introduction to ieee 802.11. ieee 802.11 a standard for wireless lans an excellent book: – matthew...
TRANSCRIPT
Introduction to IEEE 802.11
IEEE 802.11
A standard for wireless LANs
An excellent book:– Matthew S. Gast, Mike Loukides,
802.11 Wireless Networks: The Definitive Guide , O’Reilly & Associates, Inc.
History of IEEE 802.11
802.11 standard first ratified in 1997– 802.3 LAN emulation– 1 & 2 Mbps in the 2.4 GHz band
Two high rate PHY’s ratified in 1999– 802.11a: 6 to 54 Mbps in the 5 GHz band– 802.11b: 5.5 and 11 Mbps in the 2.4 GHz band
The Beat Goes On
802.11c: support for 802.11 frames 802.11d: new support for 802.11 frames 802.11e: QoS enhancement in MAC 802.11f: Inter Access Point Protocol 802.11g: 2.4 GHz extension to 22 Mbps 802.11h: channel selection and power control 802.11i: security enhancement in MAC 802.11j: 5 GHz globalization
IEEE 802 Standards
802.3 MEDIUMACCESS
(Ethernet)
PHYSICAL
802.4 MEDIUMACCESS
(token bus)
PHYSICAL
802.5 MEDIUMACCESS
(token ring)
PHYSICAL
802.11 MEDIUMACCESS
(WLAN)
PHYSICAL
802.12MEDIUMACCESS
(Gigabit
LAN)PHYSICAL
. . .
802.1 BRIDGING
802.2 LOGICAL LINK CONTROL
PHYSICAL
LAYER
DATA
LINK
LAYER
802.
1 M
AN
AG
EM
EN
T
802.11
802.11 MAC
802.11FHSS
802.11DSSS
802.11aOFDM
802.11bDSSS
802.11 BSS
Basic Service Set (BSS) --- a basic LAN Infrastructure BSS Independent BSS (Ad Hoc LAN)
Access point
802.11 ESS
Extended Service Set (ESS)
Distributed System
Major Protocols
MAC Management Operations
– Scanning– Authentication– Association– Power Saving– Timing Synchronization
Power Saving
sleep
Beacon window ATIM window
Beacon interval
timesleep
When to stay awake?
ATIM: Announcement traffic indication map.
IBSS: If a node has an outgoing data frame for B, it sends B a traffic announcement in the ATIM window.
A node stays awake for an entire beacon interval if it has incoming and/or outgoing traffic.
Beacons
Beacons carry information about the BSS.– To allow new stations to join in– Timing synchronization
Every station must listen to Beacons. Infrastructure BSS: AP sends beacons. IBSS: every station contends for beacon
generation in the beacon window.
Beacon Contention/Generation
Each station:– determines a random number k;– waits for exactly k idle slots to pass;– transmits a beacon (if no one else has done so).
Beacon: several slots in length.
window
beacon interval
Timing Sync Needed for Power Saving
sleep
Beacon window ATIM window
Beacon interval
timesleep
Timing Sync Needed for Frequency Hopping
f1
f2
f4
f3
f5
802.11 Timers (Clocks)
Timer: 64 bits, ticking in microseconds. Accuracy: within + 0.01%, or +100 ppm. Time synchronization needed for:
– Frequency hopping– Power management
∆ = max tolerable difference between clocks.
– Desired value: 25 s
802.11’s Time Sync Function
Beacon contains a timestamp. On receiving a beacon, STA adopts beacon’s
timing if T(beacon) > T(STA). Clocks move only forward.
faster adopts
12:01 12:00
slower not adopts
12:01 12:0212:01
Is IEEE 802.11 TSF Scalable?
Why or Why Not?