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Wireless and Mobile
Networks
(ELEC6219)
Session 2: Data Communication
Fundamentals
Adriana Wilde and Jeff Reeve
22 January 2015
Plan for this lecture
• At the end of this lecture (and related activities), students
should be able to :
– …discuss the need for layered models for network
architecture
– …be able to compare and contrast two layered models
– …explain how data can be encoded and consider how
errors can be detected or corrected
– … identify key theories of data communications
2
Review
Talking point
• Together
– Discuss the need for layered models for network
architecture
– Compare and contrast two layered models
– (ISO OSI is DEAD! – why do we study it then?)
https://secure.ecs.soton.ac.uk/noteswiki/w/ELEC6113-1213
4
Talking point
• Together,
– Discuss the need for layered models for network
architecture
p.26-30 (T. 4ed)
p.51-55 (T. 5ed)
– Compare and contrast two layered models
p.44-46 (T. 4ed)
p.71-73 (T. 5ed)
– (ISO OSI is DEAD! – why do we study it then?)
p.46-49 (T. 4ed)
p.73-76 (T. 5ed)
5
Protocols
Protocol Layers
p.26-30 (T. 4ed)
p.51-55 (T. 5ed)
• Protocol layering is the main structuring method used to
divide up network functionality.
•
Each protocol instance
talks virtually to its peer
•
Each layer
communicates only by
using the one below
•
Lower layer services
are accessed by an
interface
•
At bottom, messages
are carried by the
medium
7
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Protocol Layers (II)
• Example: the
philosopher-translatorsecretary architecture
• Each protocol at different
layers serves a different
purpose
p.29 (T. 4ed)
p.54 (T. 5ed)
8
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Protocol Layers (III)
• Each lower layer adds its own header (with control
information) to the message to transmit, and removes it on
reception of it
p.29-30 (T. 4e)
p.54-55 (T. 5e)
9
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Protocol Layers (IV)
• Each lower layer adds its own header (with control
information) to the message to transmit, and removes it on
reception of it
p.29-30 (T. 4e)
p.54-55 (T. 5e)
10
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Protocol Layers (IV)
• Each lower layer adds its own header (with control
information) to the message to transmit, and removes it on
reception of it
p.29-30 (T. 4e)
p.54-55 (T. 5e)
11
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Data Encoding
Data Communications Fundamentals
• We wish to communicate from A to B.
– How?
A
B
– By courier (e.g. magnetic media), optically (light and
fiber optics), electrically, electromagnetic waves, radio,
microwaves, satellite… many alternatives!
p.91 (Tanenbaum 4th ed)
p.116 (Tanenbaum 5th ed)
13
How?
• Electrical options:
– vary the voltage (the most important mechanism)
– vary the current (sometimes used in ‘noisy’
environments)
– vary the frequency (e.g. dial-up modems)
– vary the phase
• EM wave options:
– open space “wireless”
• … many alternatives!
14
Data Communications Fundamentals
• Let’s consider the electrical options
A
B
– Consider a point-to-point electrical connection between
A and B
– Usually only one wire is used to carry data – there is
always a return path
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Data Communications Fundamentals
• THIS IS SERIAL DATA TRANSMISSION
A
B
– Consider a point-to-point electrical connection between
A and B
– Usually only one wire is used to carry data – there is
always a return path
16
Case study: RS-232-C
• This is a very simple protocol used to transfer a single
character (8 bits) between computers or between a
computer terminal and a computer.
• It was supposed to be obsolete in the early 1970's, but it is
only now fading into obsolescence.
• Until very recently every computer every made would
always contain at least one RS-232-C interface (modern
laptops often do not provide the facility).
17
Data encoding
• Many alternatives to encode/decode
Data
voltage
data
• NRZ (Non-Return-to-Zero)
– Used widely, particularly in the RS-232-C interface
– the simplest encoding possible
– Data ‘0’: negative voltage
– Data ‘1’: positive voltage
p.145-150 (5e)
– note that in some literature
this is NO voltage.
True for fibre optics, where
light=1, no light=0.
18
Talking Point
• NRZ – Problems?
19
Talking Point
• NRZ – Problems?
– How are the receiver sampling times synchronized with
the bit timing used for encoding at the transmitter?
20
Talking Point
• NRZ – Problems?
– How are the receiver sampling times synchronized with
the bit timing used for encoding at the transmitter?
• RS-232-C uses a very simple mechanism: all
communication starts with a ‘1' (known as a ‘start bit’- the
idle state is a ‘0'). Per byte, 10 bits are transmitted.
• Both Tx and Rx know the expected sampling speed (it must
be arranged previously), hence the Rx can sample at the
correct time. However in practice there will be a skew, so
a +/- 5% tolerance between the two clocks is required.
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Talking Point
• NRZ – Problems?
– How are the receiver sampling times synchronized with
the bit timing used for encoding at the transmitter?
– How could we use NRZ to send a data message longer
than 8 bits?
22
A Better Alternative
• Manchester coding
– Phase encoding, allows clock recovery
• Data ‘0’: voltage changes from low to high
• Data ‘1’: voltage changes from high to low
– Guaranteed transition at the middle of every bit period
– Disadvantage?
23
Other Alternatives
• Manchester coding
– Phase encoding, allows clock recovery
• Data ‘0’: voltage changes from low to high
• Data ‘1’: voltage changes from high to low
– Guaranteed transition at the middle of every bit period
– Disadvantage?
24
Other Alternatives
• NRZ Inverted
– Similar to NRZ
– Data ‘0’: no transition
– Data ‘1’: transition
– Disadvantage?
• 4B/5B
– 4 bits encoded in 5 transition bits such that patterns with
no/few transition bits are avoided
– Disadvantage?
p.285 (4e)
p.310 (5e)
25
Talking Point
• We want reliable communication… what do we do about
data errors?
26
Talking Point
• We want reliable communication… what do we do about
data errors?
– Unavoidable
– Error detection is always an overhead
– Reduces bandwidth available for data
– Error correction can be used
– … but retransmitting on errors is often just fine!
27
Key Data
Transmission
Theories
Shannon Data Capacity
• Maximum bitrate = H log2 (1 + S/N) bits per second
where
– H is bandwidth in Hz
– S is the total signal power in watts
– N is the total noise power in watts
• Therefore, a noiseless channel would have an unbounded
data capacity!
• In practice, there is always some noise
29
Shannon Data Capacity
• Maximum bitrate = H log2 (1 + S/N) bits per second
where
– H is bandwidth in Hz
– S is the total signal power in watts
– N is the total noise power in watts
power = f(voltage)2
• Therefore, a noiseless channel would have an unbounded
data capacity!
• In practice, there is always some noise
30
Nyquist limit
• maximum bitrate = 2H log2 V bits per second
where: ‘V’ is the number of discrete levels used for encoding
– If received data is bandwidth limited to H Hz (i.e. data
channel has this bandwidth), the filtered signal can be
reconstructed using only 2H samples per second.
– (Faster sampling is pointless: all higher frequency
components have already been filtered out.)
31
Checking Learning Outcomes
• At the end of this lecture (and related activities), students
should be able to :
– …discuss the need for layered models for network
architecture
– …be able to compare and contrast two layered models
…explain how data can be encoded and consider how
errors can be detected or corrected
– … identify key theories of data communications
32