Wireless Networks 1 ECE 6160: Advanced Computer Networks Wireless Networks Instructor: Dr. Xubin (Ben) He Email: [email protected]Tel: 931-372-3462 Slides are adapted from the original slides developed by Kurose and Ross. All material copyright 1996-2007 J.F Kurose and K.W. Ross, All Rights Reserved
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handoff: mobile changes base station providing connection into wired network
Wireless Networks 9
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 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 Networks 11
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 Networks 12
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 Networks 13
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
Wireless Networks 14
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”)
Wireless Networks 15
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
Wireless Networks 16
CDMA: two-sender interference
Wireless Networks 17
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
Wireless Networks 18
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
Wireless Networks 19
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
Wireless Networks 20
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,
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
Wireless Networks 28
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
Wireless Networks 29
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.
Wireless Networks 30
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 ).
Wireless Networks 31
File Transfer In an interactive conference room
scenario, business cards and files could be exchanged among the participants.
Wireless Networks 32
Direct network access It is possible to implement LAN bridges
where one port has Bluetooth interface instead of the wired serial access.
Wireless Networks 33
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.
Wireless Networks 34
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
Wireless Networks 35
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
Wireless Networks 36
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
Wireless Networks 37
Frequency Hopping in Bluetooth
Provides resistance to interference and multipath effects
Provides a form of multiple access among co-located devices in different piconets
Wireless Networks 38
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