
H•cos(a)
cos(a+b)                   cos(90-a)cos(90-b)
cos(a)cos(b)

1/8/2003                     Inductive Logic                                          38
Spectral View of Amplitude Modulation
•   Modulating a baseband cosine onto a carrier

i(t) = cos(360wt)                (simple) baseband spectrum:
A

a cosine of frequency „w‟
w                                f

cos(360ct)
carrier spectrum:
A

a cosine of frequency „c‟
c        f
Modulated signal spectrum:
Using identity #4:
A

cos(360wt)cos(360ct) =
cos[360(c-w)t] + cos[360(c+w)t]
f
c-w c c+w            Pop Quiz: Is a modulator a TILS?

1/8/2003                           Inductive Logic                              39
Deja View of Amplitude Modulation
•   Modulating a complicated baseband signal onto a carrier

i(t)                         complicated
A

baseband spectrum
(AM radio BW = 5 kHz)
bandwidth
f

cos(360ct)
carrier spectrum
A

(AM radio carrier = 540 - 1600 kHz)

c            f
Modulated signal spectrum;
using identity #4 for each
A

frequency component
f   Notice that the modulated bandwidth is
c                twice the baseband signal bandwidth
bandwidth            (AM radio BW = 10 kHz)

1/8/2003                        Inductive Logic                                   40
Demodulation: Getting It Back
•    Given a modulated signal:            i(t)cos(360ct)
•    Multiply by the carrier again:       i(t)cos(360ct)cos(360ct)
= i(t)[cos(0) + cos(360(2ct))]
modulated                                         = i(t) + i(t)cos[360(2ct)]
spectrum
A

f
c
i(t)cos[360(2ct)]
i(t)                                                 almost
demodulated
A

spectrum
f
c                      2c
filtered and fully
filter transfer                        demodulated
A

function                               spectrum
f
c                      2c
1/8/2003                            Inductive Logic                                      41
All Together Now

A
time

Energy Efficient Signaling fsym   2fsym   3fsym

A
time
Filtered Baseband Signal fsym
A                   2fsym   3fsym

Modulated Carrier               c

1/8/2003             Inductive Logic                    42
Comparison of Modulated and
Unmodulated Carrier
+
-

unmodulated
carrier
modulated
carrier

1/8/2003           Inductive Logic        43
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]
A

f
c
i(t)cos[360(2ct) + 90]        attempted
demodulated
A

spectrum
f
c                            2c
filter transfer                        filtered signal
A

function                               spectrum
f

1/8/2003                         Inductive Logic                             44
•   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)

i(t)                   q(t)            baseband spectra
A

A
f                         f

modulated signal spectrum:
generally not symmetric
A

f
c

1/8/2003                        Inductive Logic                                45
•   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)
i(t)
demodulated
A

spectrum

f
c                          2c          carrier for q(t)
[ 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)

q(t)
A

demodulated
c                          2c                spectrum
f
1/8/2003                        Inductive Logic                                              46
•   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
upstream              downstream
A

baseband
voice
f
300 Hz        3600 Hz      N x 4312.5 Hz

1/8/2003                         Inductive Logic                                  47
•   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                       over         over                       over

1/8/2003                          Inductive Logic                                    48

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