National Diploma Unit 10 Communication Technology Assignment 2 Support Material provides information on key concepts related to communication technology including: - Data elements such as checksums, encapsulation, addresses, and sequence numbers and why they are important. - Principles of signal theory including digital signaling methods, representing data electronically, synchronous and asynchronous transmission, error detection, error correction, and issues related to bandwidth, noise, and channel types. - Different transmission methods including asynchronous and synchronous transmission, transmission over cables, frames, packets, error detection, and error correction. - Factors that affect bandwidth such as the transport medium, channel type, distance, noise, and their impact on data transmission and reception.

1. National Diploma Unit 10
Communication Technology
Assignment 2 Support Material
Created by John Mathias: 2014

2. Criteria Objectives in this Presentation
• P4 Describe what data elements are and
why they are important
• P5 Describe the principles of signal theory
• P6 Describe different transmission
methods use

3. What is Communications?
Communications is an act of transporting
messages
from a source to a destination.
A message consists of data that carries information
and is sent through a communications medium
(channel)
from one device to another

4. P4: Data Elements
• P4 Describe what data elements are and
why they are important
• checksum eg cyclic redundancy check
(CRC);
• encapsulation eg frames, packets,
datagrams; addresses;
• sequence numbers

5. P5: Signal Theory
• Criteria Objective, P5:explain the principles of
signal theory
• Signal theory: digital signalling methods;
representing data electronically (bits, bytes,
packet structures); synchronous transmission;
asynchronous transmission; error detection; error
correction; bandwidth limitation; bandwidth noise;
channel types eg telephone, high frequency (HF)
radio, microwave, satellite; other issues eg
bandwidth, data compression

6. Digital Basics
signalstate
A string of zero’s and ones form a stream of data
that can be translated by a computer to process
data and commands
Digital signals are based on two states,
Zero and One
1 0 1 0 1 0 1
Send Receive
TX RX
This is a trace you would see on an oscilloscope

7. Digital Signals
Where digital signals like the one above are sent over a cable it is
said to be a baseband system
Low
voltage/light
1 0 1 0 1 0 1
Consists of pulses of energy
signalstate
High
voltage/light
Time

8. Digital Signals
1 0 0 0 1 1
Where there are successive ones or zeros the levels
remain the same. This is called Non Return to Zero (NRZ)
Try to assemble a trace using another binary code on the next slide
This is another example
Time
High
Low

9. Digital Signals
High
Low
1 1 0 1 0 1
Time
You need to get out of presentation mode to do this
This is another example for you to do

10. Digital Signals
1 1 0 1 0 1
• Digital signals represent data in the form of binary numbers that a
computer system can understand. The stream of numbers can represent
the characters on a standard keyboard. One such code is called ASCII.
Did you get it right?
Research on the ASCII code and write out the binary for the word
DATA
High
Low
Time

11. Bits & Bytes and Packets
• The terminology relates to the data stream
• A bit represents one state (zero or one)
• A byte is 8 bits
• A packet is number of bytes structured so that it can be directed over a
network such as the Internet
A B
Internet
cloud
Router
Packets from computer A are destined for computer B
Next we will look at the structure of a packet

12. Packet Structure
Data Header
Each IP (Internet Protocol) packet consists of a header
followed by a data field. The header length can vary between
20 and 60 bytes, and the total size of the packet can be up to
65535 bytes. However, many systems cannot handle packets
as large as the protocol allows, and a working maximum size
is 576 bytes.
Direction of travel
The header contains information such as source and
destination address
Ref: http://www.comsci.us/datacom/ippacket.html

13. Data Transmission
Next we will look at the way devices are connected for data transfer
Terminology
DTE: Data Terminating Equipment
DCE: Data Communication Equipment
Terminology
DTE: usually relates to the end user equipment
DCE: relates to the devices that interface with a transmission medium
DTE DCE
Computer Modem
Telephone
Line/Free Space
USB
connection
Example

14. Transmission Modes
Next we will look at the way data is sent between two devices
There are two main ways of transmitting data: they are
Asynchronous and Synchronous
Asynchronous communications is a method that transmits data on demand
without a central clock source for timing
Synchronous communications is a method that uses a clock to transmit
data in a continuous stream
Examples are links to local printers, and modems from a computer
Examples are connections between transmission systems in telephone
exchanges and networking devices over an external connection

15. Asynchronous Mode
Modem
Are you ready to receive
Yes I am
Modem
Here is the data
I got your data thanks
Data transfer between a computer and a modem using a ‘D’ Type serial
connector is Asynchronous and involves ‘Handshaking’ or control
signalling
Task: Create a one page document showing details of RS232 (EIA/TIA 232)
using a suitable picture of the pin outs showing the designations. Can you
explain how it works? Show your references (URL)

16. Frames
7- bit ASCIIStart Bit
Parity
bit
Stop bit
The previous slide showed how asynchronous transmission was done in hardware
using control signals. This is how it is implemented using what we call a frame format.
A frame is formatted data at layer 2 of the OSI Model.
It takes care of data errors, ‘hand shaking’ and can carry a hardware identification
called a MAC address
This frame represents transmission of ASCII
characters from a terminal emulator to a
programmable device such as a router
You can set up a
terminal emulator
using the ‘Hyper
Terminal’ utility on
your computer
Router
Terminal
Emulator

17. Synchronous Transmission Physical Layer
Data transfer is physically synchronous when the transmitter and receiver
operate from the same clock source
First let us look at what a clock source looks like:
It’s a continuous repetitive waveform that looks like a digital signal except that it
does not change. Its very steady.
Clock Source
Why is this useful? Clock signals are used to synchronise data systems
so that data can be transferred and processed
exactly at the right time in the right place
Clock signals are used in a computer to synchronise the movement of data
for processing. The faster the clock the faster the processing.

18. Synchronous Transmission Physical Layer
1. Data is sent continuously from TX at a constant rate set by a clock signal
(usually regulated by a quartz crystal)
2. The RX uses the harmonic content of the data signal to regenerate the clock
signal.
3. The clock signal is used to extract the data correctly
0 1 0 10 1 0 1
Data
0101
Clock
signal
Regenerated
clock signal
reused
TX RX
Data extracted &
regenerated
Next we will look at the way data is sent synchronously between two remote devices
Definition again: Data is transfer is physically synchronous when the transmitter
and receiver operate from the same clock source

19. Synchronous Transmission in Use
Synchronous data transmission at the physical layer is a
popular method between two data devices
Type of
connections/channels:
Most radio systems
including, microwave
and Wifi
Almost all WANS such
as ISDN, leased lines,
frame relay
Between
Routers
Computers
Switches
Memory
chips and
processors
Transmission
systems
Serial
Connection
Which router will be the
source of the clock?
You decide!

20. Synchronous Transmission
Data Link Layer
Data usually represented in groups of 8 bits (byte) are organised into frames to
allow synchronous transfer of information at the logical level
Simplified Frame format based on
HDLC/SDLC
Next we will look at the way data is sent synchronously between two devices
at a logical level using a frame
I know when the
next frame is
coming when I see
the begin flag
DataFrame Ends Flag Frame Begins Flag

21. Error Detection and Correction
Modem
Modem
1 0 1 0
0 1 0 0
Bad Line
Good
signal from
modem
Bad signal
because of
the line
Digital
code is
corrupted
Next we will look at the what errors are
Example of data corruption in a
typical system
A Freeview terrestrial box should have an option to display errors in terms of bits in a
thousand or a million. If you have one find the menu option that displays it to see what it
says.

22. Error Detection Methods
Next we will look at how to detect errors
1 0 1 0 0 1 0 0
One way is to spot the difference!
How long did that take?
Next we will look at how to really detect them!
Can you spot the difference! Visually compare the
two traces below; there is one error.

23. Error Detection
Physical layer
A previous slide mentioned Line Codes.
Line coding is a way of changing the pattern of the binary code before it
gets transmitted over a line such as a telephone line.
One of the reasons to use line code is a way of detecting errors
For example a simple line code is called Alternate Mark Inversion (AMI)
This code inverts successive pulse so that the overall DC content is zero
+ve
-ve
0 Time
AMI stream of pulses for the ‘All Ones’ code
Notice that
these pulses
return to zero
Next we will look at how to detect errors

24. Error Detection at the Physical Layer
Time
Can you see the error in the code?
The receiver is expecting alternate polarity of pulses and will flag an error
if it detects two or more together of the same polarity
+ve
-ve
0
AMI stream of pulses for the ‘All Ones’ code
with an error
RX with AMI
error detector
Good code
Bad
transmission
media
Bad code
Error
indicator
Missing pulse

25. Error Detection at the Data Link Layer
A previous slide mentioned frame format.
This is a way of changing organising binary code into bytes so that data can
be received synchronously (When it ends and begins). It also has other
features such as error detection and control
Data FCSPreamble
This is another type of frame simplified for this subject of errors based on Ethernet
• Along with a start flag called a preamble is the data captured in a frame.
• At the end of the frame is what is called a Frame Check Sequence (FCS) that
is a result of a calculation made on the pattern of the data and is unique for
that pattern. If the data is corrupted along the way the receiver will detect it
because it will not match up to the expected FCS value.
A very simple FCS would count the number of ‘ones’ in the data to be transmitted

26. Error Correction at Data Link Layer 2
One way of correcting errors is to send a request to re-transmit if the FCS was
incorrect
Bad Data
Request to send data again
Good dataSending Data
again
Sending Data
TX/RX RX/TX
Line or Medium
The first frame of data was corrupted. A request was sent out to retransmit it
This method is used by Ethernet for shared access on a network.. More on that later

27. Bandwidth
• Bandwidth is defined as the amount of information that can flow
through a network connection in a given period of time.
• Factors that limit bandwidth are:
1. Transport medium eg Type of cable or medium
2. Channel type or mechanism: eg. ADSL, Leased line, Dial up, Mobile
comms, Satellite Comms, Frame Relay, ISDN
3. Distance, eg ADSL bandwidth is inversely proportional to distance
from the telephone exchange
An analogy of bandwidth is that of water flowing through a pipe
The mains water is carried in big pipes (big bandwidth) for distribution, however the
pipes to your house are of a much smaller diameter (smaller bandwidth) thus the
flow is restricted for domestic use.
Mains Water Pipe
In this slide we talk about what is bandwidth and what factors limits the flow of
information

28. Bandwidth for Analogue Signals
Analogue signals are those that vary continuously such as sound and waves in water.
Radio waves are analogue as they vary continuously
Analogue bandwidth is measured in terms of frequency
One Wavelength
One cycle
One Wavelength
One cycle
Frequency is
measured in
cycles per
second (Hertz)
Bandwidth of a telephone line is 3.4Khz
Bandwidth of an MP3 player is 20Khz
Now that we have a definition the next slides will look at analogue and digital
bandwidth and how they are related

29. Bandwidth is Related to Bit Rate
Bit rate is
measured in
bits per second
(bps)
time
1 bit
1 second
The bit rate of
this data stream
is 8 bits per
second (bps)
A fundamental (biggest amplitude) analogue frequency is
produced from a bit stream amongst other smaller harmonics
This produces
a fundamental
frequency of
4Hz (cycles per
second)The higher the bit rate for this type of signal the higher the frequency
it produces as we will see in the next slide

30. Digital Bandwidth (bit rate)
The higher the bit rate the narrower
the width of a bit and the higher the
fundamental frequency it produces
time
1 bit
1 second
Bit rate of this
data stream is
16 bits per
second (bps)
This produces
a fundamental
frequency of
8Hz (cycles per
second)
The frequency response of a cable is limited to an upper frequency and so limits
the bit rate and thus the bandwidth
As the bit rate increases so does the fundamental frequency

31. Bandwidth of Various Channel Types
Channel Type or
Service
Typical Bandwidth (bps or
bits/second)
Frame Relay 64K-8M
Dial up 56K
E1 Leased line 2M
3G Mobile Phone 512K

32. Bandwidth Exercise
Channel Type Bandwidth (bps or bits/sec)
ISDN BRI
ISDN PRI
SDH VC12 (‘VC one two’)
DAB radio channel
Now it’s your turn: Fill in the bandwidth of these channel types
Next we will look at how noise affects bandwidth

33. Noise
This is something we commonly define as unwanted sound. For me that’s most of
today’s pop music!
In the case of data communications it is unwanted electromagnetic interference
For example, data flowing in a pair of telephone wires could be subject to electrical
interference from electrical services such as motors or generators
Below is an example of what a signal could look like after being subjected to
electrical interference
Next we will look on how this affects bandwidth

34. Bandwidth and Noise
Noise will adversely affect bandwidth by distorting the data signal.
Before I say why, you need to know how data is received.
Time
Remember the AMI line code for ‘All Ones’
The receiver makes a decision as to whether the data represents logic one or zero by
referring the voltage for that expected bit against a pre-determined voltage level
called a decision level.
Pulse above
decision level = 1
anything below
decision level = 0
Decision
Levels
Next we will look at what the signal looks like after being subjected to noise and what
decisions are taken by the receiver
1 1 1 1 1 1

35. Data Reception and Noise
AMI line code ‘ALL ones’ has been affected by noise and
bandwidth limitations of the line. This has the effect of adding
or subtracting to the signal’s amplitude as well as distorting it.
Can you see the effect of the all ‘ones signal’ ?
Notice that
these pulses
are not sharp
What do you think the binary code will be now?
TimeDecision
Levels
Pulse above
decision level = 1
anything below
decision level = 0
Decision
markers
1 1 1 1 x 1

36. The Result
TimeDecision
Levels
Pulse above
decision level = 1
anything below
decision level = 0
Decision
markers
A missing pulse
1 1 1 1 0 1
Remember that
errors can be
detected with
this AMI code

37. Bandwidth Limitation
• We saw how noise can corrupt a digital signal.
• This has the effect of reducing the throughput
of data as requests to send again are made or
• causing the link to go down
For example, Freeview television signals are digital within the wireless envelope. In
areas of poor signal strength some channels do not appear or appear blocky as the
errors increase and the ‘digi’ box is unable to correct them. This is unlike weak
analogue television signals in which noise shows as ‘snow’ on the television screen
along with a poor picture

38. The Effect on Data Corruption
Freeview television signals are digital within the wireless signal envelope called a
carrier. In areas of poor signal strength some channels do not appear or appear
blocky as the errors increase and the ‘digi’ box is unable to correct them and the
picture will be completely lost. This is unlike the old analogue television signals
where the picture just gets gradually worse as the signal gets weaker.
If errors become so great this can cause the channel or link to stop working.
This is a real example of
a reception of a weak
digital TV signal with
uncorrected errors
The TV mast is only
just behind the Malvern
Hills from my house
less than a mile away!

39. There is a distinct threshold for signal
strength requirement below which the
picture will not appear.

40. Bandwidth Compression
This subject is about being aware of been able to transmit the same
amount of data in a smaller bandwidth
Here are two ways of doing this:
1. Encode the digital signal so that is takes up less bandwidth
2. Compress the original data stream
1. Encoding is can be by modulation techniques or using a line code such
as QPSK (phase modulation) and 2B1Q (multi level code). There are
other ways of encoding as well that replace repetitive binary patterns with
a short code. Eg In a text file the word ‘The’ could be replaced by a single
8 bit character code thus saving 16 bits.
2. Compressing the original data stream using a file utility such as ‘winzip’
or a file compression technique such as JPEG or MPEG
Something you can do to demonstrate data compression is to create a picture in
‘Paint’ and save it as a BMP file. Then save it as a JPEG file and compare the two
file sizes

41. Channel Types
• The next few slides cover the following
content
• channel types eg telephone, high frequency
(HF) radio, microwave, satellite;

42. Channel Types
What is a channel?
The general meaning is a guided pathway
In communications it is a way of transmitting speech and data
This includes the medium that is used to do this. eg. fibre, free space or
copper, and the method of transmission and protocols used.
So what are the channel types?….read on…

43. Channel Types: Microwave
Free Space (the air that we breath and outer space)
Terrestrial
Microwave Radio Transmission
Both systems use
dishes to transmit
and receive signals
Satellite
This dish is large
enough for relaying
telecommunications
or TV signals to a
satellite.
This tower has
two small dishes
to relay
microwave radio
signals to other
dishes placed on
land.
Ground
Station

44. Channel Types Unlicensed Radio
All three uses unlicensed short range radio frequencies
Bluetooth
WiFi
DECT Used for domestic cordless phones
Used to set up wireless LANS
Used to connect mobile phones and
PDA’s to each other or to PC’s or other
devices such as a ‘hands free’.

45. Channel Types: Cabled
Two types are Copper and Fibre

46. How Electrical Signals Travel in
Twisted Pair Copper
wires
insulation
conductors
©ikes1201
Direction of Travel

47. How Light Travels in a Glass Fibre
50 m125 m
Cladding
Core
a
b
c
©ikes1201
Sharp input pulse
Optical fibre
Output pulse
©ikes1201

48. Telephone Channels Using
Copper
Deansway Small
Office
High Street Small
Office
Worcester Telephone
Exchange (City Walls Rd)
Sometimes called ‘Plain Old
Telephone System’ (POTS)
uses twisted pairs of wires to
connect buildings in the locality
to the telephone exchange.
The telephone exchange processes
the call using the dialled number to
direct it to the correct telephone.
Each channel takes up 3.4KHz of bandwidth, good
enough for speech to be understood

49. Telephone Channels Using Fibre
Deansway Large
Office
High Street Large
Office
Worcester Telephone
Exchange (City Walls Rd)
For large offices fibre cable is
used instead of copper ones
because they have a greater
capacity or bandwidth
The telephone exchange still
processes the call using the dialled
number to direct it to the correct
telephone.
Also each speech channel still takes up 3.4KHz of
bandwidth. However data services can be used that have a
much greater bandwidth such as leased lines, Frame
Relay & ISDN PRI

50. Fibre Optic Channels
Special equipment is needed inside the office to
connect the fibre cable network to the office network.
This is called carrier network access equipment.
Carrier network
access equipment
WAN
access
Inside
the
larger
office
Telephone Exchange
Fibre Cable
connected to
Office
There can be thousands of
channels of data being carried
in one fibre if the company is
big enough. All these channels
could convey speech and data
at the same time.

51. Bandwidth of Various Channel Types
Channel Type or
Service
Typical Bandwidth (bps or
bits/second)
Frame Relay 64K-8M
Dial up 56K
E1 Leased line 2M
3G Mobile Phone 512K

52. A Fibre Optic Telecommunications
Network From Lucent
Source: http://www1.alcatel-lucent.com/com/en/appcontent/opgss/19063_1521_FL_ds_tcm228-
314931635.pdf
Fibre
Ring
PSTNInternet
Mobile
Communications
Network
Management
This shows a possible local set up for telecommunications
carrier network such as BT or Cable & Wireless

53. References
• http://www.kettering.edu/~drussell/Demos/rad2/mdq.html
{good animation of waves}
• http://en.wikipedia.org/wiki/Wifi#Wi-Fi:_How_it_works
• http://en.wikipedia.org/wiki/Dipole_antenna#Folded_dipole
• http://www.radio-electronics.com/info/wireless/index.php
{good on Wireless technologies}
• http://www.pulsewan.com/data_101.htm
{Extensive information on data comms)