Transcript Wireless Networks

Wireless Networks
Anatomy of a radio LAN
• The radio modem
– Analog transmitter
• The MAC controller
– Interface to transmitter
– At least partly in hardware
• The host interface
– How the software(driver) talks to the MAC
– PCI, PCMCIA, USB, Ethernet
• The driver
– How the App talks to the device
– Implements the part of MAC not in hardware
The Radio Modem (Physical Layer)
• ISM frequency bands (900 MHz & 2.4 GHz)
• 5 GHz frequency bands (HiperLan and UNII
band)
• Spread Spectrum techniques
• Modulations
• Interferences and noises
• Other (analog concerns)
The MAC level (link layer)
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Main channel access mechanisms
MAC techniques
Network topology
Some throughput considerations
Some Wireless LAN standards
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IEEE 802.11
802.11 HR and 802.11 at 5 GHz
HiperLan
HiperLan II
HomeRF & SWAP
BlueTooth
The radio modem (physical layer)
ISM frequency bands
• FCC/ETSI allocated
– Unlicensed but regulated
• Very different from HAM radio
– For industrial/scientific/medical use
• (900 MHz & 2.4 GHz)
• rules originally allowed around 2 Mb/s maximum bit
rate
– found a loophole and now offer 11 Mb/s systems
• Free = heavily polluted
• 2.4 GHz suffers from microwave oven interference
5 GHz frequency bands
• complicated power rules
– around 20 MHz bandwidth is optimal
• More bandwidth = more speed
– 10 – 40Mb/s
• Higher frequency
– More interference
• Obstacles
– Requires greater SNR (signal to noise ratio)
• Shorter range
Spread Spectrum
• Use increased bandwidth
– Decrease noise effects
– Shares spectrum pretty fairly
• Direct Sequence vs. Frequency Hopping
Direct Sequence
• Broadcast on many channels
– Modulate signal via a single code
• One chip per band $$
• Same chip for decoding
– Take average of decoded signals
• Interference on any narrow bands is averaged out
– What if interference is too great?
• Wide channels
– Only a few available (about 3)
• CDMA (cell phones) use something like this
– Different (orthogonal) code for each channel
Frequency Hopping
• Uses a set of narrow channels
– Changes channel every 20 - 400 ms
• If a channel is bad (interference) a new one will be
used soon
– Averages interference over time
– At least some channels should be good
• Complicates MAC level
– Performance cost of synch/init
• Co-Existance
• Ultra Secure
Modulations
• Carrier (base frequency) modulated to encode bits
• AM
– Strength
• FM
– Frequency
– Phase
2FSK vs. 4FSK (frequency shift keying)
• 2FSK
– 0, carrier – d (some offset)
– 1, carrier + d
• 4FSK
– 00, carrier – 3/2d
– 01, carrier – 1/2d
– 10, carrier + 1/2d
– 11, carrier + 3/2d
• Distance decreased from 2d to d
11Mb/s? (802.11 HR)
• Modulate code of DS to encode more data
– Not originally allowed but after showing FCC
that it causes no more harm than DS it was
allowed
• Faster = reduced range
• More complex hardware
• More sensitive to noise
OFDM
• Transmit bits in parallel
• Orthogonal sub-carriers modulated independently
Interference and Noise
• Fading
– Temporal variations
• Microwave Oven noise
– 2.4Ghz is the frequency where water molecules vibrate
• FEC
– Error correcting codes
– Not very useful since errors tend to be bursty
– Still used to correct small errors
• Multi-path/delay
– Not a problem at lower bit-rate (up to 1Mb/s)
The MAC level
Main channel access
mechanisms
• Must allocate the main resource (channel)
between nodes
• Allocated by regulating its use
– TDMA
– CSMA
– Polling
TDMA (Time Division Multiple
Access)
• Time broken up into frames
• Time slices of a frame given to nodes
• Done via mgmt. Frame
– Specified by base station
• Up slices and down slices
TDMA
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Used for cell phones
Low latency
Guarantee of bandwidth
Connection oriented
Not well suited for data network
– Inflexibility
– Does not handle bursts of traffic well
CSMA/CA
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Used by most wireless LANs (in ISM)
Connectionless
Best effort
No bandwidth or latency guarantees
Because a nodes own signal overpowers all
others collisions are not detectable
– Collision avoidance
CSMA/CA
• Listen to channel
• If idle - send one packet
• If busy - wait until idle then start contention
– Transmissions only start at beginning of slots
• Since it takes time to switch from rcv to xmit
• 20 - 50µs
Polling
• Mix of TDMA and CSMA/CA
• Base controls channel access
• Asks nodes if they want to transmit
– Connection oriented or connectionless
– Ask each node or reservation (out of channel)
MAC Techniques
• Need to improve performance of CSMA/CA
• Retransmission
– Via ack’s
• Fragmentation
– Small packets to reduce retransmissions
• RTS/CTS
– CSMA/CA only sees locally
– Ask receiver if ok to send
– One side effect is reduced collision penalty
• All add overhead
Network topology
• Ad hoc
– Isolated
– Each node provides routing
• Access points
– Similar to bridges
Some throughput
considerations
• Very low user throughput
– On a 1Mb/s system users can frequently
see as low as hundreds of bits per second
• Multi-rate systems
– Lesser bandwidth channel available with
greater range
• TCP assumes packet loss is congestion
Some Wireless LAN standards
IEEE 802.11
• One MAC
– CSMA/CA or polling
• 3 possible physical layers
– 1Mb FH
– 1 or 2 Mb DS
– Diffuse IR
• Optional APM and encryption
802.11 HR & 802.11 at 5 GHz
• Only changes physical layer
• 5Ghz
– OFDM
– 6 - 52 Mb
HiperLan
• By ETSI
• Dedicated band
– 5.1 - 5.3GHz
– Only in Europe
• 23.5 Mb
HiperLan II
• By ETSI
• Dedicated band
– 5.1 - 5.3GHz
– Only in Europe
• OFDM
– First standard based on OFDM
• 6 - 52 Mb
• Wireless ATM
• TDMA
HomeRF & SWAP
• Cheap
– MAC is in software
– Moore’s law doesn’t apply to wireless
because of analog parts
• 1 - 2 Mb FH
BlueTooth
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Not wireless LAN
Cable replacement technology
Offers point to point links
No IP support only PPP
Each channel is ~768kb FH
– 1 data, 3 voice