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Wireless Networks. Anatomy of a radio LAN The radio modem –Analog transmitter The MAC controller –Interface to transmitter –At least partly in hardware.

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Presentation on theme: "Wireless Networks. Anatomy of a radio LAN The radio modem –Analog transmitter The MAC controller –Interface to transmitter –At least partly in hardware."— Presentation transcript:

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

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