Transcript Broadband Access technologies

Broadband Access technologies
• As the multimedia content grows, we need faster internet connectivity
for SOHO users. Broadband access technologies shown below are
considered to provide integrated voice, data, and video services to
homes (broadband pipes to home):
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ADSL (asymmetrical digital subscriber loops)
HDSL (high-speed digital subscriber loop)
RADSL (rate-adaptive digital subscriber loop)
SDSL (symmetrical digital subscriber loop)
VDSL (very high-speed digital subscriber loop)
IDSL (ISDN digital subscriber loop)
• We shall concentrate on ADSL. Others are some variation of this
technology
• ADSL is an asymmetric system that makes it suitable for internet
access. The term asymmetric is used because the data rates in the two
directions are not equal. The ADSL has fast downstream channel from
the central office or ISP to the home occupies the band from 138 kHz
to 1.104 MHz, while the slow upstream channel occupies the band
from 4 to 138 kHz.[Figure 1]. Multiplexed application for backbone
network is shown in Figure 2.
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ADSL Operation
• ADSL uses BW above the 4-KHz voice band (up to 1.104 MHz) and
allows a phone call and a highspeed data connection to co-exist. A
splitter which is a highpass/low-pass filter (HPF/LPF) combination, is
used to separate the voice and data signals at each end. ATU-R is the
ADSL transceiver on the remote side, and ATU-C is at CO
(Figures 3). Discuss HP/LP filter requirements.
• In theory, ADSL can provide up to 8 Mbps (Max. More like 6 Mbps;
64-QAM @ [6bits/Hz x (1.104 –0.138)]  6.0 Mbps) of data from CO
downstream to users, and up to 800 Kbps [ 138.0-4 = 134 x 6 = 804
Kbps]from subscriber to CO. Along with these directional data is voice
communication [Fig. 6]
• Channel performance over a twisted pair can vary greatly depending
upon the length and condition of the wire. A pair is unsuitable for
ADSL if it includes a loading coil, or if it terminates in a 4-kHz band.
Perhaps one-third of all pairs are unsuitable for this reason. The
performance of the remaining pairs is highly variable depending again
upon their length and the extent of far -end and near -end cross -talk.
The concept and configuration of FEXT and NEXT is shown in Figure
4. Thus, only a subset (limited %) of all local loops qualify for ASDL
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138
KHz
Router or
Switch
DSLAM
ADSL
ADSL
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Fig. 1
ATM
network
ATU-C
ATU-C
Central office
ISP
ATM
link
Corporate
gateway
Fig. 2
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[Figure 3]
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[Fig. 4]
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ADSL Design considerations
• Figure 3 shows the system layout. Filters (HP/LP) of right type is
included in ATU-C and ATU-R so that the digital signal passes through
and voice band energy is blocked. Noise and linearity are the biggest
design consideration.
• As in Figure 3 the total line attenuation in dB integrated over the up to
l.l04 MHz band as a function of line length in kft for two types of wire
gauge AWG24 and AWG26. (AWG26 is a thinner gauge and
accordingly exhibits higher loss.)
• From the graph it is evident that a total attenuation up to 50 or 60 dB is
easily possible on long lines.
• Thus, the received signal at the end of a long line can have a very
small amplitude and thus the receiver noise performance is critical.
Therefore, noise is a huge concern during the development of the
ADSL front end.
• To counteract degradation due to noise, ADSL employs a discrete
multi tone modulation scheme. Non-linearity in the analog signal path
cause harmonic and inter-modulation distortion.
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ADSL Design considerations
• The total band of 1.104 MHZ is divided into 256 frequency bands
(bins) that are 4.3125 KHz apart [Fig. 5]. Those channels having more
noise carry less data. Channels having less noise (high S/N ratio) carry
more data.
• Before transmitting data there is a period of initialization (training)
where channel quality is measured.Test signal consists of 248 tones
over the entire 1.104 MHz band with eight missing tones or holes
where noise (IMD noise) are measured. Estimation of noise power
along with signal plus noise power measured in each bin provides S/N
ratio [ 1 + S/N]
• ANSI T1E1.413 standard specifies the maximum transmitted power
density to be - 40 dBm/Hz over a telephone line. Thus, the maximum
power level in 1.104 MHz band is  - 40 +10log10(1.104 x 106) =
60.4-40 = 20.4 dBm. Line driver at the central office must deliver this
much power.
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Fig. 5
Fig. 6
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ADSL Design considerations
• Crest factor: Variable modulation in different bins cause a very high
peak voltages over telephone lines. Independent frequency and phase
of 256 carriers, create a statistical distribution of composite signal
amplitude over the transmission line. Thus, there will be times when
the phases of multiple tones will align in a manner that causes the peak
amplitudes of the individual tones to add to each other so that the
composite waveform has a large peak amplitude. The ratio of this peak
amplitude to the RMS value is called the "crest factor" and for ADSL it
is typically chosen to be 5.6, or 15 dB (20 log10(5.6), shown in Figure
below 7). At this crest factor value, the probability of exceeding 5.6
volts is reduced to approximately 1 x 10-7. Given this crest factor, the
maximum voltage at the client end can be determined. Assuming a
maximum power level of 8 dBm (SQRT(10-2.2/100.0), the resulting
RMS voltage level on a 100 - ohm line will be 0.8 V. Converting from
RMS to peak by using a 5.6 crest factor yields a peak voltage of 4.48,
or 8.96 V, peak -to-peak.
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ADSL Design considerations
• If the A D converter is implemented using 3.3V
logic, as is often the case, then this maximum
incoming signal at the receiver must be attenuated
from 8.89V down to approximately 2V. At 2V, the
converter is able to digitize the signal. This means
that for maximum level signals, the receiver
circuitry must be capable of attenuating the signal
by about 13 dB [Fig. 8].
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Fig. 7
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Fig. 8
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