IEEE 802.11 Section14.1 http://silverscreensirens.com/ George Antheil (American composer) and Hedwig

Eva Maria Kiesler  (aka Hedy Lamarr, movie actress)

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

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

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.

More impervious 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

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]

802.11 Flavors

Add 802.11n OFDM 5.725-5.850 GHz Different 600

OFDM – Orthogonal Frequency Division Multiplexing

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

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.

14.9

Figure 14.2 Extended service sets (ESSs)

MAC protocol Space is the shared media but CSMA/CD won’t

work Hidden station problem

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.

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.

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.

14.14

Figure 14.4 CSMA/CA flowchart

14.15

Figure 14.5 CSMA/CA and NAV

802.11 addressing four different addresses field for each frame.

Will not discuss all these fields but will focus on addressing

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

14.18

Figure 14.9 Addressing mechanisms

14.19

Table 14.3 Addresses

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.

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.

Security is a real issue. People can steal your bandwidth.

See experiment conducted by Peter Shipley

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.

See[http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html] for flaws

Supported by 802.11a, b, and g. WEP2

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

SSID broadcast

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

Section 14.2 covers Bluetooth – No time to cover