Fundamentals of Communications

1
Fundamentals of Communications
2
Data Transfer
0 1 0 1 1 0 1 0
Device 1
Device 2
simple serial connection
3
Voltages
15V
0
logic
5V
1
5V
0
0V
0V
0
-5V
1
-15V
1
Internal Voltages
RS232C Voltages
4
Voltages as data
12V
-12V
0 1 0 0 1 0
1
time
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Timing of signals

• Consider that each state was of 10ms in duration
• this means that 100 such states could be set in
  1s
• or expressed as 100bits per second (100bps)
• i.e.period / number of bits

Problems However.

• How does the receiver know when to start
  listening?
• How can the receiver recognise the boundaries
  between individual bits being sent?
• Can it cope with errors arising, thereby
  corrupting bits?

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Asynchronous transfer
Start bit
Stop bit
0
1
Idle state
8 data bits
Send to receiver
7
Bits
Start bit
Stop bit
msb
lsb
0 1 0 0 0 0 0 1 0 1
parity bit
Send to receiver
1 0 1 0 0 0 0 0 1 0
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Timing

• In order to receive a data transmission correctly
  the receiver must be clocking at the same rate.
• This must be established before data is
  transferred.
• Some systems over-sample at 16 time the bit rate
  in order to avoid the transitions.
• The receiver will also check the framing i.e.
  check that the stop appears where it should.
• Consider 5,000 characters to be sent
• this is 50,000 bits since 5000 (1start 8
  data 1stop) bits
• at a data rate of 9600bps the time taken for
  transmission is 50000/9600 s or 5.2 seconds
• this assumes no time delay between characters
  being transmitted

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Some speeds for old fashioned modems

• Data rate bits/s Character rate chars/s
• 110 10
• 300 30
• 1200 120
• 2400 240
• 4800 480
• 9600 960
• 19200 1920
• 28000 2800
• 55600 5560

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Redundancy

• Note the redundant bits in the transmission
• for every 10 bits sent only 8 are data or 25
  overhead
• whenever one considers that 8th bit is parity
  this rises to 30 or the useful information is
  only 70 of that transferred.

• What are the limitations of parity?
• How many errors can it detect?

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Synchronous transfer
Synchronising bits
Data to be transferred
Start of text (STX)
End of text (ETX)
Send to receiver
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A protocol

• 2 SYN, STX, (1024 chars), ETX
• efficiency is 10248 /10278 99.7
• compared with 10248 / 102410 80 for
  asynchronous
• however as block size decreases efficiency falls.

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Band v bits/sec

• 1 baud is a change of state per second
• not to be confused with 1 bit/sec which is 1
  bit/sec
• if a change of state represents only one bit then
  1 baud is 1 bit/sec
• many cases a change of state represents 2, 3, or
  more bits.

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1 change of state 3 bits of data
Volts
7 6 5 4 3 2 1 0
010
111
Bits
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Maximum Data rate of a channel (Nyquist)

• A signal which has a maximum frequency component
  H can be reconstructed by making 2H samples per
  second.
• Nyquists theorem more generally with V discrete
  levels

bits/sec
Thus with 3kHz channel and 2 level signals the
data rate cannot exceed 6000bps
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Channels with Noise
Shannons theorem gives the following outcome
Thus a channel with a signal to noise ratio of
30dB and bandwidth of 3,000 Hz will have a
maximum capacity of 30,000 bps
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Modems

• Modulator and Demodulator.
• Modulation is the act of changing a carrier wave
  so that it can represent a signal.
• Demodulation is the act of measuring the changes
  made to a carrier wave and thereby recovering the
  original modulating signal.
• Consider AM Radio broadcast.
• RTE 567kHz Radio Ulster 1341kHz bandwidth 9kHz
• and FM Radio broadcast.
• Classic FM 102MHz bandwidth 25kHz

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Sine wave
1/3sec
X varies in time Amplitude A Frequency f time
t theta the phase
1/5sec
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Amplitude Modulation
For binary 1
For binary 0
Thus the amplitude for 0 is half that for 1, this
could be chosen to be zero if so desired.
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Frequency Modulation
For binary 1
For binary 0
Thus the frequency for 0 is different than that
for 1, called frequency shift keying (tone
dialing on telephone).
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Phase Modulation
For binary 1
For binary 0
Thus the phase for 0 is shifted 180 degrees than
that for 1, called phase shift keying
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Data rate and Baud rate for PSK
Baud rate is the number of changes of state per
second if a change of state can be made to
represent more than one bit i.e. 2 then the data
rate is twice the baud rate
Amplitude
time
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Note how the addition of 2 levels of AM enable
another bit to be encoded with each baud
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Communications telephone systems

• Existing telephones developed to carry voice.
• Data to communicated must taken on voice type
  properties
• Must use frequencies between 300 and 3,400 Hz

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Asynchronous and Synchronous Transmission

• Timing problems require a mechanism to
  synchronize the transmitter and receiver
• Two solutions
• Asynchronous
• Synchronous

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Asynchronous

• Data transmitted on character at a time
• 5 to 8 bits

• Timing only needs maintaining within each
  character
• Resync with each character

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Asynchronous - Behaviour

• In a steady stream, interval between characters
  is uniform (length of stop element)
• In idle state, receiver looks for transition 1 to
  0
• Then samples next seven intervals (char length)
• Then looks for next 1 to 0 for next char
• Simple
• Cheap
• Overhead of 2 or 3 bits per char (20)
• Good for data with large gaps (keyboard)

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Synchronous - Bit Level

• Block of data transmitted without start or stop
  bits
• Clocks must be synchronized
• Can use separate clock line
• Good over short distances
• Subject to impairments
• Embed clock signal in data
• Manchester encoding
• Carrier frequency (analog)

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Synchronous - Block Level

• Need to indicate start and end of block
• Use preamble and postamble
• e.g. series of SYN (hex 16) characters
• e.g. block of 11111111 patterns ending in
  11111110
• More efficient (lower overhead) than async

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Line Configuration

• Topology
• Physical arrangement of stations on medium
• Point to point
• Multi point
• Computer and terminals, local area network
• Half duplex
• Only one station may transmit at a time
• Requires one data path
• Full duplex
• Simultaneous transmission and reception between
  two stations
• Requires two data paths (or echo canceling)

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Traditional Configurations
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Interfacing

• Data processing devices (or data terminal
  equipment, DTE)
• Need an interface called data circuit terminating
  equipment (DCE)
• e.g. modem, NIC
• DCE transmits bits on medium
• DCE communicates data and control info with DTE
• Done over interchange circuits
• Clear interface standards required

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Characteristics of Interface

• Mechanical
• Connection plugs
• Electrical
• Voltage, timing, encoding
• Functional
• Data, control, timing, grounding
• Procedural
• Sequence of events

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V.24/EIA-232-F

• ITU-T v.24
• Only specifies functional and procedural
• References other standards for electrical and
  mechanical
• EIA-232-F (USA)
• RS-232
• Mechanical ISO 2110
• Electrical v.28
• Functional v.24
• Procedural v.24

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Mechanical Specification
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Electrical Specification

• Digital signals
• Values interpreted as data or control, depending
  on circuit
• More than -3v is binary 1, more than 3v is
  binary 0 (NRZ-L)
• Signal rate lt 20kbps
• Distance lt15m
• For control, more than-3v is off, 3v is on

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Local and Remote Loopback
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Procedural Specification

• E.g. Asynchronous private line modem
• When turned on and ready, modem (DCE) asserts DCE
  ready
• When DTE ready to send data, it asserts Request
  to Send
• Also inhibits receive mode in half duplex
• Modem responds when ready by asserting Clear to
  send
• DTE sends data
• When data arrives, local modem asserts Receive
  Line Signal Detector and delivers data

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Dial Up Operation (1)
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Dial Up Operation (2)
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Dial Up Operation (3)
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The Beginning of the Mobile Phone
It was the size of a dustbin lid and had a range
of just half a mile. The world's first mobile
phone could hardly be more different to today's
devices, which are small enough to slip inside a
pocket and can call almost anywhere in the world.
Its inventor, Nathan Stubblefield, is finally
being recognised as the father of mobile phone
technology exactly 100 years after he patented
his design for a "wireless telephone". The
melon farmer came up with his invention in 1902
after devoting every spare hour and penny he had
to establishing a telephone service in his rural
home-town of Murray, Kentucky.
Source http//www.dailymail.co.uk/pages/live/arti
cles/news/news.html?in_article_id566042in_page_i
d1770
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The Beginning of the Mobile Phone
Field test Receiver in hand, Nathan Stubblefield
demonstrates his invention in his orchard (the
mast can be seen in the centre of the picture
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The Beginning of the Mobile Phone
He constructed a 120ft mast in his orchard, which
transmitted speech from one telephone to another
using magnetic fields. However, the total
amount of wire required for the coils in the
phones was far longer than what would be required
to simply connect them - but the invention
allowed mobility. The self-taught electrician
demonstrated his device in the town's public
square on New Year's Day in 1902, broadcasting
music and speech to five receivers. And in 1908
he patented a new version designed to communicate
with moving vehicles such as stagecoaches and
boats.
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The Beginning of the Mobile Phone
Unfortunately, his phones were not commercially
successful in his lifetime and he died virtually
penniless in 1928. Sadly, Stubblefield never sold
a single unit of his design. Stubblefield it
seems just wanted to help his local community by
connecting the houses, which were some distance
from each other, with a telephone service. He
had always been obsessively secretive and never
allowed his family to leave the farm without him,
and was loath to let visitors on to his property
because he feared they might steal his
inventions. His family - he had six children -
lived in abject poverty, with any spare money
funnelled into his electrical experiments.
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The Beginning of the Mobile Phone

• His wife left him (but it is not all good
  news..)
• Stubblefield lived the last decade of his life
  as an itinerant hermit.
• He died in 1928 and was buried in an unmarked
  grave.

Source Clerkin, B (2008) Revealed The world's
first mobile phone was the size of a dustbin lid
- and had a range of just half a mile in 1902,
Daily Mail, 13th May 2008 http//www.dailymail.co
.uk/pages/live/articles/news/news.html?in_article_
id566042in_page_id1770
47
The Beginning of the Mobile Phone
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Practical Work

• Write down Shannon's formula, which describes the
  information capacity of a communications channel
  that is contaminated by noise. Describe each of
  the terms of the equation. Show that for a given
  communications rate there is a trade-off between
  two of the terms of the equation.

I is information capacity of channel in
bits/sec H is bandwidth of channel fupper -
flower Hz S/N signal to noise power ratio. Thus
the as the bandwidth is reduced the S/N ratio
must increase to maintain a given rate of
communication. And VV.