6 1 Chapter 6 Wireless and Mobile Networks Computer Networking A Top Down Approach 4 th edition Jim Kurose Keith Ross AddisonWesley July 2007 Computer Networking A Top Down Approach ID: 151319
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6: Wireless and Mobile Networks
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Chapter 6Wireless and Mobile Networks
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
Computer Networking: A Top Down Approach
5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009. Slide2
6: Wireless and Mobile Networks
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Chapter 6: Wireless and Mobile Networks
Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challengeswireless: communication over wireless linkmobility: handling the mobile user who changes point of attachment to networkSlide3
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Chapter 6 outline6.1 Introduction
Wireless6.2 Wireless links, characteristics6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards (e.g., GSM)
Mobility6.5
Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 SummarySlide4
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Elements of a wireless network
network
infrastructure
wireless hosts
laptop, PDA, IP phone
run applications
may be stationary (non-mobile) or mobile
wireless does
not
always mean mobilitySlide5
6: Wireless and Mobile Networks
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Elements of a wireless network
network
infrastructure
base station
typically connected to wired network
relay - responsible for sending packets between wired network and wireless host(s) in its “area”
e.g., cell towers, 802.11 access points Slide6
6: Wireless and Mobile Networks
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Elements of a wireless network
network
infrastructure
wireless link
typically used to connect mobile(s) to base station
also used as backbone link
multiple access protocol coordinates link access
various data rates, transmission distanceSlide7
6: Wireless and Mobile Networks
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Characteristics of selected wireless link standards
Indoor
10-30m
Outdoor
50-200m
Mid-range
outdoor
200m – 4 Km
Long-range
outdoor
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 cellular
enhanced
802.16 (WiMAX)
802.11a,g point-to-point
200
802.11n
Data rate (Mbps)
dataSlide8
6: Wireless and Mobile Networks
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Elements of a wireless network
network
infrastructure
infrastructure mode
base station connects mobiles into wired network
handoff: mobile changes base station providing connection into wired networkSlide9
6: Wireless and Mobile Networks
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Elements of a wireless network
ad hoc mode
no base stations
nodes can only transmit to other nodes within link coverage
nodes organize themselves into a network: route among themselvesSlide10
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Wireless network taxonomy
single hopmultiple hopsinfrastructure(e.g., APs)
noinfrastructure
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
host may have to
relay through several
wireless nodes to
connect to larger
Internet:
mesh net
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET
Mobile Adhoc Networks
Vehicular Adhoc NetworksSlide11
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Wireless Communication Systems & NetworkingWhat complicates wireless networking vs. wired networking?Slide12
6: Wireless and Mobile Networks
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1- Channel characteristicsfor satellite we get extended propagation delayshigh bit error rate ‘BER’ (higher than optical fiber and coax.)
asymmetry in bandwidth and delayunidirectional linkseffects of wave propagation, attenuation,… etc.2- Mobility: continuous and introduces topology dynamics3- Power constraints in lots of the wireless devicesSlide13
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Wireless Link Characteristics (1)Differences from wired link ….
decreased signal strength: radio signal attenuates as it propagates through matter (path loss)interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as wellmultipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times…. make communication across (even a point to point) wireless link much more “difficult” Slide14
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Wireless Link Characteristics (2)SNR: signal-to-noise ratiolarger SNR – easier to extract signal from noise (a “good thing”)
SNR versus BER tradeoffsgiven physical layer: increase power -> increase SNR->decrease BERgiven SNR: choose physical layer that meets BER requirement, giving highest thruputSNR 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)
BER
10
-1
10
-2
10
-3
10
-5
10
-6
10
-7
10
-4
Quadrature Amplitude Modulation (QAM)
Binary Phase Shift Keying (BPSK)Slide15
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Wireless network characteristicsMultiple wireless senders and receivers create additional problems (beyond multiple access):
A
B
C
Hidden terminal problem
B, A hear each other
B, C hear each other
A, C can not hear each other
means A, C unaware of their interference at B
A
B
C
A’s signal
strength
space
C’s signal
strength
Signal attenuation:
B, A hear each other
B, C hear each other
A, C can not hear each other interfering at BSlide16
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Chapter 6 outline6.1 Introduction
Wireless6.2 Wireless links, characteristicsCDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 cellular Internet accessarchitecturestandards (e.g., GSM)
Mobility
6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 SummarySlide17
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IEEE 802.11 Wireless LAN802.11b2.4-5 GHz unlicensed spectrum
up to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer (CDMA: code division multiple access)all hosts use same chipping code802.11a 5-6 GHz rangeup to 54 Mbps802.11g 2.4-5 GHz rangeup to 54 Mbps802.11n: multiple antennae
2.4-5 GHz rangeup to 200 Mbps
all use CSMA/CA for multiple accessall have base-station and ad-hoc network versionsSlide18
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802.11 LAN architecture
wireless host communicates with base stationbase station = access point (AP)Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:wireless hosts
access point (AP): base stationad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switch
or router
AP
APSlide19
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802.11: Channels, association802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible: channel can be same as that chosen by neighboring AP!host: must associate with an APscans channels, listening for beacon frames containing AP’s name service set ID (SSID) and MAC addressselects AP to associate withmay perform authenticationwill typically run DHCP to get IP address in AP’s subnetSlide20
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802.11: passive/active scanning
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
2
3
4
Active Scanning
:
Probe Request frame broadcast from H1
Probes response frame sent from APs
Association Request frame sent: H1 to selected AP
Association Response frame sent: selected AP to H1
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
3
1
Passive Scanning:
beacon frames sent from APs
association Request frame sent: H1 to selected AP
association Response frame sent: selected AP to H1Slide21
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IEEE 802.11: multiple accessavoid collisions: 2+
nodes transmitting at same time802.11: CSMA - sense before transmittingdon’t collide with ongoing transmission by other node802.11: no collision detection!difficult to receive (sense collisions) when transmitting due to weak received signals (fading)can’t sense all collisions in any case: hidden terminal, fadinggoal: avoid collisions: CSMA/C(ollision)A(voidance)
A
B
C
A
B
C
A’s signal
strength
space
C’s signal
strengthSlide22
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IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender
1 if sense channel idle for DIFS then transmit entire frame (no CD)2 if sense channel busy then start random backoff time
timer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff interval, repeat 2
802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem)
sender
receiver
DIFS
data
SIFS
ACK
Distributed Inter-frame Spacing (DIFS)
Short Inter-frame Spacing (SIFS)Slide23
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Hidden Terminal Problem in WLANsSlide24
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Avoiding collisions: RTS/CTS
idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMARTSs may still collide with each other (but they’re short)BS broadcasts clear-to-send (CTS) in response to RTSRTS heard by all nodessender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets!Slide25
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Collision Avoidance: RTS-CTS exchange
AP
A
B
time
RTS(A)
RTS(B)
RTS(A)
CTS(A)
CTS(A)
DATA (A)
ACK(A)
ACK(A)
reservation collision
deferSlide26
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Check Animations on-line (applet & ns)Slide27
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frame
controldurationaddress1
address
2address4
address
3
payload
CRC
2
2
6
6
6
2
6
0 - 2312
4
seq
control
802.11 frame: addressing
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 to which AP is attached
Address 4:
used only in ad hoc modeSlide28
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Internet
router
AP
H1
R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.
11
frame
R1 MAC addr AP MAC addr
dest. address
source address
802.
3
frame
802.11 frame: addressingSlide29
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frame
controldurationaddress1
address
2address4
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
802.11 frame: more
duration of reserved
transmission time (RTS/CTS)
frame seq #
(for reliable ARQ)
frame type
(RTS, CTS, ACK, data)Slide30
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hub or
switch
AP 2
AP 1
H1
BBS 2
BBS 1
802.11: mobility within same subnet
router
H1 remains in same IP subnet: IP address can remain same
switch: which AP is associated with H1?
self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1Slide31
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802.11: advanced capabilities
Rate Adaptationbase 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
40
SNR(dB)
BER
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 station
2. When BER becomes too high, switch to lower transmission rate but with lower BER
Rate adaptation can change rate from
100Mbps to 1Mbps !!
Does this affect higher protocol layers?Slide32
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802.11: advanced capabilities
Power Managementnode-to-AP: “I am going to sleep until next beacon frame”AP knows not to transmit frames to this nodenode 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 be sent; otherwise sleep again until next beacon frame (typically after 100msec)Slide33
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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 (to master)
master grants requests
802.15: evolved from Bluetooth specification
2.4-2.5 GHz radio band
up to 721 kbpsSlide34
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802.16: WiMAX
like 802.11 & cellular: base station model
transmissions to/from base station by hosts with omnidirectional antenna
base station-to-base station backhaul with point-to-point antennaunlike 802.11:range ~ 6 miles (“city rather than coffee shop”)~14 Mbps
point-to-multipoint
point-to-pointSlide35
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802.16: WiMAX: downlink, uplink scheduling
transmission framedown-link subframe: base station to node uplink subframe: node to base station
pream.
DL-MAPUL-MAP
DL
burst 1
SS #1
DL
burst 2
DL
burst n
Initial
maint.
request
conn.
downlink subframe
SS #2
SS #k
uplink subframe
…
…
…
…
base station tells nodes who will get to receive (DL map)
and who will get to send (UL map), and when
WiMAX standard provide mechanism for scheduling, but not scheduling algorithmSlide36
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Chapter 6 outline6.1 Introduction
Wireless6.2 Wireless links, characteristicsCDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards (e.g., GSM)
Mobility
6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 SummarySlide37
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Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
Components of cellular network architecture
connects cells to wide area net
manages call setup (more later!)
handles mobility (more later!)
MSC
covers geographical region
base station
(BS) analogous to 802.11 AP
mobile users
attach to network through BS
air-interface:
physical and link layer protocol between mobile and BS
cell
wired networkSlide38
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Wireless Comm. SystemsIn general a wireless communication network consists of:
1- Users (mobile station)2- Base Station (BS): connects users to MSC3- Mobile Switching Center (MSC):connects the base stations with each other, and to the PSTN (public switched telephone network)Slide39
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Cellular Comm./Networking TerminologyHand-off: the process of transferring the mobile from one base station to anotherRoamer: a mobile operating in a coverage area other than the one in which it subscribed (moving to another MSC)Slide42
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Cellular Telephone SystemsA cellular system services a large number of users over extended geographical coverage with limited frequency spectrum.
High capacity is attained by limiting the coverage of the base station to a cell, so that the same frequency can be re-used in other cellsA problem may occur when moving from one cell to another while keeping the call un-interrupted. [the hand-off problem]Slide43
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Design concepts: The Cellular Concept and Frequency Re-useThe cellular concept was introduced to solve the problem of frequency limitation (or spectral congestion) and user capacity
Replace a single high power base station with several lower power base stations, each covering a smaller geographical area, a ‘cell’.Each of the base stations is allocated a number of channels (portion of the overall system channels)Slide45
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Neighboring base stations (would in general) use different frequency channels to reduce interference.(more later on interference, channel assignment and frequency planning)Slide46
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Frequency Re-useA cell uses a set of frequenciesA ‘cluster’ holds several cells
Frequency re-use factor: 1/#cells per clusterSlide47
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F
CB
D
EA
G
F
C
B
D
E
A
G
F
C
B
D
E
A
G
F
C
B
D
E
A
G
F
C
B
D
E
A
G
Cellular frequency re-use concept: cells with the same letter use the same set of frequencies.
A cluster of cells (highlighted in bold) is replicated over the coverage area. The cluster size,
N
, is equal to 7. Since each cell contains one-seventh of the overall channels, the cell
frequency re-use factor is 1/7.
Cell
Cluster
This requires channel/frequency planning and allocation!Slide48
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Multiple Access (MA) Techniques for Wireless CommunicationsMA schemes allow multiple mobile users to share a limited frequency spectrum.
Main MA schemes: FDMA, TDMA, SSMA (FHMA, CDMA [DSMA]), SDMASlide49
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FDMASlide50
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Frequency Division Multiple Access (FDMA) Assigns individual channels to individual users on demand
Only 1 user utilizes the channel at a time. Idle times are wasted. Capacity is not shared.Communication is continuousDoes not need synchronization Costly filters at the base stationNeed guard bands to alleviate interferenceSlide51
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TDMASlide52
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Time Division Multiple Access (TDMA)In a time slot only 1 user transmits (or receives)
Several users share a single frequency channel Transmission is non-continuousPower consumption is lower than FDMA (e.g., the transmitter can be turned off when idle)During idle time, a mobile performs MAHOSynchronization is neededSlide53
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Spread Spectrum Multiple Access (SSMA)Traditional communication techniques Strive to conserve bandwidth
By contrast, Spread spectrum techniquesuse bandwidth several orders of magnitude larger than the min. required bandwidth !!Slide54
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Spread Spectrum Multiple Access (SSMA)Spread spectrum techniques use bandwidth larger than the min. required bandwidth
Modulation:Uses pseudo-noise (PN) sequence to convert the signal into widebandThe PN is random, but can be re-produced by receiverDemodulation:
Correct correlation using a PN re-produces the signalUsing wrong PN sequence produces noise, hence this scheme is ‘secure’Slide55
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Spread Spectrum (SS) uses two techniques:(1) FHMA: frequency hopped MA(1) DSMA: direct sequence MA (also called CDMA: code division multiple access)
Frequency Hopped MA (FHMA)Frequencies of individual users are varied in a pseudo-random fashion within the wideband rangeThe signal is broken into bursts and each burst is sent on a different frequencySlide56
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CDMASlide57
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Code Division Multiple Access (CDMA)used in several wireless broadcast channels (cellular, satellite, etc) standards
unique “code” assigned to each user; i.e., code set partitioningall users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode dataencoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chipping sequenceallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)Slide58
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Speading the signal power over a wide spread of the frequency spectrum reduces fading effectsonly part of the spectrum, hence only part of the signal, is affected by fadingNo frequency planning required since users use the same frequency
Soft hand-off can be provided since all the cells use the same frequency. MSC monitors signals.In soft hand-off the channel (or frequency) remains the same and the base station changesSlide59
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Space Division MA (SDMA)Controls the radiated energy for each user in space using spot beam (directional) antennasSlide60
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Hybrid Multiple Access SystemsTime division frequency hopping (TDFH): (used in some versions of GSM)
User can hop to new frequency at the start of a new TDMA frameHence reducing interference and fading effectsUser hops over pre-defined frequenciesSlide61
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FDMA/CDMA:The available bandwidth is split into subspectra. In each subspectrum CDMA is usedAllows to assign subspectra on-demandSlide62
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FDMA/CDMASlide63
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Cellular networks: the first hopTechniques for sharing mobile-to-BS radio spectrum
combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots
frequency
bands
time slotsSlide64
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Cellular standards: brief survey2G systems: voice channelsIS-136 TDMA: combined FDMA/TDMA (north america)
GSM (global system for mobile communications): combined FDMA/TDMA most widely deployedIS-95 CDMA: code division multiple access
IS-136
GSM
IS-95
GPRS
EDGE
CDMA-2000
UMTS
TDMA/FDMA
Don’t drown in a bowl
of alphabet soup: use this
for reference only
Slide65
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Cellular standards: brief survey2.5 G systems: voice and data channels
for those who can’t wait for 3G service: 2G extensionsgeneral packet radio service (GPRS)evolved from GSM data sent on multiple channels (if available)enhanced data rates for global evolution (EDGE)also evolved from GSM, using enhanced modulation data rates up to 384KCDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95Slide66
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Cellular standards: brief survey3G systems:
voice/dataUniversal Mobile Telecommunications Service (UMTS)data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 MbpsCDMA-2000: CDMA in TDMA slotsdata service: 1xEvlution Data Optimized (1xEVDO) up to 14 Mbps Slide67
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Chapter 6 outline6.1 Introduction
Wireless6.2 Wireless links, characteristicsCDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards (e.g., GSM)
Mobility
6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 SummarySlide68
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What is mobility?spectrum of mobility, from the
network perspective:no mobility
high mobility
mobile wireless user, using same access pointmobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)
mobile user, connecting/ disconnecting from network using DHCP. Slide69
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Mobility: Vocabulary
home network:
permanent “home” of mobile
(e.g., 128.119.40/24)
Permanent address:
address in home network,
can always
be used to reach mobile
e.g., 128.119.40.186
home agent:
entity that will perform mobility functions on behalf of mobile, when mobile is remote
wide area network
correspondentSlide70
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Mobility: more vocabulary
Care-of-address:
address in visited network.
(e.g., 79,129.13.2)
wide area network
visited network:
network in which mobile currently resides
(e.g., 79.129.13/24)
Permanent address:
remains constant (
e.g., 128.119.40.186)
foreign agent:
entity in visited network that performs mobility functions on behalf of mobile.
correspondent:
wants to communicate with mobileSlide71
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How do you contact a mobile friend:search all phone books?
call her parents?expect her to let you know where he/she is?
I wonder where Alice moved to?
Consider friend frequently changing addresses, how do you find her?Slide72
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Mobility: approachesLet routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
routing tables indicate where each mobile locatedno changes to end-systemsLet end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing: correspondent gets foreign address of mobile, sends directly to mobileSlide73
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Mobility: approachesLet routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
routing tables indicate where each mobile locatedno changes to end-systemslet end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing: correspondent gets foreign address of mobile, sends directly to mobile
not
scalable to millions of mobilesSlide74
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Mobility: registration
End result:
Foreign agent knows about mobile
Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home: “this mobile is resident in my network”Slide75
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Mobility via Indirect Routing
wide area network
home
network
visited
network
3
2
4
1
correspondent addresses packets using home address of mobile
home agent intercepts packets, forwards to foreign agent
foreign agent receives packets, forwards to mobile
mobile replies directly to correspondentSlide76
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Indirect Routing: commentsMobile uses two addresses:
permanent address: used by correspondent (hence mobile location is transparent to correspondent)care-of-address: used by home agent to forward datagrams to mobileforeign agent functions may be done by mobile itselftriangle routing: correspondent-home-network-mobileinefficient when correspondent, mobile are in same networkSlide77
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Indirect Routing: moving between networkssuppose mobile user moves to another network
registers with new foreign agentnew foreign agent registers with home agenthome agent update care-of-address for mobilepackets continue to be forwarded to mobile (but with new care-of-address)mobility, changing foreign networks transparent: on going connections can be maintained!Slide78
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Mobility via Direct Routing
wide area network
home
network
visited
network
4
2
4
1
correspondent requests, receives foreign address of mobile
correspondent forwards to foreign agent
foreign agent receives packets, forwards to mobile
mobile replies directly to correspondent
3Slide79
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Mobility via Direct Routing: commentsovercome triangle routing problem
non-transparent to correspondent: correspondent must get care-of-address from home agentwhat if mobile changes visited network?Slide80
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wide area network
1
foreign net visited
at session start
anchor
foreign
agent
2
4
new foreign
agent
3
5
correspondent
agent
correspondent
new
foreign
network
Accommodating mobility with direct routing
anchor foreign agent: FA in first visited network
data always routed first to anchor FA
when mobile moves: new FA arranges to have data forwarded from old FA (chaining)Slide81
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Chapter 6 outline6.1 Introduction
Wireless6.2 Wireless links, characteristicsCDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Access
architecturestandards (e.g., GSM)
Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 SummarySlide82
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Mobile IPRFC 2002, RFC 3344.Goals:Attempts to provide support for host mobility while maintaining ‘transparency’:
the correspondent node need not know the location of the mobile nodethe connection already established should be maintained during movement even if the mobile node changes its network point of attachmentSlide83
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Mobile IPhas many features we’ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)three components to standard:
indirect routing of datagramsagent discoveryregistration with home agentSlide84
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Mobile IPEach mobile node has a home network, home address and home agent
Home Agent (HA)
Home Network
Mobile Node
Correspondent NodeSlide85
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Home Agent
Home Network
Correspondent Node
Foreign Agent (FA)
Foreign Network
Mobile Node
When mobile node (MN) moves to a foreign network it obtains a
care-of-address (COA) from the foreign agent (FA) that registers
it with the home agent (HA)
COA is used by HA to forward packets destined to MN
Solicitation
Advertisement (FA,COA)
Register (HA)
RegisterSlide86
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Mobile IP: registration exampleSlide87
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Mobile IP: indirect routing
Permanent address: 128.119.40.186
Care-of address: 79.129.13.2
dest: 128.119.40.186
packet sent by correspondent
dest: 79.129.13.2
dest: 128.119.40.186
packet sent by home agent to foreign agent: a
packet within a packet
dest: 128.119.40.186
foreign-agent-to-mobile packetSlide88
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Home Agent (HA)
Correspondent
Node (CN)
Mobile Node (MN)
Packets to MN are
picked up by the HA
and tunneled to MN
Packets sent by MN go
directly to CN
Triangle Routing in Mobile-IPSlide89
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Home Agent (HA)
Correspondent
Node (CN)
Mobile Node (MN)
Triangle Routing in Mobile-IP
C
A
B
Triangular routing can be very inefficient, especially when
C << B+A, where A (as shown) is the shortest path from
CN to MNSlide90
6: Wireless and Mobile Networks
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Drawbacks of Mobile IPOther than (the main problem) of triangular routingMobile IP incurs lots of communication with the home agent with every movement
so, may not be fit for ‘micro’ mobility [e.g., move between rooms or buildings within the same network domain]handoff delays are significant since registration/packets need to go through the home agent firstSlide91
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Suggested solutionsTo avoid triangular routinguse ‘route optimization’use micro-mobility architectures
Cellular IP (CIP)HawaiiMulticast-based Mobility (M&M)Slide92
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Home Agent (HA)
Correspondent
Node (CN)
Mobile Node (MN)
(2) Initial packets
to MN are sent
through HA to MN
(3) When MN gets packets from CN
it sends a
Binding Update
to CN with
its new address
Route Optimization (simple illustration)
(1) MN registers with HA as in
basic Mobile IP.
(4) CN changes the destination
address of the packets to go to
MN’s new addressSlide93
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With route optimizationTriangular routing is avoidedStill have problems with micro mobility and smooth hand-off Need additional mechanisms to deal with these issues, which makes the protocol complex.Slide94
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Micro-MobilityHierarchical approach to mobility:During frequent, intra-domain, movement only local efficient handoff is performed without notifying the home agent (HA) or the correspondent node (CN)For inter-domain mobility use Mobile IP. Notify HA or CN only during inter-domain movementSlide95
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Distribution tree dynamics while roaming
Domain Root
Wireless link
Mobile Node
FA or CNSlide96
6: Wireless and Mobile Networks
6-96
M&M: Join/Prune dynamics to modify distribution
Domain Root
Wireless link
Mobile NodeSlide97
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Components of cellular network architecture
correspondent
MSC
MSC
MSC
MSC
MSC
wired public telephone
network
different cellular networks,
operated by different providers
recall:Slide98
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Handling mobility in cellular networkshome network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)
home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)visited network: network in which mobile currently residesvisitor location register (VLR): database with entry for each user currently in networkcould be home networkSlide99
6: Wireless and Mobile Networks
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Public switched
telephone
network
mobile
user
home
Mobile
Switching
Center
HLR
home
network
visited
network
correspondent
Mobile
Switching
Center
VLR
GSM: indirect routing to mobile
1
call routed
to home network
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
3
home MSC sets up 2
nd
leg of call
to MSC in visited network
4
MSC in visited network completes
call through base station to mobileSlide100
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Mobile
Switching
Center
VLR
old BSS
new BSS
old
routing
new
routing
GSM: handoff with common MSC
Handoff goal: route call via new base station (without interruption)
reasons for handoff:
stronger signal to/from new BSS (continuing connectivity, less battery drain)
load balance: free up channel in current BSS
GSM doesn’t mandate why to perform handoff (policy), only how (mechanism)
handoff initiated by old BSSSlide101
6: Wireless and Mobile Networks
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Mobile
Switching
Center
VLR
old BSS
1
3
2
4
5
6
7
8
GSM: handoff with common MSC
new BSS
1. old BSS informs MSC of impending handoff, provides list of 1
+
new BSSs
2. MSC sets up path (allocates resources) to new BSS
3. new BSS allocates radio channel for use by mobile
4. new BSS signals MSC, old BSS: ready
5. old BSS tells mobile: perform handoff to new BSS
6. mobile, new BSS signal to activate new channel
7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call
8 MSC-old-BSS resources releasedSlide102
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home network
Home MSC
PSTN
correspondent
MSC
anchor MSC
MSC
MSC
(a) before handoff
GSM: handoff between MSCs
anchor MSC:
first MSC visited during call
call remains routed through anchor MSC
new MSCs add on to end of MSC chain as mobile moves to new MSC
IS-41 allows optional path minimization step to shorten multi-MSC chainSlide103
6: Wireless and Mobile Networks
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home network
Home MSC
PSTN
correspondent
MSC
anchor MSC
MSC
MSC
(b) after handoff
GSM: handoff between MSCs
anchor MSC:
first MSC visited during call
call remains routed through anchor MSC
new MSCs add on to end of MSC chain as mobile moves to new MSC
IS-41 allows optional path minimization step to shorten multi-MSC chainSlide104
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Mobility: GSM versus Mobile IP
GSM element
Comment on GSM element
Mobile IP element
Home system
Network to which mobile user’s permanent phone number belongs
Home network
Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR)
Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information
Home agent
Visited System
Network other than home system where mobile user is currently residing
Visited network
Visited Mobile services Switching Center.
Visitor Location Record (VLR)
Visited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user
Foreign agent
Mobile Station Roaming Number (MSRN), or “roaming number”
Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent.
Care-of-addressSlide105
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Wireless, mobility: impact on higher layer protocolslogically, impact should
be minimal …best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile… but performance-wise:packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoffTCP interprets loss as congestion, will decrease congestion window un-necessarilydelay impairments for real-time trafficlimited bandwidth of wireless linksSlide106
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Chapter 6 SummaryWireless
wireless links:capacity, distancechannel impairmentsCDMAIEEE 802.11 (“wi-fi”)CSMA/CA reflects wireless channel characteristicscellular accessarchitecturestandards (e.g., GSM, CDMA-2000, UMTS)
Mobilityprinciples: addressing, routing to mobile usershome, visited networks
direct, indirect routingcare-of-addressescase studiesmobile IPmobility in GSMimpact on higher-layer protocolsSlide107
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Code Division Multiple Access (CDMA)used in several wireless broadcast channels (cellular, satellite, etc) standards
unique “code” assigned to each user; i.e., code set partitioningall users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode dataencoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chipping sequenceallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)Slide108
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CDMA Encode/Decode
slot 1
slot 0
d1 = -1
1
1
1
1
1
-
1
-
1
-
1
-
Z
i,m
= d
i
.
c
m
d
0
= 1
1
1
1
1
1
-
1
-
1
-
1
-
1
1
1
1
1
-
1
-
1
-
1
-
1
1
1
1
1
-
1
-
1
-
1
-
slot 0
channel
output
slot 1
channel
output
channel output Z
i,m
sender
code
data
bits
slot 1
slot 0
d
1
= -1
d
0
= 1
1
1
1
1
1
-
1
-
1
-
1
-
1
1
1
1
1
-
1
-
1
-
1
-
1
1
1
1
1
-
1
-
1
-
1
-
1
1
1
1
1
-
1
-
1
-
1
-
slot 0
channel
output
slot 1
channel
output
receiver
code
received
input
D
i
=
S
Z
i,m
.
c
m
m=1
M
MSlide109
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CDMA: two-sender interferenceSlide110
6: Wireless and Mobile Networks
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Direct Sequence Spread SpectrumOriginal signal is m(
t)The spreading signal is p(t) [the PN sequence]The spread spectrum signal is Sss(t)A single pulse or symbol of the PN waveform is called a chipSlide111
6: Wireless and Mobile Networks
6-111
S
ss(t) ~ m(t)p(t)cos(2
fct+
)B: is the bandwidth of m(t)cos(2f
c
t+
)
W
ss
: is the bandwidth of
S
ss
(
t
)
W
ss
>>
B
Chip Clock
PN Code
Generator
Oscillator
f
c
S
ss
(
t
)
Transmitted Signal
Data
m(t)
Phase modulation
Block diagram of a DS-SS system with binary phase modulation
Transmitter
p(t)Slide112
6: Wireless and Mobile Networks
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Channel
encoder
(A)
(B)
(C)
f
(B,C)
Symbol duration for
m(t)
:
Ts
Chip duration for
p(t)
:
Tc
Processing Gain
PG=W
ss
/B=Ts/Tc
, a measure of interference rejection capability
Symbol
ChipSlide113
6: Wireless and Mobile Networks
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Bit stream
(A)
Encoded
stream
(B)
Pseudo-noise
sequence
(C)
m
(
t
)
p
(
t
)
Tc
TsSlide114
6: Wireless and Mobile Networks
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Example:f(B,C)=BC, where1 1= 0
1 0 = 10 0 = 0if we have received f(B,C) and we are able to re-generate the PN (C), then we can get B.