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Chuah Winter 2006
EEC173B/ECS152C, Winter 2006
Wireless LANs
802.11 Frame format
802.11 MAC management
‐ Synchronization, Handoffs, Power
Acknowledgment: Selected slides from Prof. Schiller
2
Key Points from Last Lecture
MAC methods: DCF & PCF
CSMA/CA with positive ACK
‐ Exponential backoff ‐ “Prioritized” access via different IFS values
Hidden/exposed terminal problems
‐ RTS/CTS clearing ‐ Virtual sensing using received NAV
3
802.11 ‐ Frame format
Frame Types
‐ Data: unicast (ACKed); broadcast/multicast (not ACKed) ‐ Control: RTS/CTS, ACKs ‐ Management (beacon, probe request/response, authentication, association, etc)
Sequence numbers
‐ Important against duplicated frames due to lost ACKs
Addresses
‐ Receiver, transmitter (physical), BSS identifier, sender (logical)
Miscellaneous
‐ Sending time, checksum, frame control, data
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802.11 ‐ Frame format
Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4 Data^ CRC
bytes 2 2 6 6 6 2 6 0-2312 4
Protocol version Type Subtype^
To DS
More Frag Retry
Power Mgmt
More Data WEP
2 2 4 1 From DS
1 Order
bits 1 1 1 1 1 1
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MAC Address Format
scenario to DS from DS
address 1 address 2 address 3 address 4
ad-hoc network 0 0 DA SA BSSID - infrastructure network, from AP
0 1 DA BSSID SA -
infrastructure network, to AP
1 0 BSSID SA DA -
infrastructure network, within DS
1 1 RA TA DA SA
DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address
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Special Frames: ACK, RTS, CTS
Acknowledgement
Request To Send
Clear To Send
Frame Control Duration^
Receiver Address
Transmitter Address CRC
bytes 2 2 6 6 4
Frame Control Duration^
Receiver Address CRC
bytes 2 2 6 4
Frame Control Duration^
Receiver Address CRC
bytes (^2 2 6 ) ACK
RTS
CTS
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802.11 ‐ MAC Management Sublayer
Registration/Synchronization
‐ Try to find a LAN, try to stay within a LAN ‐ Timer, etc.
Handoff: Association/Reassociation
‐ Integration into a LAN ‐ Roaming, i.e. change networks by changing access points ‐ Scanning, i.e. active search for a network
Power management
‐ Sleep‐mode without missing a message ‐ Periodic sleep, frame buffering, traffic measurements
Security
MIB ‐ Management Information Base
‐ Managing, read, write 8
Registration
A management frame called beacon is transmitted periodically by the AP to establish the timing synchronization function (TSF) TSF contains: BSS id, timestamp, traffic indication map (TIM), power management, and roaming information RSS measurements are done on the beacon message Association: process by which an MS registers with an AP
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Management Operations: Scanning
Passive scanning
‐ Listen to BS beacons
Active scanning
‐ MS sends probe request ‐ BS responds to probe
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Power Management (1)
How to power‐off during idle periods? Idea: switch the transceiver off if not needed States of a station: sleep and awake IEEE 802.11 buffers data at the AP, and sends the data when the MS is awakened Timing Synchronization Function (TSF) ‐ Using TSF, all MSs are synchronized – they wake up at the same time to listen to beacon With every beacon a Traffic Indication Map (TIM) is sent that has a list of stations having buffered data An MS learns that it has buffered data by checking beacon and TIM
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Power management (2)
Infrastructure ‐ Traffic Indication Map (TIM)
- List of unicast receivers transmitted by AP ‐ Delivery Traffic Indication Map (DTIM)
- List of broadcast/multicast receivers transmitted by AP Ad‐hoc ‐ Ad‐hoc Traffic Indication Map (ATIM)
- Announcement of receivers by stations buffering frames
- More complicated ‐ no central AP
- Collision of ATIMs possible (scalability?)
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Power saving with wake‐up patterns (infrastructure)
TIM interval
t
medium
access point busy
D
busy busy busy
T T D
T (^) TIM D (^) DTIM
DTIM interval
B B
B (^) broadcast/multicast
station awake
p PS poll
p
d
d
d data transmissionto/from the station
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Power saving with wake‐up patterns (ad‐hoc)
awake
A transmit ATIM D transmit data
t
station 1 B^1 B^1
B (^) beacon frame
station 2 B^2 B^2
random delay
A
a
D
d
ATIM window beacon interval
a acknowledge ATIM d acknowledge data
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Wifi ‐ IEEE 802.11b (1)
Data rate ‐ 1, 2, 5.5, 11 Mbit/s, depending on SNR ‐ User data rate max. approx. 6 Mbit/s Transmission range ‐ 300m outdoor, 30m indoor ‐ Max. data rate ~10m indoor Frequency ‐ Free 2.4 GHz ISM‐band Security ‐ Limited, WEP insecure, SSID Cost ‐ $20‐$100 base station, dropping Availability ‐ Many products, many vendors
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Wifi ‐ IEEE 802.11b (2)
Connection set‐up time ‐ Connectionless/always on ‐ Typ. Best effort, no guarantees (unless polling is used, limited support in products) Quality of Service Manageability ‐ Limited (no automated key distribution, sym. Encryption) Special Advantages ‐ Many installed systems, lot of experience, available worldwide, free ISM‐band, many vendors, integrated in laptops, simple system Disadvantages ‐ Heavy interference on ISM‐band, no service guarantees, slow relative speed only
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Channel Selection (Non‐overlapping)
2400 [MHz]
2412 2442 2472 2483.
channel 1 channel 7 channel 13
Europe (ETSI)
US (FCC)/Canada (IC)
2400 [MHz]
2412 2437 2462 2483.
channel 1 channel 6 channel 11
22 MHz
22 MHz
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OFDM in IEEE 802.11a
Orthogonal FDM with 52 used subcarriers (64 in total)
48 data + 4 pilot
(plus 12 virtual subcarriers)
312.5 kHz spacing
subcarrier number
-26 -21 -7 -1 1 7 21 26 channel center frequency
pilot 312.5 kHz
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Concluding Remarks
IEEE 802.11 WLAN is becoming real popular
these days
There is still a big room to improve the current
802.11 systems
Important to consider how any improved system
co‐exists with legacy systems
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Characteristics of Wireless LANs
Advantages
‐ Very flexible within the reception area ‐ Ad‐hoc networks without previous planning possible ‐ (Almost) no wiring difficulties (e.g. historic buildings, firewalls) ‐ More robust against disasters like, e.g., earthquakes, fire ‐ or users pulling a plug...
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Characteristics of Wireless LANs
Disadvantages
‐ Typically very low bandwidth compared to wired networks (1‐ 10 Mbit/s) ‐ Many proprietary solutions, especially for higher bit‐ rates, standards take their time (e.g. IEEE 802.11) ‐ Products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT‐ 2000
