Chapter 7 – Data CommunicationsAims:
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 ‘girl-less, 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)
Convertto Digital
Convertto Digital
MPEGmovie
Convertto Digital
Convertto Digital
MP-3sound file
WAV file
Convertto Digital
Convertto Digital
JPEG/GIFpicture file
BMPfile
Red,Green,Blue
Compression reducesredundancy in the data
MPEG-1or MPEG-2compression
MPEG-1or MPEG-2compression
MPEG Audio(MP-3)
compression
MPEG Audio(MP-3)
compression
JPEG/GIFcompression
JPEG/GIFcompression
IntegratedDigital
Network
IntegratedDigital
Network
Local areanetwork
Local areanetwork
Telephoneexchange
Telephoneexchange
Ne
two
rkC
on
nect
ion
Ne
two
rkC
on
nect
ion
Convertto Digital
Convertto Digital
MPEGmovie
Convertto Digital
Convertto Digital
MP-3sound file
WAV file
Convertto Digital
Convertto Digital
JPEG/GIFpicture file
BMPfile
Red,Green,Blue
Compression reducesredundancy in the data
MPEG-1or MPEG-2compression
MPEG-1or MPEG-2compression
MPEG Audio(MP-3)
compression
MPEG Audio(MP-3)
compression
JPEG/GIFcompression
JPEG/GIFcompression
IntegratedDigital
Network
IntegratedDigital
Network
Local areanetwork
Local areanetwork
Telephoneexchange
Telephoneexchange
Ne
two
rkC
on
nect
ion
Ne
two
rkC
on
nect
ion
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
Time domain Frequency domain
V max
f
T (1/f)
(1/T)
-V max
0
V max
AmplitudeAmplitude
Am
plit
ude
(or
sign
al p
ow
er)
Frequency (Hz)
Lower frequency
Upper frequency
Bandwidth
Am
plit
ude
(or
sign
al p
ow
er)
Frequency (Hz)
Lower frequency
Upper frequency
Bandwidth
V
tV
t
f
V
f
V
f1 f2 f3 f4
f1 f2 f3 f4 f5 f6
Faster rateof change
V
tV
t
f
V
f
V
f1 f2 f3 f4
f1 f2 f3 f4 f5 f6
Faster rateof change
64 Mbps64 Mbps 100 Mbps100 Mbps 64 kbps64 kbps 200 Mbps200 Mbps
Source Destination
64 kbps64 kbpsDestinationSource
Overall bandwidth islimited by the slowest element of the transmissionsystem
Bandwidth of transmission system elements
64 Mbps64 Mbps 100 Mbps100 Mbps 64 kbps64 kbps 200 Mbps200 Mbps
Source Destination
64 kbps64 kbpsDestinationSource
Overall bandwidth islimited by the slowest element of the transmissionsystem
Bandwidth of transmission system elements
Analoguesystem
Digitalsystem
Noise
• Thermal noise. Thermal noise occurs from the random movement of electrons in a con ductor 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.
PowerNoise
PowerSignaldB
N
S
log10)( 10
bits/sec 1log. 2
NS
BCapacity
Bit capacityof a channeldepends onthe signal-to-noise ratio
Modulation
Voltage-to- frequency converter
Frequency- to-voltage converter
Frequency modulated signal
Received signal
Information signal
Amplitudemodulation
Frequencymodulation
Digital modulation
1 1 0 1 0
ASK
PSK
FSK
Phase and amplitude modulation
Amplitude 1
Amplitude 2
Amplitude 3
Amplitude 4
Phase 1
Phase 2Phase 3
90°
0°180°
270°
Frequency modulation
Radiostation 3
Radioreceiver
Receiver tuned topick-up only in a rangeof carrier frequencies
Radiostation 1
Radiostation 2
Time-division multiplexing
Time slot 1
Time slot 2
Time slot 3
Time-division multiplexor
Source 1
Source 2
Source 3
Electromagnetic waves
Cosmic
rays
Gamma
rays
X-rays Ultra-
violet
Light Infra-
red
Micro-
waves
Radio
waves
0.3-3 GHz UHF (ultra-high frequencies)
3-30 MHz SHF (super-high frequencies)
30-300 GHz EHF (extremely high frequencies)
400 nm 700 nm3 nm
30-300 Hz ELF (extremely low frequencies)
0.3-3 kHz VF (voice frequencies)
3-30 kHz VLF (very low frequencies)
30-300 kHz LF (low frequencies)
0.3-3 MHz MF (medium frequencies)
3-30 MHz HF (high frequencies)
30-300 MHz VHF (very high frequencies)
30 Hz300 MHz300 GHz3 pm
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
Circuit-
switching
Packet-
switching
possible routes
fixed routePSE
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
