BROADBAND TRANSMISSION STANDARDS

1
BROADBAND TRANSMISSION STANDARDS

• DSL, ADSL and other flavors

2
Background

• For the longest time it was taught that analog
  phone lines are bandlimited to 4 KHz
• Nyquists first criterion then states that
  maximum theoretical data rate on such lines is
  8000 pulses/sec.
• So what happened?

3
Historical trends

• Modems started out at 300 bits/sec using binary
  FSK modulation
• Over time speed grew to 1200, 2400, 9600 and
  finally 56K bits/sec.
• This increase was made possible by using high
  density digital modulations
• And yes, this was all done within the confines of
  4 KHz bandwidth
• 56K, V.90, was the first standard taking
  advantage of digital backbone at 64 Kbps

4
26 Mb/sec over ordinary phone lines?

• DSL in its various flavors, allows for up to 26
  MB/sec over twisted pair. How is that possible?
• The answer is that it is the switch that has
  limited us not the phone lines
• Solution avoid the switch by extracting digital
  signals before they get to the switch and
  re-route them to a broadband network

5
Original objectives

• Cover a serving distance of 18000 feet on a
  single twisted pair telephone subscriber loop at
  T1 rate of 1.544 Mb/s. Motivation
  video-on-demand (meets H.261 MPEG rate)
• Other features
• POTS availability 300 Hz-4KHz
• Upstream control channel 16-64Kb/s in 10 KHz-50
  KHz
• Downstream 1.544 Mb/s in 100-500 KHz

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DSL vs. voice band modems
Old Way
upstream
downstream
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DSL vs. voice band modems
internet
CO
DSL
split
split
DSL
Broadband
Local loop lt1 mile
Broadband
26 Mb/s
3-26
Narrowband network
CO
Splitter allows for the coexistence of POTS and
DSL on the same line
120 KHz 300 KHz
30 MHz
8
Spectrum picture
ADSL
POTS
Upstream
Downstream
8 Mb/s
1.5 Mb/s
G.Lite
4 20 140
552
1104
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Why the A in ADSL?

• Asymmetric DSL rates are
• Downstream 1.544 Mb/s for distances less than
  18000 feet (to CO) up to 6.144 Mb/s up to 12000
  feet
• Upstream 16 to 640 Kb/s
• Asymmetry is due to the bundling of the twisted
  pair telephone wires. Symmetric rates interfere
  with each other
• Symmetric rates must be much slower and cover
  shorter distances

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DSL flavors

• ADSL
• Modulation CAP or DMT
• Data rate 8 Mb/sec downstream (in 240KHz-2 MHz)
  and up to 1 Mb/sec upstream in the 25--200 KHz
  simultaneously with POTS
• HDSL
• Four wire access for achieving symmetrical rates
  at T1 (1.544 Mb/sec) or E1(2.048 Mb/sec).
• Modulation CAP64(passband) or 2B1Q( baseband)
• Spectrum in copper loop0-300 KHz or 0-425 KHz
• Fractional T1 rates supported (i.e. nx64 Kb/sec)
• No simultaneous voice data, HDSL2 will do that

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More DSL flavors

• SDSL
• Single-pair (two wire) implementation of 2B1Q or
  CAP modem
• Echo cancellation and adaptive equalization
• Symmetric data rates of 384 Kb/s, 768 Kb/s, 1
  Mb/s or 2 Mb/s.
• SDSL is expected to eclipse HDSL due to its
  single pair, symmetrical data rate property
• VDSL
• Supports much shorter distances
• Very high-speed DSL 6.5-51.8 Mb/s downstream.
• 1.6-6.5 Mb/s upstream for asymmetrical services
  and 6.5-25.9 Mb/s for symmetrical services(short
  lt1000 ft, two-wire loops)
• Modulation M-ary CAP, DMT

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DSL Modems

• Discrete Multi Tone (DMT)
• or
• Orthogonal Frequency Division Multiplexing
• (OFDM)

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The general problemmultipath

• Transmitted signal arrives at the receiver
  through a number of paths

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Effect on pulse transmission

• Multiple copies of the signal interfere with each
  other(ISI). Let T be bit duration. Pulses are
  delayed differently forming a delay spread
• Largest delay is called maximum delay spread

?max
A received symbol can be influenced by ?max/T
previous symbols We want max delay spread to be
less than T, i.e. bit duration
T
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What are the choices?Single carrier approach

• Either reduce delay spread or increase T
• Increasing T means reducing bitrate. Dont like
  that.
• Example (DVB)
• Transmission rate R1/T7.4 Msymbols/sec
• Maximum channel delay ?max224 ?sec
• For a single carrier modulation
• ?max/T1600
• The complexity of removing this much interference
  is enormous

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Multi-carrier approach

• Split data at rate R into N parallel stream of
  rate Rmc1/TmcR/N
• Each slower data stream is modulated by a
  different carrier frequency and transmitted in
  the same band
• ISI is reduced by a factor of N
• ?max/Tmc ?max/NT
• For DVB, we have N8192 streams resulting in
• ?max/Tmc0.2

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What are we talking about?Multicarrier modulation

• Modulation method used in DSL is of a frequency
  division multiplexing flavor.
• Channel is divided into many subchannels
• This approach bundles bits and transmits them
  over different frequency bands to counter channel
  characteristics

Bits per hertz
line gain
Bits per hertz
frequency
frequency
frequency
18
Channel allocation

• DMT in current ADSL creates 256 downstream
  subbands, 4 kHz each
• Each channel can be modulated with QAM at up to
  15 bits/sec/hz.
• Theoretical transmission rate is then 15.36
  Mb/sec over a zero length line. How?

15(b/s/Hz) x 4000(Hz/channel) x
256(channels)15.36 Mb/sec
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Bit allocation

• Instead of using equalizers to correct channel
  response, DMT spreads data over all channels
  according to the SNR in each one.

Ideal channel
Sweet spot (up to 15 b/s/hertz)
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Orthogonal Frequency Division Multiplexingfirst
step

• Take a bitstream running and divide it into N
  parallel channels each at a reduced rate

1 0 1 ...
On each line, group Bits into a symbol. The
symbols are much wider than the original
bitstream thus defeating possible ISI
0 1 1 .
1 1 1.
11001010110101
N
1 0 0...
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Modulation in OFDM

• Each symbol is mapped to a QAM constellation.
  Channels are modulated by orthogonal frequencies

f1
1 0 1 1
1 0 1 0
f2
0 0 0 1
f3
16 lines 16 frequencies
1 1 1 0
f16
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OFDM signal model

• The OFDM signal can be written as the sum of N
  pulses modulating N orthogonal carriers
• Fixed k means summing symbols from different
  channels, each modulating a different frequency

Symbol coming from a M-ary QAM const. Msqrt(N)
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OFDM epoch

• Lets look at epoch k0. This is taking N
  symbols one from each channel and adding them up.

1 0 1 1
1 0 1 0
0 0 0 1
T
1 1 1 0
Assigned to One of these
k0
k1
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Generating OFDM by IFFT

• Sample v(t) at tkTs
• OFDM can be generated by an inverse FFT

X 0,0
X 0,1
X 0,N-1
IFFT
V 0
V 1
V N-1
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Variable bit loading

• Input data at Mfs bits /sec grouped into blocks
  of M bits at a block (or symbol) rate of fs then
  ...

mn bits
Serial to parallel conversion-M bits at a time
Mfs bits /sec
m3
m2
m1
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Bringing in the multitones

• Each group of bits then modulate a separate
  carrier the mnth group modulates a carrier with
  carrier frequency fc,n
• There are Nc carriers spaced ?f Hz apart. The
  total number of carriers where Ncn.
• For the previous slide, we need 5 carriers
  because we broke up 8 bits into 5 groups of bits

1 bit
fcn
?
3 bits
2 bits
fc1
1 bit
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Selecting modulations
2 bits
3 bits
When a block contains 2 bits, they represent 4
states. The 4 states come from a 4-QAM
modulation. Same goes for 3 bits