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

159.253 Computer Systems

Data Communications

“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. Do you understand this?

And radio operates in exactly the same way; you send signals here, they receive them there

The only difference is

that there is no cat”

Albert Einstein

Media

Tanenbaum 3rd edition: 82-100

TYPICAL PARAMETERS

Twisted pair (wire)

16-600Mbps

A few km

Coax

100Mbps

500m

Fibre optic cable

10Gbps

1000km

Radio

10kbps – 100Mbps

Satellite

100Mbps

Voice grade telephone

56kbps

ADSL (phone wire)

8Mbps

10km

Bit Encoding

VARIOUS TECHNOLOGIES

0

1

Voltage levels:

Inside CPU

0V

5V

(or 3.3V or 2.8V)

RS232 (V.24)

-12V

+12V

Current flow

20mA current generators

transmitter

transmitter

Bit Encoding

VARIOUS TECHNOLOGIES

0

1

Voltage levels:

Inside CPU

0V

5V

(or 3.3V or 2.8V)

RS232 (V.24)

-12V

+12V

Current flow

0mA

20mA

Direction of voltage change at clock edge

1

1

1

0

0

0

1

Bit Encoding

VARIOUS TECHNOLOGIES

0

1

Voltage levels:

Inside CPU

0V

5V

(or 3.3V or 2.8V)

RS232 (V.24)

-12V

+12V

Current flow

0mA

20mA

Direction of voltage change at clock edge

Amplitude and frequency modulation

light

dark

Light transmission

Parallel Transmission

CENTRONICS INTERFACE

Normally between a computer and printer.

8 wires transmit a data byte + control wires

Sender (computer) puts the correct voltages on the wire asserts a STROBE line to say that valid data is present.

Receiver (printer)

receives data byte puts TRUE on ACK (acknowledgment) control line

Works over distances of ~ 5m

Serial Transmission

PROBLEMS WITH PARALLEL DATA PATHS

Parallel cables are expensive! Generally preferable to transmit bits serially Common local speeds: 300 bps 600 bps 1200 bps 9600 bps

Serial transmission used exclusively over long distances ATM Internet

Bit Synchronisation

START AND STOP BITS

Receiver should sample signal in the middle of each bit

Needs to know Duration of bits (by “agreement” between source and destination) Time when bits start and stop

Asynchronous frame

20% overhead (2 bits in 10 are “administration”)

Bit Synchronisation

UART

(UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER)

Inputs serial data

Remembers 10-bit sequences (more if stop “bit” lasts for more than 1 bit time)

Detects 8-bit sequences surrounded by start and stop bits Delivers these in parallel to the processor

UART’s clock typically runs at 16 x serial bit rate.

UART synchronises on start bit’s hi -> lo transition

Samples incoming signal: once after 24 clock-cycles (1.5 bit times) 9 times after 16 clock-cycles

Up to 7 clock cycles clock drift without loss of synchronisation

Non-Data Signals

BREAK SIGNAL

Logic 0 state > ~ 1/5 sec is interpreted as a BREAK signal. (For current loop this occurred if the wire was disconnected or broken) Length of break is not related to the transmission rate

Can be used as an auto speed detect feature - The user generates a break signal The remote system goes to a new bit rate and sends “login:” User stops sending Breaks when the correct message appears

Bit Encoding

FIBRE OPTIC TRANSMISSION

Light transmitted by total internal reflection at glass/glass boundary Glass is highly transparent for 3 IR light wavelengths: 850, 1300, 1550 nm 1 encoded as a light pulse; 0 encoded as a dark period

Speed limit > 25,000GHz for all three frequencies Actual transmission speeds (~10GHz) limited by computer switching technology

Transmitting Data over Phone Lines

MODULATION

Sine waves

Amplitude

3600

00

Phase

Wavelength (= 1/frequency)

Transmitting Data over Phone Lines

THE TELEPHONE SYSTEM

Frequency range optimised for voice signals

Direct transmission of digital data not possible Long sequences of 1's or 0's look like a signal with frequency  0Hz Frequency for 9600bps sequences of alternating 0's and 1's > 3400Hz.

Modulate an audio-frequency carrier signal with digital data Carrier always above low-f cutoff To get high data rates at <3400Hz, squeeze multiple bits into a signal element

Transmitting Data over Phone Lines

FREQUENCY MODULATION

Used for bit rates of 300bps or below.

logic 0

f = 1180Hz

010=

Note: exaggerated frequency difference in this diagram

Transmitting Data over Phone Lines

FREQUENCY MODULATION

Frequency shift keying

Simplex- transmission only in one direction Half-duplex - alternating transmission Full duplex - both sides talk at once.

Full-duplex requires another pair of frequencies logic 1 = 1650Hz logic 0 = 1850Hz

The side that started the call is in originate mode uses the first set of frequencies (980 and 1180Hz) The side that answers the call is in answer mode uses the second set of frequencies