6: Wireless and Mobile Networks - PowerPoint Presentation

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6: Wireless and Mobile Networks

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Chapter 6Wireless and Mobile Networks

Computer Networking: A Top Down Approach Featuring the Internet, 6th edition. Jim Kurose, Keith RossAddison-Wesley

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2009J.F Kurose and K.W. Ross, All Rights Reserved 11/8/2014

Modified by John Copeland,Georgia Tech,for use in ECE3600

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Chapter 6: Wireless and Mobile NetworksBackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! (2008)computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challengescommunication over wireless link

handling mobile user who changes point of attachment to network

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Chapter 6 outline6.1 Introduction Wireless6.2 Wireless links, characteristicsCDMA (Carrier Detect)

6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards (e.g., GSM)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

network

infrastructure

wireless hosts

laptop, PDA, IP phone

run applications

may be stationary (non-mobile) or mobile

wireless does

not

always mean mobility

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Elements of a wireless network

network

infrastructure

base station

typically connected to wired network

relay - responsible for sending packets between wired network and wireless host(s) in its

“

area

”

e.g., cell towers 802.11 access points

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Elements of a wireless network

network

infrastructure

wireless link

typically used to connect mobile(s) to base station

also used as backbone link

multiple access protocol coordinates link access

various data rates, transmission distance

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Characteristics of selected wireless link standards

Bluetooth

Indoor10-30mOutdoor50-200m

Mid-rangeoutdoor200m – 4 Km

Long-range

outdoor

5Km – 20 Km

.056

.384

1

4

5-11

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 (4G?))

802.11a,g point-to-point

200

802.11n

Data rate (Mbps)

data

NFR

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Wireless Access Point (WAP) and Directional Antenna

("Cantenna") to extend range in one direction (4x).

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Elements of a cellular wireless network

network

infrastructure

infrastructure mode

base station

(

or Wireless Access Point, WAP)

connects mobiles into wired network

handoff: mobile changes base station providing connection into wired network

Slide10

http://upload.wikimedia.org/

wikipedia/en/5/57/CellTowersAtCorners.gif

1

1 7 Types of Cells,21 Sets of Frequencies (3 for each cell type)

Cellular Network Frequency Reuse151588

Cell Tower

Freq. B

Freq. C

Freq. A

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Elements of a wireless network

Ad hoc mode

no base stationsnodes can only transmit to other nodes within link coverage

nodes organize themselves into a network: route among themselves

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Wireless Link CharacteristicsDifferences from 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 ad destination at slightly different times…. make communication across (even a point to point) wireless link much more “difficult”

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

larger SNR – easier to extract signal from noise (a “good thing”)SNR versus BER tradeoffsgiven physical layer: increase power -> increase SNR->decrease BERgiven SNR: choose physical layer that meets BER requirement, giving highest thruputSNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)

10

20

30

40

QAM256 (8 Mbps) (1M symbols/s)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BER

10

-1

10

-2

10

-3

10

-5

10

-6

10

-7

10

-4

Unusable

Weak <- SIGNAL -> Strong

BER = bit error rate

10

-4

means 1 error per 10,000 bits

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Wireless network characteristicsMultiple wireless senders and receivers create additional problems (beyond multiple access):

A

B

C

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

A

’

s signal

strength

space

C

’

s signal

strength

Signal fading:

B, A hear each other

B, C hear each other

A, C can not hear each other interfering at B

Slide15

Network Core: Circuit Switching

network resources (e.g., bandwidth)

divided into “pieces”pieces allocated to callsresource piece idle if not used by owning call (no sharing)

dividing link bandwidth into “pieces”frequency division (FDM)time division (TDM)code division (CDM)

FDMfrequency

time

TDM

frequency

time

4 users

6: Wireless and Mobile Networks

6-

15

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Code Division Multiple Access (CDMA)used in several wireless broadcast channels (cellular, satellite, etc) standardsunique “code” assigned to each user; i.e., code set partitioningall users share same frequency, but each user has own

“chipping” sequence (i.e., code) to encode dataencoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chipping sequenceallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

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CDMA Encode/Decode

slot 1slot 0

d1 = -11

1

1

1

1

-

1

-

1

-

1

-

Z

i,m

= d

i

.

c

m

d

0

= 1

1

1

1

1

1

-

1

-

1

-

1

-

1

1

1

1

1

-

1

-

1

-

1

-

1

1

1

1

1

-

1

-

1

-

1

-

slot 0

channel

output

slot 1

channel

output

channel output Z

i,m

sender

code

data

bits

slot 1

slot 0

d

1

= -1

d

0

= 1

1

1

1

1

1

-

1

-

1

-

1

-

1

1

1

1

1

-

1

-

1

-

1

-

1

1

1

1

1

-

1

-

1

-

1

-

1

1

1

1

1

-

1

-

1

-

1

-

slot 0

channel

output

slot 1

channel

output

receiver

code

received

input

D

i

=

S

Z

i,m

.

c

m

m=1

M

M

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CDMA: two-sender interference

Orthogonal Codes

Sum over iAi = 0 Bi = 0 Ai x Bi = 0 Ai x(-Bi) = 0

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Chapter 6 outline6.1 Introduction Wireless6.2

Wireless links, characteristicsCDMA6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)6.4 Cellular Internet Accessarchitecturestandards (e.g., GSM)Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7

Handling mobility in cellular networks6.8 Mobility and higher-layer protocols6.9 Summary

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IEEE 802.11 Wireless LAN (WiFi)802.11b2.4-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layerall hosts use same chipping code

802.11a 5-6 GHz rangeup to 54 Mbps802.11g 2.4-5 GHz rangeup to 54 Mbps802.11n: multiple antennae2.4-5 GHz rangeup to 200 Mbpsall use CSMA/CA for multiple accessall have base-station and ad-hoc network versions

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802.11 LAN architecture

wireless host communicates with base stationbase station = access point (AP)Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:wireless hostsaccess point (AP or WAP): base stationad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switch

or router

AP

AP

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802.11: Channels, association802.11b: 2.4GHz - 2.485GHz spectrum divided into 11 channels at different frequencies (only 1, 6, 11 don't overlap)AP admin chooses frequency for APinterference possible: channel can be same as that chosen by neighboring AP!host: must associate

with an APscans channels, listening for beacon frames containing AP’s name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in AP’s subnet

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IEEE 802.11: multiple accessavoid collisions: 2+ nodes transmitting at same time802.11: CSMA - sense before transmittingdon’t collide with ongoing transmission by other node

802.11: no collision detection!difficult to receive (sense collisions) when transmitting due to weak received signalscan’t sense all collisions in any case: hidden terminal, fadinggoal: avoid collisions: CSMA/C(ollision)A(voidance)

A

B

C

A

B

C

A

’

s signal

strength

space

C

’

s signal

strength

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

then transmit entire frame (no CD)2 if sense channel busy then start random back-off timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff interval, repeat 2802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem)

senderreceiver

DIFS

data

SIFS

ACK

(without CA - no collision avoidance)

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Avoiding collisions (more)idea: 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 (WAP) using CSMARTSs may still collide with each other (but they’re short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes within range of the receiver.sender transmits data frameother stations defer transmissions Avoid data frame collisions completely*

using small reservation packets!(*for static wireless hosts)BS-base station, WAP-wireless access point, CSMA-carrier sense multiple access

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Collision Avoidance: RTS-CTS exchange

AP

A

B

time

RTS(A)

RTS(B)

RTS(A)

CTS(A)

CTS(A)

DATA (A)

ACK(A)

ACK(A)

reservation collision

defer

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framecontrol

durationaddress1address2

address4address3payloadCRC

2

2

6

6

6

2

6

0 - 2312

4

seq

control

802.11 frame: addressing

Address 2:

MAC address

of wireless host or AP

transmitting this frame

[Serves as E'net source address]

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 [Serves as E'net destination address]

Sequence No.:

needed for ARQ (ACK required) mode.

Address 4:

MAC address

of wireless relay host (ad hoc networks only)

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Internet

router

AP

H1

R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1

1st dest (AP)

address 2

source

address 3

2nd dest (eth)

802.

11

(WiFi) frame

R1 MAC addr

H1

MAC addr

dest. address

source address

802.

3

(Eth)

frame

802.11 frame: addressing

Ethernet

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framecontrol

durationaddress1address2

address4address3payloadCRC

2

2

6

6

6

2

6

0 - 2312

4

seq

control

Type

From

AP

Subtype

To

AP

More

frag

WEP

More

data

Power

mgt

Retry

Rsvd

Protocol

version

2

2

4

1

1

1

1

1

1

1

1

802.11 frame: more

duration of reserved

transmission time (RTS/CTS)

frame seq #

(for reliable ARQ)

frame type

(RTS, CTS, ACK, data)

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hub or

switch

AP 2

AP 1

H1

BBS 2

BBS 1

802.11: mobility within same subnet

router

H1 remains in same IP subnet: IP address can remain same

switch: which AP is associated with H1?

self-learning (Ch. 5): switch will see frame from H1 and

“

remember

”

which switch port can be used to reach H1

[What about a Hub?]

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802.11: advanced capabilities

Rate Adaptationbase station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10

20

30

40

SNR(dB)

BER

10

-1

10

-2

10

-3

10

-5

10

-6

10

-7

10

-4

operating point

1. SNR decreases, BER increase as node moves away from base station

2. When BER becomes too high, switch to lower transmission rate but with lower BER

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M

radius of

coverageS

S

S

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)

P

802.15: personal area network

(like BlueTooth)

less than 10 m diameter

replacement for cables (mouse, keyboard, headphones)

ad hoc: no infrastructure

master/slaves:

slaves request permission to send (to master)

master grants requests

802.15: evolved from Bluetooth specification

2.4-2.5 GHz radio band

up to 721 kbps

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802.16: WiMAX

like 802.11 & cellular: base station modeltransmissions to/from base station by hosts with omni-directional antennabase station-to-base station backhaul with point-to-point antennaunlike 802.11:range ~ 6 miles (

“city rather than coffee shop”)~14 Mbps

point-to-multipoint

point-to-point

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iStumbler (Mac)

ISS

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Chapter 6 outline6.1 Introduction Wireless6.2 Wireless links, characteristics

CDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Accessarchitecturestandards (e.g., GSM)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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Mobile

Switching

Center

Public telephone

network, and

Internet

Mobile

Switching

Center

Components of cellular network architecture

connects cells to wide area net

manages call setup (more later!)

handles mobility (more later!)

MSC

covers geographical region

base station

(BS) analogous to 802.11 AP

mobile users

attach to network through BS

air-interface:

physical and link layer protocol between mobile and BS

cell

wired network

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Cellular networks: the first hopTwo techniques for sharing mobile-to-BS radio spectrumcombined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slotsCDMA: code division multiple access

frequency

bands

time slots

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Cellular standards: Generations

1G

systems: Cellular, FM, voice, 1 mobile per frequency2G systems: Digital, voice, many mobiles per freq.3G systems: Digital, voice & data Universal Mobile Telecommunications Service (UMTS)GSM next step, but using CDMACDMA-2000: CDMA in TDMA slots (HSPA) 28 Mbps (AT&T)1xEvolution Data Optimized (1xEVDO) 14 Mbps (Verizon)4G systems: Higher speed, like 802.17 WiMAX As of August 2011, U.S. carriers currently use the following (most will go to LTE): AT&T = HSPA+ (63 Mbps) (2011 -> LTE) Sprint = WiMax (47 Mbps) (2013 -> LTE) Verizon = LTE (93 Mbps) T-Mobile = HSPA+ (75 Mbps) (2013 -> LTE)

LTE – 300 Mbps max. (700 MHz band, OFDMA) LTE Advanced – 1000 Mbps max. (8x8 MIMO and 128 QAM)

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Cell Tower Coverage The 450 MHz band was made available by the switch to digital (HiDef) TV and the release of higher frequency TV channels for other services (first responders, universal Internet, ...)

.Frequency bands in the 700 MHz range were auctioned by the FCC in 2012, and there are plans to buy back TV channels in the 600 MHz band.The gain of a half-wave antenna varies as 1/f2. Lower frequencies also penetrate walls better.

30 mi16 mi 9 mi 8 mi

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Wireless, mobility: impact on higher layer protocolslogically, impact should be minimal …best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile… but performance-wise:packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff

TCP interprets loss as congestion, will decrease congestion window un-necessarilydelay impairments for real-time trafficlimited bandwidth (bits/second) of wireless links