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CCM 4300 Lecture 4
Computer Networks, Wireless
and Mobile Communication
Systems
Dr E. Ever
School of Computing Science
1
Session Content
 Recap of last session
 Lesson Objectives
 Network Topologies
 Introduction – an example of human-to-human interaction
 What is a protocol?
 ISO OSI Reference Model
- TCP/IP protocol
 Physical layer
Data link layer and access control
2
Recap of Last Session
defined a computer network and identified some of
the basic hardware components.
 explored the network, transport and application layers.
 identified some of the advantages and disadvantages
of Ethernet technologies.
 Virtual circuits vs. datagrams
3
Lesson objectives
 At the completion of this lesson you should be able to
- understand the basic of wireless networks
- Support for mobile systems and applications:
•physical connectivity
•network layer issues
•transport layer issues
Understand and analyse the problems:
-with wireless networks compared to wired networks
-in running existing protocols in a mobile
scenario
Understand the need for specific support for mobile and
wireless scenarios
4
•Connectivity
•transmission
7 application
•modulation
6 presentation
•media access
5 session
•Support in the network infrastructure
•connectivity between the wireless
and the wired world
•Protocols
Application
layer
Supporting wireless and mobile systems
4 transport
3 network
2 data link
1 physical
•specifically for dealing with mobility
5
Wireless Networks: Why?
Mobility:Users can access files, network resources, and the Internet
without being physically connected to the network with wires. Users
can be mobile while maintaining high speed and real-time access to
the network. This increases productivity of the users.
Minimise required infrastructure
Disaster recovery: Continuity of Operations Network Services
Re-route wired network thru wireless to data vault/ ID
Long distance, low data rate links
6
Performance Evaluation of WLAN depending on
Number of Workstations and Protocols
Wireless
Networks:
Why?
A. Ipatovs,E. Petersons (Throughput)
Less installation costs: Installation cost is reduced due to less
wiring.
Rapid installation: The time required for installation is reduced
because network connections can be made without moving or adding
wires, or pulling through walls or ceilings, or making modifications to
the infrastructure cable plant.
Flexibility: Networks can easily be installed. For example, users can
quickly install a small WLAN for temporary needs such as a
conference, trade show, or meeting (ad hoc networks). Wireless local
loop (WLL)
Scalability: Network topology can easily be configured to meet
specific application and installation needs and to scale from small to
large networks.
7
Mobile and Wireless Networks: Background
What is wireless? Brief history…..
•The physical phenomena known as radio waves were first known as
‘Hertzian Waves’. Hertz showed that the electromagnetic phenomena
(under study by Tesla) could be used to transfer energy between
locations without a physical connection.
•Guglielmo Marconi began work in 1894 to reproduce the Hertz
laboratory experiment over greater distances. His study and efforts
brought about the first radio link in the form of wireless telegraph.
•The combined works of Tesla, Hertz, and Marconi proved that
electromagnetic phenomena (such as a large spark) generated at
one location could be detected at another location without a direct
physical connection between locations. Thus, the ability to
communicate without wires i.e. ‘Wireless’.
8
Mobile and Wireless networks: Key Concepts
 Wireless links inherently are more complex than wireline
links
 Wireless links suffer from unfavorable channel
characteristics
 There is a very limited spectrum for wireless
communication
 Wireless communication is susceptible to interception
High error rates (electrical noise, signal reflections)
Wireless networks generate electrical interference
themselves
Power range is low, why? To minimise interference
intrinsically insecure, (authentication)
9
Mobile and Wireless networks: Key Concepts
10
Mobile and Wireless Networks : Concept
Next Generation Internet (NGI)
MPLS, QoS, multimedia support,
group communication, accounting
Telematics (TM): Protocols,
services, standards, LAN, Internet,
TCP/IP, WWW, security, ISDN,
management, interworking units
Microprocessor Lab (MPP) Practical
assignments for RO (also RA and
TM)
Wireless/mobile System
Networked System
Computer
Processor
Logic
Physics
Mobile Communications Lab (MCL)
Practical assignments for MC, NGI and
TM
Mobile Communications (MC):
Wireless transmission, medium
access, GSM, 3G, WLAN, Mobile IP,
Ad-hoc-networks, WAP
Computer Architecture (RA)
Multi processor systems, pipelining, vector
processing, interconnections,
multithreading
Computer Organization (RO): CPU,
RISC/CISC, assembler, I/O, bus, controller,
PIO, DMA, interrupt, memory, peripherals
Computer structures: Boolean algebra,
combinational and sequential circuits,
computer arithmetic, von Neumann
machine
Physical electrical basics: semiconductors,
TTL, CMOS, gates, memory, programmable
logic, discrete elements
11
Wireless Connectivity
 Transmission
 radio-based systems
 (IR currently of limited use)
 noise: modulation techniques and error correction
Available wireless networks: of interest are IEEE 802.11x
c) Bridging between networks, d) standard harmonisation, e) QoS, f) supports roaming, h)
spectrum management, a) specific frequencies (EU), i) updates for security
 Access network
 provide connectivity between mobile devices
 cell-based systems: Mobility support
 Infrastructure wireless
 Mobile and ad hoc network (MANET)
12
Key questions … wireless networks….
 How can we provide connectivity for mobile systems?
 What kind of network structure do we need to support
wireless mobility?
 What changes might we need to make to the existing
infrastructure (mechanisms and systems) to support
wireless mobility?
13
Wireless Networking Technologies
 Personal Area Network (WPAN) -802.15
 Bluetooth
Infra-red
Local Area Network (WLAN) – 2.4 GHz waveband
 IEEE 802.11 (WaveLAN)
 HomeRF
WMAN – 802.16
Wide Area Network (WAN) (Wired/national)
 GSM (current 2G) (Global System for Mobile Communications)
 GPRS (2.5G) (General Packet Radio Service)
 UMTS (3G) (Universal Mobile Telecommunications System)
Wide Area Network (International)
 Satellite systems
LEO, MEO, HEO and GEO
14
Wireless Networking Technologies – con…
 Transmission:
 Spread spectrum
 TDMA
 FDMA
 CDMA
 Networking:
 Mobile IP
 MANET
15
Wireless Transmission
• Radio
•Spectrum regulated
•ISM channels
•used by other
applications
• Radio broadcast
•Signal confinement
• Propagation problems
•loss
•interference
•multi-path - fading
industrial, scientific and medical (ISM)
• Infra-red
•Not regulated
•Line of site
•scattering
•diffusion satellite
•LEDs
•Low power:
 10 Mb/s
•Laser diode:
 10 Mb/s
•more complex transceiver
16
Infra-red and radio diffusion modes
Point-to-point
passive satellite
active satellite
typical 1Mb/s
•Non active requires
high transmitter power
Directional antennas, detectors
and emitters
•power limited?
•reduce multi-path effects
17
Radio broadcast connectivity
•Multiple host, multiple channels?
•TDM and FDM (fixed allocation)
•impractical
•not scaleable
•Many hosts, single channel?
•Shared Media:
•but
•when to transmit?
•was there a collision?
•is the receiver listening?
host
host
host
host
•can we ensure Rx listening?
18
Radio Systems I
 Easy to set-up network:
• High wiring
data rates possible
 Mature technology:
• mobile still maturing
 Local and nationwide
 Global – satellites
 Interference
 Security
 Spectrum regulation
 Safety
Radio spectrum is subject to international
regulatory control, so it is not possible to use
just any part of the spectrum at will – you
have to obtain a license.
Description
Frequency
Wavelength
High frequency 3 -30MHz
100 -10m
VHF
50 -100MHz
6 -3m
UHF
400 -1000MHz
75 -30cm
Microwaves
3´109 -1011Hz
10cm -3mm
Millimetre waves 1011 -1012Hz
3mm -0.3mm
Infra-red
1012 -6´1014Hz
0.3mm -0.5mm
Visible light
6´1014 -8´1014Hz 0.5mm -0.4mm
Ultra-violet
8´1014 -1017Hz 0.4mm -10-9 m
X-rays
1017 -1019Hz
10-9 m -10-13 m
Gamma rays
> 1019Hz
< 10-13 m
Note: For each doubling of the distance between the
source and receiver, a 6dB loss is experienced.
19
Radio Systems II
Different propagation characteristics
reflection
 Effects:
1- reflection (meeting plan object)
2- refraction (medium with different wave speed)
3- diffraction (wave encounters an edge) LF
4- scattering (any other waves other than the above)
refraction
 Interface - multi-path
Effects:
diffraction
e.g. TV “ghosting
20
RF Behaviour (key for slide 21)
Reflection: occurs when a propagating electromagnetic wave strikes an
object that has very large dimensions in comparison to the wavelength of the
propagating wave. Reflection occurs from the surface of earth, buildings,
walls, and many other obstacles (this reflection is referred to as multipath)

Refraction:

Diffraction:

Scattering:
describes the bending of the wave as it passes through a
medium of different density. i.e., As an RF wave passes into a denser
medium the wave will be bent such that its direction changes where some of
it will be reflected and some will be bent through the medium in different
direction. Eg, atmospheric conditions change
occurs when the radio path between the TX and RX is
obstructed by a surface that has sharp irregularities or an otherwise a rough
surface, i.e., the wave is bending around an obstacle
here if an RF wave strikes an even surface and is reflected
in many directions with small amplitude reflections and destroys the main RF
signal or if it encounters heavy dust it gets reflected into tiny particals. 21
Radio - ISM
• Existing spectrum allocation
•e.g. radio and TV, (mobile), telecommunication,
satellite
• ISM
• Industrial, scientific and medical (ISM)
• 3 bands
• some frequencies already occupied
• uses include military
• Bands available
• 902 - 928 MHz (26 MHz)
•2.400 - 2.4835 GHz (83.5 MHz), unlicensed, 100mW
• 5.725 - 5.850 GHz (125MHz), licence required, 2W
•Typical high noise:
•interference from other users
22
Modulation and Media Access techniques
• Spread Spectrum
bits are not transmitted over a single frequency because
of electrical interference in 2.4GHz frequency band
•transmitted bandwidth >> minimum bandwidth that the signal
requires
•Therefore the source signal bandwidth must be spread across
a much wider frequency range.
• very low signal to noise ratio (SNR) possible
• typical < 1 (0dB)
Good noise immunity
•Overall signal bandwidth:
Hard to jam & snoop
•“spread” source signal
Works with low S/N
•C = Blog2(1 + S/N)
× Complex
 C  B [ln(1 + S/N) / ln 2] = 1.44 B S/N
B  0.7 CN/S
Hartley-Shannon Law
32Kb/s, +30dB  B  32KHz
32Kb/s, -30dB  B  22MHz
23
Direct sequence spread spectrum DS-SS
 Square pulse train:
smaller, T  higher signal
bandwidth
 Combine data with
pseudorandom binary sequence:
pseudorandom noise (PN)
spreading sequence
Combine with carrier:
e.g. BPSK, QPSK
Chip - bit in PN sequence
Chipping rate
tb
user data
0
1
XOR
tc
chipping
sequence
0 1 1 0 1 0 1 0 1 1 0 1 0 1
=
resulting
signal
0 1 1 0 1 0 1 1 0 0 1 0 1 0
tb: bit period
tc: chip period
24
DS - SS
 At Tx:
synchronisation bits all 1’s
 At Rx:
local copy of PN
XOR with Rx
autocorrelation
correct sync generates
preamble signal
 Synchronisation:
 preamble
periodically
DSSS typically fixed 22MHz,
that makes about 14 channels
avalaible to users (varies!)
preamble
PN:1011011100
Tx
No sync
Rx
PN
XOR
in sync
Rx
PN
XOR
25
Frequency hopping spread spectrum
0236 A’s code Frequency channel numbers
 Bandwidth split into:
6320 B’s code
channels
 Hopping sequence:
N-bits
Tx hops between channels
psuedorandom hop code
N-bits
 chip period:
hold time on a channel
N-bits
chipping rate:
hopping rate
N-bits
Good Tx/Rx sync required
0
a
b
a
b
a
b
a
1 2
3
4
a b
b a
a b
b a
a b
b a
a b
b a
b
time
802.11 uses 79 1MHz channels, it hops 400 ms or less
(2.5 hops or more per second), min hop size 6MHz
5 6
b
a
b
a
b
a
b
a
26
Frequency hopping spread spectrum
Fast frequency hopping
• Multiple chips per bit
• Good noise immunity
• More expensive than slow
frequency hopping
• Hard to sync Tx and Rx
Slow frequency hopping
• Multiple bits per chip
• Easier to sync than fast
frequency hopping
• Not as good immunity to
noise as fast frequency
hopping
Noisy channel can be dropped from hopping
sequence
27
FHSS vs. DSSS

DSSS







Ease of implementation
High data rates 1, 2, 5.5 and 11 Mbps in 2.4 GHz ISM band
has better immunity to noise
has less latency, no pause while channel hops
supplies a large per network bandwidth 11Mb/S
allows just 3 networks to coexist
FHSS





allows 26 networks to coexist
has aggregate bandwidth of 52Mb/s, supplies 2Mb/s
uses less power, better for portable devices
cheaper to build
degrades more gracefully under heavy load
28
Wireless LANs
• Infrastructure Wireless
•wireless connectivity to a fixed network, e.g., PDA
•fixed wire replacement e.g. laptops
•portable access unit (PAU)
•Ad hoc wireless
AP
AP: Access Point
AP wired network
AP
•totally wireless network
•communication only between portable devices
29
Radio frequency usage.
Infrastructure wireless
LANs
WANs
Limited frequency use
Limited frequency allocation
LAN: ISM band
WAN: regulatory controls
How to support large number of users
limited radio/(electrical bandwidth
shared media? Bandwidth
30
Cell-based network
•Radio-based mobile communication
•Digital mobile telephones:
•privacy
BS
•data/voice/X
BS
BS
•extendable network
BS
BS
BS
•network topology
BS
BS
BS
BS
BS
•cells
BS
•base-stations
•LAN/PAN vs. WAN:
•Connectionless, shared media vs. circuit switched
•3G wireless – connectionless
•Base Stations
•interconnected by terrestrial network
31
QoS issues of cellular networks
0.7
For E[V]=1.8km/hr, R=100
•Handoff MRM
Cell-based
networks
SPX For E[V]=1.8km/hr, R=100
MRM
For E[V]=1.8km/hr, R=300
•Channel
failures
0.6
SPX For E[V]=1.8km/hr, R=300
f E[V]=40km/hr, R=100
For
d
•MobilityMRM
SPX For E[V]=40km/hr, R=100
Blocking Probability
3
•
0.5
0.4
r
f4
f5
f2
f1
f4
MRM For E[V]=40km/hr, R=300
f6 E[V]=40km/hr,
f5
SPX For
R=300
f3
f7
f1
f2
0.3
(infrastructure
wireless)
Queue
0.2
+ reuse (d/r)
OC•Frequency
•Network
scaling:
H
0.1
•reduce sell-size
 HC
0
•increase
of cells
0.2
0.25 number
0.3
0.35
0.4
0.45
0.5
mean arrival rate,OC (calls/sec)
Handoff
region
1
.
.
.
.
S
0.55

0.6
32
Frequency planning I


Frequency reuse only with a certain distance
between the base stations
f
Standard model using 7 frequencies:
f
3
5
f4
f6
f1
f3

Fixed frequency assignment:

f5
f4
f7
f1
f2
certain frequencies are assigned to a certain cell


f2
problem: different traffic load in different cells
Dynamic frequency assignment:

base station chooses frequencies depending on the
frequencies already used in neighbor
33
Frequency planning II
f3
f3
f2
f1
f2
f1
f3
f2
f1
f3
f2
f2
3 cell cluster
f3
f2
f3
f5
f4
f1
f1
f3
f3
f2
f6
f1
f3
f3
f5
f4
f7
f1
f3
f2
f6
f7
f5
f2
7 cell cluster
f2
f2
f2
f1 f
f1 f
f1 f
h
h
3
3
3
h1 2
h1 2
g2 h3 g2 h3
g2
g1
g1
g
1
g3
g3
g3
3 cell cluster
with 3 sector antennas
34
Cell-based networks
•Problems - fading:(shadowing,
multipath)
• interference due to
scattering of signal
• BER:
• ~10 -3 possible
• FEC for data
• Network planning:
• surveys of propagation
characteristics
• Some “fading factors”:
Any solution to signal fading?
increase the transmitter power, is not available in mobile communication where
transmitter power is limited.
• free space loss
• street orientation
•Variations of up
to20dB
• foliage
•Variations of 18dB
between summer
and winter
• tunnels
•signal attenuation
of up to 30dB
35
Cell-based networks
•LAN/PAN technology:
• usually ISM (IR possible)
• a handful of high bandwidth channels
• media-access control
• Smaller cell-size:
• micro-cells
• pico-cells
• use power detection to select “best” base-station
36
Media access control in WLANs
•Distributed and centralised MACs
•MAC – wireless LANs
•Hidden terminal and exposed terminal problems
•Pls. Read chapters 2 and 3 from Schiller 3rd edition
•Key Questions:
•How to deal with connection in wireless LANs?
•How can you ensure that a terminal can receive a
transmission?
37
Centralised vs Distributed
•MAC schemes can be centralised or decentralised.
Centralised
Distributed
•Central controller:
 low latency
•Signalling channel
•general data application
•Connection based system
•ad-hoc networks
 Coordination
 Better network utilisation
•synchronisation
 Reliability
•relay: full connectivity
•no single point of failure
 Resource control:
 Increased complexity
•allocation of capacity
•coordination mechanisms
 Additional latency
•connectivity handshakes
 Single point of failure
•QoS ?
Recovery protocol possible
38
Wireless MAC methods
•ALOHA:
•Pure Aloha
•Slotted Aloha
•R-ALOHA – Portable Access Unit (PAU) controls
reservations
•CSMA/CA:
•Non-persistent, persistent and p-persistent
•CSMA/CD:
•Modification – collision detection comb
•TDMA, FDMA, CDMA
•DFWMAC
39
Pure ALOHA
•Packet radio:
• 1 packet per unit time
•Simple algorithm
•Transmit
•If collision, wait random time then re-transmit
•S: packet generation rate
•G: packet transmission rate (note that G includes re-transmission)
• poisson distribution for packet generation and
transmission S = G.P{0pkts in time 2}
40
ALOHA continues
• p{n packets in time t} = (Gt)n e-Gt /n!
t0-1
t0
t0+1
t0+2
time
2
P{0 pkts in time 2} = (2G)0 e-2G /0!
= e-2G
Since S = GP  S = Ge-2G
dS/dG = 0  e-2G – 2G e-2G
G = 0.5
Smax = 18.4% from graphs
41
Pure ALOHA: recap
The topology: All stations send frames to a central node, which
broadcasts the frames to all stations.
The protocol: 1. whenever a station has data, it transmits
2. Sender finds out whether transmission was
successful or not by listening to the broadcast from the central node
3. If collision occurs (partial, full), sender retransmits
after some random time.
Then Pure Aloha:
•very simple
•low utilisation
•light loads only
42
Pure ALOHA: recap
43
Slotted ALOHA
The slotted Aloha protocol:
•Aloha with an additional constraint
•Time is divided into discrete time intervals (slots)
•A station can only transmit at the beginning of the frame
Then slotted Aloha:
•transmission synchronized to start of time slots
•window of vulnerability: 1 time unit not 2
•36.8% utilisation  S = Ge-G
•requires timing mechanism
•no partial collisions
44
Reservation ALOHA
Slot user
R-Aloha:
Slot allocation
A
A
A
B
C
C
D
D
A
A
BD
B
•Simple
C
C
•Possibility of sending without
collisions
D
D
A
A
B
B
C
C
D
D
•slots arranged in frames
•TDM
channels: reservation
WHY Collision?
Unused
slots
Because B has not used and more than one other claimed the slot
•unused slots up for grabs
•80% efficiency
 R-Aloha:
collisions
Slots
reclaimed
•high latency
Now after the backoff
time
B has something to send and it can reclaim the reserved slot
45
Reservation ALOHA
46
CSMA
•Carrier Sense Multiple Access (CSMA):
•if channel is free, transmit
•persistent
S1
•At the receiver:
•checksum detects collision
•Non-persistent CSMA:
Tp
•random time back-off
TF
•increased delay
•CSMA/CA
•P-persistent CSMA:
•transmit with probability P
•increased delay (1 - p)
D
S2
bit1
bit1
time
Tp = D/V
47
CSMA/CD: comb
A
•Pseudo-random bit pattern
B
•Comb
•Station(s) to transmit:
A, B, C in
•First transmits comb
B, C, in A out
•For a 1, transmit
C in A, B out
•For a 0, listen
C
1000
1100
1110
C can transmit
•Stations in contention
•“drop out” as they listen during a 0
48
TDMA: Time Division Multiple Access
•Channel allocation:
• time-frame with fixed
number of time-slots
Time slot
• signalling time-slot
• source requests a time-slot
• PAU:
• listens on signalling time-slot (0)
for requests
• assigns channel to source
• source uses time-slot for a
single frame
• S-ALOHA with demand
assignment time-slot
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time
frame
time
49
FDMA: Frequency Division Multiple Access
• Channel allocation:
• fixed number of frequency
channels
• signalling channel
• source requests a channel
• PAU:
• listens on signalling
channel (0) for requests
• assigns channel to source
• source uses channel for a
single frame
Frequency channel numbers
0
1 2
3
4
5
Note: can use CSMA/CA or Aloha For signalling channel
50
CDMA: Code division Multiple access
• Frequency hopping:
0236 A’s code
• multiple frequency
3542 B’s code
channels
• part of message transmitted
N-bits
on each channel
• channel hopping sequence
is a code
N-bits
• each station has a different
code
N-bits
• Slow frequency-hopping:
• transmit N bits then hop
Frequency channel numbers
0
1 2
a
3
4
5 6
b
a
b
a b
b
N-bits
a
time
51
CDMA continues
•DS CDMA also possible:
• code is pseudorandom number (PN)
• controller allocates station allocates PN
• Rx and Tx use same PN for a transmission
 Good noise immunity
 Soft hand-off using two codes
Needs very good synchronisation:
• large overhead to synchronisation mechanism
 Complex to use than FDMA and TDMA
52
Hidden terminal and exposed terminal
• A  B: OK
• A  C: OK
• B  C: not OK
• If C transmits to A, B
could also transmit
A
B
• A  B: OK
• C  D: OK
• C can “overhear” B:
• C will not transmit
when B transmits
A
C
B is hidden to C
B
C
D
C is exposed to B
53
Near and far terminal

Signal drowning!


single strength decreases proportional to the
square of the distance
Consider terminals A, B send and C receive

the signal of terminal B therefore drowns out A’s
signal as a consequence

C cannot receive A
A
B
C
54
Multiple Access with Collision Avoidance

MACA (Multiple Access with Collision Avoidance) uses
short signaling packets for collision avoidance



Signaling packets contain




RTS (request to send): a sender request the right to send from a
receiver with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as
soon as it is ready to receive
sender address
receiver address
packet size
Variants of this method can be found in IEEE802.11 as
DFWMAC (Distributed Foundation Wireless MAC)
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Distribution Foundation Wireless MAC
• Source and destination in
contact?
• DFWMAC:
• four-way handshake
• src: RTS
• dst: RxBUSY or CTS
• src: DATA
• dst: ACK
• Used with any MAC
transmission method
• Also called RTS-CTS
PAU
RTS
PAU
CTS
data
ACK
Portable device
or PAU
RTS
Rx busy
PAU
Time-out
RTS
CTS
data
RTS: request to send
CTS: clear to send
ACK
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MACA variant: DFWMAC in IEEE802.11
sender
receiver
idle
idle
packet ready to send; RTS
RxBusy
ACK
time-out 
NAK;
RTS
wait for the
right to send
time-out;
RTS
data;
ACK
RTS;
CTS
time-out 
data;
NAK
CTS; data
wait for
data
wait for ACK
RTS; RxBusy
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
RTS: request to send
CTS: clear to send
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Can MACA avoid hidden/exposed trmnl?

MACA avoids the problem of hidden terminals



A and C want to
send to B
A sends RTS first
C waits after receiving
CTS from B
RTS
CTS
A

CTS
B
C
MACA avoids the problem of exposed terminals


B wants to send to A, C
to another terminal
now C does not have
to wait for it cannot
receive CTS from A
RTS
RTS
CTS
A
B
C
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Summary
• Centralised wireless MACs:
• TDMA
• FDMA
• CDMA
• S-ALOHA, R-ALOHA
• Distributed wireless MACs:
• ALOHA
• CSMA/CA
• CSMA/CD comb
• DFWMAC
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