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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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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

CS118/Spring05

elements of a wireless network
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

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”
  • Infrastructure mode:
  • basestations connect mobiles to wired networks
  • when moving mobiles change basestations to keep Internet access (handoff)
  • Ad hoc mode:
  • no basestations
  • each node helps forward packets to other node

CS118/Spring05

wireless link characteristics
Wireless Link Characteristics

C

B

A

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

CS118/Spring05

ieee 802 11 wireless lan
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

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

CS118/Spring05

802 11 lan architecture
802.11 LAN architecture

AP

AP

Internet

  • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies; 3 non-overlapping
    • AP admin chooses frequency for AP
    • neighboring APs may choose same channel–interference
  • AP sends beacon frame periodically
    • 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 AP’s subnet

hub, switch

or router

BSS 1

BSS 2

BSS: Basic Service Set

SSID: Service Set Identifier

CS118/Spring05

ieee 802 11 multiple access
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)

CS118/Spring05

ieee 802 11 mac protocol csma ca 1
IEEE 802.11 MAC Protocol: CSMA/CA (1)

DIFS

data

SIFS

ACK

802.11 sender

1 if sense channel idle for DIFSthen

- transmit entire frame (no CD)

2 ifsense 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: distributed inter-frame spacing

SIFS: short inter-frame spacing

CS118/Spring05

ieee 802 11 mac protocol csma ca 2
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

CS118/Spring05

collision avoidance rts cts exchange
Collision Avoidance: RTS-CTS exchange

RTS(B)

RTS(A)

reservation collision

RTS(A)

CTS(A)

CTS(A)

DATA (A)

ACK(A)

ACK(A)

A

B

AP

defer

time

CS118/Spring05

802 11 frame addressing
802.11 frame: addressing

6

4

2

2

6

6

6

2

0 - 2312

frame

control

duration

address

1

address

2

address

3

address

4

payload

CRC

seq

control

Address 3: used only in ad hoc mode

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 2: MAC address

of wireless host or AP

transmitting this frame

CS118/Spring05

slide11

802.11 frame: addressing

router

AP

Internet

R1 MAC addr H1 MAC addr

source address

dest. address

802.3frame

AP MAC addr H1 MAC addr R1 MAC addr

address 3

address 2

address 1

802.11 frame

H1

R1

CS118/Spring05

slide12

802.11 frame: more

6

4

2

2

6

6

6

2

0 - 2312

frame

control

duration

address

1

address

2

address

3

address

4

payload

CRC

seq

control

2

2

4

1

1

1

1

1

1

1

1

Protocol

version

Type

Subtype

To

AP

From

AP

More

frag

Retry

Power

mgt

More

data

WEP

Rsvd

frame seq #

(for reliable ARQ)

duration of reserved

transmission time (RTS/CTS)

frame type

(RTS, CTS, ACK, data)

CS118/Spring05

slide13
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

802.11: mobility within same subnet

router

hub or

switch

BBS 1

AP 1

AP 2

H1

BBS 2

CS118/Spring05

mobility vocabulary
Mobility: Vocabulary

home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote

home network: permanent “home” of mobile (e.g., 128.119.40/24)

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

CS118/Spring05

mobility more vocabulary
Mobility: more vocabulary

visited network: network in which mobile currently resides (e.g., 79.129.13/24)

Permanent address: remains constant (e.g., 128.119.40.186)

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

CS118/Spring05

mobility approaches
Mobility: approaches

not

scalable

to millions of

mobiles

  • 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: correspondent sends packets to to home agent, which forwards to mobile
    • direct routing: correspondent gets foreign address of mobile, sends directly to mobile

CS118/Spring05

mobility registration
Mobility: registration

mobile contacts foreign agent on entering visited network

foreign agent contacts home agent home: “this mobile is resident in my network”

1

2

End result:

  • Foreign agent knows about mobile
  • Home agent knows location of mobile

visited network

home network

wide area network

CS118/Spring05

mobility via indirect routing
Mobility via Indirect Routing

foreign agent receives packets, forwards to mobile

home agent intercepts packets, forwards to foreign agent

correspondent addresses packets using home address of mobile

mobile replies directly to correspondent

Q: Which address will mobile use as source address?

3

2

4

1

visited

network

home

network

wide area network

CS118/Spring05

mobility via indirect routing further movement
Mobility via Indirect Routing: further movement

4

1

2

3

visited

network

home

network

wide area network

Q: Will the correspondence be aware of mobile's move?

CS118/Spring05

indirect routing comments
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

CS118/Spring05

indirect routing moving between networks
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!

CS118/Spring05

mobility via direct routing
Mobility via Direct Routing

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

4

2

4

1

3

correspondent forwards to foreign agent

visited

network

home

network

wide area network

correspondent requests, receives foreign address of mobile

CS118/Spring05

mobility via direct routing comments
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 moves to another visited network?

CS118/Spring05

accommodating mobility with direct routing
Accommodating mobility with direct routing

1

2

4

3

5

  • 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

anchor

foreign

agent

wide area network

new

foreign

network

correspondent

agent

new foreign

agent

correspondent

CS118/Spring05