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 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

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 • Moore’s law doesn’t 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