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The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
ISM Unlicensed Frequency Bands
ExtremelyLow
VeryLow
Low Medium High VeryHigh
UltraHigh
SuperHigh
Infrared VisibleLight
Ultra-violet
X-Rays
AudioAM Broadcast
Short Wave Radio FM BroadcastTelevision Infrared wireless LAN
Cellular (840MHz)NPCS (1.9GHz)
902-928 MHz26 MHz
5 GHz(IEEE 802.11)
HyperLANHyperLAN2
2.4 – 2.4835 GHz83.5 MHz
(IEEE 802.11)
102210161014
1012
900 MHz vs. 2.4 GHz vs. 5GHz
900 MHz 2.4 GHz
PROs
CONs
Greater Range than 2.4 GHz ( For in- Building LANs)
Maximum Data Rate 1 Mbps
Limited Bandwidth
Crowded Band
Global Market
IEEE 802.11b/g
Higher Data Rates (10+ Mbps)
Less Range than
900 MHz (For In-Building LANs)
5 GHz
Global Market
IEEE 802.11a
Higher Data Rates (20+ Mbps, up to 50M)
Much Less Range than 900 or 2.4GHz
Higher Cost RF Components
Large Antenna required
The 802.11 Standard
199719971–2 Mbps1–2 Mbps
2.4G2.4G
1999199954 Mbps54 Mbps
5G5GOrthogonal Orthogonal
FDMFDM
1999199911 Mbps11 Mbps
2.4G2.4G
2001200154 Mbps54 Mbps
2.4G2.4G
Power Management (awake, doze)Timing Management (beacon, sync within 4usec)
Frequency Hopping
• 79 Channels, 1 MHz Each• Maximum time on any one freq is .4 sec in any 30 sec period• Changes frequency (Hops) at least every 0.4 seconds (dwell
time – adjustable), Lost packets are re-transmitted on next hop
• Synchronized hopping sequences required by same random number generators in every station
• 26 hopping patterns in three different sets with minimum interference with each other – called orthogonal patterns
2.400 GHz 2.483 GHz
Tim
e
12
34
56
78
9
Direct Sequence
Similar to CDMA 22 MHz wide stationary channels (11 chips) Each data bit becomes a string of chips (Barker sequence) transmitted
in parallel across a wide frequency range, data may be decoded from redundant bits
• 3 non-overlapping channels, can move to an alternate channel to avoid interference
• 3 Access Points can occupy same area• 802.11b
• DSSS 1 M baud data rate with 1 bit (1Mbps), 2 bit (2 Mbps)• HR-DSSS 1.375 M baud, 4 bit (5.5M), 8 bit (11 Mbps)
IEEE 802.11 Architecture
ap
apap
Ad-Hoc Mode
Infrastructure Mode
Ad-Hoc Mode
Computers are brought together to form a network. There is no structure to the network; there are no fixed points; and usually every node is able to communicate with every other node.
No administration and pre configuration
IETF MANET (Mobile Ad hoc Networks) working group
Ad-Hoc Mode
Infrastructure Mode
It uses fixed network access points (AP) with which mobile nodes can communicate. These network APs are connected to wired network to widen the LAN's capability by bridging wireless nodes to other wired nodes.
All communications between mobiles and wired network clients go through the AP.
Mobiles can roam between APs and seamless wide area coverage is possible.
ap
apap
Infrastructure Mode
Wireless LANs Issues (CSMA)
The range of a single radio may not cover the entire system Hidden station problem (A->B, C->B since C does not hear A,
collision) Exposed station problem (B->A, C hears B, C won’t send to D,
reduced efficiency)
Multiple Access with Collision Avoidance
IDEA: having a short frame transmitted from both sender and receiver before the actual transfer
A sending a short RTS (30 bytes) to B with length of LB responding with a CTS to A, whoever hears CTS shall remain silent
for the duration of LA sends data (length L) to BFurther optimization as MACAW (MACA for Wireless)
RTS, RTS, not CTSnot CTSTx okTx ok
CTS, CTS, not RTSnot RTS
Keep silentKeep silent
CTS, CTS, RTSRTS
Keep silentKeep silent
Medium Access ControlDistributed Coordination Function
Distributed Control, Ethernet-like CSMACSMA/CA (collision avoidance)
• Physical channel sensing• Sense channel, transmit entire frame, retry if necessary
• Virtual channel sensing (MACAW)• Add ACK frame
Network Allocation Vector Network Allocation Vector (quiet time)(quiet time)
Short frame (30B)Short frame (30B)Contains data Contains data
lengthlength
data length data length copied from RTScopied from RTS
Fragmentation for Throughputfragment burst
Unreliable ISM bandsError rate p = 10-4,
success rate for full Ethernet frame (12,144 bit) <30%, (1-p)**n
Error rate p = 10-6, 1% will be damaged.
Medium Access ControlPoint Coordination Function (PCF)
• Central Control• Base polls other stations
• Broadcast a beacon frame periodically (10ms to 100ms) with system parameters (hopping sequence, dwell time, clock synchronization)
• Base determines the transmission priority• QoS guarantee
• Can Coexist with DCF
Short Short InterFrame InterFrame
SpacingSpacing
RTS/CTS/ACKRTS/CTS/ACK Fragment burstFragment burst
The 802.11 Data Frame Structure
WEPWEP
More More FrameFrame
ssOrdered Ordered framesframes
Data Data ControControlMgmtlMgmt
To APTo AP
RTSRTSCTSCTSACKACK
Intercell Intercell traffic traffic
addressesaddresses
Sleep / Sleep / AwakeAwake
Frame length Frame length plus ack plus ack
(used for NAV)(used for NAV)
Intercell Intercell traffic traffic
addressesaddresses
Fragment Fragment sequencesequence
802.11 AP Services
Distribution Services• Association
• station reports identity, data rate, power
• Disassociation• Reassociation
• handover
• Distribution• routing
• Integration• format conversion
IntraCell Services• Authentication
• conducted after association
• Deauthentication• Privacy
• Wired-Equivalent Privacy WEP RC4
• Data Deliver
Steps to Association:
Client evaluates APresponse, selects best AP.
AP sends Probe ResponseAccess Point A
Access Point
B
Initial connection to an Access Point
Client sends probe
Client sends authenticationrequest to selected AP (A).
AP A confirms authenticationand registers client.
Client sends associationrequest to selected AP (A).
AP A confirms associationand registers client.
Association Process-- Passive Scanning
Steps to Re-association:
Adapter listens for beaconsfrom APs.
Adapter evaluates APbeacons, selects best AP.
Adapter sends associationrequest to selected AP (B).
AP B confirms associationand registers adapter.
Access Point
A
Access Point
B
Roaming from Access Point A to Access Point B
AP B informs AP A of re-association with AP B.
AP A forwards buffered packetsto AP B and de-registers adapter.
Re-association Process
Cellular and 802.11b
Cellular True mobility Secure Roaming & Handoff Integration with SP Voice
networks Data to 384Kbps with 2.5G Max at 2Mbps with 3GPP Higher network costs/user Licensed spectrum
802.11b 11Mbps today Integration with Enterprise
data network Unlicensed spectrum Higher client costs/user Limited mobility/roaming QoS and Security, voice in
development
Many Enterprises see the need for both solutions in the medium-term
Bluetooth
Frequency: 2.4 GHz ISM Band - FHSS 2.4 - 2.48G, 79Mhz = 79x1Mhz, 1600 hops per sec
Range: 10m (100m), Omni-directional, save power Low Power: 1mW (100mW with amplifier) Speed: 1Mbps gross Network: 8 devices (1master+7slave) in a piconet 3 Simultaneous voice plus data Data -- asymmetric @723.2k+57.6k, or symmetric @433.9k 10 piconet can form a scatternet Low Cost Ericsson, IBM, Nokia, Intel, Toshiba, etc. Founded Feb 98, www.bluetooth.com