Agenda

1. Agenda • Wireless LANs • Cellular Telephones • Satellite Communications • LAB: Wireless IP Phones

3. 802.11 Wireless LANs • Wireless Technologies for LANs • - Radio • - Infrared light (as in remote control) • - Ideal for mobile devices • - Useful when wiring would be costly • - Excellent Disaster Recovery Plan

4. 802.11 Wireless LAN Standards • Standards come from the IEEE 802.11 Working Group • - Have already defined 1 Mbps and 2 Mbps standards for radio • - Higher speeds will be coming • - Up to 10 Mbps (??)

5. 802.11 Wireless LANs • Mode • Direct: peer-to-peer • Control point to govern interactions Access/Control Point

6. 802.11 Wireless LANs • Media Access Control • CSMA/CA • CSMA with Collision Avoidance • Tries to avoid collisions • When line is clear, station may send (CSMA), • but before it sends, must wait a random amount of time • This prevents stations that have been waiting to transmit from all transmitting at once

7. 802.11 Wireless LANs • Media Access Control • When a frame is received correctly, the receiver immediately sends back an acknowledgement • This allows the sender to know if it needs to resend • - Adds Overhead / Slows Throughput Frame ACK

8. Wireless LAN Applications • Disaster Recovery • Burnt / Destroyed Cabling System • Quick way to restore network • Battlefield • Wireless LANs in the tents • Less setup reaquirements • No wires to deal with • Hospitals • Doctors and Nurses move around a lot • Look up patient records

9. 802.11 IEEE Working Group • Created MAC layer acces • Two Modes as of 1997 • 1 and 2 Mbps

10. Wireless LAN Physical Layer • All of the following may operate at 1/2 Mbps • - Infared • - Frequency Hopping/Spread Spectrum (FHSS) • - Direct Sequence Spread Spectrum (DSSS)

11. Transmit Speed in Wireless LANs • Review: There are two variables that affect the Transmission Speed (Nyquist/Shannon)z • Signal Strength (divided by noise) • Bandwidth • If either are increased - Data Rate may be incresed • Traditionally - Operate in Channels of small bandwidth • Signal Strength was very high • Resulting in battery drain / EMF Exposure

12. Spread Spectrum • Use very wide bandwidth channels • 2.4 GHz - 2.48 GHz • Bandwidth = 83.5 MHz • Result: Transmit with reduced power

13. Frequency Hopping Spread Spectrum • Constantly Jump to new frequency • Frequency hopping • Provides Protection in Wireless medium (air waves) • Before Station Transmits - It’s given a code by the peer station

14. Frequency Hopping Spread Spectrum • Frequency changes every 20-50ms • Two stations transmiiting at the same time at the same frequency will cause lost frames • By nature - FHSS reduces collisions

15. Direct Sequence Spread Spectrum • Uses Chips • Chip : Brief Radio Bursts • One Bit may use 11 Chips (ie. 11 chips per bit period) • They are spaced out in time (I’ll draw graph - pg322) • Chip sets are determined by specific time code • used between two stations only

16. Direct Sequence Spread Spectrum • Each Chip may be “in phase” or 180 Degrees out of phase • In reference with some radio signal

17. PART II • Cellular Telephones Transmitter/ Receiver Mobile Phone

18. Cellular Telephones • Original Mobile Telephones • One transmitter/receiver • Limited number of channels • For good service can support about 20 subscribers per channel (rough rule of thumb) Transmitter/ Receiver Mobile Phone

19. Cellular Telephones • Divide Region into Cells • One cellsite (transmitter/receiver) per cell • Channels can be reused in non-adjacent cells Yes No Can Reuse Ch. 232? Uses Channel 232 No No Yes Yes No No Channel 232 Used in 4 cells No

20. Cellular Telephones • Channel Reuse • Without channel reuse, you can serve only about 20 subscribers per channel for good service • Rough rule of thumb • Otherwise, the system will not be available too often when people want to call or receive calls • If reused 3 times, 60 subscribers per channel

21. Handoffs • When you move to another cell within the same system • You are transferred automatically to that cell’s cellsite

22. Control • Mobile Telephone Switching Office • Controls cellsites, handoffs, etc. • Calls go to/from MTSO • Connects to POP to link to traditional telephone (wireline) carriers POP MTSO LEC, ICX, etc.

23. Placing a Call • Enter number, hit send • Cellphone broadcasts request • Several cellsites receive, send to MTSO • MTSO assigns cellphone to cellsite where signal is loudest • MTSO sends message to cellphone via that cellsite, telling the phone what incoming, outgoing channels to use

24. Receiving a Call • MTSO has each cellsite broadcast cellphone’s ID number • Cellphone transmits a response • Responses from cellsites go to MTSO • MTSO selects cellsite where signal is loudest • MTSO sends message via the cellsite to cellphone, giving channels and telling the cellphone to ring

25. First Generation Cellular • Analog Operation • Limits services (no paging, etc.) • Limits signal quality • Large Cells • Usually only 20-40 per city • Limits channel reuse • Limited Number of Channels • In U.S., 832 two-way channels

26. Third-Generation Cellular • Personal Communication Service (PCS) • Or Personal Communication Network (PCN) • More channels • About 2,500 • Smaller cells permit more channel reuse • Supports more subscribers per channel • Possible because signal is digital

27. Third-Generation Cellular • Digital, like 2d generation • Paging and other digital services • Cleaner signal

28. Potential System Capacity (Roughly) • Category 1st Gen 3nd Gen • Cells/City 30 100 • Channel reuse ~4 ~14 • Channels 800 2,500 • Effective channels 3,200 35,000 • With compression *3,200 105,000 • Subscribers 64,000 2,000,000 • *No compression in 1st generation

29. Third-Generation Cellular • PCS Cellphones • Do not have to transmit as far • Inverse cube law--if triple distance, 33 or 27 times the power required • Cellphones can be less expensive because use less power • Large number of possible subscribers removes scarcity cost penalties • Vendors offer more services made possible by digital technology

30. Part III • Satellites

31. Traditional Communications Satellites • In geosynchronous orbit • Appear to be stationary in sky • Far from the ground • 36 km (22,300 miles) • Need much power to send/receive • Need dish antennas to concentrate signals • Must point dish at the satellite • Impractical for portable telephony

32. LEO Satellites • Low Earth Orbit Satellites • Only 500 to 2,000 km (300 to 500 miles) above the earth • Need far less power to reach than 22,300 mile geosynchronous satellites • Can get by with omnidirectional antenna • Can use phone of reasonable size, cost • Access anywhere Omnidirectional Antenna

33. LEO Satellites • Satellites circle the earth every 90 minutes • Handoffs between satellites serving you. Now #2; soon #3 • Like cellular, except you are (relatively) stationary and the transmitter/receiver moves 2 1 3

34. MEO Satellites • Medium Earth Orbit Satellites • 5,000 km to 15,000 km (3,000-9,000 miles) • Farther away than LEOs, so need more power • But orbital period is longer, so serving satellites can serve a user longer before handoff