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

PSTN wiring

Old (analog) PSTN subscriber line subscriber line

Voice-grade modems UTP modem modem

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

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

Unshielded Twisted Pair

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

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

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)

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

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!

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

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

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 (25 - 1000 UTP) lack of intermanufacturer consistency

xDSL

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 COST: @ 0 (just equipment price) TIME: @ 0 (just setup time)

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

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

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

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

Attenuation vs. frequency

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

xDSL - Maximum Reach

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

Interference for xDSL

Examples 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)

xDSL flavors

xDSL flavors

ITU 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)

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

xDSL types

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 1.544 Mbps to customer private lines Use two UTP in half duplex Requires expensive line conditioning One T1 per binder group

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

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

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

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)

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 512-640 kbps US, 6-8 Mbps DS G.lite 512 Kbps US, 1.5 Mbps DS ADSL could mean All Data Subscribers Living

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)

ADSL2 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

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

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)

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!

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

Modem Theory

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)

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)

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!

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

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

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

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

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!

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 - 9600 fax! 8PSK V.27 4W 4800bps

QAM constellations (cont)

DMT - continued frequency time

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

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

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