Wireless Networks

1
Wireless Networks
2
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

3
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)

4
The MAC level (link layer) 

• Main channel access mechanisms 
• MAC techniques 
• Network topology 
• Some throughput considerations 

5
Some Wireless LAN standards 

• IEEE 802.11 
• 802.11 HR and 802.11 at 5 GHz 
• HiperLan 
• HiperLan II 
• HomeRF SWAP 
• BlueTooth 

6
The radio modem (physical layer) 
7
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 

8
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

9
Spread Spectrum

• Use increased bandwidth
• Decrease noise effects
• Shares spectrum pretty fairly
• Direct Sequence vs. Frequency Hopping

10
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

11
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

12
Modulations

• Carrier (base frequency) modulated to encode bits
• AM
• Strength
• FM
• Frequency
• Phase

13
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

14
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

15
OFDM

• Transmit bits in parallel
• Orthogonal sub-carriers modulated independently

16
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)

17
The MAC level
18
Main channel access mechanisms 

• Must allocate the main resource (channel) between
  nodes
• Allocated by regulating its use
• TDMA
• CSMA
• Polling

19
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

20
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

21
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

22
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

23
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)

24
MAC Techniques 

• Need to improve performance of CSMA/CA
• Retransmission
• Via acks
• 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

25
Network topology 

• Ad hoc
• Isolated
• Each node provides routing
• Access points
• Similar to bridges

26
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

27
Some Wireless LAN standards 
28
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

29
802.11 HR 802.11 at 5 GHz

• Only changes physical layer
•  5Ghz
• OFDM
• 6 - 52 Mb

30
HiperLan 

• By ETSI
• Dedicated band
• 5.1 - 5.3GHz
• Only in Europe
• 23.5 Mb

31
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

32
HomeRF SWAP 

• Cheap
• MAC is in software
• Moores law doesnt apply to wireless because of
  analog parts
• 1 - 2 Mb FH

33
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