1. Network Concepts Shannon’s Communications Model Channels Networks
Network Topologies
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2. Shannon’s Communication Model
Trans-mitter
Info
Source
Channel
Re-ceiver
Destin-ation
Message
Signal
Received Signal
Noise
Source
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3. Types of Signal
A signal is a means of transmitting an element of data (usually a bit or a number of bits) across distances.
Signals can be:
Analogue (vary continuously across a range of values)
Digital (vary discretely between a number of distinct levels)
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4. Types of Channel
Analogue. Signals varies continuously and can take an infinite number of values.
Digital. Signal varies discontinuously between a discrete number of values
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5. Types of Channel
Guided. Signals are constrained within the medium of the channel
E.g. copper cable or fibre optic cable
Unguided. Signals are unconstrained or have few constraints
E.g radio
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6. Types of Channel
Serial. Each bit is transmitted in sequence. Used in virtually all data communications
Parallel. A number of bits are transmitted simultaneously. Used internally in computer buses or in cables from computers to nearby peripherals (typically printers)
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7. Types of Channel
Synchronous. The receiver must always know the arrival time and duration of each signal it receives. The clock at the receiver must be synchronised with the clock at the transmitter.
Asynchronous. The receiver can receive a signal at any time without its clock being continually synchronised with the clock of the transmitter.
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8. Types of Channel Types of Channel
Simplex. Operates in one direction only (E.g. Stock Market Feed or TV Broadcast)
Half Duplex. Operates in one direction at a time (E.g. Walkie-Talkies)
Full Duplex. Operates in both directions, possibly at the same time (E.g. Telephone Conversation)
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9. Types of Channel
Point-to-Point. A single receiver and a single transmitter
Point-to-Multipoint. A specified number of receivers and a single transmitter
Broadcast. An unspecified number of receivers and possibly an unspecified number of transmitters.
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10. Types of Channel
Baseband. The channel is filled with just one signal (E.g. Ethernet)
Broadband. The channel carries multiple signals at the same time, usually via Frequency Division Multiplexing (E.g. Cable TV)
Note: Broadband can also be used as a term to describe a channel that has high capacity (E.g. ADSL), as opposed to narrowband (E.g. Telephony)
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11. Types of Channel
Dedicated. The channel is dedicated for use by a single end-to-end communication
Shared. The channel is shared by a number of different end-to-end communications
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12. Types of Shared Channel
Switched. The channel is established for use by one end-to-end communication, but once this has been closed, the capacity can be used by another end-to-end communication. (E.g. a telephone call)
Multiplexed. A number of simultaneous different end-to-end communications are permanently allocated capacity on a channel
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13. Types of Switched Channel
There are 3 basic types of switched channels
Circuit Switched
Message Switched
Packet Switched
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14. Circuit Switching
A physical communications channel across the network must be set up before communication can take place and this path cannot be shared even when it is poorly utilised
Advantage
Guarantied Quality of Service (constant delay)
Disadvantage
Inefficiency
Example
Telephone call
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15. Message Switching
Each arbitrary length message is routed as a single unit through a shared communication channel, but only one messages can be sent at a time
Advantages
Simplicity
Disadvantages
Very unpredictable Quality of Service (long delays)
Example
as an application
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16. Packet Switching
Data is sent as packets up to a fixed maximum length through a shared communications channel one at a time, but packets from different end-to-end communications can be interleaved
Advantages
Efficiency
Disadvantages
Unpredictable Quality of Service (variable delay)
Example
Internet Protocol
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17. Types of Multiplexed Channel
Frequency Division (FDM). Capacity on the channel is shared by allocating a different range of frequencies to each end-to-end communication. For optical fibres, the term used is Wavelength Division Multiplexing (WDM).
Time Division (TDM). Capacity on the channel is shared by interleaving data in individual timeslots for each end-to-end communication serially on a round-robin basis, but each end-to-end communication is allocated its own permanent capacity.
Statistical Time Division. Similar to TDM, but there is no fixed allocation of capacity to each end-to-end communication. They are allocated timeslots only when needed and can, if necessary use extra spare timeslots.
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18. Frequency Division Multiplexing
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19. Time Division MultiplexingA3 A2 A1
MUX
MUX A3 A2 A1 B3 B2 B1 B3 B2 B1 B3 A3 B2 A2 B1 A1
E.g. Data from several 64 kbit/s lines is interleaved onto a 2 Mbit/s line
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20. Statistical Time Division Multiplexing
MUX
MUX A1 B3 B2 B1 B3 B2 B1 B3 B2 B1 A1
After A1 and B1 have been transmitted, B2 and B3 can be allocated
consecutive timeslots, as there is no other data waiting.
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21. Characteristics of Channels
Bandwidth. The range of frequencies that can be effectively carried by the channel [Measured in Hertz (Hz)]
Capacity. The maximum number of bits that can be carried by the channel in one second [Measured in bit/s]
Throughput. The practical maximum number of bits that can be carried by the channel in one second as experienced by an end user [Also measured in bit/s]
Utilisation. The proportion of time that the channel is fully occupied [Measured in %]
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22. Characteristics of Channels (Negative)
Noise. Spurious unwanted random signals generated in a channel [Measured in Decibels (dB)]
Attenuation. The loss of strength of a signal over distance [Measured in Decibels (dB) per km]
Delay. The difference between the time a signal enters the channel at the transmitter and the time it exits the channel at the receiver [Measured in milliseconds (ms)]
Jitter. The variability of delays. [Measured in standard deviations]
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23. Noise
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24. Types of Noise Galactic Noise (E.g. pulsars)
Atmospheric Noise(E.g. Lightening)
Quantum Noise (Statistical random affect on photons)
Man-made Noise (Power lines, electric motors, switches etc.)
Thermal/White/Gaussian Noise (Effect of heat on the flow of electrons)
Intermodulation Noise (Specific combinations of frequencies causing interference)
Crosstalk Noise (Interference from other circuits)
Quantisation Noise (Approximation in coding samples)
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25. Signal to Noise Ratio SNR = Power of Signal
Noise is normally measured relative to the strength of the signal being transmitted
Signal to Noise Ratio
SNR = Power of Signal
Power of the Noise
For convenience, SNR is usually measured on a logarithmic scale in a unit called the deciBel (dB)
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26. Decibels SNR (in deciBels) For example:
= 10 log10 (Signal Power / Noise Power)
For example:
if signal power = 300 milliWatts
and noise power = 3 mW
SNR = 10 log10 (300 / 3)
= 10 log10 (100)
= 20 dB
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27. Decibels
The deciBel is not an absolute unit. It is a relative unit that compares two quantities
Even when the deciBel is used to measure the power of sound, it is measured against a reference level
Engineers choose to measure powers in deciBels, so that cumulative gains and losses can be calculated by addition and subtraction rather than multiplication and division
The deciBel was chosen rather than the Bel, for convenience, as practical quantities can be represented as whole numbers between 0 and 100
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28. Common Signal to Noise Ratios
Signal : Noise
Signal to Noise Ratio (dB)
10,000:1
40 dB
1,000:1
30 dB
100:1
20 dB
10:1
10 dB
1:1
0 dB
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29. Types of Delay Propagation delay Transmission delay Queuing delay
Caused by limit imposed by the speed of light (3x108 m/s in free space). Typically copper and fibre optics cables limit speed of signal to 2x108 m/s
Transmission delay
Caused by the time it takes to transmit data onto a serial channel operating at a particular rate = L/R seconds (where L bits are transmitted at R bit/s)
Queuing delay
Caused by queues in multiplexers, switches and transmission equipment.
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