1. Chief Scientist RAD Data Communications
xDSL Introduction
Yaakov J. Stein
Chief Scientist RAD Data Communications 1

2. PSTN wiring

3. Old (analog) PSTN
subscriber line
subscriber line

4. Voice-grade modems
UTP
modem
modem

5. New (digital) PSTN analog digital “last mile” TDM TDM “last mile” PSTN
CO SWITCH
“last mile”
TDM
analog
digital
PSTN
TDM
“last mile”
CO SWITCH

6. Voice-grade modems over new PSTN
CO SWITCH
PSTN
UTP subscriber line
modem
CO SWITCH
modem
network/
ISP
router
Modem technology is basically unchanged
Communications speeds do not increase

7. Unshielded Twisted Pair

8. 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

9. Why twisted? n a V = (a+n) - (b+n) b 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 n a
V = (a+n) - (b+n) b

10. Why 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)

11. Loading 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

12. Bridge taps 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!

13. Other problems 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
Gauge changes
Binders typically 26 AWG
Change to 24 after 10 Kft
In rural areas change to 19 AWG after that

14. CSA 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

15. Present 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 ( UTP)
lack of intermanufacturer consistency

16. xDSL

17. Alternatives 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
0 (just equipment price)
0 (just setup time)

18. xDSL 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

19. xDSL System Reference Model
POTS
SPLITTER
UTP
CO SWITCH
DSLAM
xTU-C
network/
ISP
router
xTU-R
PSTN
PDN
POTS-R
POTS-C
WAN
x = H, A, V, ...
Analog modem

20. Splitter 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 modem
No 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

21. Why is DSL better 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

22. Attenuation vs. frequency

23. Maximum 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

24. xDSL - Maximum Reach

25. Sources of Interference
XMTR RCVR
RCVR XMTR
FEXT
NEXT
RF INGRESS
THERMAL
NOISE

26. Interference for xDSL

27. Examples of Realistic Reach
More realistic design goals (splices, some xtalk)
1.5 Mbps 18 Kft km (80% US loops)
2 Mbps 16 Kft 5 km
6 Mbps 12 Kft km (CSA 50% US loops)
10 Mbps 7 Kft 2 km
13 Mbps 4.5 Kft km
26 Mbps 3 Kft m
52 Mbps 1 Kft m (SONET STS-1 = 1/3 STM-1)

28. xDSL flavors

29. xDSL flavors

30. ITU G.99x standards G.991 HDSL (G.991.1 HDSL G.991.2 SHDSL)
G.992 ADSL (G ADSL G splitterless ADSL
G ADSL2 G splitterless ADSL2
G ADSL2+)
G.993 VDSL (G VDSL G VDSL2)
G.994 HANDSHAKE
G.995 GENERAL (INFO)
G.996 TEST
G.997 PLOAM
G.998 bonding (G ATM G Ethernet G TDIM)

31. Bonding 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

32. xDSL types

33. T1 service 1963: Coax deployment of T1 1971: UTP 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 Mbps to customer private lines
Use two UTP in half duplex
Requires expensive line conditioning
One T1 per binder group

34. T1 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

35. The first xDSL! 1984,88: IDSL 1991: HDSL 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

36. HDSL 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

37. HDSL2 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)

38. 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 10:1
full rate ADSL kbps US, 6-8 Mbps DS G.lite 512 Kbps US, 1.5 Mbps DS
ADSL could mean All Data Subscribers Living

39. G.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)

40. ADSL2 ADSL uses BW from 20 kHz to 1.1 MHz
ADSL2 Increases rate/reach of ADSL by using 20 kHz 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

41. VDSL 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

42. VDSL2
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)

43. 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!

44. Competition - Cable modems
CMTS
CABLE
MODEM
OPTICAL
FIBER
NODE
CATV
HEADEND
fiber
coax
COAXIAL
AMPLIFIER
CABLE
MODEM
CABLE
MODEM

45. Modem Theory

46. 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)

47. The 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)

48. The 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!

49. The simplest modem - DC T1 uses AMI (Alternate Mark Inversion)
Absolutely no DC!
No bandwidth increase!

50. The simplest modem - DC Even better - use OOK (On Off Keying)
Absolutely no DC!
Based on sinusoid (“carrier”)
Can hear it (morse code)

51. PSK BPSK 1 bit / symbol or QPSK
Even better to use sinusoids with different phases!
BPSK
1 bit / symbol
or QPSK
2 bits / symbol
Bell 212 2W 1200 bps
V.22

52. QAM 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

53. Star watching For QAM we can draw a diagram with
x and y as axes
A is the radius, f the angle
For example, QPSK can be drawn (rotations are time shifts)
Each point represents 2 bits!

54. QAM constellations 16 QAM V.29 (4W 9600 bps) Adaptive equalizer
V.22bis 2400 bps Codex 9600 (V.29) 2W
first non-Bell modem (Carterphone decision)
Adaptive equalizer
Reduced PAR constellation
Today fax!
8PSK
V.27 4W
4800bps

55. QAM constellations (cont)

56. DMT - continued
frequency
time

57. xDSL 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

58. Duplexing 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

59. ADSL FDD Duplexing US uses tones 8 - 32 (below 30 KHz reserved)
DS uses 256 tones (FDM from tone 33, EC from tone 8)
POTS US DS 8 32
256