Intro to xdsl part 1

xDSL Introduction

• Yaakov J. Stein
• Chief Scientist RAD Data Communications

PSTN wiring

Old (analog) PSTN

• subscriber line

• subscriber line

Voice-grade modems

• UTP

• modem

• modem

New (digital) PSTN

• “last mile”

• CO SWITCH

• “last mile”

• PSTN

• CO SWITCH

• TDM

• TDM

• digital

• analog

Voice-grade modems over 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

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?

• 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

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