Wireless LAN and Bluetooth

Wireless Networks 1

ECE 6160: Advanced Computer Networks

Wireless Networks

Instructor: Dr. Xubin (Ben) HeEmail: [email protected]

Tel: 931-372-3462

Slides are adapted from the original slides developed by Kurose and Ross.All material copyright 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved

mailto:[email protected]

Wireless Networks 2

Background: # wireless (mobile) phone subscribers now

exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDAs,

Internet-enabled phone promise anytime untethered Internet access

two important (but different) challenges wireless: communication over wireless link mobility: handling the mobile user who changes

point of attachment to network

Wireless Networks 3

Topics about wireless networksWireless Wireless links,

characteristics CDMA

IEEE 802.11 wireless LANs (“wi-fi”)

Bluetooth Cellular Internet

Access architecture standards (e.g., GSM)

Mobility Principles: addressing

and routing to mobile users

Mobile IP Handling mobility in

cellular networks Mobility and higher-

layer protocols

Wireless Networks 4

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

Wireless Networks 5



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

Wireless Networks 6



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

Wireless Networks 7

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

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 3G

802.15

802.11b

802.11a,g

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

802.16 (WiMAX)

802.11a,g point-to-point

200 802.11n

Dat

a ra

te (

Mbp

s)

data

Wireless Networks 8



infrastructure mode base station

connects mobiles into wired network

handoff: mobile changes base station providing connection into wired network

Wireless Networks 9


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 Networks 10

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


Wireless Link Characteristics (1)

Differences 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”


Wireless Link Characteristics (2)

SNR: signal-to-noise ratio larger SNR – easier to

extract signal from noise (a “good thing”)

SNR versus BER tradeoffs given physical layer:

increase power -> increase SNR->decrease BER

given SNR: choose physical layer that meets BER requirement, giving highest throughput

• SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4


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

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 “code” assigned to each user; i.e., code

set partitioning all users share same frequency, but each user

has own “chipping” 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 “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)


CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.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 Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 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 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M


CDMA: two-sender interference


IEEE 802.11 Wireless LAN

802.11b 2.4-5 GHz unlicensed

spectrum up to 11 Mbps direct sequence spread

spectrum (DSSS) in physical layer

• all hosts use same chipping code

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 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


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

into 11 channels at different frequencies 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


IEEE 802.11: multiple access avoid collisions: 2+ nodes transmitting at same

time 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


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 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

802.11 receiver- if frame received OK

return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK


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 data frame collisions completely using small reservation packets!


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


framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame

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

duration of reserved transmission time (RTS/CTS)

frame seq #(for reliable ARQ)


Bluetooth

A standard permitting for wireless connection of:

Personal computers Printers Mobile phones Handsfree headsets LCD projectors Modems Wireless LAN devices Notebooks Desktop PCs PDAs


Bluetooth Devices

NOKIA 9110 + FUJIDIGITAL CAMERA

ERICSSONCOMMUNICATOR

ERICSSON R520GSM 900/1800/1900

ALCATELOne TouchTM 700

GPRS, WAPERICSSON

BLUETOOTHCELLPHONE

HEADSET

http://www.cellular.co.za/ericsson_r520.htm

http://www.cellular.co.za/alcatel_one_touch_700_wap.htm


Bluetooth Application Areas

Bluetooth provides support for 3 general application areas using short range wireless connectivity: Data and voice access points

• Real-time voice and data transmissions

Cable replacement• Eliminates need for numerous cable attachments for connection

Ad hoc networking• Device with Bluetooth radio can establish connection with another

when in range


Profiles: Bluetooth usage models

Set of protocols to implement a particular Bluetooth-based application, defined as profile.

File transfer Internet bridge LAN access Synchronization: phone book, task lists… Three-in-one phone Headset: act as a remote device’s audio I/O


The ultimate headset Bluetooth removes the cable between the headset and the

telephone. The user of a headset is not physically tied to the audio device

and is free to roam about the area. The same headset can be used with multiple devices. The same headset used with a telephone might also be usedwith a cordless telephone base station and could be used foraudio interaction with computers. Using voice recognition it may be possible to place telephone calls using only headset as the user interface.


The three in one phone The three in one usage model allows a mobile

telephone to be used: as a cellular phone in the standard manner. As a cordless phone connecting to a voice access point. As a an walkie-talkie for direct phone-to-phone

communication with another device in proximity (why to use it when you can shout ).


File Transfer In an interactive conference room

scenario, business cards and files could be exchanged among the participants.


Direct network access It is possible to implement LAN bridges

where one port has Bluetooth interface instead of the wired serial access.


Piconets and Scatternets

Piconet Basic unit of Bluetooth networking Master and one to seven slave devices Master determines channel and phase

Scatternet: When two or more piconets at least partially overlap in time and space a scatternet is formed. Device in one piconet may exist as master or slave in another

piconet Allows many devices to share same area Makes efficient use of bandwidth

Each piconet has its own hopping pattern determined by the master.


Scatternet

Master A Master B

Active slave A1

Active slave A3

Active slave B1

Active slave B2

Active slave A2Active slave B3

Piconet A Piconet B


Master and Slave roles in piconet

A given master may communicate with up to 7 active slaves and up to 255 parked slaves.

All slaves communicating with a single master form a piconet.

There can be only one master in a single piconet. In the higher protocol layers, devices operate as peers

Master

Slave1

Slave2

Slave3

piconet


Communications topology The Bluetooth network model uses peer-to-peer

communications based on proximity. The slaves within the area of proximity can form a

piconet.

Proximity sphere

Piconet

Master

Active slave

Parked slave

sniff slave

hold slave

Active slave

Standby slave

Standby slave


Frequency Hopping in Bluetooth

Provides resistance to interference and multipath effects

Provides a form of multiple access among co-located devices in different piconets


Frequency Hopping

Total bandwidth divided into 1MHz physical channels

FH occurs by jumping from one channel to another in pseudorandom sequence

Hopping sequence shared with all devices on piconet (FH channel)

Piconet access: Bluetooth devices use time division duplex (TDD) Access technique is TDMA FH-TDD-TDMA


ARQ Scheme Elements

Error detection – destination detects errors, discards packets

Positive acknowledgment – destination returns positive acknowledgment

Retransmission after timeout – source retransmits if packet unacknowledged

Negative acknowledgment and retransmission – destination returns negative acknowledgement for packets with errors, source retransmits

Wireless LAN and Bluetooth

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Transcript of Wireless LAN and Bluetooth