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