Chapter 7 – Data
Communications
Aims:
Outline the history of data communications, especially the main
events.
Define the main parameters involved in the transmission of data.
Outline methods that are used to carrier data.
Define method of routing data through networks.
• Automated telephone switching. In 1889, Almon Strowger, a
Kansas City undertaker, patented an automatic switching system. In
one of the least catchy advertising slogans, it was advertised as a ‘girlless, cuss-less, out-of-orderless, wait-less telephone system.’
• Radio transmission. One of the few benefits of war (whether it be a
real war or a cold war) is the rapid development of science and
technology. Radio transmission benefited from this over World War I.
• Trans-continental cables. After World War II, the first telephone
cable across the Atlantic was laid from Oban, in Scotland to Clarenville
in Newfoundland. Previously, in 1902, the first Pacific Ocean cable was
laid.
• Satellites. The first artificial satellite was Sputnik 1, which was
launched by the USSR in 1957. This was closely followed in the
following year by the US satellite, Explorer 1. The great revolution
when the ATT-owned Telstar satellite started communicating over
large distances using microwave signals.
• Digital transmission and coding. Most information transmitted is
now transmitted in the form of digital pulses. A standard code for this
transmission, called pulse code modulation (PCM), was invented by
A.H. Reeves in the 1930s, but was not used until the 1960s .
• Fibre-optic transmissions. Satellite communications increased the
amount of data that could be transmitted over a channel, but in 1965
Charles Kao laid down the future of high-capacity communication
with the proof that data could be carried using optical fibres.
Communication types
• Bandwidth contention, bandwidth sharing or reserved
bandwidth. Some communication systems reserve bandwidth for a
connection (such as ISDN and ATM), while others allow systems to
contend for it (such as Ethernet).
• Virtual path, dedicated line or datagram. Some communication
systems allow for a virtual path to be setup between the two
connected systems, while others support a dedicated line between the
two systems.
• Global addressing, local addressing or no addressing. An
addressing structure provides for individual data packets to have an
associated destination address. Each of the devices involved in the
routing of the data read this address and send the data packet off on
the optimal path.
Integrated digital network (IDN)
MPEG-1
MPEG-1
or
orMPEG-2
MPEG-2
compression
compression
Convert
Convert
totoDigital
Digital
MPEG
MPEGAudio
Audio
(MP-3)
(MP-3)
compression
compression
Convert
Convert
totoDigital
Digital
WAV
file
BMP
file
Local
Localarea
area
network
network
Telephone
Telephone
exchange
exchange
MPEG
movie
MP-3
sound file
JPEG/GIF
JPEG/GIF
compression
compression
Compression reduces
redundancy in the data
JPEG/GIF
picture file
Integrated
Integrated
Digital
Digital
Network
Network
Network
Network
Connection
Connection
Red,
Green,
Blue
Convert
Convert
totoDigital
Digital
Frequencies and banwidth
Octave 1
Octave 2
Octave 3
Octave 4
Octave 5
Octave 6
Octave 7
C
32.70
65.41
130.81
261.63
523.25
1046.50
2093.00
C#,Db
34.65
69.30
138.59
277.18
554.36
1100.73
2217.46
D
36.71
73.42
146.83
293.66
587.33
1174.66
2349.32
D#,Eb
38.89
77.78
155.56
311.13
622.25
1244.51
2489.02
E
41.20
82.41
164.81
329.63
659.26
1318.51
2367.02
F
43.65
87.31
174.61
349.23
698.46
1396.91
2637.02
F#,Gb
46.25
92.45
185.00
369.99
739.99
1474.98
2959.96
G
49.00
98.00
196.00
392.00
783.99
1567.98
3135.96
G#,Ab
51.91
103.83
207.65
415.30
830.61
1661.22
3322.44
A
55.00
110.00
220.00
440.00
880.00
1760.00
3520.00
A#,Bb
58.27
116.54
233.08
466.16
932.33
1664.66
3729.31
B
61.74
123.47
246.94
493.88
987.77
1975.53
3951.07
Amplitude
Bandwidth
Amplitude
T (1/f)
V max
0
Amplitude
(or signal power)
V max
Lower
frequency
Upper
frequency
-V max
f (1/T)
Time domain
Frequency domain
Frequency (Hz)
V
V
Digital
system
t
V
Faster rate
of change
f1 f2 f3 f4
f
f1 f2 f3 f4 f5 f6
f
V
t
Bandwidth of transmission
system elements
64
64Mbps
Mbps
100
100Mbps
Mbps
64
64kbps
kbps
200
200Mbps
Mbps
Source
Analogue
system
Destination
64
64kbps
kbps
Source
Overall bandwidth is
limited by the slowest
element of the transmission
system
Destination
Noise
• Thermal noise. Thermal noise occurs from the random movement
of electrons in a conductor and is independent of frequency.
• Cross-talk. Electrical signals propagate with an electric and a
magnetic field. If two conductors are laid beside each other then the
magnetic field from one couples into the other.
• Impulse noise. Impulse noise is any unpredictable electromagnetic
disturbance, such as from lightning or from energy radiated from an
electric motor.
Signal Power
S
(dB)  10 log 10
N
Noise Power
S

Capacity  B. log 2 1  
 N
bits/sec
Bit capacity
of a channel
depends on
the signal-tonoise ratio
Modulation
Amplitude
modulation
Information
Frequency
Received
signal
modulated
signal
Voltage-to-
Frequency
modulation
signal
Frequency-
frequency
to-voltage
converter
converter
Digital modulation
1
ASK
PSK
FSK
1
0
1
0
Phase and amplitude modulation
90°
Amplitude 4
Phase 3
Phase 2
Amplitude 3
Phase 1
180°
0°
Amplitude 2
Amplitude 1
270°
Frequency modulation
Radio
station 1
Radio
station 2
Radio
station 3
Receiver tuned to
pick-up only in a range
of carrier frequencies
Radio
receiver
Time-division multiplexing
Source
1
Time slot
1
Source
2
Time-division
multiplexor
Source
3
Time slot
2
Time slot
3
Electromagnetic waves
3 pm
Cosmic
rays
Gamma
rays
X-rays
3 nm
Ultraviolet
400 nm
Light
700 nm 300 GHz
Infrared
300 MHz
Microwaves
30 Hz
Radio
waves
30-300 Hz ELF (extremely low frequencies)
0.3-3 kHz VF (voice frequencies)
3-30 kHz VLF (very low frequencies)
0.3-3 GHz UHF (ultra-high frequencies)
30-300 kHz LF (low frequencies)
3-30 MHz SHF (super-high frequencies)
30-300 GHz EHF (extremely high frequencies)
0.3-3 MHz MF (medium frequencies)
3-30 MHz HF (high frequencies)
30-300 MHz VHF (very high frequencies)
Routing of data
• Circuit switching. This type of switching uses a dedicated line to
make the connection between the source and destination, just as a
telephone line makes a connection between the caller and the
recipient.
• Packet switching. This type of switching involves splitting data into
data packets. Each packet contains the data and a packet header which
has the information that is used to route the packet through the
network.
– Datagram. This is where the data packets travel from the source to the
destination, and can take any path through the interconnected network.
– Virtual circuit. This is where all the data packets are routed along the
same path. It differs from circuit switching in that there is no dedicated
path for the data.
• Multirate circuit switching. Traditionally TDM (time division
multiplexing) is used to transmit data over a PSN (public switched
network). This uses a circuit switching technology with a fixed data
rate, and has fixed channels for the data.
• Frame relay. This method is similar to packet switching, but the data
packets (typically known as data frames in frame relays) have a variable
length and are not fixed in length. This allows for variable bit rates.
• Cell relay. This method uses fixed packets (cells), and is a progression
of the frame relay and multirate circuit switching.
Circuit-switching v. packet switching
Circuitswitching
PSE
fixed route
Packetswitching
possible routes
Circuit-switching
Packet-switching
Investment in
equipment
Minimal as it uses existing
connections
Expensive for initial investment
Error and flow
control
None, this must be supplied by
the end users.
Yes, using the FCS in the data link
layer
Simultaneous
transmissions and
connections
No
Yes, nodes can communicate with
many nodes at the same time and over
many different routes
Allows for data to be
sent without first
setting up a
connection
No
Yes, using datagrams
Response time
Once the link is setup it
provides a good reliable
connection with little
propagation delay
Response time depends on the size of
the data packets and the traffic within
the network