ccn chapter6 4th ed june 18 2007

8/3/2019 CCN Chapter6 4th Ed June 18 2007

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Wireless and MobileNetworks



Wireless and Mobile Networks

Background: # wireless (mobile) phone subscribers now

exceeds # wired phone subscribers!

computer nets: laptops, palmtops, PDAs,Internet-enabled phone promise anytimeuntethered Internet access

two important (but different) challenges wireless: communication over wireless link

mobility: handling the mobile user who changes pointof attachment to network



Elements of a wireless network

networkinfrastructure

wireless hosts laptop, PDA, IP phone run applications may be stationary

(non-mobile) or mobile wireless does not

always mean mobility





base station typically connected to

wired network relay - responsible

for sending packets

between wirednetwork and wirelesshost(s) in its ´areaµ e.g., cell towers,

802.11 accesspoints





wireless link typically used to

connect mobile(s) tobase station

also used as backbone

link multiple access

protocol coordinateslink access

various data rates,transmission distance



Characteristics of selected wireless linkstandards

Indoor 10-30m

Outdoor 50-200m

Mid-range

outdoor 200m ± 4 Km

Long-range

outdoor 5Km ± 20 Km

.056

.384

1

45-11

54

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 3G

802.15

802.11b

802.11a,g

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellular

enhanced

802.16 (WiMAX)

802.11a,g point-to-point

200 802.11n

D a t a r a t e ( M b p s ) data





infrastructure mode base station connects

mobiles into wirednetwork

handoff: mobile

changes base stationproviding connectioninto wired network




ad hoc mode no base stations nodes can only

transmit to othernodes within link

coverage nodes organize

themselves into anetwork: route amongthemselves



Wireless network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects tobase station (WiFi,WiMAX, cellular)which connects to

larger Internet

no base station, noconnection to larger

Internet (Bluetooth,ad hoc nets)

host may have torelay through several

wireless nodes to

connect to largerInternet: mesh net

no base station, noconnection to larger

Internet. May have to

relay to reach othera given wireless nodeMANET , VANET



Wireless Link Characteristics (1)

Differences from wired link «.

decreased signal strength: radio signalattenuates as it propagates through matter(path loss)

interference from other sources: standardizedwireless 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 ad destination atslightly different times

«. make communication across (even a point to point)wireless link much more ´difficultµ



Wireless Link Characteristics (2)

SNR: signal-to-noise ratio

larger SNR ² easier toextract signal from noise (a´good thingµ)

SNR versus BE

R tradeoffs given physical layer:

increase power -> increaseSNR->decrease BER

given SNR: choose physicallayer that meets BERrequirement, giving highestthruput

SNR may change withmobility: dynamically adaptphysical layer (modulation

technique, rate)

10 20 30 40

SNR(dB)

B E

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4



Wireless network characteristics

Multiple wireless senders and receivers createadditional problems (beyond multiple access):

AB

C

Hidden terminal problem

B, A hear each other B, C hear each other A, C can not hear each othermeans A, C unaware of their

interference at B

A B C

A·s signalstrength

space

C·s signalstrength

Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other

interfering at B



Code Division Multiple Access (CDMA)

used in several wireless broadcast channels(cellular, satellite, etc) standards

unique ćodeµ assigned to each user; i.e., code setpartitioning

all users share same frequency, but each user hasown ćhippingµ sequence (i.e., code) to encode data encoded signal = (original data) X (chipping

sequence) decoding: inner-product of encoded signal and

chipping sequence allows multiple users to ćoexistµ and transmit

simultaneously with minimal interference (if codesare órthogonalµ)



IEEE 802.11 Wireless LAN

802.11b 2.4-5 GHz unlicensed spectrum

up to 11 Mbps

direct sequence spread

spectrum (DSSS) in physicallayer

all hosts use same chippingcode

802.11a 5-6 GHz range

up to 54 Mbps

802.11g 2.4-5 GHz range up to 54 Mbps

802.11n: multiple antennae

2.4-5 GHz range

up to 200 Mbps

all use CSMA/CA for multiple access

all have base-station and ad-hoc network versions



802.11 LAN architecture

wireless host communicateswith base station

base station = accesspoint (AP)

Basic Service Set (BSS)(aka ´cellµ) in infrastructuremode contains:

wireless hosts

access point (AP): base

station ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor router

AP

AP



802.11: Channels, association

802.11b: 2.4GHz-2.485GHz spectrum divided into11 channels at different frequenciesAP admin chooses frequency for AP

interference possible: channel can be same asthat chosen by neighboring AP!

host: must associate with an AP scans channels, listening for beacon frames

containing AP·s name (SSID) and MAC address selects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in AP·s

subnet



802.11: passive/active scanning

AP 2AP 1

H1

BBS 2BBS 1

122

34

Active Scanning : (1) Probe Request frame broadcast

from H1

(2) Probes response frame sent from APs

(3) Association Request frame sent:

H1 to selected AP

(4) Association Response frame

sent: H1 to selected AP

AP 2AP 1

H1

BBS 2BBS 1

1

23

1

P assive Scanning:(1) beacon frames sent from APs

(2) association Request frame sent:

H1 to selected AP(3) association Response frame sent:

H1 to selected AP



IEEE 802.11 MAC Protocol: CSMA/CA

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 timetimer counts down while channel idle

transmit when timer expires

if no ACK, increase random backoffinterval, repeat 2

802.11 receiver

- if frame received OK

return ACK after SIFS (ACK needed dueto hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK



Avoiding collisions (more)

idea: allow sender to ´reserveµ channel rather than randomaccess of data frames: avoid collisions of long data frames

sender first transmits small request-to-send (RT S) packetsto BS using CSMA

RT Ss may still collide with each other (but they·re short) BS broadcasts clear-to-send CT S in response to RT S

RT S heard by all nodes

sender transmits data frame

other stations defer transmissions

avoid data frame collisions completelyusing small reservation packets!



Collision Avoidance: RT S-CT S exchange

APA B

time

DAT A (A)

reservation collision

defer



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 4: used onlyin ad hoc mode



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 AP MAC addr

dest. address source address

802.3 frame

802.11 frame: addressing



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: moreduration of reservedtransmission time (RT S/CT S)

frame seq #(for reliable ARQ)

frame type(RT S, CT S, ACK, data)



hub orswitch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router H1 remains in same IP

subnet: IP addresscan remain same

switch: which AP isassociated with H1?self-learning (Ch. 5):

switch will see framefrom H1 and

´rememberµ whichswitch port can beused to reach H1



802.11: advanced capabilities

Rate Adaptation

base station, mobiledynamically changetransmission rate

(physical layermodulation technique)as mobile moves, SNRvaries

10 20 30 40SNR(dB)

B E R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

1. SNR decreases, BERincrease as node moves

away from base station

2. When BER becomes toohigh, switch to lowertransmission rate but withlower BER



802.11: advanced capabilities

Power Management node-to-AP: ´I am going to sleep until next

beacon frameµ

AP knows not to transmit frames to this

nodenode wakes up before next beacon frame

beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent

node will stay awake if AP-to-mobile framesto be sent; otherwise sleep again until nextbeacon frame



M

radius of

coverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)P

802.15: personal area network

less than 10 m diameter replacement for cables

(mouse, keyboard,headphones)

ad hoc: no infrastructure master/slaves:

slaves request permission tosend (to master)

master grants requests 802.15: evolved from

Bluetooth specification 2.4-2.5 GHz radio band

up to 721 kbps



802.16: WiMAX

like 802.11 & cellular:base station model transmissions to/from

base station by hostswith omnidirectional

antenna base station-to-base

station backhaul withpoint-to-point antenna

unlike 802.11: range ~ 6 miles (´city

rather than coffeeshopµ)

~14 Mbps

point-to-multipoint

point-to-point



802.16: WiMAX: downlink, uplink scheduling

transmission frame down-link subframe: base station to node

uplink subframe: node to base station

p r e a m .

DL-

MAPUL-

MAP

DL

burst 1SS #1

DL

burst 2

DL

burst n

Initial

maint.

request

conn.

downlink subframe

SS #2 SS #k

uplink subframe

«

«

«

«

base station tells nodes who will get to receive (DL map)and who will get to send (UL map), and when

WiMAX standard provide mechanism forscheduling, but not scheduling algorithm



What is mobility?

spectrum of mobility, from the network perspective:

no mobility high mobility

mobile wireless user,using same access

point

mobile user, passingthrough multiple

access point whilemaintaining ongoingconnections (like cellphone)

mobile user,connecting/

disconnectingfrom networkusing DHCP.



Mobility: Vocabulary

home network: permanent´homeµ of mobile(e.g., 128.119.40/24)

Permanent address:

address in homenetwork, can always beused to reach mobilee.g., 128.119.40.186

home agent: entity that willperform mobility functions onbehalf of mobile, when mobileis remote

wide areanetwork

correspondent



Mobility: more vocabulary

Care-of-address: address

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

wide areanetwork

visited network: networkin which mobile currentlyresides (e.g., 79.129.13/24)

Permanent address: remainsconstant (e.g., 128.119.40.186)

foreign agent: entityin visited networkthat performsmobility functions onbehalf of mobile.

correspondent: wantsto communicate with

mobile



How do you contact a mobile friend:

search all phone

books? call her parents?

expect her to let youknow where he/she is?

I wonder whereAlice moved to?

Consider friend frequently changingaddresses, how do you find her?



Mobility: approaches

Let routing handle it: routers advertise permanentaddress of mobile-nodes-in-residence via usualrouting table exchange.

routing tables indicate where each mobile located no changes to end-systems

Let end-systems handle it:

indirect routing: communication from

correspondent to mobile goes through homeagent, then forwarded to remote

direct routing: correspondent gets foreignaddress of mobile, sends directly to mobile



Mobility: approaches

Let routing handle it: routers advertise permanentaddress of mobile-nodes-in-residence via usualrouting table exchange.

routing tables indicate where each mobile located no changes to end-systems

let end-systems handle it:

indirect routing: communication from

correspondent to mobile goes through homeagent, then forwarded to remote

direct routing: correspondent gets foreignaddress of mobile, sends directly to mobile

notscalable

to millions of

mobiles



Mobility: registration

End result:

Foreign agent knows about mobile

Home agent knows location of mobile

wide areanetwork

home network visited network

1

mobile contactsforeign agent onentering visitednetwork

2

foreign agent contacts homeagent home: ´this mobile isresident in my networkµ



Mobility via Indirect Routing

wide areanetwork

homenetwork

visitednetwork

3

2

41

correspondentaddresses packetsusing home addressof mobile

home agent interceptspackets, forwards toforeign agent

foreign agentreceives packets,forwards to mobile

mobile repliesdirectly tocorrespondent



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 whencorrespondent, mobile

are in same network



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!

ccn chapter6 4th ed june 18 2007

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