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Chapter 7 outline7.1 Introduction
Mobility 7 5 Principles:Wireless
7.2 Wireless links, characteristics
7.5 Principles: addressing and routing to mobile users7 6 M bil IPcharacteristics
CDMA 7.3 IEEE 802.11
wireless LANs (“wi fi”)
7.6 Mobile IP 7.7 Handling mobility in
cellular networkswireless LANs ( wi-fi ) 7.4 Cellular Internet
Access 7.8 Mobility and higher-
layer protocols architecture standards (e.g., GSM) 7.9 Summary
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Characteristics of selected i l ss li k st d dswireless link standards450 802.11n
802.11ac
5-11
54
802.11b
802.11a, g4G: LTE
802.11a, g point-to-point
bps)
1
4
802.15
Enhanced 3G: HSPA
a ra
te (M
b
056
.384
2G: GSM CDMA
3G: UMTS/WCDMA, CDMA2000
Dat
a
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
Long-rangeoutdoor
.056 2G: GSM, CDMA
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200m – 4 Km 5Km – 20 Km
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Elements of a wireless network
wireless hosts laptops, phones, sensors run applications may be stationary or
b lnetwork
infrastructure
mobile wireless does not always
mean mobility
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Elements of a wireless networkf d
base station typically connected to
Infrastructure mode
yp ywired network
forwards packets between wired
t k d i lnetwork
infrastructure
network and wireless hosts in its “area” e.g., 802.11 access
point (AP)point (AP)
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802.11: mobility within same subnet it h hi h AP i
switch
router switch: which AP is associated with H1? self-learning (Ch. 6) - switchg ( )
switch will see frame from H1 and “remember” which switch port can be
d h H1
AP 2
AP 1
used to reach H1 H1 moves but remains
in same IP subnet - IP AP 2
H1
in same IP subnet IP address can remain the same (See subsection 7.3.4, page 546 of textbook)page 546 of textbook)
Suppose the two APs are connected directly to a router instead of to a switch. Mobility ?
Not covered in this class:
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Not covered in this class: mobile IP (section 7.6) and mobility in cellular networks (section 7.7)
6
Elements of a wireless network
ad hoc mode no base stations no base stations nodes can only transmit
to other nodes within link coveragelink coverage
nodes organize themselves into a network: need protocol pto route packets among themselves
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Wireless network taxonomy
single hop multiple hops
lots of research on routing protocols
single hop mult ple hops
infrastructuremode (e g APs)
hosts connect to base station (WiFi,
ll l WiMAX)
wireless nodes serve as relays towards
Internet e gmode (e.g., APs) cellular, WiMAX) which connects to
Internet
Internet, e.g.,community mesh net
no base station, no
noinfrastructure
no base station, noconnection to Internet
(Bluetooth 802 11
,connection to Internet;
wireless nodes serve as relays to
(Bluetooth, 802.11ad hoc mode)
reach some nodes:MANET, VANET
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Wireless Link Characteristics (1)W L ( )Differences from wired link ….
decreased signal strength: radio signal attenuates more as it propagates (path loss)
int f n f m th s u s: i l ss n t k interference from other sources: wireless network frequencies (e.g., 2.4 GHz) shared by cordless phones, sensors, etc.; devices (motors) interfere as well
multipath propagation: radio signal reflects off objects multipath propagation: radio signal reflects off objects, reaching destination at slightly different times
…. make communication across a wireless link more “difficult” – may need link layer protocol to provide reliable delivery
Wireless and Mobile Networks (SSL) 7-8
y
12/5/2017
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Wireless Link Characteristics (2) SNR (signal-to-noise ratio) tradeoffs
for a given modulation technique:increase power or reduce noise ->
10-1
10-2
increase power or reduce noise increase SNR
-> decrease BER for a fixed bit error rate (BER): R
10-3
10-4
for a fixed bit error rate (BER): larger SNR –> higher bit rate for the same bandwidth
for a given SNR: choose
BER 10-5
10-6
for a given SNR: choose modulation technique that meets BER requirement to get highest bit rate QAM256 (8 Mbps)
10 20 30 40SNR(dB)
10-7
SNR may change with mobility dynamically adapt modulation
technique (rate)
QAM16 (4 Mbps)
BPSK (1 Mbps)
Wireless and Mobile Networks (SSL) 7-9
technique (rate) 12/5/2017
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Wireless network characteristics Multiple wireless senders and receivers create
additional problems (carrier sensing is unreliable):
CA B C
A’ i l C’s signal
AB
dd l bl
A’s signalstrength
C s signalstrength
Hidden terminal problem B, A hear each other B, C hear each other
space
Signal attenuation (fading): B, A hear each other
A, C cannot hear each other-> A, C unaware of interference at receiver B
B, C hear each other A, C cannot hear each other,
unaware of interference at B
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IEEE 802.11: multiple accessb f CSMA - sense before transmitting
can’t sense all transmissions: hidden terminal, fading llisi d t ti is diffi lt l collision detection is difficult – strong transmit signal,
weak receive signal
New goal: avoid collisions hence CSMA/CANew goal: avoid collisions, hence CSMA/CA
CA B C
B
C
A’s signalstrength
C’s signalstrength
A
space
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IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if channel sensed idle for DIFS then
ftransmit entire frame2 if channel sensed busy then
choose random backoff time
sender receiver
DIFS whenever channel sensed idle, timer
counts down transmit when timer reaches zero
data
if no ACK, increase random backoff interval, repeat 2
802.11 receiverSIFS
ACK- if frame received OKreturn ACK after SIFS (no collision detection due to hidden terminal and
f di bl )
ACK
Wireless and Mobile Networks (SSL) 7-12
fading problems)
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Collision Avoidance - idea senders transmit small request-to-send (RTS)
packets to AP using CSMA RTS packets may collide with each other (but
they’re short) AP b d t l t d (CTS) i t AP broadcasts clear-to-send (CTS) in response to
received RTS CTS heard by all nodesS heard by all nodes
the sender selected then transmits data frame other stations defer transmissions
Try to avoid long data frame collision by using small reservation packets
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RTS-CTS exchange (use is optional)
APA B
reservation collision
DATA (A)defer
time
defer
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CS 356 Fall 2017 - The end
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