2. Criteria Objectives in this
Presentation
• P2 explain the principles of signal
theory
• P3 describe communication protocols
used and explain why they are
important
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. 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
5. Communication Protocols
We are going to cover what a protocol is a and describe some common
ones in digital communications
A protocol is a set of rules to follow when two or more devices are
communicating
An analogy is that of a life of a telephone call.
Both parties put their phones back in the ready to answer mode
The phone rings
The caller replies ‘Hi may I speak to John’The called person says ‘I am John’
The called person answers with a ‘Hello’
The conversation starts The conversation ends with a ‘goodbye’ usually from
the caller
6. Protocol Exercise
Now it is your turn to sort out the order of a transaction in a shop such as
Argos ie. the protocol to follow when obtaining goods.
Enter shop
Select item and write down code and
quantity on a slip
Pay for the item
Look at catalogue
Go to the till and hand
over slip
Collect item
Hand over the receipt to the
Collection point when called
Receive receipt and details
of collection point
1
2
3
4
5
6
7
8
7. Protocol Exercise Answer
Was that easy?
Enter shop
Select item and write down code and
quantity on a slip
Pay for the item
Look at catalogue
Go to the till and hand
over slip
Collect item
Hand over the receipt to the
Collection point when called
Receive receipt and details
of collection point
1
2
3
4
5
6
7
8
8. TCP/IP
Next we are going to look at two protocols commonly used on a
network such as the Internet
These two protocols usually work together as a ‘Stack’ inside a
computer.
TCP stands for Transport Control Protocol and
IP stands for Internet Protocol
They have different roles
•TCP creates and ensures that data transfer is reliable
• IP ensures that data is addressed so that it can reach its proper
destination
Let’s have look at IP first
9. Internet Protocol (IP)
We previously looked at a packet structure; Remember?
Data IP Header
Data is the user information broken up into chunks and given a layer 4 headers such
as for TCP (we will look at that afterwards).
The IP Header above contains information such as source and destination addresses.
For simplification we shall only look at the addressing information
A B
Internet
cloud
Router
Packets from computer A are destined for computer B
10. Internet Protocol
Source
Address
Destination
Address
Data 211.410.11.3 202.20.10.1
Every network communication device will have an IP address allocated in
the form of four groups of numbers. This called a dotted decimal format.
Packets are labelled with the source and destination IP addresses so
that they can be directed by a process known as routing and so that the
receiver can return data to the sender forming a two way connection
A B
Router
202.20.10.1 211.410.11.3
Internet
cloud
11. Transport Control Protocol (TCP)
IP HeaderData
We previously looked at a packet structure; this is how it is structured
We will now look at the data part of that IP packet in which could
contain a segment formatted for TCP:
In order for data to be sent over an IP internetwork it has to be chopped up
into segments first. This is done inside the computer by the TCP/IP stack.
Data
Data
Seg 5Seg 1 Seg 2 Seg 3 Seg 4
12. TCP Process, Sequence Numbering
Data
Seg 5Seg 1 Seg 2 Seg 3 Seg 4
In this example a string of data is chopped up into five segments and each
of these segments is given a sequence number. This forms part of the TCP
formatting process.
Segmented Data Seq
1
Segmented Data Seq
2
Segmented Data Seq
4
Segmented Data Seq
3
Segmented Data Seq
5
13. TCP Segment Format
Simplified segment format based on TCP
Each segment is also allocated source and destination port numbers
Segmented Data Sequen-
ce no
Source
Port
Destination
Port
Ports are numbers together with ip addresses are used to set up many
simultaneous communication sessions on a computer
WWW
Peer to
Peer
email
Peer to Peer
Email
server
Web
Server
End
User
Port 80 Port 110 Port 2124
Checksum
14. TCP Flow Control
Each segment is also allocated flow control bytes called ACK and Window
Simplified segment format based on TCP
Segmented Data Sequ-
ence
no
Source
Port
Destination
Port
Flow
control
Flow control together with the sequence numbering is used to ensure a reliable
delivery of data by the acknowledgment of receipt of segments in the right order
Now give
me the
next 5
segments
email
End
User
I am
sending 5
segments
Email
ServerI have
received
the 5
segments
I expected
15. TCP Flow Control
Remember that when data is sent out to an internetwork it has an IP header
to tell the network devices in the internetwork its destination.
email
End
User
I am
sending 5
segments
Email
Server
I have
received
only 3 of
the 5
segments
I expected
Now give
me the
last 2
segments
again
Now what if the email server did not receive some of the segments properly
TCP will confirm the number of segments received and ask for the last
missing ones again. It will adjust the number of segments it requests to a
smaller number until its satisfied that the errors are reduced. This is called
‘windowing’.
16. Wireless Technology in Use
• Mobile Phones
• Point to Point
• Point to multipoint
• Infrared
• Laser
• Blue Tooth
• WIFI
17. Wireless communication
• The increasing use of wireless networks in
local area networks (LANs), access points,
• infrared (line of sight, scatter and
reflective), LASER, narrow band radio,
spread-spectrum radio, point to point and
extended LANs with wireless bridges,
• the increased availability of mobile
computing
18. Wireless Protocols
We looked at two important protocols in some depth. This was to give you an idea
of what is involved in data transfer over an internetwork such as the Internet.
Next we will look at some wireless protocols because these are becoming more
important.
First of all a little lesson in radio basics….
WiFi
GSM/GPRS
UMTS
Blue
Tooth
IrDA
19. Bluetooth
• Low cost short range unlicensed radio standard for
interconnecting devices
• Constantly channel hops (swaps frequencies) for
reliable connection
• Uses the frequency band around 2.4GHz with up
to 2 Mbit/s bit rate.
Q:You know
what LAN is.
What is PAN?
Notes
20. More on Bluetooth
Serious flaws in Bluetooth security lead to disclosure of personal data
There is a
problem with
security
What is
‘bluejacking’
and
‘bluesnarfing’?
The
Viking!
Task: Make a list of Bluetooth products that a company might find
useful
Can you name all
of these items?
Notes
21. Bluetooth Technical
A short range network connecting devices such as computer peripherals and PDAs
is said to be a “piconet”. Bluetooth is a standard for such a piconet which specifies
radio access and frequency hopped single carrier modulation techniques which are
similar in principle to those used by the original 1 Mb/s FHSS IEEE802.11 standard
over wider distances. Bluetooth is not an IEEE standard and is not easily described
in terms of the seven-layer OSI model. It operates in the 2.40-2.48 GHz band over
ranges of about 10 metres .The band is divided into 79 channels each 1MHz wide
and Bluetooth frequency hops its carrier with 1600 hops/second and a dwell time of
625 microseconds. There is a master unit controlling the hopping sequence used
by each slave. All devices hop together. Frequency shift keying (FSK) modulation is
used with a bandwidth efficiency of 1 bit/s per Hz to achieve 1Mb/s. Much of this
capacity is consumed by a rather elaborate overhead of control information. The
fact that Bluetooth uses the same 2.40-2.48 GHz frequency band as IEEE802.11b
and that it is non-compatible with IEEE802.11 is a serious problem since Bluetooth
is capable of wiping out IEEE802.11b communication over its short range. The
IEEE 802.11 committees are considering this problem and most likely will propose
a new standard to rival Bluetooth.
Ref: 802.11 Wireless LAN Fundamentals, Pejman Roshan, Jonathan Leary, Cisco
Press, 2003
22. Infrared
Infrared
Data
Association
IrDA
Uses infrared for very short
distance communications
Speeds of up to 16Mbit/s with further
developments to 100Mbit/s
Look for some products using this
standard: http://www.irda.org/
Q: What products could help an
organisation?
Notes:
23. Mobile Phone History
• First introduced in the early 70’s
• The heavy battery pack version lasted an
hour at best
A Invented by
Martin Cooper in
1973
The long talk time
version was very
heavy!
http://inventors.about.com/cs/inventorsalphabet/a/martin_cooper.htm
24. Mobile Phone History
• Denoted now by the letter G for generation
http://www.techweb.com/encyclopedia/defineterm.jhtml?term=cellulargenerations
1G: Analogue
system. First
mobile phones
1990’s Late1990’s
Now 4G: The near
future
2G: Changes to a
digital system.
More efficient
and secure Q: What
features does
3G give the
user?
1970’s
2.5G: Adds email &
internet access using
Wireless Application
Protocol (WAP)
25. UK Mobile Phone Operators
• Yes there are five that actually own
infrastructure, the rest are third party providers.
What is
infrastructure?
Masts,
switching
centres and
interconnects
to the PSTN
BT TowerMobile Mast
installation
Three standards in use
GSM, Global System for
Mobile communications
GPRS, General Packet Radio
Service
and UMTS, Universal Mobile
Telecommunications System
Q: What does
PSTN stand
for?
http://www.hillsfar.demon.co.uk/Site/Cellnet.html
https://styx.uwaterloo.ca/~jscouria/GSM/trio.html
http://www.webopedia.com/TERM/G/GPRS.html
28. Wireless LAN History
A number of manufacturers approached the Institute of Electrical and Electronics
Engineers (IEEE) in the USA. Having successfully launched the Ethernet standard
under the 802.3 committee, they set up another committee with the designation 802.11,
but it took until 1997 before a recommendation could be agreed. It allowed for data
transfer at a rate of 2 Mbps using either frequency hopping or direct sequence spread
spectrum. The standard was published in 1997 to enable development to start. Even at
this early stage there were two variants, 802.11a (operating in the 5.8 GHz ISM band)
and 802.11b (operating in the 2.4 GHz ISM band). These standards were ratified in
December 1999 and January 2000 respectively.
Ref: http://www.radio-electronics.com/info/wireless/overview/wireless_dev.php
Further developments led to the 802.11g standard giving
54mbit/s in the 2.4 GHz band
Q: Manufacturers claim 108Mbit/s bit
rate on 802.11g systems. How does this
happen?
Notes
29. Wireless LAN Configurations
A wireless access point allows remote devices such as laptops
to access the network
Q: Can you name
these devices?
Switch Modem Laptop Wireless Access
Point
Desktop ComputerTelephone
Socket with filter
Wireless Adaptor Network Cable
30. Wireless LAN Configuration Continued
A wireless router allows remote devices such as laptops to
access the network and has a router and modem inside with
sometimes more than one Ethernet port
Q: Where do you
Definitely need a
cable?
Wireless Router
Centrino
Laptop Desktop Computer
Wireless Adaptor
Telephone
Socket with filter
Notes
31. Security Protocols Intro
Wireless communications is a security risk because of the
broadcast nature of the signal
An example of how open wireless was before digital systems
was its use in mobile communications by the emergency
services.
Channels could be monitored by using a device called a
scanner that could tune to a wide range of frequencies and
modulation schemes
Recently the police has replaced the old analogue radio
system with a digital one called TETRA that is very secure.
Next to adopt the
new radio system is
the ambulance
service.
32. Wireless LAN Security Protocols
The two main protocols in use are WEP and WPA
WEP: Wired Equivalency Privacy. As specified by IEEE 802.11b.
Data that is to be sent and received wirelessly is encrypted using a
combination of the data to be sent and a private and public key. The
encryption method uses a cipher called RC4.
WPA: Wi-Fi Protected Access, as specified by the Wi-Fi Alliance
This also uses the RC4 cipher method in addition to another protocol
called TKIP (Temporal Key Integrity Protocol) that provides a robust
encryption method of the public key. This makes it harder for intrusion
attacks to be successful.
A cipher is a method of converting plain text into a code that is designed to
only be read by the authenticated receiver.
A key is a number from a very large range used as a base for
the encryption process calculation (algorithm).
33. Wireless LAN Security Process
For access to the wireless router a password is required from the laptop. This
is encrypted and sent to the wireless router for authentication. If this is the
only level of security used then a wireless connection is made and data will
be transferred in encrypted form.
Encrypted
Password
Acknowledgement
Encrypted data flow
using key and a
cipher method
34. Setting up Wireless Security
Task: Investigate and report how you would set up a
wireless router for home use.
To do this task: Make a list of software and hardware you
will need and then describe the procedure you could take
to install and set up a working wireless connection
There are many wireless systems available for a home network.
BT, Netgear, Dlink, Cisco, Belkin, 3-Com, Linksys,
US Robotics are some makes.
Q. Does a router need extra software on your PC
to function?
35. 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)
