E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
1
EE 586 Communication andSwitching Networks
Lecture 17
E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Ch. 6: Wireless and Mobile Networks
Background:
# wireless (mobile) phone subscribers now exceeds #wired phone subscribers (5-to-1)!
# wireless Internet-connected devices exceedswireline Internet-connected devices
laptops, Internet-enabled phones promise anytime untetheredInternet access
two important (but different) challenges
wireless: communication over wireless link
mobility: handling the mobile user who changes point ofattachment to network
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(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Chapter 6 outline
6.1 Introduction
Wireless
6.2 Wireless links,characteristics
CDMA
6.3 IEEE 802.11 wirelessLANs (Wi-Fi)
6.4 Cellular Internet Access
architecture
standards (e.g., GSM)
Mobility
6.5 Principles: addressing androuting to mobile users
6.6 Mobile IP
6.7 Handling mobility incellular networks
6.8 Mobility and higher-layerprotocols
6.9 Summary
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(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
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wireless hosts
laptop, smartphone
run applications
may be stationary (non-mobile) or mobile
wireless does notalways mean mobility
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Elements of a wireless network
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network
infrastructure
(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
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 base station
typically connected towired network
relay - responsible forsending packets betweenwired network andwireless host(s) in itsarea
e.g., cell towers,802.11 access points
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Elements of a wireless network
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network
infrastructure
(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
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 wireless link
typically used to connectmobile(s) to base station
also used as backbonelink
multiple access protocolcoordinates link access
various data rates,transmission distance
Elements of a wireless network
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network
infrastructure
(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
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 infrastructure mode
base station connectsmobiles into wirednetwork
handoff: mobile changesbase station providingconnection into wirednetwork
Elements of a wireless network
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network
infrastructure
(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
ad hoc mode
no base stations
nodes can onlytransmit to othernodes within linkcoverage
nodes organizethemselves into anetwork: routeamong themselves
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Elements of a wireless network
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(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Characteristics of selected wireless links
Indoor
10-30m
Outdoor
50-200m
Mid-range
outdoor
200m – 4 Km
Long-range
outdoor
5Km – 20 Km
.056
.384
1
4
5-11
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2G: IS-95, CDMA, GSM
2.5G: UMTS/WCDMA, CDMA2000
802.15
802.11b
802.11a,g
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
4G: LTWE WIMAX
802.11a,g point-to-point
200
802.11n
Data rate (Mbps)
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(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Wireless Link Characteristics
important differences from wired link ….
decreased signal strength: radio signal attenuates as itpropagates through matter (path loss)
interference from other sources: standardized wirelessnetwork frequencies (e.g., 2.4 GHz) shared by otherdevices (e.g., phone); devices (motors) interfere aswell
multipath propagation: radio signal reflects off objectsground, arriving ad destination at slightly differenttimes
…. make communication across (even a point to point)wireless link much more difficult
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(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
(modified by Cheung for EE586; based on K&R original)
Single User: Modulation Schemes
Modulation Schemes
BPSK = Binary Phase Shift Keying
QAM = Quadrature Amplitude Modulation
Map bits to sinusoids with different phases &amplitudes
Impact of wireless medium:
Path loss lowers the amplitude
Interference & Multipath corrupt the phase
200px-BPSK_Gray_Coded
BPSK
QAM16
Use more levels & phases
 Higher Bandwidth
 Smaller separation:
 Higher BER
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"1"=𝐴 cos (𝜔𝑡+ 0 𝑜 )  
"0"=𝐴 cos (𝜔𝑡+ 180 𝑜 )
200px-BPSK_Gray_Coded
Higher SNR
 Bigger separation
 Lower bit error rate
       (BER)
BPSK with higher power
E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Tradeoff among SNR, Speed and Bit Error Rate (BER)
SNR: signal-to-noise ratio
larger SNR – easier toextract signal from noise (agood thing)
SNR versus BER tradeoffs
given physical layer: increasepower -> increase SNR   ->decrease BER
given SNR: choose physical layerthat meets BER requirement,giving highest thruput
SNR may change withmobility: dynamically adaptphysical layer (modulationtechnique, rate)
10
20
30
40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
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(modified by Cheung for EE586; based on K&R original)
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
802.11 LAN architecture
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wireless host communicateswith base station
base station = access point(AP)
Basic Service Set (BSS) (akacell) in infrastructuremode contains:
wireless hosts
access point (AP): base station
ad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switch
or router
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11channels at different frequencies
AP admin chooses frequency for AP
interference possible: channel can be same as thatchosen by neighboring AP!
host: must associate with an AP
scans channels, listening for beacon frames containingAPs name (SSID) and MAC address
selects AP to associate with
may perform authentication [Chapter 8]
will typically run DHCP to get IP address in APssubnet
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
802.11: passive/active scanning
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AP 2
AP 1
H1
BBS 2
BBS 1
1
2
3
1
passive scanning:
(1)beacon frames sent from APs
(2)association Request frame sent:H1 to selected AP
(3)association Response frame sentfrom  selected AP to H1
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AP 2
AP 1
H1
BBS 2
BBS 1
1
2
2
3
4
active  scanning:
(1)Probe Request frame broadcast fromH1
(2)Probe Response frames sent from APs
(3)Association Request frame sent: H1 toselected AP
(4)Association Response frame sent fromselected AP to H1
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
frame
control
duration
address
1
address
2
address
4
address
3
payload
CRC
2
2
6
6
6
2
6
0 - 2312
4
seq
control
802.11 frame: addressing
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Address 2: MAC address
of wireless host or AP
transmitting this frame
Address 1: MAC address
of wireless host or AP
to receive this frame
Address 3: MAC address
of router interface towhich AP is attached
Address 4: used only inad hoc mode
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Internet
router
H1
R1
AP MAC addr  H1 MAC addr  R1 MAC addr
address 1
address 2
address 3
802.11 frame
R1 MAC addr  H1 MAC addr
dest. address
source address
802.3 frame
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802.11 frame: addressing
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
frame
control
duration
address
1
address
2
address
4
address
3
payload
CRC
2
2
6
6
6
2
6
0 - 2312
4
seq
control
Type
From
AP
Subtype
To
AP
More
frag
WEP
More
data
Power
mgt
Retry
Rsvd
Protocol
version
2
2
4
1
1
1
1
1
1
1
1
duration of reserved
transmission time (RTS/CTS)
frame seq #
(for RDT)
frame type
(RTS, CTS, ACK, data)
802.11 frame: more
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
Multiple Access
Channel access mechanism
Distributed Coordination Function (DCF)
Carrier sense multiple access (CSMA) with immediateMAC-level ACK
RTS/CTS(4-way handshaking) for collision avoidance
RTS stands for Request-to-Send
CTS stands for Clear-to-Send
Point Coordination Function (PCF)
Polled access through AP and distributed access
Contention-free period (CFP) and contention period (CP)
Seldom implemented in practice
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
802.11: mobility within same subnet
H1 remains in sameIP subnet: IP addresscan remain same
switch: which AP isassociated with H1?
self-learning (Ch. 5):switch will see framefrom H1 andremember whichswitch port can beused to reach H1
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H1
BBS 2
BBS 1
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
802.11: advanced capabilities
Rate adaptation
base station, mobiledynamically changetransmission rate(physical layer modulationtechnique) as mobilemoves, SNR varies
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QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
10
20
30
40
SNR(dB)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1. SNR decreases, BERincrease as node movesaway from base station
2. When BER becomestoo high, switch to lowertransmission rate but withlower BER
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
power management
node-to-AP: I am going to sleep until nextbeacon frame
AP knows not to transmit frames to this node
node wakes up before next beacon frame
beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to besent; otherwise sleep again until next beacon frame
802.11: advanced capabilities
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
M
radius of
coverage
S
S
S
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)
P
802.15: personal area network
less than 10 m diameter
replacement for cables (mouse,keyboard, headphones)
ad hoc: no infrastructure
master/slaves:
slaves request permission to send (tomaster)
master grants requests
802.15: evolved from Bluetoothspecification
802.15.1 – Bluetooth
802.15.2 – Coexistence
802.15.3 – High Rate WPAN (11-55 Mbps)
802.15.4 – Low Rate WPAN
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E l e c t r i c a l    &   C o m p u t e r
Department of
Electrical & Computer Engineering
IEEE 802.15.4 (Low Rate WPAN)
Important standard for home networking,industrial control and building automation
Three PHY modes
20 kbps at 868 MHz
40 kbps at 915 MHz
250 kbps at 2.4 GHz (DSSS)
Pure ad-hoc mode
Simple CSMA algorithm
Coordinator mode with superframe
Hybrid CSMA + CSMA/CD algorithm
Up to 64k nodes with 16-bit addresses
Extensions to the standard
IEEE 802.15.4a, 802.15.4e, 802.15.5
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802
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