IEEE 802.11 Section14.1

1. IEEE 802.11 Section14.1 • http://silverscreensirens.com/ • George Antheil (American composer) and Hedwig Eva Maria Kiesler (aka HedyLamarr, movie actress)

2. They conceived of a scheme to control armed torpedoes over long distances in such a way that the enemy could not detect or jam the transmissions (spread spectrum) because regular signals are subject to interference

3. Frequency-Hopping Spread Spectrum (FHSS) defines a set of frequencies f1, f2, f3, …fn that all lie in its broadcast range. More secure, more resistant to noise • Used in Bluetooth: 2.4 GHz divided into 79 1 MHz channels. • Part of the 802.11 standard • Book section 6.2 Spread Spectrum

4. Direct Sequence Spread Spectrum (DSSS) • DSSS spreads a signal out over a large bandwidth, making the transmission appears as background noise to conventional narrow band transmitters and receivers. • Moreimpervious to interference since the signal’s energy is spread over a much wider bandwidth. • Part of the 802.11 standard • See also book, section 6.2 and • http://www.cs.clemson.edu/~westall/851/spread-spectrum.pdf

5. Wireless IEEE 802.11 • Wireless IEEE 802.11: Wi-Fi – short for wireless fidelity – nickname for 802.11 • Infrared waves and radio waves (most common) • White paper of 802.11 (and MIMO, or smart antenna technology) at Cisco. • Also [http://www.computerworld.com/s/article/109410/MIMO] and • [http://www.computerworld.com/s/article/9019472/FAQ_802.11n_wireless_networking]

6. 802.11 Flavors Add 802.11n OFDM 5.725-5.850 GHz Different 600 • OFDM – Orthogonal Frequency Division Multiplexing

7. NOTE: Microwaves, Bluetooth devices, cordless telephones, and garage door openers also operate at 2.4 GHz • 802.11n: reportedly achieves rates of up to 140 Mbps • There’s also an 802.11i: which specifies security mechanisms

8. Wi-FI components • Access point (AP) – base station used to connect with wireless devices • The set of wireless devices with which a single AP communicates defines a Basic Service Set (BSS) • multiple BSS’s may be connected via a distribution systems (DS). A DS may be a LAN, multiple LANs or something else. 802.11 does not define it.

9. Figure 14.2 Extended service sets (ESSs)

10. MAC protocol • Space is the shared media but CSMA/CD won’t work • Hidden station problem

11. 802.11 uses Distributed Coordination Function (DCF) • Implements Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). • Definitions • Short Inter-Frame space (SIFS) – a period of time a device might wait. • Distributed Inter-Frame Spacing (DIFS) – another period of time a device might wait. • SIFS < DIFS • SIFS or DIFS helps to prioritize devices and specifies wait time in certain situations. • Waiting less time implies higher priority.

12. Contending for the shared media occurs as follows: • Do CSMA & persistence strategy and wait an amount of time defined by SIFS or DIFS. • NOTE: A device wanting to send something waits DIFS • one that is responding waits SIFS (higher priority since SIFS is shorter). • In order to send, a device will • send a Request to Send (RTS) frame to the destination (AP) • also specify time needed to send data. • Destination responds by sending a Clear to Send (CTS) frame.

13. What happens if two RTS frames collide? • Simple - destination sends no CTS frame and devices resend RTS frames later after a timeout. • Once a source has received a CTS frame, it sends a data frame. • Any OTHER device within range of destination also senses CTS and will not send (thus collision avoidance) – at least for a specified period of time. • This period of time implemented by a timer called a network allocation vector (NAV) and gives the sending device time to send.

14. Figure 14.4 CSMA/CA flowchart

15. Figure 14.5 CSMA/CA and NAV

16. 802.11 addressing • four different addresses field for each frame. • Will not discuss all these fields but will focus on addressing

17. 4 possibilities • The frame stays within a BSS. Example: A sends to B (case 1 in the next slide and in table 14.3) • The frame travels from a BSS to a DS. Example: A sends to B, but the frame goes to an AP first(case 3) • The frame travels from a DS to a BSS. Example: A sends to B and the frame arrives at B from an AP (case 2) • The frame travels across a DS between two APs. A sends to B and the frame goes from AP1 to AP2

18. Figure 14.9 Addressing mechanisms

19. Table 14.3 Addresses

20. Finding the appropriate AP: • Device sends a Probe Request Frame. • Any AP in range responds with a Probe Response Frame. • If more than one respond, device selects the one with the strongest signal.

21. Roaming: Moving from one BSS to another BSS. • Device sends Reassociate Request Frame. • An AP responds with a Reassociate Response frame. • Sent when a device is moved and detects a weakened signal. • Disassociate frame disassociates a device from a previously associated AP. • If device moved out of range the new AP may have to send the old AP the frame.

22. Security is a real issue. People can steal your bandwidth. • See experiment conducted by Peter Shipley

23. WEP - Wire Equivalent Privacy: encryption/authentication schemes that come with an AP. • relatively weak encryption. • Allows the use of a default encryption key (BAD!!!) • can dynamically change keys, but the method often results in keys being repeated.

24. See[http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html] for flaws • Supported by 802.11a, b, and g. • WEP2

25. WPA - Wi-Fi Protected Access: better encryption than WEP • More secure • WPA2 allows the use of AES. • SSID broadcast

26. IEEE 802.16 WiMAX • Another of the 802 standards • Connection oriented MAC. • Cell phones. • http://www.wimax.com/education • 802.11 vs 802.16. • WiMAXvs 3g • Another reference. • 2g vs 3g vs 4g hype.

27. Section 14.2 covers Bluetooth – No time to cover