week9 wireless

7/28/2019 Week9 Wireless

1/24

6/2/05 CS118/Spring051

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


2/24

6/2/05 CS118/Spring052

Elements of a wireless network

Wireless host: may bestationary (non-mobile) ormobile

wireless link

typically connectingmobile(s) to base station

can also be used asbackbone link

multiaccess protocol:coordinates link access

Infrastructure mode:

basestations connect mobiles towired networks

when moving mobiles change

basestations to keep Internet

access (handoff)

Wired networkinfrastructure

Ad hoc mode:

no basestations

each node helps

forward packetsto other node

Base station:

typically connected to wired network

relay - responsible for sending packets

between wired network and wirelesshost(s) in its area


3/24

6/2/05 CS118/Spring053

Wireless Link Characteristicscommunication across a point to point wireless link is much more

difficult than wired link .

decreased signal strength: radio signal attenuates as it propagatesthrough matter (path loss)

interference from other sources: standardized wireless networkfrequencies (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

A B

C


4/24

6/2/05 CS118/Spring054

IEEE 802.11 Wireless LAN

802.11b 2.4-5 GHz unlicensed

radio spectrum

Data rate up to 11 Mbps

direct sequence spreadspectrum (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 formultiple access

All have infrastructureand ad-hoc networkversions


5/24

6/2/05 CS118/Spring055

802.11 LAN architecture 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

channelinterference

AP sends beacon frame periodically

SSID, MAC address

host: must associate with an AP

scan channels, listening forbeaconframescontaining APs name (SSID)

and MAC address

selects AP to associate with; initiates

association protocol

typically run DHCP to get IP address inAPs subnet

BSS 1

BSS 2

Internet

hub, switch

or routerAP

AP

BSS: Basic Service Set

SSID: Service Set Identifier


6/24

6/2/05 CS118/Spring056

IEEE 802.11: multiple access

Like Ethernet, uses CSMA:

random access

carrier sense: dont collide with ongoing transmission

Unlike Ethernet:

no collision detectiontransmit all frames to completion

acknowledgmentbecause without collision detection, you dont

know if your transmission collided or not

Why no collision detection?

difficult to receive (sense collisions) when transmitting due to weak

received signals (fading) cant sense all collisions in any case: hidden terminal, fading

Goal: avoid collisions: CSMA/C(ollision)A(voidance)


7/24

6/2/05 CS118/Spring057

IEEE 802.11 MAC Protocol: CSMA/CA (1)

802.11 sender

1 if sense channel idle forDIFS then- 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 backoffinterval, repeat 2

802.11 receiverif frame received OK

- return ACK afterSIFS (ACK needed dueto hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DIFS: distributed inter-frame spacing

SIFS: short inter-frame spacing


8/24

6/2/05 CS118/Spring058

IEEE 802.11 MAC Protocol: CSMA/CA (2)

Dealing with hidden terminal:

idea: allow sender to reserve channel: avoid collisions of longdata frames

sender first transmits small request-to-send (RTS) packets to APusing CSMA

RTSs may still collide with each other (but theyre short)

AP broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

sender transmits data frame

other stations defer transmissions


9/246/2/05 CS118/Spring059

Collision Avoidance: RTS-CTS exchange

APA B

time

DATA (A)

reservation collision

defer


10/246/2/05 CS118/Spring0510

frame

controlduration

address

1

address

2

address

4

address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq

control

802.11 frame: addressing

Address 2: MAC addressof wireless host or APtransmitting this frame

Address 1: MAC addressof wireless host or APto receive this frame

Address 3: MAC addressof router interface towhich AP is attached

Address 3: used onlyin ad hoc mode


11/246/2/05 CS118/Spring0511

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

R1 MAC addr H1 MAC addr

dest. address source address

802.3frame

802.11 frame: addressing


12/246/2/05 CS118/Spring0512

frame

controlduration

address

1

address

2

address

4

address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq

control

TypeFrom

APSubtype

To

AP

More

fragWEP

More

data

Power

mgtRetry Rsvd

Protocol

version

2 2 4 1 1 1 1 1 11 1

802.11 frame: more

duration of reservedtransmission time (RTS/CTS)

frame seq #

(for reliable ARQ)

frame type(RTS, CTS, ACK, data)


13/246/2/05 CS118/Spring0513

hub orswitch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router

H1 detects weakeningsignal from AP1, scan and

find AP2 to attach to

H1 remains in same IP

subnet: IP address canremain 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


14/246/2/05 CS118/Spring0514

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 mobilee.g., 128.119.40.186

home agent: entity that

will perform mobility

functions on behalf ofmobile, when mobile is

remote

wide area

network

correspondent

correspondent: wants to

communicate with mobile


15/246/2/05 CS118/Spring0515

Mobility: more vocabulary

Care-of-address: address in

visited network. (e.g., 79,129.13.2)

wide area

network

visited network: network in

which mobile currently resides(e.g., 79.129.13/24)

Permanent address: remains

constant (e.g., 128.119.40.186)

foreign agent: entity in

visited network that

performs mobility

functions on behalf of

mobile.

correspondent: wants to

communicate with

mobile


16/246/2/05 CS118/Spring0516

Let routing handle it: routers advertise permanentaddress 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

Mobility: approaches

not

scalable

to millions of

mobiles


17/246/2/05 CS118/Spring0517

Mobility: registration

End result:

Foreign agent knows about mobile

Home agent knows location of mobile

wide area

network

home network visited network

1

mobile contactsforeign agent onentering visitednetwork

2

foreign agent contacts homeagent home: this mobile isresident in my network


18/246/2/05 CS118/Spring0518

Mobility via Indirect Routing

wide area

networkhomenetwork

visited

network

3

2

41

correspondent addresses

packets using home

address of mobile

home agent interceptspackets, forwards to foreign

agent

foreign agent receives

packets, forwards to

mobile

mobile repliesdirectly tocorrespondent

Q: Which address will

mobile use as source

address?


19/246/2/05 CS118/Spring0519

Mobility via Indirect Routing: further movement

wide area

networkhomenetwork

visited

network

2

3

1

4

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


20/246/2/05 CS118/Spring0520

Indirect Routing: comments

Mobile uses two addresses:

permanent address: used by correspondent (hencemobile location is transparentto 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


21/246/2/05 CS118/Spring0521

Indirect Routing: moving between networks

suppose mobile user moves to anothernetwork

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 (butwith new care-of-address)

mobility, changing foreign networkstransparent: on going connections can bemaintained!


22/246/2/05 CS118/Spring0522

Mobility via Direct Routing

wide area

network

homenetwork

visitednetwork

4

2

41correspondentrequests, receives

foreign address ofmobile

correspondent forwardsto foreign agent

foreign agentreceives packets,forwards to mobile

mobile repliesdirectly tocorrespondent

3


23/246/2/05 CS118/Spring0523

Mobility via Direct Routing: comments

overcome triangle routing problemnon-transparent to correspondent:

correspondent must get care-of-address from

home agentwhat if mobile moves to another visited network?


24/24

wide areanetwork

1

foreign net visited

at session startanchor

foreign

agent2

4

new foreign

agent

35

correspondent

agentcorrespondent

new

foreign

network

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 dataforwarded from old FA (chaining)

week9 wireless

Documents