You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York, and his head is meowing in Los Angeles, страница 2

Transmitting Data over Phone Lines

PHASE MODULATION

Used for 1200bps and above

00 phase angle at start of coding element (a.k.a. in-phase, or sin signal)

1800 phase angle at start of coding element (a.k.a. out-of-phase, or cos signal)

1

0

1

So we represent

as

Transmitting Data over Phone Lines

DIFFERENTIAL PHASE MODULATION

PHASE MODULATION

^

But absolute phase angles are not easy to measure.

Insert a phase change between adjacent signal elements logic 0  a 900 (1/4 cycle) phase shift logic 1  a 2700 (3/4 cycle) phase shift

So 0100 becomes

0

0

1

0

Data is only encoded at boundaries between signalling elements Why do the the signalling elements have to be 11/2 wavelengths long?

Transmitting Data over Phone Lines

BAUD RATE

Each signal element is a “baud” After its inventor Baudot (1845-1903)

Emile Baudot

No of signal elements/s is called the baud rate

In our example, each bit occupies 1.5 cycles So 1200bps data requires 1800Hz carrier What is the baud rate? 1200 baud

Transmitting Data over Phone Lines

BAUD RATE vs. BIT RATE

BAUD RATE

BAUD RATE vs. BIT RATE

For higher speeds Keep the same carrier frequency (i.e., fcarrier = 1800Hz, still) But encode more bits per signalling element To get 2400 bps encode two bits at a time using: 00 00 phase shift 01 900 shift 11 1800 shift 10 2700 shift

carrier: 1800Hz baud rate: 1200baud bit rate: 2400bps

so 00 01 10 11 becomes:

2700

Transmitting Data over Phone Lines

DIFFERENTIAL PHASE CODING

(Back to) DIFFERENTIAL PHASE ENCODING

Coding scheme using phase shifts can be represented by phase diagrams (or constellation charts)

00 00 phase shift 01 900 shift 11 1800 shift 10 2700 shift

Angle represents phase Radial distance represents amplitude of carrier wave (only 1 amplitude used here)

Transmitting Data over Phone Lines

QUADRATURE AMPLITUDE MODULATION

QAM (QUADRATURE AMPLITUDE MODULATION)

For higher speeds, increase the number of values per signalling element For 4800bps, encode 3 bits per signal element at 1600 baud fcarrier = ?

2400Hz, if 1.5 cycles per signalling element

QAM uses both phase shifts AND amplitude

Transmitting Data over Phone Lines

Theoretical Limitations

THEORETICAL LIMITATIONS

Nyquist theorem: Max bitrate = 2 x carrier frequency = 2 x carrier frequency x log2(possible values/signal element)

x no of bits encoded by a signalling element

e.g., Max bitrate = 2 x 3000 x log2(8) = 18000bps

Fixed by legacy technology

Indefinitely incrementable?

In theory, perhaps. In practice, noise interferes

Transmitting Data over Phone Lines

Shannon-Hartley Law

SHANNON-HARTLEY LAW takes noise into account

Public Switched Telephone Network

Max bitrate = bandwidth x log2(1 + S/N)

S/N for analogue PSTN with multiple exchanges is ~1000 Depending on complexity of connection

Max bitrate = 3000 x log2(1000) = 30000bps

33.3kbps modems reached this limit

Transmitting Data over Phone Lines

56K Modems

HOW CAN MODEMS RUN AT 56K?

analogue

digital

56.6kbps modems “expect” to be used with mainly digital PSTNs only 1 analogue link - user’s modem to local exchange S/N ratio better than 1000 In fact, they often fail to achieve 56.6kbps

Transmitting Data over Phone Lines

Modem Commands

MODEM COMMANDS

Most modems obey the (de facto standard) Hayes commands All start with AT All finish with a Return

switches the modem into command mode, rather than pass-through mode.

Transmitting Data over Phone Lines

RS232

RS232

Data Circuit Terminating Equipment

Data Terminal Equipment

≤ 25 wires

Pin Circuit Mnemonic Function 2 103 TD Transmit data 3 104 RD Receive data 4 105 RTS Request to send 5 106 CTS Clear to send 6 107 DSR Data set ready 7 102 SIG Signal ground 8 109 CD Carrier detect 20 108 DTR Data terminal ready 22 125 RI Ring indicator

Transmitting Data over Phone Lines

RS232

RS232

RS232 is not meant for interconnecting DCEs directly but is commonly used for this type of communication

Minimal RS232 connection (“Null modem”)

Often connect CTS and DTR (or RTS) with a crossover, too

ADSL

ADSL

ASYNCHRONOUS DIGITAL SUBSCRIBER LINK

Asynchronous Download speed and upload speeds differ

Digital Because an analogue link is transferring digital data

Subscriber Uses the telephone network

ADSL

ADSL

BASIC TECHNOLOGY

Extends the useful life of POTS copper wires Bandwidth increased from 3kHz to 1.1MHz Divided into separate frequency bands (Discrete MultiTone signalling) Each frequency band has its own carrier, coded using QAM (2 phase values, 4 amplitudes) System adapts to S/N ratio across the frequency range Upload/download channel allocation variable

(in practice, 26 upstream channels)

upload channels

download channels