Chapter 6 slides, Computer Networking, 3rd edition
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Transcript Chapter 6 slides, Computer Networking, 3rd edition
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