Transcript Chapter 6: Wireless and Mobile Networks

Chapter 6: Wireless and Mobile Networks
 Cover the following sections only:
 6.3:
802.11 wireless LANs
 6.5: mobility management: principles
 two important (but different) new challenges
 communication
over wireless link
 handling mobile user who changes point of
attachment to network
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Elements of a wireless network
wireless link
 typically connecting
mobile(s) to base station
 can also be used as
backbone link
 multiaccess protocol:
coordinates link access
Infrastructure mode:
• basestations connect mobiles to
wired networks
• when moving mobiles change
basestations to keep Internet
access (handoff)
Wired network
infrastructure
Wireless host: may be
stationary (non-mobile) or
mobile
Base station:
• typically connected to wired network
• relay - responsible for sending packets
between wired network and wireless
host(s) in its “area”
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Ad hoc mode:
• no basestations
• each node helps
forward packets
to other node
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Wireless Link Characteristics
communication across a point to point wireless link is much more
“difficult” than 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 at destination at slightly different times
 Multiple wireless senders and receivers create additional problems
(beyond multiple access): 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

C
A
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B
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IEEE 802.11 Wireless LAN
 802.11b
 2.4-5 GHz unlicensed
radio spectrum
 Data rate up to 11 Mbps
 direct sequence spread
spectrum (DSSS) in
physical layer
 widely deployed, using
base stations
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 802.11a
 5-6 GHz range
 up to 54 Mbps
 802.11g
 2.4-2.5 GHz range
 up to 54 Mbps
 All use CSMA/CA for
multiple access
 All have infrastructure
and ad-hoc network
versions
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802.11 LAN architecture
 802.11b: 2.4GHz-2.485GHz
spectrum divided into 11 channels
at different frequencies; 3 nonoverlapping
Internet


hub, switch
or router 
AP
AP
BSS 2
BSS: Basic Service Set
SSID: Service Set Identifier
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AP sends beacon frame periodically

BSS 1
AP admin chooses frequency for AP
neighboring APs may choose same
channel–interference
SSID, MAC address
 host: must associate with an AP
 scan channels, listening for beacon
frames containing AP’s name (SSID)
and MAC address
 selects AP to associate with; initiates
association protocol
 typically run DHCP to get IP address in
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AP’s subnet
IEEE 802.11: multiple access
 Like Ethernet, uses CSMA:
 random access
 carrier sense: don’t collide with ongoing transmission
 Unlike Ethernet:
 no collision detection – transmit all frames to completion
 acknowledgment – because without collision detection, you don’t
know if your transmission collided or not
 Why 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)
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IEEE 802.11 MAC Protocol: CSMA/CA (1)
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
sender
receiver
DIFS
data
SIFS
802.11 receiver
ACK
if frame received OK
- return ACK after SIFS (ACK needed due
to hidden terminal problem)
DIFS: distributed inter-frame spacing
SIFS: short inter-frame spacing
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IEEE 802.11 MAC Protocol: CSMA/CA (2)
Dealing with hidden terminal:
 idea: allow sender to “reserve” channel: avoid collisions of long
data frames
 sender first transmits small request-to-send (RTS) packets to AP
using CSMA

RTSs may still collide with each other (but they’re short)
 AP broadcasts clear-to-send CTS in response to RTS
 CTS heard by all nodes
 sender transmits data frame
 other stations defer transmissions
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Collision Avoidance: RTS-CTS exchange
A
B
AP
reservation collision
DATA (A)
defer
time
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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 3: 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
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802.11 frame: addressing
R1 router
H1
Internet
AP
R1 MAC addr H1 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
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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)
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802.11: mobility within same subnet
 H1 detects weakening
signal from AP1, scan and
find AP2 to attach to
 H1 remains in same IP
subnet: IP address can
remain same
 switch: which AP is
associated with H1?
 self-learning:
switch will
see frame from H1 and
“remember” which interface
can be used to reach H1
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router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
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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
correspondent
correspondent: wants to
communicate with mobile
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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
foreign agent: entity in
visited network that
performs mobility
functions on behalf of
mobile.
correspondent: wants to
communicate with
mobile
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Mobility: approaches
 Let routing handle it: routers advertise permanent
address of mobile-nodes-in-residence
via
usual
not
routing table exchange.
scalable
to millions
of each mobile located
tables indicate
where
mobiles
 no changes to end-systems
 routing
 Let end-systems handle it:
 indirect
routing: correspondent sends packets to to
home agent, which forwards to mobile
 direct routing: correspondent gets foreign address of
mobile, sends directly to mobile
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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
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Mobility via Indirect Routing
home agent intercepts
packets, forwards to foreign
agent
home
network
foreign agent receives
packets, forwards to
mobile
visited
network
3
wide area
network
1
2
4
correspondent addresses
packets using home
address of mobile
mobile replies
directly to
correspondent
Q: Which address will
mobile use as source
address?
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Mobility via Indirect Routing: further movement
visited
network
home
network
wide area
network
3
4
2
1
Q: Will the correspondence be aware of mobile's move?
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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-network-mobile
 inefficient when
Correspondent & mobile
are in same network
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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!
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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
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visited
network
3
1
4
mobile replies
directly to
correspondent
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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
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if mobile moves to another visited network?
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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
new foreign
agent
correspondent
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24
new
foreign
network
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