Chapter 6
Wireless and Mobile
Networks
Computer Networking:
A Top Down Approach
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.
6: Wireless and Mobile Networks
6-1
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 access
 two important (but different) challenges


wireless: communication over wireless link
mobility: handling the mobile user who changes point
of attachment to network
6: Wireless and Mobile Networks
6-2
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-3
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 mobility
6: Wireless and Mobile Networks
6-4
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
6: Wireless and Mobile Networks
6-5
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 distance
6: Wireless and Mobile Networks
6-6
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX)
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
.384
UMTS/WCDMA, CDMA2000
.056
3G
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
6: Wireless and Mobile Networks
6-7
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 network
6: Wireless and Mobile Networks
6-8
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
themselves
6: Wireless and Mobile Networks
6-9
Wireless network taxonomy
single hop
infrastructure
(e.g., APs)
no
infrastructure
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)
Mobile Adhoc Networks
multiple hops
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
Vehicular Adhoc Networks
6: Wireless and Mobile Networks
6-10
Wireless Communication Systems & Networking
- What complicates wireless networking vs.
wired networking?
6: Wireless and Mobile Networks
6-11
- 1- Channel characteristics
-
for satellite we get extended propagation delays
high bit error rate ‘BER’ (higher than optical fiber and
coax.)
asymmetry in bandwidth and delay
unidirectional links
effects of wave propagation, attenuation,… etc.
- 2- Mobility: continuous and introduces topology
dynamics
- 3- Power constraints in lots of the wireless
devices
6: Wireless and Mobile Networks
6-12
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 well
 multipath 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”
6: Wireless and Mobile Networks
6-13
Wireless Link Characteristics (2)
10-1
 SNR: signal-to-noise ratio
larger SNR – easier to
extract signal from noise (a
“good thing”)
 SNR versus BER tradeoffs
 given physical layer:
increase power -> increase
SNR->decrease BER
 given SNR: choose physical
layer that meets BER
requirement, giving highest
thruput
10-2

• SNR may change with
mobility: dynamically adapt
physical layer (modulation
technique, rate)
BER
10-3
10-4
10-5
10-6
10-7
10
20
30
40
SNR(dB)
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
Quadrature Amplitude Modulation (QAM)
Binary Phase Shift Keying (BPSK)
6: Wireless and Mobile Networks
6-14
Wireless network characteristics
Multiple wireless senders and receivers create
additional problems (beyond multiple access):
C
A
B
A
B
Hidden terminal problem
C
C’s signal
strength
A’s signal
strength
space
 B, A hear each other
Signal attenuation:
 A, C can not hear each other
 B, C hear each other
 B, C hear each other
 B, A hear each other
means A, C unaware of their
interference at B
 A, C can not hear each other
interfering at B
6: Wireless and Mobile Networks
6-15
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 cellular Internet
access


architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-16
IEEE 802.11 Wireless LAN
 802.11a
 802.11b
 5-6 GHz range
 2.4-5 GHz unlicensed spectrum
 up to 54 Mbps
 up to 11 Mbps
 802.11g
 direct sequence spread
spectrum (DSSS) in physical
 2.4-5 GHz range
layer (CDMA: code division
 up to 54 Mbps
multiple access)
 802.11n: multiple antennae
• all hosts use same chipping
 2.4-5 GHz range
code
 up to 200 Mbps
 all use CSMA/CA for multiple access
 all have base-station and ad-hoc network versions
6: Wireless and Mobile Networks
6-17
802.11 LAN architecture
 wireless host communicates
Internet
AP
hub, switch
or router
BSS 1
AP
BSS 2
with base station
 base station = access
point (AP)
 Basic Service Set (BSS)
(aka “cell”) in infrastructure
mode contains:
 wireless hosts
 access point (AP): base
station
 ad hoc mode: hosts only
6: Wireless and Mobile Networks
6-18
802.11: Channels, association
 802.11b: 2.4GHz-2.485GHz spectrum divided into 11
channels at different frequencies
 AP admin chooses frequency for AP
 interference possible: channel can be same as that
chosen by neighboring AP!
 host: must associate with an AP
 scans channels, listening for beacon frames containing
AP’s name service set ID (SSID) and MAC address
 selects AP to associate with
 may perform authentication
 will typically run DHCP to get IP address in AP’s subnet
6: Wireless and Mobile Networks
6-19
802.11: passive/active scanning
BBS 1
AP 1
BBS 2
1
1
2
AP 2
BBS 1
BBS 2
AP 1
AP 2
1
2
3
2
3
4
H1
H1
Passive Scanning:
Active Scanning:
(1) beacon frames sent from APs
(2) association Request frame sent:
H1 to selected AP
(3) association Response frame sent:
selected AP to H1
(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: selected AP to H1
6: Wireless and Mobile Networks
6-20
IEEE 802.11: multiple access
 avoid collisions: 2+ nodes transmitting at same time
 802.11: CSMA - sense before transmitting
 don’t collide with ongoing transmission by other node
 802.11: 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, fading
 goal: avoid collisions: CSMA/C(ollision)A(voidance)
C
A
B
A
B
C
C’s signal
strength
A’s signal
strength
space
6: Wireless and Mobile Networks
6-21
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 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 idle
transmit when timer expires
if 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)
6: Wireless and Mobile Networks
6-22
Hidden Terminal Problem in WLANs
6: Wireless and Mobile Networks
6-23
Avoiding collisions: RTS/CTS
idea: allow sender to “reserve” channel rather than random
access of data frames: avoid collisions of long data frames
 sender first transmits small request-to-send (RTS) packets
to BS using CSMA
 RTSs may still collide with each other (but they’re short)
 BS broadcasts clear-to-send (CTS) in response to RTS
 RTS heard by all nodes
 sender transmits data frame
 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
A
B
AP
reservation collision
DATA (A)
defer
time
6: Wireless and Mobile Networks
6-25
Check Animations on-line (applet & ns)
6: Wireless and Mobile Networks
6-26
802.11 frame: addressing
2
2
6
6
6
frame
address address address
duration
control
1
2
3
Address 1: MAC address
of wireless host or AP
to receive this frame
2
6
seq address
4
control
0 - 2312
4
payload
CRC
Address 4: used only
in ad hoc mode
Address 3: MAC address
of router interface to
which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame
6: Wireless and Mobile Networks
6-27
802.11 frame: addressing
R1 router
H1
Internet
AP
R1 MAC addr AP MAC addr
dest. address
source address
802.3 frame
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.11 frame
6: Wireless and Mobile Networks
6-28
802.11 frame: more
frame seq #
(for reliable ARQ)
duration of reserved
transmission time (RTS/CTS)
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
Protocol
version
2
4
1
Type
Subtype
To
AP
6
2
1
seq address
4
control
1
From More
AP
frag
1
Retry
1
0 - 2312
4
payload
CRC
1
Power More
mgt
data
1
1
WEP
Rsvd
frame type
(RTS, CTS, ACK, data)
6: Wireless and Mobile Networks
6-29
802.11: mobility within same subnet
 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 H1
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
6: Wireless and Mobile Networks
6-30
802.11: advanced capabilities
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
operating point
Rate adaptation can change rate from
100Mbps to 1Mbps !!
Does this affect higher protocol layers?
10-1
10-2
10-3
BER
Rate Adaptation
 base station, mobile
dynamically change
transmission rate
(physical layer
modulation technique)
as mobile moves, SNR
varies
10-4
10-5
10-6
10-7
10
20
30
SNR(dB)
40
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
6: Wireless and Mobile Networks
6-31
802.11: advanced capabilities
Power Management
 node-to-AP: “I am going to sleep until next
beacon frame”
 AP knows not to transmit frames to this
node
 node wakes up before next beacon frame
 beacon frame: contains list of mobiles with APto-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)
6: Wireless and Mobile Networks
6-32
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 kbps
P
S
P
radius of
coverage
M
S
P
S
P
M Master device
S Slave device
P Parked device (inactive)
6: Wireless and Mobile Networks
6-33
802.16: WiMAX
 like 802.11 & cellular:
point-to-point
base station model
transmissions to/from
base station by hosts
with omnidirectional
antenna
 base station-to-base
station backhaul with
point-to-point antenna

point-to-multipoint
 unlike 802.11:
 range ~ 6 miles (“city
rather than coffee
shop”)
 ~14 Mbps
6: Wireless and Mobile Networks
6-34
802.16: WiMAX: downlink, uplink scheduling
 transmission frame
down-link subframe: base station to node
 uplink subframe: node to base station

pream.
…
DL- ULMAP MAP
DL
burst 1
DL
burst 2
downlink subframe
…
…
DL
burst n
Initial request
SS #1 SS #2
maint. conn.
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 algorithm
6: Wireless and Mobile Networks
6-35
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-36
Components of cellular network architecture
MSC
cell
 connects cells to wide area net
 manages call setup (more later!)
 handles mobility (more later!)
 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
Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network
6: Wireless and Mobile Networks
6-37
Wireless Comm. Systems
- In general a wireless communication network
consists of:
- 1- Users (mobile station)
- 2- Base Station (BS): connects users to MSC
- 3- Mobile Switching Center (MSC):
-
connects the base stations with each other, and
to the PSTN (public switched telephone network)
6: Wireless and Mobile Networks
6-38
6: Wireless and Mobile Networks
6-39
6: Wireless and Mobile Networks
6-40
Cellular Comm./Networking
Terminology
- Hand-off: the process of transferring the
mobile from one base station to another
- Roamer: a mobile operating in a coverage
area other than the one in which it
subscribed (moving to another MSC)
6: Wireless and Mobile Networks
6-41
Cellular Telephone Systems
- A 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 cells
- A problem may occur when moving from one
cell to another while keeping the call uninterrupted. [the hand-off problem]
6: Wireless and Mobile Networks
6-42
6: Wireless and Mobile Networks
6-43
Design concepts: The Cellular
Concept and Frequency Re-use
- The 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)
6: Wireless and Mobile Networks
6-44
- Neighboring base stations (would in
general) use different frequency channels
to reduce interference.
- (more later on interference, channel
assignment and frequency planning)
6: Wireless and Mobile Networks
6-45
Frequency Re-use
- A cell uses a set of frequencies
- A ‘cluster’ holds several cells
- Frequency re-use factor: 1/#cells per
cluster
6: Wireless and Mobile Networks
6-46
B
G
C
A
F
B
G
D
E
C
A
F
B
G
C
A
F
B
G
D
E
G
E
A
B
D
C
F
Cluster
Cell
D
E
C
A
F
D
E
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.
This requires channel/frequency planning and allocation!
6: Wireless and Mobile Networks
6-47
Multiple Access (MA) Techniques
for Wireless Communications
- MA schemes allow multiple mobile users to
share a limited frequency spectrum.
- Main MA schemes: FDMA, TDMA, SSMA
(FHMA, CDMA [DSMA]), SDMA
6: Wireless and Mobile Networks
6-48
FDMA
6: Wireless and Mobile Networks
6-49
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 continuous
Does not need synchronization
Costly filters at the base station
Need guard bands to alleviate interference
6: Wireless and Mobile Networks
6-50
TDMA
6: Wireless and Mobile Networks
6-51
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-continuous
Power consumption is lower than FDMA (e.g.,
the transmitter can be turned off when idle)
During idle time, a mobile performs MAHO
Synchronization is needed
6: Wireless and Mobile Networks
6-52
Spread Spectrum Multiple
Access (SSMA)
- Traditional communication techniques
-
Strive to conserve bandwidth
- By contrast, Spread spectrum techniques
- use bandwidth several orders of magnitude
larger than the min. required bandwidth !!
6: Wireless and Mobile Networks
6-53
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 wideband
- The PN is random, but can be re-produced by receiver
- Demodulation:
- Correct correlation using a PN re-produces the signal
- Using wrong PN sequence produces noise, hence this
scheme is ‘secure’
6: Wireless and Mobile Networks
6-54
- 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 range
- The signal is broken into bursts and each burst is
sent on a different frequency
6: Wireless and Mobile Networks
6-55
CDMA
6: Wireless and Mobile Networks
6-56
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
partitioning
all users share same frequency, but each user has
own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping
sequence)
decoding: inner-product of encoded signal and
chipping sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes
are “orthogonal”)
6: Wireless and Mobile Networks
6-57
- Speading the signal power over a wide spread
of the frequency spectrum reduces fading
effects
-
only part of the spectrum, hence only part of the
signal, is affected by fading
- No 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
changes
6: Wireless and Mobile Networks
6-58
Space Division MA (SDMA)
 Controls the radiated energy for each user in
space using spot beam (directional) antennas
6: Wireless and Mobile Networks
6-59
Hybrid Multiple Access
Systems
- Time division frequency hopping (TDFH):
(used in some versions of GSM)
- User can hop to new frequency at the start
of a new TDMA frame
- Hence reducing interference and fading
effects
- User hops over pre-defined frequencies
6: Wireless and Mobile Networks
6-60
- FDMA/CDMA:
- The available bandwidth is split into
subspectra. In each subspectrum CDMA is
used
- Allows to assign subspectra on-demand
6: Wireless and Mobile Networks
6-61
FDMA/CDMA
6: Wireless and Mobile Networks
6-62
Cellular networks: the first hop
Techniques for sharing
mobile-to-BS radio
spectrum
 combined FDMA/TDMA:
divide spectrum in
frequency channels, divide
each channel into time
slots
frequency
time slots
bands
6: Wireless and Mobile Networks
6-63
Cellular standards: brief survey
2G systems: voice channels
 IS-136 TDMA: combined FDMA/TDMA (north
america)
 GSM (global system for mobile communications):
combined FDMA/TDMA

most widely deployed
 IS-95 CDMA: code division multiple access
GSM
Don’t drown in a bowl
of alphabet soup: use this
for reference only 
6: Wireless and Mobile Networks
6-64
Cellular standards: brief survey
2.5 G systems: voice and data channels
 for those who can’t wait for 3G service: 2G extensions
 general 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 384K
 CDMA-2000 (phase 1)
 data rates up to 144K
 evolved from IS-95
6: Wireless and Mobile Networks
6-65
Cellular standards: brief survey
3G systems: voice/data
 Universal Mobile Telecommunications Service (UMTS)
 data service: High Speed Uplink/Downlink packet
Access (HSDPA/HSUPA): 3 Mbps
 CDMA-2000: CDMA in TDMA slots
 data service: 1xEvlution Data Optimized (1xEVDO)
up to 14 Mbps
6: Wireless and Mobile Networks
6-66
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-67
What is mobility?
 spectrum of mobility, from the network perspective:
no mobility
mobile wireless user, mobile user,
using same access
connecting/
point
disconnecting
from network
using DHCP.
high mobility
mobile user, passing
through multiple
access point while
maintaining ongoing
connections (like cell
phone)
6: Wireless and Mobile Networks
6-68
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
correspondent
6: Wireless and Mobile Networks
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Mobility: more vocabulary
Permanent address: remains
constant (e.g., 128.119.40.186)
visited network: network
in which mobile currently
resides (e.g., 79.129.13/24)
Care-of-address: address
in visited network.
(e.g., 79,129.13.2)
wide area
network
correspondent: wants
to communicate with
mobile
foreign agent: entity
in visited network
that performs
mobility functions on
behalf of mobile.
6: Wireless and Mobile Networks
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How do you contact a mobile friend:
Consider friend frequently changing
addresses, how do you find her?
I wonder where
Alice moved to?
 search all phone
books?
 call her parents?
 expect her to let you
know where he/she is?
6: Wireless and Mobile Networks
6-71
Mobility: approaches
 Let routing handle it: routers advertise permanent
address of mobile-nodes-in-residence via usual
routing table exchange.
 routing tables indicate where each mobile located
 no changes to end-systems
 Let end-systems handle it:
 indirect routing: communication from
correspondent to mobile goes through home
agent, then forwarded to remote
 direct routing: correspondent gets foreign
address of mobile, sends directly to mobile
6: Wireless and Mobile Networks
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Mobility: approaches
 Let routing handle it: routers advertise permanent
not
address of mobile-nodes-in-residence
via usual
scalable
routing table exchange.
to millions of
 routing tables indicate
mobiles where each mobile located
no changes to end-systems
 let end-systems handle it:
 indirect routing: communication from
correspondent to mobile goes through home
agent, then forwarded to remote
 direct routing: correspondent gets foreign
address of mobile, sends directly to mobile

6: Wireless and Mobile Networks
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Mobility: registration
visited network
home network
1
2
wide area
network
foreign agent contacts home
agent home: “this mobile is
resident in my network”
mobile contacts
foreign agent on
entering visited
network
End result:
 Foreign agent knows about mobile
 Home agent knows location of mobile
6: Wireless and Mobile Networks
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Mobility via Indirect Routing
foreign agent
receives packets,
forwards to mobile
home agent intercepts
packets, forwards to
foreign agent
home
network
visited
network
3
wide area
network
correspondent
addresses packets
using home address
of mobile
1
2
4
mobile replies
directly to
correspondent
6: Wireless and Mobile Networks
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Indirect Routing: comments
 Mobile 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 mobile
 foreign agent functions may be done by mobile itself
 triangle routing: correspondent-home-networkmobile
 inefficient when
correspondent, mobile
are in same network

6: Wireless and Mobile Networks
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Indirect Routing: moving between networks
 suppose mobile user moves to another
network
registers with new foreign agent
 new foreign agent registers with home agent
 home agent update care-of-address for mobile
 packets continue to be forwarded to mobile (but
with new care-of-address)

 mobility, changing foreign networks
transparent: on going connections can be
maintained!
6: Wireless and Mobile Networks
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Mobility via Direct Routing
correspondent forwards
to foreign agent
foreign agent
receives packets,
forwards to mobile
home
network
4
wide area
network
2
correspondent
requests, receives
foreign address of
mobile
visited
network
1
3
4
mobile replies
directly to
correspondent
6: Wireless and Mobile Networks
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Mobility via Direct Routing: comments
 overcome triangle routing problem
 non-transparent to correspondent:
correspondent must get care-of-address
from home agent

what if mobile changes visited network?
6: Wireless and Mobile Networks
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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)
foreign net visited
at session start
wide area
network
anchor
foreign
agent
1
2
4
5
correspondent
agent
correspondent
3
new foreign
agent
new
foreign
network
6: Wireless and Mobile Networks
6-80
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
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Mobile IP
 RFC 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 node
 the connection already established should be
maintained during movement even if the mobile
node changes its network point of attachment
6: Wireless and Mobile Networks
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Mobile IP
 has 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 datagrams
 agent discovery
 registration with home agent
6: Wireless and Mobile Networks
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Mobile IP
 Each mobile node has a home network,
home address and home agent
Correspondent Node
Home Agent (HA)
Home Network
Mobile Node
6: Wireless and Mobile Networks
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• 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
Foreign Agent (FA)
Advertisement (FA,COA)
Solicitation
Foreign Network
Correspondent Node
Register
Mobile Node
Register (HA)
Home Agent
Home Network
6: Wireless and Mobile Networks
6-85
Mobile IP: registration example
home agent
HA: 128.119.40.7
foreign agent
COA: 79.129.13.2
visited network: 79.129.13/24
ICMP agent adv.
COA: 79.129.13.2
….
registration req.
COA: 79.129.13.2
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 9999
identification: 714
encapsulation format
….
Mobile agent
MA: 128.119.40.186
registration req.
COA: 79.129.13.2
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 9999
identification:714
….
registration reply
time
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 4999
Identification: 714
encapsulation format
….
registration reply
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 4999
Identification: 714
….
6: Wireless and Mobile Networks
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Mobile IP: indirect routing
foreign-agent-to-mobile packet
packet sent by home agent to foreign
agent: a packet within a packet
dest: 79.129.13.2
dest: 128.119.40.186
dest: 128.119.40.186
Permanent address:
128.119.40.186
dest: 128.119.40.186
Care-of address:
79.129.13.2
packet sent by
correspondent
6: Wireless and Mobile Networks
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Packets sent by MN go
directly to CN
Mobile Node (MN)
Correspondent
Node (CN)
Packets to MN are
picked up by the HA
and tunneled to MN
Home Agent (HA)
• Triangle Routing in Mobile-IP
6: Wireless and Mobile Networks
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Triangular routing can be very inefficient, especially when
C << B+A, where A (as shown) is the shortest path from
CN to MN
C
Mobile Node (MN)
Correspondent
Node (CN)
A
B
Home Agent (HA)
• Triangle Routing in Mobile-IP
6: Wireless and Mobile Networks
6-89
Drawbacks of Mobile IP
 Other than (the main problem) of triangular
routing
Mobile 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 first

6: Wireless and Mobile Networks
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Suggested solutions
 To avoid triangular routing
use ‘route optimization’
 use micro-mobility architectures

• Cellular IP (CIP)
• Hawaii
• Multicast-based Mobility (M&M)
6: Wireless and Mobile Networks
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(4) CN changes the destination
address of the packets to go to
MN’s new address
(3) When MN gets packets from CN
it sends a Binding Update to CN with
its new address
Mobile Node (MN)
Correspondent
Node (CN)
(2) Initial packets
to MN are sent
through HA to MN
(1) MN registers with HA as in
basic Mobile IP.
Home Agent (HA)
• Route Optimization (simple illustration)
6: Wireless and Mobile Networks
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 With route optimization
Triangular routing is avoided
 Still have problems with micro mobility and
smooth hand-off
 Need additional mechanisms to deal with these
issues, which makes the protocol complex.

6: Wireless and Mobile Networks
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Micro-Mobility
 Hierarchical 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 movement

6: Wireless and Mobile Networks
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Distribution tree dynamics while roaming
Domain Root
FA or CN
Wireless link
Mobile Node
6: Wireless and Mobile Networks
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M&M: Join/Prune dynamics to modify distribution
Domain Root
Wireless link
Mobile Node
6: Wireless and Mobile Networks
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Components of cellular network architecture
recall:
correspondent
wired public
telephone
network
MSC
MSC
MSC
MSC
MSC
different cellular networks,
operated by different providers
6: Wireless and Mobile Networks
6-97
Handling mobility in cellular networks
 home 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
resides
 visitor location register (VLR): database with
entry for each user currently in network
 could be home network
6: Wireless and Mobile Networks
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GSM: indirect routing to mobile
home
network
HLR
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
correspondent
home
Mobile
Switching
Center
1
3
VLR
Mobile
Switching
Center
4
Public
switched
telephone
network
call routed
to home network
home MSC sets up 2nd leg of call
to MSC in visited network
mobile
user
visited
network
MSC in visited network completes
call through base station to mobile
6: Wireless and Mobile Networks
6-99
GSM: handoff with common MSC
 Handoff goal: route call via
new base station (without
interruption)
 reasons for handoff:
VLR Mobile
Switching
Center
old
routing
old BSS

new
routing

new BSS

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 BSS
6: Wireless and Mobile Networks 6-100
GSM: handoff with common MSC
VLR Mobile
Switching
Center 2
4
1
8
old BSS
5
7
3
6
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 released
6: Wireless and Mobile Networks 6-101
GSM: handoff between MSCs
 anchor MSC: first MSC
visited during call
home network
correspondent
Home
MSC

call remains routed
through anchor MSC
 new MSCs add on to end
anchor MSC
PSTN
MSC
MSC
MSC
(a) before handoff
of MSC chain as mobile
moves to new MSC
 IS-41 allows optional
path minimization step
to shorten multi-MSC
chain
6: Wireless and Mobile Networks 6-102
GSM: handoff between MSCs
 anchor MSC: first MSC
visited during call
home network
correspondent
Home
MSC

call remains routed
through anchor MSC
 new MSCs add on to end
anchor MSC
PSTN
MSC
MSC
MSC
(b) after handoff
of MSC chain as mobile
moves to new MSC
 IS-41 allows optional
path minimization step
to shorten multi-MSC
chain
6: Wireless and Mobile Networks 6-103
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-ofaddress
6: Wireless and Mobile Networks 6-104
Wireless, mobility: impact on higher layer protocols
 logically, 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
handoff
 TCP interprets loss as congestion, will decrease
congestion window un-necessarily
 delay impairments for real-time traffic
 limited bandwidth of wireless links

6: Wireless and Mobile Networks 6-105
Chapter 6 Summary
Wireless
 wireless links:



capacity, distance
channel impairments
CDMA
 IEEE 802.11 (“wi-fi”)
 CSMA/CA reflects
wireless channel
characteristics
 cellular access
 architecture
 standards (e.g., GSM,
CDMA-2000, UMTS)
Mobility
 principles: addressing,
routing to mobile users



home, visited networks
direct, indirect routing
care-of-addresses
 case studies
 mobile IP
 mobility in GSM
 impact on higher-layer
protocols
6: Wireless and Mobile Networks 6-106
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
partitioning
all users share same frequency, but each user has
own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping
sequence)
decoding: inner-product of encoded signal and
chipping sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes
are “orthogonal”)
6: Wireless and Mobile Networks 6-107
CDMA Encode/Decode
sender
d0 = 1
data
bits
code
Zi,m= di.cm
-1 -1 -1
1
-1
1 1 1
-1 -1 -1
slot 1
-1
slot 1
channel
output
1
-1
1 1 1 1 1 1
1
d1 = -1
1 1 1
channel output Zi,m
-1 -1 -1
slot 0
1
-1
-1 -1 -1
slot 0
channel
output
M
Di = S Zi,m.cm
m=1
received
input
code
receiver
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 1
M
1
1
-1
-1 -1 -1
slot 0
d0 = 1
d1 = -1
slot 1
channel
output
slot 0
channel
output
6: Wireless and Mobile Networks 6-108
CDMA: two-sender interference
6: Wireless and Mobile Networks 6-109
Direct Sequence Spread
Spectrum
 Original 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 chip
6: Wireless and Mobile Networks 6-110
Sss(t) ~ m(t)p(t)cos(2fct+)
B: is the bandwidth of m(t)cos(2fct+)
Wss: is the bandwidth of Sss(t)
Wss >> B
Phase modulation
Data m(t)
Sss(t)
Transmitted Signal
p(t)
PN Code
Oscillator
Generator
fc
Chip Clock
Block diagram of a DS-SS system with binary phase modulation
Transmitter
6: Wireless and Mobile Networks 6-111
(A)
Symbol
Channel (B)
encoder
f(B,C)
(C)
Chip
Symbol duration for m(t): Ts
Chip duration for p(t): Tc
Processing Gain PG=Wss/B=Ts/Tc, a measure of interference rejection capability
6: Wireless and Mobile Networks 6-112
Bit stream
(A)
Ts
Encoded
stream
(B)
m(t)
Tc
Pseudo-noise
sequence
p(t)
(C)
6: Wireless and Mobile Networks 6-113
 Example:

f(B,C)=BC, where
• 1  1= 0
• 10=1
• 00=0

if we have received f(B,C) and we are able to
re-generate the PN (C), then we can get B.
6: Wireless and Mobile Networks 6-114