Stein Intro xDSL 1.1 x DSL Introduction Yaakov J. Stein Chief Scientist RAD Data Communications.
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Stein Intro xDSL 1.1
xxDSLDSL
Introduction Introduction
xxDSLDSL
Introduction Introduction
Yaakov J. Stein
Chief ScientistRAD Data Communications
Stein Intro xDSL 1.5
New (digital) PSTNNew (digital) PSTN
“last mile”CO SWITCH
“last mile”
PSTN
CO SWITCH
TDM
TDM
digitalanalog
Stein Intro xDSL 1.6
Voice-grade modems Voice-grade modems over new PSTNover new PSTN
UTP subscriber line
CO SWITCH
network/ISP
router
modem
PSTN
modem
CO SWITCH
Modem technology is basically unchanged
Communications speeds do not increase
Stein Intro xDSL 1.8
What is UTP?What is UTP?
Two plastic insulated copper wires
Two directions over single pair
Twisted to reduce crosstalk
Supplies DC power and audio signal
Due to physics attenuation increases with frequency
Stein Intro xDSL 1.9
Why twisted?Why twisted?
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)
n
a
b
Stein Intro xDSL 1.10
Why twisted? - continuedWhy twisted? - continued
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 V1 where Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)
a
d
c
b
C bc C bd
L bc L ad
Stein Intro xDSL 1.11
Loading coilLoading coil
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
Stein Intro xDSL 1.12
I forgot to mention bridged taps!
Parallel run of unterminated UTP unused piece left over from old installation placed for subscriber flexibility
Signal are reflected from end of a BT
A bridged tap can act like a notch filter!
Bridge tapsBridge taps
Stein Intro xDSL 1.13
Subscriber lines are seldom single runs of cableUS UTP usually comes in 500 ft lengths
Splices must be made
Average line has >20 splices
Splices corrode and add to attenuation
Gauge changesBinders typically 26 AWG
Change to 24 after 10 Kft
In rural areas change to 19 AWG after that
Other problemsOther problems
Stein Intro xDSL 1.14
CSA guidelinesCSA guidelines
1981 AT&T Carrier Service Area guidelines
No loading coils Maximum of 9 Kft of 26 gauge (including bridged taps)
Maximum of 12 Kft of 24 gauge (including bridged taps)
Maximum of 2.5 Kft bridged taps Maximum single bridged tap 2 Kft Suggested: no more than 2 gauges
In 1991 more than 60% of US lines met CSA requirements
Stein Intro xDSL 1.15
Present US PSTNPresent US PSTN
UTP only in the last mile (subscriber line) 70% unloaded < 18Kft 15% loaded > 18Kft 15% optical or digital to remote terminal + DA (distribution area)
PIC, 19, 22, 24, 26 gauge
Built for 2W 4 KHz audio bandwidth
DC used for powering
Above 100KHz: severe attenuation cross-talk in binder groups (25 - 1000 UTP) lack of intermanufacturer consistency
Stein Intro xDSL 1.17
Alternatives for data servicesAlternatives for data services
Fiber, coax, HFC
COST: $10K-$20K / mile
TIME: months to install
T1/E1
COST: >$5K/mile for conditioning
TIME: weeks to install
DSL
COST: 0 (just equipment price)
TIME: 0 (just setup time)
Stein Intro xDSL 1.18
xDSLxDSL
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
Stein Intro xDSL 1.19
xDSLxDSL System Reference Model System Reference Model
POTSSPLITTER
UTP
CO SWITCH
DSLAM
xTU-C
network/ISP
router xTU-R
POTSSPLITTER
PSTN
PDN
POTS-RPOTS-C
WAN
x = H, A, V, ...
Analog modem
Stein Intro xDSL 1.20
SplitterSplitter
Splitter separates POTS from DSL signals Must guarantee lifeline POTS services! Hence usually passive filter Must block impulse noise (e.g. ring) from phone into DSL
ADSLforum/T1E1.4 specify that splitter be separate from modemNo interface specification yet (can’t buy splitter and modem from different
vendors)
Splitter requires installation Costly technician visit is the major impediment to deployment G.lite is splitterless ADSL
Stein Intro xDSL 1.21
Why is DSL better Why is DSL better than a voice-grade modem?than a voice-grade modem?
Analog telephony modems are limited to 4 KHz bandwidth
Shannon’s theorem tells us that the maximum transfer rate
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?
S
N
Stein Intro xDSL 1.22
Attenuation vs. frequencyAttenuation vs. frequency
0 2 4 6 8 10-90
-80
-70
-60
-50
-40
-30
-20
-10
024 and 26 AWG Cables
Freq [MHz]
Atte
nua
tion
[dB
/Km
]
Stein Intro xDSL 1.23
Maximum reachMaximum reach
Length of cable for reliable communications
ASSUMING ONLY THERMAL NOISE
Bellcore study in residential areas (NJ) found -140 dBm / Hz white (i.e. independent of frequency)
is a good approximation
Real systems have other sources of noise,
and thus have lower reach (Shannon!)
We can compute the maximum reach from UTP attenuation
Stein Intro xDSL 1.24
xDSL - Maximum ReachxDSL - Maximum Reach
0 10 20 30 40 50 600
1
2
3
4
5
6DSL MAXIMUM REACH
Rate[Mbps]
Re
ach
[Km
]
Stein Intro xDSL 1.25
Sources of InterferenceSources of Interference
XMTR RCVR
RCVR XMTR FEXT
NEXT
RCVR XMTR
XMTR RCVR
RF INGRESS
THERMAL NOISE
Stein Intro xDSL 1.26
Interference for xDSLInterference for xDSL
0 0.5 1 1.5 2-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0ISDN NEXT, AM INGRESS, SELF FEXT
Freq [MHz]
Inte
rfe
renc
e [d
Bm
/Hz]
Stein Intro xDSL 1.27
Examples of Realistic ReachExamples of Realistic Reach
More realistic design goals (splices, some xtalk)
1.5 Mbps 18 Kft 5.5 km (80% US loops)
2 Mbps 16 Kft 5 km
6 Mbps 12 Kft 3.5 km (CSA 50% US loops)
10 Mbps 7 Kft 2 km
13 Mbps 4.5 Kft 1.4 km
26 Mbps 3 Kft 900 m
52 Mbps 1 Kft 300 m (SONET STS-1 = 1/3 STM-1)
Stein Intro xDSL 1.28
xDSL flavorsxDSL flavors
modem speed reach main applications
IDSL 160 (144) Kbps 5.5 km POTSreplacement,videoconferencing,Internet access
HDSL 2 Mbps (4-6W) 3.6-4.5 km T1/E1 replacementPBX interconnect,FR
HDSL2 2 Mbps (2W) 3 km same as HDSL
SDSL <= 2 Mbps 3 km same as HDSL
Stein Intro xDSL 1.29
xDSL flavorsxDSL flavors
modem speed reach main applications
ADSL 6 Mbps DS 640 Kbps US
3.5-5.5 km residential Internet, video-on-demand
ADSL2 8 Mbps DS 800 Kbps US
> ADSL Internet access, VoIP
ADSL2+ 16 Mbps DS 800 Kbps US
< 2 km “
VDSL <= 52 Mbps 300m - 1 km LAN interconnect, HDTV, combined services
VDSL2 200 Mbps (aggregate)
up to 1.8 km “
cable modem 10-30Mbps DS shared
50 km residential Internet
HPNA 1, 10 Mbps home wiring residential networking
Stein Intro xDSL 1.30
ITU G.99x standardsITU G.99x standards
G.991 HDSL (G.991.1 HDSL G.991.2 SHDSL)
G.992 ADSL (G.992.1 ADSL G.992.2 splitterless ADSL G.992.3 ADSL2 G.992.4 splitterless ADSL2 G.992.5 ADSL2+) G.993 VDSL (G.993.1 VDSL G.993.2 VDSL2)
G.994 HANDSHAKE
G.995 GENERAL (INFO)
G.996 TEST
G.997 PLOAM G.998 bonding (G.998.1 ATM G.998.2 Ethernet G.998.3 TDIM)
Stein Intro xDSL 1.31
BondingBonding
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: ATM - extension of IMA (may be different rates per pair)
cells marked with SID and sent on any pair Ethernet - based on 802.3(EFM) frames are fragmented, marked with SN, and sent on many pairs Time division inverse mux Dynamic Spectral Management (Cioffi) Ethernet link aggregation
Stein Intro xDSL 1.33
T1 serviceT1 service
1963: Coax deployment of T1 2 groups in digital TDM RZ-AMI line code Beyond CSA range should use DLC (direct loop carrier) Repeaters every 6 Kft Made possible by Bell Labs invention of the transistor
1971: UTP deployment of T1 Bring 1.544 Mbps to customer private lines Use two UTP in half duplex Requires expensive line conditioning One T1 per binder group
Stein Intro xDSL 1.34
T1 line conditioningT1 line conditioning
In order for a subscriber’s line to carry T1
Single gauge CSA range No loading coils No bridged taps Repeaters every 6 Kft (starting 3 Kft) One T1 per binder group Labor intensive (expensive) process Need something better … (DSL) Europeans already found something better
Stein Intro xDSL 1.35
The first xDSL!The first xDSL!
1984,88: IDSL BRI access for ISDN 2B1Q (4 level PAM) modulation Prevalent in Europe, never really caught on in US 144 Kbps over CSA range
1991: HDSL Replace T1 line code with IDSL line code (2B1Q) 1 UTP (3 in Europe for E1 rates) Full CSA distance without line conditioning Requires DSP
Stein Intro xDSL 1.36
HDSLHDSL
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
Stein Intro xDSL 1.37
HDSL2HDSL2
Customers request HDSL service that is single UTP HDSL at least full CSA reach spectrally compatible w/
HDSL, T1, ADSL, etc.
Variously called
HDSL2 (ANSI)
SDSL Symmetric DSL (ETSI)
Now called
SHDSL Single pair HDSL (ITU)
Stein Intro xDSL 1.38
ADSL (full rate)ADSL (full rate)
Asymmetric - high rate DS lower rate US
Originally designed for video on demand
Almost retired due to lack of interest
…but then came the Internet
Studies show DS:US should be about 10:1full rate ADSL 512-640 kbps US, 6-8 Mbps DS G.lite 512 Kbps US, 1.5 Mbps DS
ADSL could mean All Data Subscribers Living
Stein Intro xDSL 1.39
G.liteG.lite
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)
Stein Intro xDSL 1.40
ADSL2ADSL2
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 better modulation reduced framing overhead stronger ECC reduced power mode misc. algorithmic 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
Stein Intro xDSL 1.41
VDSLVDSL
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
Stein Intro xDSL 1.42
VDSL2VDSL2
VDSL uses BW of 1.1 MHz - 12 MHz (spectrally compatible with ADSL)
VDSL2 uses 20 KHz - 30 MHz
new band-plans increased DS transmit power various algorithmic improvements borrowed improvements from ADSL2 3 user data types - STM, ATM and packet (pure Ethernet)
Stein Intro xDSL 1.43
HPNA (G.PNT)HPNA (G.PNT)
Studies show that about 50% of US homes have a PC
30% have Internet access, 20% have more than one PC!
Average consumer has trouble with cabling
HomePNA de facto industry standard for home networking Computers, peripherals interconnect (and connect to Internet?)
using internal phone wiring (user side of splitter) Does not interrupt lifeline POTS services Does not require costly or messy LAN wiring of the home Presently 1 Mbps, soon 10 Mbps, eventually 100 Mbps!
Stein Intro xDSL 1.44
Competition -Competition - Cable modems Cable modems
CATV
HEADEND
OPTICAL
FIBER
NODE
COAXIAL
AMPLIFIER
CABLE
MODEM
CABLE
MODEM
CABLE
MODEM
CABLE
MODEM
fiber coax
CMTS
Stein Intro xDSL 1.46
How do modems work?How do modems work?
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)
1 1 1 00 1 10
Stein Intro xDSL 1.47
The simplest modem - DCThe simplest modem - DC
So what about transmitting -1/+1?
This is better, but not perfect! DC isn’t exactly zero Still can have a long run of +1 OR -1 that will decay Even without decay, long runs ruin timing recovery (see below)
1 1 1 00 1 10
Stein Intro xDSL 1.48
The simplest modem - DCThe simplest modem - DC
What about RZ?
No long +1 runs, so DC decay not important Still there is DC Half width pulses means twice bandwidth!
1 1 1 00 1 10
Stein Intro xDSL 1.49
The simplest modem - DCThe simplest modem - DC
T1 uses AMI (Alternate Mark Inversion)
Absolutely no DC! No bandwidth increase!
1 1 1 00 1 10
Stein Intro xDSL 1.50
The simplest modem - DCThe simplest modem - DC
Even better - use OOK (On Off Keying)
Absolutely no DC! Based on sinusoid (“carrier”) Can hear it (morse code)
1 1 1 00 1 10
Stein Intro xDSL 1.51
PSKPSK
Even better to use sinusoids with different phases!
BPSK 1 bit / symbol
or QPSK
2 bits / symbol
Bell 212 2W 1200 bps
V.22
1 1 1 0 0 1 0 1
11 10 01 01 00 11 01
Stein Intro xDSL 1.52
QAMQAM
Finally, best to use different phases and amplitudes
2 bits per symbol
V.22bis 2W full duplex 2400 bps used 16 QAM (4 bits/symbol)
This is getting confusing
11 10 01 01 00 11 01
Stein Intro xDSL 1.53
Star watching Star watching
For QAM we can draw a diagram with x and y as axes A is the radius, the angle
For example, QPSK can be drawn (rotations are time shifts)
Each point represents 2 bits!
Stein Intro xDSL 1.54
QAM constellations QAM constellations
16 QAM V.29 (4W 9600 bps)
V.22bis 2400 bps Codex 9600 (V.29) 2W
first non-Bell modem (Carterphone decision)
Adaptive equalizer
Reduced PAR constellation
Today - 9600 fax!
8PSKV.27
4W
4800bps
Stein Intro xDSL 1.57
xDSL Line CodesxDSL Line Codes
PAM IDSL (2B1Q) HDSL2 (with TCM and optionally OPTIS) SDSL
QAM/CAP proprietary HDSL/ADSL/VDSL
DMT ADSL G.lite
VDSL line code war is still raging
Stein Intro xDSL 1.58
DuplexingDuplexing
How do we send information in BOTH directions? Earliest modems used two UTP, one for each direction (4W) Next generation used 1/2 bandwidth for each direction (FDD) Alternative is to use 1/2 the time (TDD)
More advanced DSP uses adaptive echo canceling
m odulator
dem odulator
4W to 2W
H YBR ID
U TPLEC
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