Post on 03-May-2018
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6: Wireless and Mobile Networks 6-1
Chapter 6Wireless and Mobile Networks
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All material copyright 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
6: Wireless and Mobile Networks 6-2
Chapter 6: Wireless and Mobile Networks
Background:r # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers!r computer nets: laptops, palmtops, PDAs,
Internet-enabled phone promise anytime untethered Internet access
r two important (but different) challengesm wireless: communication over wireless linkm mobility: handling the mobile user who changes point
of attachment to network
2
6: Wireless and Mobile Networks 6-3
Chapter 6 outline
6.1 Introduction
Wirelessr 6.2 Wireless links,
characteristicsm CDMA
r 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
r 6.4 Cellular Internet Accessm architecturem standards (e.g., GSM)
Mobilityr 6.5 Principles:
addressing and routing to mobile users
r 6.6 Mobile IPr 6.7 Handling mobility in
cellular networksr 6.8 Mobility and higher-
layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-4
Elements of a wireless network
network infrastructure
wireless hostsr laptop, PDA, IP phoner run applicationsr may be stationary
(non-mobile) or mobilem wireless does not
always mean mobility
3
6: Wireless and Mobile Networks 6-5
Elements of a wireless network
network infrastructure
base stationr typically connected to
wired networkr relay - responsible
for sending packets between wired network and wireless host(s) in its “area”m e.g., cell towers,
802.11 access points
6: Wireless and Mobile Networks 6-6
Elements of a wireless network
network infrastructure
wireless linkr typically used to
connect mobile(s) to base station
r also used as backbone link
r multiple access protocol coordinates link access
r various data rates, transmission distance
4
6: Wireless and Mobile Networks 6-7
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m – 4 Km
Long-rangeoutdoor
5Km – 20 Km
.056
.384
1
4
5-11
54
IS-95, CDMA, GSM 2G
UMTS/WCDMA, CDMA2000 3G
802.15
802.11b
802.11a,g
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellularenhanced
802.16 (WiMAX)
802.11a,g point-to-point
200 802.11n
Dat
a ra
te (M
bps)
data
6: Wireless and Mobile Networks 6-8
Elements of a wireless network
network infrastructure
infrastructure moder base station connects
mobiles into wired network
r handoff: mobile changes base station providing connection into wired network
5
6: Wireless and Mobile Networks 6-9
Elements of a wireless networkad hoc moder no base stationsr nodes can only
transmit to other nodes within link coverage
r nodes organize themselves into a network: route among themselves
6: Wireless and Mobile Networks 6-10
Wireless network taxonomy
single hop multiple hops
infrastructure(e.g., APs)
noinfrastructure
host connects to base station (WiFi,WiMAX, cellular) which connects to
larger Internet
no base station, noconnection to larger Internet (Bluetooth,
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet: mesh net
no base station, noconnection to larger
Internet. May have torelay to reach other a given wireless node
MANET, VANET
6
6: Wireless and Mobile Networks 6-11
Chapter 6 outline
6.1 Introduction
Wirelessr 6.2 Wireless links,
characteristicsm CDMA
r 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
r 6.4 Cellular Internet Accessm architecturem standards (e.g., GSM)
Mobilityr 6.5 Principles:
addressing and routing to mobile users
r 6.6 Mobile IPr 6.7 Handling mobility in
cellular networksr 6.8 Mobility and higher-
layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-12
Wireless Link Characteristics (1)Differences from wired link ….
m decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
m interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
m multipath propagation: radio signal reflects off objects and the ground, arriving at destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”
7
6: Wireless and Mobile Networks 6-13
Wireless Link Characteristics (2)r SNR: signal-to-noise ratio
m larger SNR – easier to extract signal from noise (a “good thing”)
r SNR versus BER tradeoffsm given physical layer:
increase power -> increase SNR->decrease BER
m given SNR: choose physical layer that meets BER requirement, giving highest thruput
• SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
6: Wireless and Mobile Networks 6-14
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access):
AB
C
Hidden terminal problemr B, A hear each otherr B, C hear each otherr A, C can not hear each othermeans A, C unaware of their
interference at B
A B C
A’s signalstrength
space
C’s signalstrength
Signal attenuation:r B, A hear each otherr B, C hear each otherr A, C can not hear each other
interfering at B
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6: Wireless and Mobile Networks 6-15
Code Division Multiple Access (CDMA)
r used in several wireless broadcast channels (cellular, satellite, etc) standards
r unique “code” assigned to each user; i.e., code set partitioning
r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data
r encoded signal = (original data) X (chipping sequence)
r decoding: inner-product of encoded signal and chipping sequence
r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
6: Wireless and Mobile Networks 6-16
Chapter 6 outline
6.1 Introduction
Wirelessr 6.2 Wireless links,
characteristicsm CDMA
r 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
r 6.4 cellular Internet accessm architecturem standards (e.g., GSM)
Mobilityr 6.5 Principles:
addressing and routing to mobile users
r 6.6 Mobile IPr 6.7 Handling mobility in
cellular networksr 6.8 Mobility and higher-
layer protocols
6.9 Summary
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6: Wireless and Mobile Networks 6-17
IEEE 802.11 Wireless LANr 802.11b
m 2.4-5 GHz unlicensed spectrumm up to 11 Mbpsm direct sequence spread
spectrum (DSSS) in physical layer
• all hosts use same chipping code
r 802.11am 5-6 GHz rangem up to 54 Mbps
r 802.11gm 2.4-5 GHz rangem up to 54 Mbps
r 802.11n: multiple antennaem 2.4-5 GHz rangem up to 200 Mbps
r all use CSMA/CA for multiple accessr all have base-station and ad-hoc network versions
6: Wireless and Mobile Networks 6-18
802.11 LAN architecture
r wireless host communicates with base stationm base station = access
point (AP)r Basic Service Set (BSS)
(aka “cell”) in infrastructure mode contains:m wireless hostsm access point (AP): base
stationm ad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switchor routerAP
AP
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6: Wireless and Mobile Networks 6-19
802.11: Channels, association
r 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesm AP admin chooses frequency for APm interference possible: channel can be same as
that chosen by neighboring AP!r host: must associate with an APm scans channels, listening for beacon frames
containing AP’s name (SSID) and MAC addressm selects AP to associate withm may perform authentication [Chapter 8]m will typically run DHCP to get IP address in AP’s
subnet
6: Wireless and Mobile Networks 6-20
802.11: passive/active scanning
AP 2AP 1
H1
BBS 2BBS 1
1223 4
Active Scanning: (1) Probe Request frame broadcast
from H1(2) Probes response frame sent from
APs(3) Association Request frame sent:
H1 to selected AP (4) Association Response frame
sent: H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning:(1) beacon frames sent from APs(2) association Request frame sent:
H1 to selected AP (3) association Response frame sent:
H1 to selected AP
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6: Wireless and Mobile Networks 6-21
IEEE 802.11: multiple accessr avoid collisions: 2+ nodes transmitting at same timer 802.11: CSMA - sense before transmitting
m don’t collide with ongoing transmission by other noder 802.11: no collision detection!
m difficult to receive (sense collisions) when transmitting due to weak received signals (fading)
m can’t sense all collisions in any case: hidden terminal, fadingm goal: avoid collisions: CSMA/C(ollision)A(voidance)
AB
CA B C
A’s signalstrength
space
C’s signalstrength
6: Wireless and Mobile Networks 6-22
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff
interval, repeat 2802.11 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
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6: Wireless and Mobile Networks 6-23
Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames
r sender first transmits small request-to-send (RTS) packets to BS using CSMAm RTSs may still collide with each other (but they’re short)
r BS broadcasts clear-to-send CTS in response to RTSr CTS heard by all nodes
m sender transmits data framem other stations defer transmissions
avoid data frame collisions completely using small reservation packets!
6: Wireless and Mobile Networks 6-24
Collision Avoidance: RTS-CTS exchange
APA B
time
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
RTS(A)
RTS(B)
reservation collision
RTS(A)
defer
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6: Wireless and Mobile Networks 6-25
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
802.11 frame: addressing
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 4: used only in ad hoc mode
6: Wireless and Mobile Networks 6-26
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
802.11 frame
R1 MAC addr H1 MAC addr dest. address source address
802.3 frame
802.11 frame: addressing
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6: Wireless and Mobile Networks 6-27
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
TypeFromAP
SubtypeToAP
More frag
WEPMoredata
Powermgt
Retry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
802.11 frame: moreduration of reserved transmission time (RTS/CTS)
frame seq #(for reliable ARQ)
frame type(RTS, CTS, ACK, data)
6: Wireless and Mobile Networks 6-28
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
802.11: mobility within same subnet
routerr H1 remains in same IP subnet: IP address can remain same
r switch: which AP is associated with H1?m self-learning (Ch. 5):
switch will see frame from H1 and “remember” which switch port can be used to reach H1
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6: Wireless and Mobile Networks 6-29
802.11: advanced capabilities
Rate Adaptationr base station, mobile
dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1. SNR decreases, BER increase as node moves away from base station2. When BER becomes too high, switch to lower transmission rate but with lower BER
6: Wireless and Mobile Networks 6-30
802.11: advanced capabilitiesPower Managementr node-to-AP: “I am going to sleep until next
beacon frame”mAP knows not to transmit frames to this
nodemnode wakes up before next beacon frame
r beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sentmnode will stay awake if AP-to-mobile frames
to be sent; otherwise sleep again until next beacon frame
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6: Wireless and Mobile Networks 6-31
Mradius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
802.15: personal area network
r less than 10 m diameterr replacement for cables
(mouse, keyboard, headphones)
r ad hoc: no infrastructurer master/slaves:
m slaves request permission to send (to master)
m master grants requestsr 802.15: evolved from
Bluetooth specificationm 2.4-2.5 GHz radio bandm up to 721 kbps
6: Wireless and Mobile Networks 6-32
802.16: WiMAXr like 802.11 & cellular:
base station modelm transmissions to/from
base station by hosts with omnidirectionalantenna
m base station-to-base station backhaul with point-to-point antenna
r unlike 802.11:m range ~ 6 miles (“city
rather than coffee shop”)
m ~14 Mbps
point-to-multipoint
point-to-point