6: Wireless and Mobile Networks 6-1

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

6: Wireless and Mobile Networks 6-2

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” cell towers 802.11 access

points

6: Wireless and Mobile Networks 6-3

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

6: Wireless and Mobile Networks 6-4

Characteristics of selected wireless link standards

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

Dat

a ra

te (

Mbp

s)

Mesh

6: Wireless and Mobile Networks 6-5

WiMax Vs. Wireless Mesh

WiMax Similar to cellular network infrastructure Use licensed spectrum 10 Mbit/s at 10 km in good environment Is under development by many companies

Wireless Mesh Extension of 802.11 Wireless LAN Use unlicensed public spectrum 802.11’s access routers interconnect together

• Ad Hoc (usually non-mobile) networking and routing Currently used in some places

• Town & small city’s government agents (firefighter, police)

– More popular in Europe than in US• Challenges: complex routing, high error rate, QoS

6: Wireless and Mobile Networks 6-6

Elements of a wireless network

network infrastructure

infrastructure mode base station

connects mobiles into wired network

handoff: mobile changes base station providing connection into wired network

6: Wireless and Mobile Networks 6-7

Elements of a wireless network

Ad hoc mode no base stations nodes can only

transmit to other nodes within link coverage

nodes organize themselves into a network: route among themselves

Wireless active research area:

Ad hoc networkSensor network

6: Wireless and Mobile Networks 6-8

Ad Hoc Vs. Sensor Networks

Ad Hoc network Challenge Mobility of nodes Good features: Plenty of power, computation

resource Applications

• Mostly mobile laptops or PDAs• Vehicular network

Sensor network Challenge limited power, computing resource Good features:

• Usually stationary, dense network Applications

• Military battlefield, civil engineering, environmental monitoring

Wireless, Mobile Networks 6-9

Wireless network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects to base 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 larger Internet: mesh net

no base station, noconnection to larger Internet. May have torelay to reach other a given wireless node

MANET, VANET

6: Wireless and Mobile Networks 6-10

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 at destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

6: Wireless and Mobile Networks 6-11

IEEE 802.11 Wireless LAN

802.11b 2.4-2.485 GHz

unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

• all hosts use same chipping code

widely deployed, using base stations

802.11a 5.1-5.8 GHz range up to 54 Mbps

802.11g 2.4-2.485 GHz range up to 54 Mbps Use OFDM in physical

layer All use CSMA/CA for

multiple access All have base-station

and ad-hoc network versions

6: Wireless and Mobile Networks 6-12

802.11 LAN architecture

wireless host communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka “cell”) in infrastructure mode contains: wireless hosts access point (AP):

base station ad hoc mode: hosts

only

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP

6: Wireless and Mobile Networks 6-13

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

channels at different frequencies 11 channels are partial overlapping (1, 6, 11 non-

overlapping) AP admin chooses frequency for AP interference possible: channel can be same as that

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 with may perform authentication [Chapter 8] will typically run DHCP to get IP address in AP’s

subnet

6: Wireless and Mobile Networks 6-14

IEEE 802.11: multiple access 802.11: CSMA - sense before transmitting

don’t collide with ongoing transmission by other node

802.11: 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)

AB

CA B C

A’s signalstrength

space

C’s signalstrength

6: Wireless and Mobile Networks 6-15

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 backoff timetimer counts down while channel idletransmit when timer expiresif (no ACK)

increase random backoff interval, repeat 2

else /* received ack */return back to 2 (why?) to transmit next frame

802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due to

hidden terminal problem) (no ack in ethernet!!)

sender receiver

DIFS

data

SIFS

ACK

DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing

6: Wireless and Mobile Networks 6-16

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 using CSMA RTSs may still collide with each other (but they’re

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

sender transmits data frame other stations defer transmissions

Avoid long data frame collisionsusing small reservation packets!

6: Wireless and Mobile Networks 6-17

Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DIFS

CIFS

CIFS

CIFS

6: Wireless and Mobile Networks 6-18

RTS/CTS in Practice

RTS/CTS introduces delay, consume channel resource. Benefit when the data frame is much larger

than RTS/CTS. APs set threshold of data frame length

in order to use RTS/CTS If > threshold, use RTS/CTS

Many APs skip RTS/CTS by using a threshold larger than the Max frame length

6: Wireless and Mobile Networks 6-19

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame: addressing

Address 2: MAC addressof wireless host or AP transmitting this frame

Address 1: MAC addressof wireless host or AP to receive this frame

Address 3: MAC addressof router interface to which AP is attached

Address 4: used only in ad hoc mode

6: Wireless and Mobile Networks 6-20

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

6: Wireless and Mobile Networks 6-21

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

802.11 frame: moreduration of reserved transmission time (data, RTS/CTS)

frame type(RTS, CTS, ACK, data)

6: Wireless and Mobile Networks 6-22

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

AP2 broadcast H1’s MAC to switch

6: Wireless and Mobile Networks 6-23

802.15 MAC and Bluetooth

802.11 MAC 11 Mbps – 54 Mbps Up to 100 meters range

802.15 MAC Wireless personal area network (WPAN) < 10 meters range Simple (cheap) device, low power

assumption Cable, wire replacement

• E.g., mouse, keyboard, headphone Example: Bluetooth

6: Wireless and Mobile Networks 6-24

Bluetooth

Physical layer properties: 2.4GHz unlicensed spectrum Frequency-hopping spread spectrum

• 79 channels with different frequencies• TDM transmit: jump among channels with preset

sequences (coding) Up to 721bps (802.11 is 11 Mbps to 54

Mbps)

6: Wireless and Mobile Networks 6-25

Bluetooth Ad hoc network

structure One master, <=7

slaves Odd time slot: master Even time: slaves

Parked: inactive devices

Problem: slow speed can be achieved by RF device Much cheaper, simpler

6: Wireless and Mobile Networks 6-26

CDMA Principle (6.2.1)

Code Division Multiple Access Wide spectrum technique All users use the full spectrum Users with different codings not interfere

Each bit is encoded by much high rate signal (code) Receiver can recover the bit with the

corresponding code

6: Wireless and Mobile Networks 6-27

CDMA example

6: Wireless and Mobile Networks 6-28

Working with multiple users

How to extract data when multiple users transmit at the same time?

Assumptions: Interfering signals are additive Signal 1+1+1+(-1) = 2

New signals in the air (N senders):

Same decoding formula!

6: Wireless and Mobile Networks 6-29

Why extract correctlyBy each user?

A: user codes are orthogonal

Wireless, Mobile Networks 6-30

Mobile Switching

Center

Public telephonenetwork

Mobile Switching

Center

Components of cellular network architecture

connects cells to wired tel. 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

Wireless, Mobile Networks 6-31

Cellular networks: the first hopTwo techniques for sharing

mobile-to-BS radio spectrum

combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots

CDMA: code division multiple access

frequencybands

time slots

Wireless, Mobile Networks 6-32

BSCBTS

Base transceiver station (BTS)

Base station controller (BSC)

Mobile Switching Center (MSC)

Mobile subscribers

Base station system (BSS)

Legend

2G (voice) network architecture

MSC

Public telephonenetwork

GatewayMSC

G

Wireless, Mobile Networks 6-33

3G (voice+data) network architecture

radionetwork controller

MSC

SGSN

Public telephonenetwork

GatewayMSC

G

Serving GPRS Support Node (SGSN)

Gateway GPRS Support Node (GGSN)

Public Internet

GGSN

G

Key insight: new cellular datanetwork operates in parallel (except at edge) with existing cellular voice network voice network unchanged in core data network operates in parallel

Wireless, Mobile Networks 6-34

radionetwork controller

MSC

SGSN

Public telephonenetwork

GatewayMSC

G

Public Internet

GGSN

G

radio access networkUniversal Terrestrial Radio Access Network (UTRAN)

core networkGeneral Packet Radio Service

(GPRS) Core Network

publicInternet

radio interface(WCDMA, HSPA)

3G (voice+data) network architecture