Intro to xdsl part 1

Competition - Cable modems

• CATV
• HEADEND

• OPTICAL
• FIBER
• NODE

• COAXIAL
• AMPLIFIER

• CABLE
• MODEM

• CABLE
• MODEM

• CABLE
• MODEM

• CABLE
• MODEM

• fiber

• coax

• CMTS

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

• 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, the angle
• For example, QPSK can be drawn (rotations are time shifts)
• Each point represents 2 bits!