x DSL Introduction . Yaakov J. Stein Chief Scientist RAD Data Communications. PSTN wiring. subscriber line. subscriber line. Old (analog) PSTN. UTP. Voice-grade modems. modem. modem. New (digital) PSTN. CO SWITCH. “last mile”. TDM. analog. digital. PSTN. TDM.
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Yaakov J. Stein
Chief ScientistRAD Data Communications
subscriber lineOld (analog) PSTN
routerVoice-grade modems over new PSTN
UTP subscriber line
Modem technology is basically unchanged
Communications speeds do not increase
from Bell’s 1881 patent
To place the direct and return lines close together.
To twist the direct and return lines around one another so that they
should be absolutely equidistant from the disturbing wires
V = (a+n) - (b+n)
But even UTP has some cross-talk
George Cambell models UTP crosstalk (see BSTJ 14(4) Oct 1935)
Cross-talk due to capacitive and/or inductive mismatch
|I2| = Q f V1where Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)
What does a loading coil do?
Flattens response in voice band
Attenuates strongly above voice frequencies
loops longer than 18 Kft need loading coils
88 mH every 6kft starting 3kft
I forgot to mention bridged taps!
Parallel run of unterminated UTP
Signal are reflected from end of a BT
A bridged tap can act like a notch filter!
Subscriber lines are seldom single runs of cable
US UTP usually comes in 500 ft lengths
Splices must be made
Average line has >20 splices
Splices corrode and add to attenuation
Binders typically 26 AWG
Change to 24 after 10 Kft
In rural areas change to 19 AWG after that
1981 AT&T Carrier Service Area guidelines
In 1991 more than 60% of US lines met CSA requirements
UTP only in the last mile (subscriber line)
PIC, 19, 22, 24, 26 gauge
Built for 2W 4 KHz audio bandwidth
DC used for powering
Fiber, coax, HFC
COST: $10K-$20K / mile
TIME: months to install
COST: >$5K/mile for conditioning
TIME: weeks to install
COST: @ 0 (just equipment price)
TIME: @ 0 (just setup time)
Need higher speed digital connection to subscribers
Not feasible to replace UTP in the last mile
Older voice grade modems assume 4KHz analog line
Newer (V.90) modems assume 64Kbps digital line
DSL modems don’t assume anything
Use whatever the physics of the UTP allows
x = H, A, V, ...xDSL System Reference Model
Splitter separates POTS from DSL signals
ADSLforum/T1E1.4 specify that splitter be separate from modem
No interface specification yet (can’t buy splitter and modem from different vendors)
Splitter requires installation
SWhy is DSL better than a voice-grade modem?
for SNR >> 1
C = BW log2 ( SNR + 1 ) C(bits/Hz) = SNR(dB) / 3
So by using more BW we can get higher transfer rates!
But what is the BW of UTP?
ASSUMING ONLY THERMAL NOISE
Bellcore study in residential areas (NJ) found
is a good approximation
and thus have lower reach (Shannon!)
NOISESources of Interference
More realistic design goals (splices, some xtalk)
G.992.3 ADSL2 G.992.4 splitterless ADSL2
If we need more BW than attainable by Shannon bounds
we can use more than one UTP pair (although XT may reduce)
this is called bonding or inverse multiplexing
There are many ways of using multiple pairs:
cells marked with SID and sent on any pair
frames are fragmented, marked with SN, and sent on many pairs
1963: Coax deployment of T1
1971: UTP deployment of T1
In order for a subscriber’s line to carry T1
Replace T1/E1 DS1 service
Use 2B1Q line code, DFE
Full duplex on each pair with echo cancellation
CSA reach w/o conditioning/repeaters
more complex DSP
ANSI: 2 pairs for T1 (each 784 Kbps)
ETSI: 1, 2, 3 or 4 pairs
Most mature of DSL technologies
Customers request HDSL service that is
HDSL, T1, ADSL, etc.
SHDSL Single pairHDSL (ITU)
Asymmetric - high rate DS lower rate US
Originally designed forvideo on demand
Almost retired due to lack of interest
…but then came the Internet
Studies show DS:US should be about 10:1
full rate ADSL 512-640 kbps US, 6-8 Mbps DS G.lite 512 Kbps US, 1.5 Mbps DS
ADSLcould meanAll Data Subscribers Living
Splitterless ADSL, UAWG
ADSL compatible DMT compatible using only 128 tones
512 Kbps US / 1.5 Mbps DS
Still much faster than V.34 or V.90 modems
No splitter required!
Certain features removed for simplicity
simpler implementation (only 500 MIPS < 2000 MIPS for full rate)
ADSL uses BW from 20 kHz to 1.1 MHz
ADSL2 Increases rate/reach of ADSL by using 20 kHz - 4.4 MHz
Also numerous efficiency improvements
for given rate, reach improved by 200 m
3 user data types - STM, ATM and packet (Ethernet)
ADSL2+ dramatically increased rate at short distances
Optical network expanding (getting closer to subscriber)
Optical Network Unit ONU at curb or basement cabinet
FTTC (curb), FTTB (building)
These scenarios usually dictates low power
Rates can be very high since required reach is minimal!
Proposed standard has multiple rates and reaches
VDSL uses BW of 1.1 MHz - 12 MHz (spectrally compatible with ADSL)
VDSL2 uses 20 KHz - 30 MHz
30% have Internet access, 20% have more than one PC!
HomePNA de facto industry standard for home networking
using internal phone wiring (user side of splitter)
The simplest attempt is to simply transmit 1 or 0 (volts?)
This is called NRZ (short serial cables, e.g. RS232)
Information rate = number of bits transmitted per second (bps)
So what about transmitting -1/+1?
This is better, but not perfect!
What about RZ?
T1 uses AMI (Alternate Mark Inversion)
Even better - use OOK (On Off Keying)
Even better to use sinusoids with different phases!
1 bit / symbol
2 bits / symbol
Bell 212 2W 1200 bps
Finally, best to use different phases and amplitudes
2 bits per symbol
This is getting confusing
For QAM we can draw a diagram with
For example, QPSK can be drawn (rotations are time shifts)
Each point represents 2 bits!
16 QAMV.29 (4W 9600 bps)
V.22bis 2400 bps Codex 9600 (V.29)
first non-Bell modem(Carterphone decision)
Reduced PAR constellation
Today - 9600 fax!
VDSL line code war is still raging
How do we send information in BOTH directions?
More advanced DSP uses adaptive echo canceling
256ADSL FDD Duplexing
US uses tones 8 - 32 (below 30 KHz reserved)
DS uses 256 tones (FDM from tone 33, EC from tone 8)