Data Nets Wireless LL and mobility

8/10/2019 Data Nets Wireless LL and mobility

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Wireless, Mobile Networks 6-1

Ch. 6: Wireless and Mobile Networks

Background: ! # wireless (mobile) phone subscribers now exceeds #

wired phone subscribers (5-to-1)!! # wireless Internet-connected devices equals #

wireline Internet-connected devices" laptops, Internet-enabled phones promise anytime untetheredInternet access

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

" mobility: handling the mobile user who changes point ofattachment to network


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Chapter 6 outline

6.1 Introduction

Wireless6.2 Wireless links,

characteristics" CDMA

6.3 IEEE 802.11 wirelessLANs (Wi-Fi)

6.4 Cellular Internet Access

Mobility6.5 Principles: addressing and

routing to mobile users6.6 Mobile IP6.7 Handling mobility in

cellular networks6.8 Mobility and higher-layer

protocols

6.9 Summary


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Elements of a wireless network

networkinfrastructure

cell tower in cellular network (e.g. 3G or 4G)

access pointin 802.11wireless LAN


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wireless hosts! laptop, smartphone! run applications! may be stationary (non-

mobile) or mobile" wireless does not alwaysmean mobility




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base station! typically connected to

wired network! responsible for sending

packets between wired

network and wirelesshost(s) in its area " e.g., cell towers,

802.11 access points




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wireless link! typically used to connect

mobile(s) to base station! multiple access protocol

coordinates link access! various data rates,

transmission distance




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Characteristics of selected wireless links

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m 4 Km

Long-rangeoutdoor

5Km 20 Km

.056

.384

1

4

5-11

54

2G: IS-95, CDMA, GSM

2.5G: UMTS/WCDMA, CDMA2000

802.15

802.11b

802.11a,g

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

4G: LTWE WIMAX

802.11a,g point-to-point

200 802.11n

D a

t a r a

t e ( M b p s

)


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hosts operate ininfrastructure mode

! all services via base station(connects mobiles into wirednetwork)

! handoff: mobile changes basestation providing connectioninto wired network




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ad hoc mode! no base stations! nodes can only

transmit to othernodes within link

coverage! nodes organize

themselves into anetwork: routeamong themselves



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Wireless network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects tobase station (WiFi,

cellular)which connects to

larger Internet

no base station, noconnection to larger

Internet (e.g. Bluetooth)

host may have torelay through several

wireless nodes toconnect to larger

Internet: mesh net

no base station, noconnection to largerInternet. May have torelay to reach other

wireless nodeMANET (mobile ad hoc netw)


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Chapter 6 outline

6.1 Introduction






routing to mobile users6.6 Mobile IP

6.7 Handling mobility incellular networks

6.8 Mobility and higher-layerprotocols

6.9 Summary


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Wireless Link Characteristics (1)

important differences from wired link ." decreased signal strength: radio signal strength

decreases as it propagates through matter ( path loss )" interference from other sources: standardized wireless

network frequencies (e.g., 2.4 GHz) shared by otherdevices (e.g., phone); devices (e.g. microwave)interfere as well

" multipath propagation: radio signals reaching the receivingantenna by two or more paths because radio signalreflects off objects and the ground

. make communication across (even a point to point)wireless link much more difficult => powerful CRCerror detection & reliable data transfer protocols


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Wireless Link Characteristics (2)! SNR: signal-to-noise ratio

" measures signal strength relative tonoise => larger SNR gives easier toextract signal from background noise

! three different modulation schemes:" amplitude modulation of a signal: change

the amplitude of periodic waveform toencode information (QAM =Quadrature amplitude modulation)

! SNR versus Bit-Error-Rate (BER)" for given modulation scheme:

increase transmission power -> increaseSNR->decrease BERbut: little practical gain beyond certainthreshold, stronger signal costs moreenergy (bad for battery-powered users)

10 20 30 40

QAM256 (8)

QAM16 (4)

BPSK (1)

SNR(decibels)

B E R

10 -1

10 -2

10 -3

10 -5

10 -6

10 -7

10 -4


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Wireless Link Characteristics (2)! SNR versus Bit-Error-Rate (BER)

" for given SNR: choose modulationscheme that meets BERrequirement and gives highestthroughput

" SNR may change with mobility:dynamically adapt modulationtechnique and rate to keep BERlow! (e.g. with SNR of 10 dB we haveto choose BPSK to avoid high BER)

" when sending at high bit rate,differences in amplitude are low=> errors while we sample thesignal

10 20 30 40

QAM256 (8)

QAM16 (4)

BPSK (1)

SNR(decibels)

B E R

10 -1

10 -2

10 -3

10 -5

10 -6

10 -7

10 -4


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Wireless network characteristicsMultiple wireless senders and receivers create additional

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 other

=> A, C unaware of theirinterference at B

A B C

A s signalstrength

space

C s signalstrength

Fading problem:! B, A hear each other!

B, C hear each other! A, C can not hear each other assignal strength decreases =>interference at B


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Code Division Multiple Access (CDMA)!

uses: 3G mobile networks (UMTS, etc.)! channel partitioning protocol! unique code assigned to each user! all users share same frequency, but each user uses

his own code to encode data=> allows multiple users to coexist and transmitsimultaneously with minimal interference (if codes areorthogonal )

! encoded signal = (original data) x (code)! decoding: sum of product of encoded signal and code! divide time slot for transmitting a bit into many mini

slots! view 0 are -1


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CDMA encode/decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= d i.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Z i,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 11- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = ! Zi,m .cmm=1

M

M8x1/8-8x1/8


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CDMA: two-sender interference

using same code as sender

1, receiver recovers sender1s original data fromsummed channel data!

Sender 1

Sender 2

channel sums together

transmissions by sender 1and 2

(-2-2-2-2)/8 (2+2+2+2)/8


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CDMA: Code Division MA Drawbacks:

- Synchronization of all users is required - Multipath propagation: delayed copies of signal may be received whichare not orthogonal any longer!

- Near-far problem: nearby users will completely swamp far away users(signal of users far away cannot be received correctly) => accuratepower control needed

- having J more codes, the bandwidth must J times larger Main advantage: - when using FDMA or TDMA for cellular systems, frequencies used in acell cannot be reused in adjacent cells (to avoid interference); in CDMAone can reuse the frequencies

more details:A. Viterbi, CDMA: Principles of Spread Spectrum Communication,

Addison Wesley, 1995.


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Chapter 6 outline

6.1 Introduction







6.7 Summary


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IEEE 802.11 Wireless LAN

! different standards (802.11a/b/g/n)! differ in modulation techniques used (=> different maximal

transmission rate) and in frequency range (2.4 or 5 GHz)! all use CSMA/CA (carrier sense multiple access with collision

avoidance)! all have base-station and ad-hoc network versions


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802.11 LAN architecture! wireless host communicates

with base station" base station = access point (AP)

! Basic Service Set (BSS)contains:

" wireless hosts" access point (AP): base

station" ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor router


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802.11: Channels, association! 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels

at different (partly overlapping) frequencies" admin chooses frequency for AP" interference possible: channels can be same or overlapping

! host: must associate with an AP" scans channels, listening for beacon frames (beacon =

Blinklicht) containing AP s name (service set identifier =SSID) and MAC address

" selects AP to associate with" may perform authentication" will typically run DHCP to get IP address in AP s subnet


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802.11: passive/active scanning

AP 2 AP 1

H1

BSS 2BSS 1

12 3

1

passive scanning: (1) beacon frames sent from APs(2) association Request frame sent: H1 to

selected AP(3) association Response frame sent from

selected AP to H1

AP 2 AP 1

H1

BSS 2BSS 1

122

3 4

active scanning :(1) Probe Request frame broadcast

from H1(2) Probe Response frames sent

from APs(3) Association Request frame sent:

H1 to selected AP(4) Association Response frame sent

from selected AP to H1


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IEEE 802.11: multiple access! random access protocol as for Ethernet (carrier sense MAC)! sense before transmitting

" refrain from transmitting when channel is sensed busy! no collision detection during transmission!

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

received signals that gets overwhelmed by sending signal" can t sense all collisions in any case: hidden terminal, fading" => once transmission is running, it is never aborted (as opposed to

Ethernet when collision is detected)

! optional feature: try to avoid collisions when severalnodes are transmitting at the same time

" goal: avoid collisions: CSMA/C(ollision)A(voidance)


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IEEE 802.11 MAC Protocol: CSMA/CA! SIFS= Short Inter-frame Spacing

" Time required for a receivingstation to sense the end of aframe and start transmitting anACK (compute CRC etc.)

! DIFS= Distributed Inter-frameSpacing" sender waits before transmission" If the medium is continuously idle for

this duration, only then a node is

supposed to transmit a frame." DIFS = SIFS + (2 * Slot time) where

slot time is 20 s in 802.11b

sender receiver

DIFS

data

SIFS

ACK


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IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for time period 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

(binary exponential), repeat step 2

802.11 receiver

- if frame received OKreturn ACK after time period SIFS (ACK

needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK


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IEEE 802.11 MAC Protocol: CSMA

sender receiver

DIFS

data

SIFS

ACK

Why not send directly when channelis sensed idle?Assume three senders, one istransmitting and finishes=> other senders both start sending

=> collision With random backoff time, one ofthe senders might send directlywhile the other waits

When do collisions occur: same

backoff time chosen or senders arehidden from each other (cant sensewhether channel idle)


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decrement byone slot time


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!"#$%& ()*+,-./0122 3%1/&4$%&

IEEE 802.11 MAC Protocol: CSMA

source: IEEE Std 802.11TM-2012 (Revision of IEEE Std 802.11-2007)


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Optional avoidance mechanismidea: allow sender to reserve channel rather than random

access of data frames: avoid collisions of long data frames! sender first transmits small request-to-send (RTS) packets

to BS using CSMA" RTSs may still collide with each other (but they re short)

! BS broadcasts clear-to-send (CTS) in response to RTS! CTS heard by all nodes

" sender transmits data frame" other stations defer transmissions

avoid data frame collisions completelyusing small reservation packets!


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Collision Avoidance: RTS-CTS exchange

AP A B

time

RT S ( A) R TS(B )

RT S ( A)

C TS( A ) CT S ( A)

DATA (A)

AC K( A ) ACK ( A)

reservation collision

defer


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framecontrol duration

address1

address2

address4

address3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame: four address fields!

Address 2: MAC addressof wireless host or APtransmitting this frame(sender : wireless host or AP)

Address 1: MAC addressof wireless host or APto receive this frame(destination within subnet)

Address 3: MAC addressof router interface towhich AP is attached(=> send to Internet; seeexample on next slide)

Address 4: used only inad hoc mode

bytes


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InternetrouterH1 R1

H1 MAC addr AP MAC addr R1 MAC addraddress 1 address 2 address 3

802. 11 frame

H1 MAC addr R1 MAC addr

dest. address source address

802. 3 Ethernet frame

802.11 frame: addressing

from the routersperspective, theAP is invisible(ethernet subnet!)

now, H1 knows MAC adressof source router


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InternetrouterH1 R1

AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3

802. 11 frame

R1 MAC addr H1 MAC addr

dest. address source address

802. 3 Ethernet frame

802.11 frame: addressing

address 3 allows theAP to determinecorrect destinationMAC address


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framecontrol duration

address1

address2

address4

address3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFrom APSubtype

To AP

Morefrag WEP

Moredata

PowermgtRetry Rsvd

Protocolversion

2 2 4 1 1 1 1 1 11 1

expected duration of plannedtransmission (including ACKs)when RTS, CTS or data is sent

frame seq #(since frames may be

retransmitted if not ACKed)

frame type(RTS, CTS, ACK, data)

802.11 frame: more

frame sent by access point

detailed specification: http://standards.ieee.org/about/get/802/802.11.html


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802.11: mobility within same subnet

How does switch know which AP isassociated with H1?

self-learning (Ch. 5): switch will see

frame from H1 and

remember

whichswitch port can be used to reach H1

H1 BSS 2BSS 1

H1 detects weakening signal from AP1" scans and finds beacon frames of AP2" dissociates with AP1, associates with AP2" remains in same IP subnet: keep IP

address and all ongoing TCP

connections

better solutions are currently being developed (so that switch is informedwhen H1 disassociates/associates)


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802.11: advanced capabilities

Rate adaptation! base station, mobile

dynamically changetransmission rate (physicallayer modulation technique)as mobile moves, SNR varies

! 2 unackd frames in a row =>next lower modul. scheme

! 10 frames ackd in a row =>

next higher modul. scheme

QAM256QAM16BPSK

10 20 30 40 SNR(dB)

B E R

10-1

10 -2

10 -3

10 -5

10 -6

10 -7

10 -4 operating point

1. SNR decreases, BERincrease as node movesaway from base station

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


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power management! node-to-AP: I am going to sleep until next beacon

frame (power manag. bit in header)" AP knows not to transmit frames to this node (buffers

frames!)" node wakes up before next beacon frame (every 100 msec)! beacon frame: contains list of mobiles with buffered

AP-to-mobile frames" node will stay awake if AP-to-mobile frames to be sent (=>

has to send request to AP)" otherwise sleep again until next beacon frame

802.11: advanced capabilities


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Chapter 6 outline

6.1 Introduction




6.4 Cellular Internet access



6.7 Summary


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MobileSwitching

Center

Public telephonenetwork

MobileSwitching

Center

Components of cellular network architecture

! connects cells to wired tel. net.! manages call setup (more later!)! handles mobility (more later!)

MSC

! covers geographicalregion! base station (BS)analogous to 802.11 AP! mobile users attach tonetwork through BS! air-interface: physicaland link layer protocolbetween mobile and BS

cell

wired network


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Cellular networks: the first hop

Two techniques for sharingmobile-to-BS radio spectrum(1) combined FDMA/TDMA:divide spectrum in frequencychannels, divide each channel into

time slots (used in 2G systems)(Having F different sub-bands andT time slots, how manysimultaneous calls can besupported?)

(2) CDMA: code division multipleaccess (used in 3G UMTS)

frequencybands

time slots


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Various degrees of mobility?! spectrum of mobility, from the network perspective:

no mobility high mobility

mobile wireless user,using same accesspoint

mobile user, passingthrough multipleaccess point while

maintaining ongoingconnections ( e.g.cellular access in anICE)

mobile user,connecting/disconnecting from

network usingDHCP (e.g. movingto different room).


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wide areanetwork


Mobility: vocabularyhome network: permanenthome 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 will perform mobility functions onbehalf of mobile, when mobile isremote


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Mobility: more vocabulary

wide areanetwork

care-of-address: addressin visited network.(e.g., 79,129.13.2)

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

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

foreign agent : entity invisited network that

performs mobilityfunctions on behalf ofmobile.

correspondent: wantsto communicate withmobile


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How do you contact a mobile friend:

! search all phone books?! call her parents?! expect her to let you

know where he/she is?

I wonder where Alice moved to?

Consider friend frequently changingaddresses, how do you find her?


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Mobility: approaches

! let routing handle it: routers advertise permanent address ofmobile-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: communication from correspondent to

mobile goes through home agent, then forwarded toremote

" direct routing: correspondent gets foreign address ofmobile, sends directly to mobile

search all phone books

call her parents

expect her to let you know


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! let routing handle it: routers advertise permanent address ofmobile-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: communication from correspondent to

mobile goes through home agent, then forwarded toremote

" direct routing: correspondent gets foreign address ofmobile, sends directly to mobile

notscalable

to millions of mobiles

Mobility: approaches


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wide areanetwork


Mobility: registration

end result:! foreign agent knows about mobile! home agent knows location of mobile

home networkvisited network

1

mobile contactsforeign agent onentering visitednetwork

2

foreign agent contacts homeagent: this mobile is resident inmy network


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Mobility via indirect routing

wide areanetwork

homenetwork

visitednetwork

3

2

41

correspondentaddresses packets

using home address ofmobile

home agent interceptspackets, forwards toforeign agent

foreign agentreceives packets,forwards to mobile

mobile replies

directly tocorrespondentsource address is

permanent address

=> usage: see Mobile IP (later)


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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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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! inefficient when correspondent and mobile are in

same network


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1 23

4


Mobility via direct routing

homenetwork

visitednetwork

correspondentrequests, receives

foreign address ofmobile

correspondent forwardsto foreign agent

foreign agentreceives packets,forwards to mobile

mobile repliesdirectly to

correspondent

used for routing telephone calls tomobile users in several mobile telephonenetwork standards, including GSM


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Mobility via direct routing: comments

! more efficient if correspondent is in same orclose network! non-transparent to correspondent: correspondent

must get care-of-address from home agent

" what if mobile changes visited network?" how to inform correspondent?

12

3

4


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wide areanetwork

1

foreign net visitedat session start

anchorforeignagent 2

4

new foreignagent

3

correspondentagent

correspondent

newforeignnetwork

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)! requires additional coordination among the foreign agents

5


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Chapter 6 outline

6.1 Introduction







6.7 Summary


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Mobile IP: indirect routing

Permanent address:128.119.40.186

Care-of address:79.129.13.2

dest: 128.119.40.186packet sent bycorrespondent

dest: 79.129.13.2 dest: 128.119.40.186

packet sent by home agent to foreignagent: a packet within a packet

dest: 128.119.40.186

foreign-agent-to-mobile packetRFC 5944


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Mobile IP: agent discovery

agent advertisement: foreign/home agents advertiseservice for mobile nodes by broadcasting ICMPmessages (type field = 9)=> mobile node gets care-of-address


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Mobile IP: registration examplevisited network: 79.129.13/24

home agentHA: 128.119.40.7

foreign agentCOA: 79.129.13.2

mobile agentMA: 128.119.40.186

registration req.COA: 79.129.13.2HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification:714! .

registration replyHA: 128.119.40.7MA: 128.119.40.186Lifetime: 4999Identification: 714! .

registration reply

HA: 128.119.40.7MA: 128.119.40.186Lifetime: 4999Identification: 714! .

time

ICMP agent adv.COA: 79.129.13.2! .

registration req.COA: 79.129.13.2

HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification: 714! .

now, FA knowsthat it should

look fordatagrams withCOA andforward themdecapsulated

moredetails:RFC 5944


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Current state of Mobile IPnot widely deployed yet because:! need for seamless handover is given for few

applications only (e.g. VoIP, video conferencing)! handover is nice to have, re-connection is mainly

sufficient! GSM (telephone) networks have their own mobility

solutions (indirect routing; phone number ispermanent, roaming number is COA)

! deployed in WiMAX networks (IEEE 802.16)



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Chapter 6 summary

Wireless! wireless links:

" capacity, distance" channel impairments" CDMA

! IEEE 802.11 (Wi-Fi)" CSMA/CA reflects wireless

channel characteristics! cellular access

Mobility! principles: addressing,

routing to mobile users" home, visited networks" direct, indirect routing" care-of-addresses

! case studies" mobile IP

Documents

Data Nets Wireless LL and mobility