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) 

• Main channel access mechanisms 

• MAC techniques 

• Network topology 

• Some throughput considerations 

Some Wireless LAN standards 

• 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

• 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

• 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 

• 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