1DT066
Distributed Information Systems
Chapter 6
Wireless, WiFi and mobility
Adapted from: Computer Networking, Kurose/Ross
Elements of a wireless
network
wireless hosts



network
infrastructure
Adapted from: Computer Networking, Kurose/Ross
pp 539-544

laptop, smartphone
run applications
may be stationary (nonmobile) or mobile
May roam or migrate
Wireless, Mobile Networks
6-2
Elements of a wireless
network
base station


network
infrastructure
Adapted from: Computer Networking, Kurose/Ross
pp 539-544
connected to wired
network
relaying- responsible
for sending packets
between wired network
and wireless host(s) in
its “radio area” (blue
circle)
 e.g., 3G/cell
towers, 802.11
access points
Wireless, Mobile Networks
6-3
Infrastructure mode
infrastructure mode


network
infrastructure
Adapted from: Computer Networking, Kurose/Ross
pp 539-544
base station connects
mobiles into wired
network
handoff: mobile
changes base station
Wireless, Mobile Networks
6-4
Ad hoc mode
ad hoc mode
 No (wired) base
stations
 nodes can only
transmit to other nodes
within radio reach
 nodes organize
themselves into a
network: route only
among themselves
Adapted from: Computer Networking, Kurose/Ross
pp 539-544
Wireless, Mobile Networks
6-5
Wireless Link Characteristics
important 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) are
shared by other devices (e.g., microwave oven).
Electrical devices, such as electrical motors,
interfere as well.
 multipath propagation: radio signal reflects off
objects, the ground, atmosphere, etc. Reflections
arrive at destination at slightly different times
…. make communication across (even a Wireless,
pointMobile
to Networks
point)6-6
wireless link much more “difficult” compared to a
Adapted from: Computer Networking, Kurose/Ross
pp 545
Characteristics of selected wireless
links
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11a,g point-to-point
802.11b
4
4G: LTWE WIMAX
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
1
802.15
.384
2.5G: UMTS/WCDMA, CDMA2000
.056
2G: IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Adapted from: Computer Networking, Kurose/Ross
pp 543
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
Wireless, Mobile Networks
6-7
IEEE 802.11 Wireless LAN
802.11b
 2.4 GHz unlicensed spectrum
 up to 11 Mbps
802.11a
 5-6 GHz range
 up to 54 Mbps


802.11g
 2.4-5 GHz range
 up to 54 Mbps
802.11n: multiple antenna
 2.4-5 GHz range
 up to 200 Mbps
all use the radio sharing protocol CSMA/CA for
multiple access
all have base-station and an ad-hoc network
configuration mode
Adapted from: Computer Networking, Kurose/Ross
pp 552
Wireless, Mobile Networks
6-8
802.11 LAN architecture

Internet
wireless host
communicates with base
station
 base station = access
point (AP)
hub, switch
or router
BSS 1

Basic Service Set (BSS)
(aka “cell”) in
infrastructure mode
contains:
 wireless hosts
 access point (AP): base
station
BSS 2
Adapted from: Computer Networking, Kurose/Ross
pp 553-554
Wireless, Mobile Networks
6-9
802.11: Host association to
AP

host: must associate with an Access Point
 Host scans radio channels, listening for beacon
frames containing AP’s name (SSID) and MAC
address
 Host selects (the best) AP to associate with
 may perform authentication to get access
 will typically run DHCP to get IP address in AP’s
subnet
Adapted from: Computer Networking, Kurose/Ross
pp 555-557
Wireless, Mobile Networks 6-10
IEEE 802.11: Sharing the radio
channel


Many nodes can independently chose to send at the
same time
802.11: Carrier Sense Multiple Access – host senses
(listen) radio channel if busy before transmitting
 Don’t transmit and collide with ongoing transmission by other
node

802.11: difficult to detect a collision!
 difficult to receive (sense collisions) when transmitting due to
weak received signals
 can’t sense all collisions in any case: hidden terminal,
B
A
C
C
 goal: avoid collisions:
CSMA/C(ollision)A(voidance)
A
C’s signal
strength
A’s signal
strength
B
space
Adapted from: Computer Networking, Kurose/Ross
pp 557-560
Wireless, Mobile Networks 6-11
IEEE 802.11 MAC Protocol:
CSMA/CA
802.11 sender
1 if sense channel idle for DIFS then
sender
transmit entire frame (no CD)
2 if sense channel busy then
DIFS
start random backoff time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff interval,
Draw this figure!
repeat 2
802.11 receiver
- if frame received OK
receiver
data
SIFS
ACK
return ACK after SIFS (ACK needed due to
hidden terminal problem)
Adapted from: Computer Networking, Kurose/Ross
pp 557-560
Wireless, Mobile Networks 6-12
What is mobility?

spectrum of mobility, from the network perspective:
no mobility
mobile wireless user,
using same access
point
Roaming - high mobility
mobile user,
connecting/
disconnecting from
network using
DHCP.
mobile user, passing
through multiple
access point while
maintaining ongoing
connections (like cell
phone)
Adapted from: Computer Networking, Kurose/Ross
pp 581
Wireless, Mobile Networks 6-13
Mobility: vocabulary
home network: permanent
“home” of mobile
(e.g., 128.119.40/24)
home agent: entity that will
perform mobility functions on
behalf of mobile, when mobile is
remote
wide area
network
permanent address:
address in home
network, can always be
used to reach mobile
e.g., 128.119.40.186
Adapted from: Computer Networking, Kurose/Ross
pp 582
Wireless, Mobile Networks 6-14
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
foreign agent: entity in
visited network that
performs mobility
functions on behalf of
mobile.
correspondent: wants
to communicate with
mobile
Adapted from: Computer Networking, Kurose/Ross
pp 582
Wireless, Mobile Networks 6-15
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 visiting mobile
 home agent knows location of mobile
Adapted from: Computer Networking, Kurose/Ross
pp 583-588
Wireless, Mobile Networks 6-16
Mobility via indirect routing
home agent intercepts
packets, forwards to
foreign agent
foreign agent
receives packets,
forwards to mobile
visited
network
home
network
3
1
wide area
network
2
4
correspondent
addresses packets
using home address of
mobile
Adapted from: Computer Networking, Kurose/Ross
mobile replies
directly to
correspondent
pp 583-588
Wireless, Mobile Networks 6-17
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

triangle routing:
correspondent2home2network2mobile
 inefficient when
Correspondent and Mobile
are in same network.
Adapted from: Computer Networking, Kurose/Ross
pp 583-588
Wireless, Mobile Networks 6-18