Transcript IEEE 802.11 and Bluetooth

Wireless Networking and Communication
IEEE 802.11
BlueTooth
The IEEE 802.11 standard covers:
Physical (PHY) and
Medium access control (MAC) layers.
808.2 Logical Link Control (LLC)
Data-Link Layer
MAC
FHSS (radio)
802.11 Wireless LAN (WLAN)
DSSS (radio)
Infra-Red (light)
PHY Layer
FHSS: Frequency-Hop Spread Spectrum
DSSS: Direct-Sequence Spread-Spectrum
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Types: Ad-hoc and Client/Server
Ad-hoc
•Simple network between multiple nodes.
•A given coverage area Basic Service Set (BSS).
•Without an Access Point (A/P) or server.
•Each station observes ‘netiquette’.
•All units have fair access to the wireless media.
•Methods for arbitrating medium access.
•Maximise throughput for all of the users.
Client/Server (Infrastructure)
•An access point controls the allocation of transmit time for stations
•Mobile stations may roam from cell (BSS) to cell.
•The access point is used to handle traffic from the mobile to the wired
or wireless network backbone.
•Allows point coordination of all the stations in the BSS and ensures
proper handling of the data traffic.
•The access point routes data between the stations, servers.
•A/P networks generally provide better throughput performance.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Wireless Networking
IEEE 802.11
3
IEEE 802.11 PHY Layer
The PHY layer handles the transmission of data between nodes.
Can use one of three transmission techniques:
• Direct-Sequence Spread Spectrum – DSSS, Spreads signal across the band,
according to a spreading function,
• Frequency-Hopping Spread Spectrum – FHSS, Single frequency used at a time.
• Infrared (IR) pulse position modulation (light).
IEEE 802.11 originally made provision for data rates of either 1 Mbps or 2 Mbps.
Radio transmissions are in the 2.4 - 2.4835 GHz radio frequency band
(an unlicensed band for industrial, scientific, and medical (ISM) applications),
and in the 5GHz band.
The 3 - 428 THz (312 – 4.2814 Hz) light frequency band is used for IR transmission.
Infrared is considered to be more secure to eavesdropping, since IR transmissions
require line-of-sight links as opposed to radio frequency transmissions, which can
penetrate walls and be intercepted by third parties unknowingly.
However, infrared transmissions can be adversely affected by sunlight.
The spread-spectrum protocol of 802.11 provides some rudimentary security.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
The MAC layer is a set of protocols to control access to, and sharing of, the
medium.
802.11 is based on the Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA) protocol.
CSMA/CD (Collision Detection), as employed in Ethernet, cannot be used
with radio transmissions because it requires the transmitter to continuously
listen on the carrier whilst transmitting.
As radio signals (in 802.11 at least) are unguided, they dissipate in all
directions, and only a very small fraction of the output power reaches the
receiver.
The transmitted signal is so much stronger than the received signal that it is
not possible to detect a collision whilst transmitting.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
When a node has a packet to be transmitted, it first listens to the medium
→ If the channel is clear, it then transmits the packet.
→ Otherwise, it chooses a random "backoff factor" which determines the amount of time
the node must wait until it is allowed to transmit its packet.
(probabilistically, the longer the busy period, more nodes will be waiting to Tx)
During periods in which the channel is clear, the transmitting node decrements its
backoff counter (It does not decrement its backoff counter when the channel is busy).
When the backoff counter reaches zero, the node transmits the packet.
The probability that two nodes will choose the same backoff factor is small, so collisions
between packets are minimized.
Choose random backoff
Receive packet
(from host)
to send
Detect channel clear, start
decrementing counter
Counter reaches zero,
transmit the packet
Time
Listening
Busy
Richard Anthony
The packet
Network Design and Implementation – level 3
Carrier
University of Greenwich
802.11a
Up to 54 Mbps in the 5GHz band.
Uses an OFDM encoding scheme rather than FHSS or DSSS.
OFDM is an FDM modulation technique for transmitting large amounts of digital data
using a radio wave. Splits the radio signal into multiple smaller sub-signals that are
then transmitted simultaneously at different frequencies to the receiver.
a uses 12 separate non-overlapping channels. Up to 12 access points set to different
channels can operate in the same area without interference.
This makes access point channel assignment much easier and significantly increases
the throughput the wireless LAN can deliver within a given area.
RF interference is much less likely (than with b and g) because the 5 GHz band is
less-crowded than the 2.4 GHz band.
a delivers up to 54 Mbps, with extensions to even higher data rates possible by
combining channels. Due to higher frequency, range (around 80 feet) is lower than
lower frequency systems (b and sometimes g). This increases cost as it requires
more access points, but enables a much greater capacity in smaller areas via a higher
degree of channel reuse.
A significant problem with a is that it's not directly compatible with b or g.
Eventually, multimode NICs will become the norm, thus solving this problem.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
802.11b
802.11b (aka 802.11 High Rate or Wi-Fi) provides 11 Mbps transmission (with a
fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band.
(The term Wi-Fi was formerly used to describe the 2.4GHz 802.11b standard).
The term Wireless Fidelity is now used generically when referring of any type
of 802.11 network. The term is promoted by the Wi-Fi Alliance, who test and
approve products as "Wi-Fi Certified“, i.e. interoperable with each other, even if
from different manufacturers.
Typically, however, any Wi-Fi product using the same radio frequency (for
example, 2.4GHz for b or g, 5GHz for a) will work with any other, even if not
"Wi-Fi Certified“).
The fallback Tx rates are used to maintain signal quality at greater distances or
in noisy environments.
b uses only the DSSS transmission technique.
b was a 1999 ratification to the original 802.11 standard, allowing wireless
functionality comparable to Ethernet (10 MHz).
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
DSSS – 802.11b coding (1)
Data stream of ‘0’s and ‘1’s is modulated with a ‘chipping sequence’.
One such code is the Barker code, an 11-bit sequence (10110111000)
that has certain mathematical properties (Autocorrelation / low correlation
sidelobes) making it ideal for modulating radio waves.
(A correlation sidelobe is the correlation of a codeword with a time-shifted
version of itself. Autocorrelation is useful for finding repeating patterns in a
signal, e.g. determining the presence of a periodic signal that is distorted by
noise).
The data stream is exclusive OR'd (XOR) with the Barker code to
generate a series of data objects called chips.
XOR
0
0
1
1
0
1
0
1
=
=
=
=
0
1
1
0
Each bit is encoded by the 11-bit Barker code.
Each group of 11 chips encodes one bit of data – see next slide.
The wireless radio generates 2.4-GHz carrier wave (2.4 to 2.483 GHz).
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
DSSS – 802.11b coding (2)
Basic problem - If there is noise, some bits might be flipped
during transmission.
Using barker code:
• Sender side, modulates data with Barker code pattern, so one
data bit becomes eleven bits of modulated signal
• Input data, e.g. 01
• XOR with Barker code 10110111000 gives
1011011100001001000111
• At the receiver side, basically reverse all operations.
• Receiver XOR with Barker code, Get
0000000000011111111111
Notice, if bit-shifted, the result would be very different
• Add up each 11 signal bits of same value to create each data
bit, get 0 and then 1
Wireless Networking
IEEE 802.11
9 (802.11b) Phase-Shift Keying
For 1-Mbps transmission, BPSK (Binary Phase Shift Keying) is
used (one phase shift for each bit, 0 and 180 degree phases used).
To accomplish 2-Mbps transmission, QPSK (Quadrature Phase Shift
Keying) is used.
QPSK uses four rotations (0, 90, 180 and 270 degrees) and can thus
encode 2 bits of information in the same space as BPSK encodes 1.
Trade-off: power must be increased or range decreased to maintain
signal quality:
The FCC regulates output power of portable radios to 1 watt, so range
is the only remaining factor that can change.
Thus, on 802.11 devices, as you move away from the radio, the radio
adapts and uses a simpler (and slower) encoding mechanism to send
data.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
802.11g
Provides 20+ Mbps in the 2.4 GHz band.
Commonly now provides 54Mbps.
‘Super G’ provides up to 108Mbps.
g is an extension to b, and is basis of majority of wireless LANs in existence.
g uses the 2.4 GHz band using OFDM technology (as used in a).
Backward compatibility ensures a b card will interface directly with a g
access point (and vice versa) at 11 Mbps or lower depending on range.
At 54 Mbps, range will likely be less than b access points operating at 11 Mbps.
b and g both use 2.4GHz band, and the transmitted signal uses approx 30MHz,
which is one third of the band, limiting the number of non-overlapping g access
Points to three. The solution is to lower the power of each access point, which
enables closer placement of access points.
A major issue with g (as with b) is RF interference from other 2.4 GHz devices,
such as the newer cordless phones.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
802.11n
Maximum raw data rate from 54 Mbit/s to 600 Mbit/s.
802.1n uses smart antenna technology to extend performance.
Multiple-Input and Multiple-Output (MIMO) uses multiple antennas at both
transmitter and receiver. Can achieve significant increases in data throughput
and link range without additional bandwidth or transmit power. It has higher
spectral efficiency (more bits per second per hertz of bandwidth) and better
link reliability (reduced fading).
802.1n uses up to 4 spatial streams (each has a channel width of 40 MHz,
which is twice the channel width (20 MHz) of previous 802.11 physical layer
mechanisms).
Each spatial stream requires a separate antenna pair, i.e. at both transmitter
and receiver.
Each MIMO antenna also requires a separate radio frequency chain
(analogue electronics) and A-D converter – representing higher
implementation costs.
The Hidden-Node Problem 1
Nodes A, B and C can all hear wireless access point P.
A and B can hear one another, B and C can hear one another, but A can't hear C.
When A sends to B, C might also be transmitting!
This is a common occurrence in real-world wireless environments
(walls and other structures create obscure radio coverage areas).
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Wireless Networking
Solution
IEEE 802.11
12 The Hidden-Node Problem 2
RTS/CTS (request to send/clear to send) is specified as an optional
feature of the IEEE 802.11b standard.
1. When A wants to transmit to B, it first sends an RTS packet. This includes
identification of the receiver of the data transmission ensuing, and the duration of the
whole transmission (including the ACK related to it). B hears this request.
(A uses the standard transmission method to obtain access to send the RTS packet).
2. Once the packet is received
by the intended (data packet)
recipient, that host replies
with a CTS message that
includes the same duration
of the session about to happen.
3. C hears B’s CTS reply,
and is made aware of
the potential collision.
4. C hold its data for the
appropriate amount of
time, preventing a
collision.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
BlueTooth
Bluetooth wireless was originally devised by Ericsson in 1994.
Named after Harald Blatand (or Bluetooth), a 10th century Danish Viking king who had
united and controlled large parts of Scandinavia (today Denmark and Norway).
Key features:
Mainly based on the ad-hoc mode of the IEEE 802.11 standard, this means that
stations must observe "netiqette" and give other units fair access to the wireless media.
General features include:
robustness,
low complexity,
low power,
low cost.
Range:
Frequency band:
Spectrum Allocation:
Interference avoidance:
10 meters
2.4 GHz (Unlicensed ISM band)
83.5 MHz (79 RF channels, spaced 1 MHz apart)
Frequency hopping
Data bandwidth:
V1.2
v2.0
1Mbit/s (721Kbit/s actual)
3 Mbit/s (2.1 Mbit/s actual)
encoding based on FSK and PSK.
WiMedia Alliance
Richard Anthony
Proposed 53 - 480 Mb/s
Network Design and Implementation – level 3
University of Greenwich
The 2.4 GHz band used by Bluetooth:
The Industrial Scientific Medical (ISM) frequency band is
unlicensed
The ISM band is used by a mix of applications which include:
CCTV, Microwaves ovens, Medical equipment,
Wireless doorbells, Home automation schemes etc.
Some ‘interesting’ schemes that have used this band include:
Wireless Power Transportation (WPT) (Bill Brown early 60’s)
Space Solar Power (SSP) Glaser 1968
Bluetooth uses frequency hopping to avoid interference with /
from these applications.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Concepts and Definitions
MAC address:
`
3-bit address used to identify units participating in the piconet.
Maximum of 8 addresses per piconet,
Only issued to active devices.
Master unit:
The device in a piconet whose clock and hopping sequence are used
to synchronize all other devices in the piconet.
The master also numbers the communication channels.
Slave units:
All devices in a piconet that are not the master
(up to 7 active units for each master).
Sniff mode and hold mode:
Devices that are synchronized to a piconet, and which have temporarily entered powersaving modes in which device activity is lowered. They keep their MAC-addresses.
Parked units:
Devices in a piconet which are regularly synchronized but do not have MAC addresses.
Must monitor the Beacon-channel (woken up by the master with a beacon signal).
The Beacon-channel:
The master establishes a beacon channel when one or more slaves are parked.
This channel consists of one beacon slot, or a train of equidistant beacon slots
transmitted at constant time interval (periodic).
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Piconets 1
A piconet is a group of devices connected via Bluetooth technology in an ad-hoc
fashion.
A piconet starts with two connected devices, e.g. portable PC and a mobile phone.
All Bluetooth devices are peer units and have identical implementations.
One unit will act as a master for synchonization & clocking
Other unit(s) will be slave(s) for the duration of the piconet connection.
Types of connections in Bluetooth:
This node would have the
a) Single-slave
role of master in one
b) Multi-slave (up to 7)
piconet and slave in the
other.
c) Scatternet (inter-connected piconets)
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Wireless Communication: BlueTooth
5
Piconets 2
Scatternet:
Two or more independent and non-synchronized piconets that communicate with
each other.
A slave or a master in one piconet can establish this connection by becoming a slave in
the other piconet. It will relay communications between the piconets as required.
Master
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Communication Strategy
Basically peer to peer
All communication is done via the master unit.
There is no direct communication between slave units.
The master doesn’t route messages between slave units.
If slave units need to talk to each other, they form a new
piconet, with one of them acting as master.
This does not mean that they have to leave the previous
piconet. More likely, they will be parked in the old net unless
they decide to quit the old net.
 Reconfiguration in Bluetooth is dynamic and very fast.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Wireless Communication: BlueTooth
vCard / vCal
WAE
Object Exchange
Protocol (OBEX)
WAP
UDP
TDP
7
AT –
Commands
Layered Architecture 1
TCS BIN
Service Discovery
Protocol (SDP)
IP
Point-to-Point
Protocol (PPP)
RFCOMM
Logical Link Control and Adaptation Protocol (L2CAP)
Link Manager Protocol (LMP)
Baseband
Bluetooth Radio
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Layered Architecture
vCard:
vCal:
The electronic business-card exchange format.
The electronic calendaring and scheduling exchange format.
AT-Commands: Telephony control - profiles for (e.g.) Dial-up, Fax and Headsets.
TCS-BIN:
Telephony Control Specification – Binary.
RFCOMM:
Serial Cable Emulation Protocol
(Wireless emulation of RS232 serial interface).
Wireless Application Environment (WAE): Uppermost layer in the WAP1 software
stack, this layer provides basic components on mobile applications are developed.
Logical Link Control and Adaptation Protocol (L2CAP):
Supports higher level protocol multiplexing, packet segmentation and reassembly, and
the conveying of quality of service information.
Link Manager Protocol (LMP):
Used by Link Managers (on either side) for link set-up and control.
Baseband: Describes the specification of the Bluetooth Link Controller (LC) which
carries out the baseband protocols and other low-level link routines.
Bluetooth Radio:
Defines requirements of a Bluetooth transceiver operating in the 2.4 GHz ISM band.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Types of Address
BD_ADDR: Bluetooth Device Address.
Each Bluetooth transceiver has a globally unique 48-bit device address.
AM_ADDR (aka MAC address): Active Member Address (3 bits)
It is only valid as long as the slave is active on the channel.
PM_ADDR: Parked Member Address
An 8-bit member (master-local) address that separates the parked slaves.
(which piconet they are in – needed since they have no MAC address when
parked)
The PM_ADDR is only valid as long as the slave is parked.
AR_ADDR: Access Request Address (Identifies a slot, not a device).
This is used by a parked slave to determine the slave-to-master half slot in
the access window it is allowed to send access request messages in.
It is only valid as long as the slave is parked and is not necessarily unique.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Physical Channel
The channel is represented by a pseudo-random hopping sequence hopping through
the 79 RF channels.
Two or more Bluetooth devices using the same channel form a piconet.
There is one master and one or more slave(s) in each piconet.
The master establishes a frequency-hopping scheme when a piconet is established.
The frequency selection scheme consists of two steps:
1. Selecting a sequence.
2. Mapping the sequence onto the hop frequencies.
The hopping sequence is unique for the piconet - determined by the Bluetooth device
address (BD_ADDR) of the master.
The phase in the hopping sequence is determined by the clock of the master.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Transmission
Frequency Bands and Channel Arrangement
Spectrum spreading accomplished by frequency hopping in 79 channels
displaced by 1 MHz, starting at 2.402GHz and finishing at 2.480GHz.
A guard band is used at the lower and upper band edge
Upper Guard Band
2.480 GHz
.
.
.
.
2.404 GHz
2.403 GHz
2.402 GHz
83.5 MHz
79 channels,
spaced 1 MHz
apart
Lower Guard Band
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Frequency-hopping 1
Bluetooth is designed to operate in noisy radio frequency environments.
Uses a fast ACK and frequency-hopping scheme to make the link robust.
Hops to a new frequency after transmitting or receiving a packet.
Hops faster and uses shorter packets than other systems in the same band, to
limit the impact of ISM band equipment and other sources of disturbances.
Forward Error Correction (FEC) further limits the impact of random noise.
Frequency hopping is ineffective if the noise is spread over the whole
frequency range but sources of electro-magnetic noise usually give the pattern
illustrated below right.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Channels and Links
Bluetooth Channels (three different contexts)
• 79 RF-channels, on individual frequencies 1 MHz apart.
• Communications channels, consisting of a pseudo-random hopping
sequence through the RF-channels (like a "session" in the OSI-model
context).
• Five logical channels, which are used for control purposes.
Physical links (two types are defined):
SCO (Synchronous Connection-Oriented)
A point-to-point between master and slave.
The master maintains the link by using reserved timeslots at regular
intervals.
Packet retransmissions are not allowed.
ACL (Asynchronous Connection-Less)
Packet-switched connections between the master and all active slaves.
Packet retransmissions are usually applied to assure data integrity.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Channels and Links 2
SCO
ACL
SCO
ACL ACL SCO
SCO
ACL
Master
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Physical layer - Baseband
•Manages physical channels and links
•Also error correction, hop selection and security.
•Implemented as a link controller,
•Works with the link manager for carrying out link connection and power
control.
•Manages asynchronous and synchronous links, handles packets
•Paging and inquiry to discover and access bluetooth devices in the area.
The baseband transceiver applies a Time-Division Duplex (TDD) scheme.
(alternate transmit and receive). The Bluetooth baseband protocol is a
combination of circuit and packet switching.
Time slots can be reserved for synchronous packets.
A frequency hop is done for each packet that is transmitted.
A packet normally covers one time slot, but can be extended to cover up to 5
slots.
Bluetooth can support:
• an asynchronous data channel, or
• up to 3 simultaneous synchronous voice channels, or
• a channel which simultaneously supports asynchronous data and
synchronous voice.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Time-Division Duplex (TDD)
Duplex means both ends can transmit at the same time –
this is achieved by fine-grained multiplexing.
The channel is divided into timeslots, each of 625 micro seconds
(there are 1600 time slots in one second).
The timeslots are numbered, using the bluetooth clock of the master of the
piconet.
As the timeslots are numbered it is easy to implement duplexing based on
alternation –
• The master transmits in the even-numbered timeslots only.
• The slave transmits in odd-numbered timeslots only.
Thus there are no collisions between master and slave devices, and since the
timeslots
are so small it (effectively) achieves duplex communication.
Slot 6 (M)
Slot 7 (S) Slot 8 (M) Slot 9 (S) Slot 10 (M) Slot 11 (S) Slot 12 (M) Slot 13 (S)
625 μs
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Error Correction Schemes
Bluetooth uses checksum calculations to detect errors.
Two FEC schemes are also used:
• 1/3 rate Forward Error Correction (FEC) - every bit repeated 3
times - redundancy.
• 2/3 rate FEC -10 data bits encoded into 15-bit (modified Hamming)
self-correcting code.
The purpose of the FEC scheme on the data payload is to reduce the
number of re-transmissions, but in a low-error environment, FEC
reduces throughput unnecessarily so may be omitted.
Packet headers are always protected by 1/3 rate FEC, they contain
valuable link info and thus must be able to sustain some bit errors.
•
ARQ unnumbered scheme (Automatic Repeat Request)
- ACK signals receipt of a good packet.
- NACK signals receipt of a corrupted packet, and implies a repeat
request.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Error Correction Schemes 2
The ARQ-scheme illustrated:
The first error occurs when the master transmits packet B to slave1.
Slave 1 returns a NAK piggybacked onto packet G.
The master then retransmits packet B (as well as an ACK for packet G).
The second error occurs when slave 2 sends packet Y to the master.
(Master does not get ACK or NACK for X so assumes that X, or the response, was lost).
The master returns a NAK, and slave 2 retransmits packet Y.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Connection setup 1
Bluetooth supports point-to-point and point-to-multi-point connections.
Piconets can be linked together (each has a different hop sequence).
All users on the same piconet are synchronized to this hopping sequence.
Before any connections are created, all devices are in STANDBY mode.
Unconnected units listen (on a set of 32 hop frequencies defined for that unit)
every 1.28 seconds.
Power saving modes can be used for connected units if no data needs to be
transmitted:
The master can put slaves (or they can ask to be put) into HOLD mode,
where only an internal timer is running.
Data transfer restarts instantly when units transition out of HOLD mode.
The HOLD is used when:
Connecting several piconets or
Managing a low power device such as a temperature sensor.
In the SNIFF mode, a slave device listens at reduced rate, thus reducing its
duty cycle.
In the PARK mode, a device is still synchronized to the piconet but does not
participate in the traffic. Parked devices have given up their MAC address
and occasionally listen to the traffic of the master to re-synchronize and
check on broadcast messages.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich
Wireless Communication: BlueTooth
Richard Anthony
20
Connection setup 2
Network Design and Implementation – level 3
University of Greenwich
Security
Information Integrity in Bluetooth is based on:
• Random Number Generation
• Encryption
• Encryption Key Management
• Authentication.
Software controls and identity coding built into each microchip ensure that
only those units preset by their owners can communicate.
Additional features which increase the difficulty of eavesdropping:
The frequency-hopping scheme (with 1600 hops/sec) is far quicker than any
other competing system.
Output power can be automatically adapted, to reduce the range to
requirement. Two power levels are defined:
Low - covers the shorter personal area within a room,
High - covers a medium range, such as within a home.
The combined effect of these two measures improves security.
Richard Anthony
Network Design and Implementation – level 3
University of Greenwich