Chapter 7 – Data CommunicationsAims:

Outline the history of data communications, especially the mainevents.

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 tocontend 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 betweenthe 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-2

compression

MPEG-1or MPEG-2

compression

MPEG Audio(MP-3)

compression

MPEG Audio(MP-3)

compression

JPEG/GIFcompression

JPEG/GIFcompression

IntegratedDigital

Network

IntegratedDigital

Network

Local areanetwork

Local areanetwork

Telephoneexchange

Telephoneexchange

Net

wor

kC

onne

ctio

nN

etw

ork

Con

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-2

compression

MPEG-1or MPEG-2

compression

MPEG Audio(MP-3)

compression

MPEG Audio(MP-3)

compression

JPEG/GIFcompression

JPEG/GIFcompression

IntegratedDigital

Network

IntegratedDigital

Network

Local areanetwork

Local areanetwork

Telephoneexchange

Telephoneexchange

Net

wor

kC

onne

ctio

nN

etw

ork

Con

nect

ion

Frequencies and banwidth

3951.071975.53987.77493.88246.94123.4761.74B

3729.311664.66932.33466.16233.08116.5458.27A#,Bb

3520.001760.00880.00440.00220.00110.0055.00A

3322.441661.22830.61415.30207.65103.8351.91G#,Ab

3135.961567.98783.99392.00196.0098.0049.00G

2959.961474.98739.99369.99185.0092.4546.25F#,Gb

2637.021396.91698.46349.23174.6187.3143.65F

2367.021318.51659.26329.63164.8182.4141.20E

2489.021244.51622.25311.13155.5677.7838.89D#,Eb

2349.321174.66587.33293.66146.8373.4236.71D

2217.461100.73554.36277.18138.5969.3034.65C#,Db

2093.001046.50523.25261.63130.8165.4132.70C

Octave 7Octave 6Octave 5Octave 4Octave 3Octave 2Octave 1

Time domain Frequency domain

V max

f

T (1/f)

(1/T)

-V max

0

V max

AmplitudeAmplitude

Am

plitu

de(o

r sig

nal p

ower

)

Frequency (Hz)

Lower frequency

Upper frequency

Bandwidth

Am

plitu

de(o

r sig

nal p

ower

)

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 MbpsSource 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 MbpsSource 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 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.

PowerNoisePowerSignaldB

NS

log10)( 10=

bits/sec 1log. 2

+=N

SBCapacity

Bit capacityof a channeldepends onthe signal-to-noise ratio

Modulation

Voltage-to-frequencyconverter

Frequency-to-voltageconverter

Frequencymodulated signal

Receivedsignal

Informationsignal

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 frequenciesRadio

station 1

Radiostation 2

Time-division multiplexing

Time slot1

Time slot2

Time slot3

Time-divisionmultiplexor

Source 1

Source2

Source 3

Electromagnetic waves

Cosmicrays

Gammarays

X-rays Ultra-violet

Light Infra-red

Micro-waves

Radiowaves

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

Response time depends on the size of the data packets and the traffic within the network

Once the link is setup it provides a good reliable connection with little propagation delay

Response time

Yes, using datagramsNoAllows for data to be sent without first setting up a connection

Yes, nodes can communicate with many nodes at the same time and over many different routes

NoSimultaneous transmissions and connections

Yes, using the FCS in the data link layer

None, this must be supplied by the end users.

Error and flow control

Expensive for initial investmentMinimal as it uses existing connections

Investment in equipment

Packet-switchingCircuit-switching