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2001/3/22 802.11 by Yu-Chun Pan 1
ANSI/IEEE 802.11
Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) Specifications
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802.11 Design issue The media impact the design
Neither absolute nor readily observable boundaries Unprotected from outside signals Less reliable than wired PHYs Dynamic topologies Lack full connectivity Time-varing and asymmetric propagation properties
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802.11 ISO Model
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802.11 Componets(1) Wireless Medium(WM)
The medium used to implement a wireless LAN Station(STA)
Any device that contains an 802.11 conformant MAC and PHY interface to the wireless medium Station Service
The set of services that support transport of MSDU(Mac Service Data Units) between Stations within a BSS Basic Service Set(BSS)
The BSS is the basic building block of an 802.11 LAN DS
A system used to interconnect a set of BSSs to create an ESS
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802.11 Componets(2) Distribution System Services(DSS)
The set of services provided by the DS which enable the MAC to transport MSDUs between BSSs within an ESS Access Point(AP):
Any entity that has STA functionality and provides access to the DS An AP is a STA which provides access to the DS by providing DS services in addition to Station Services.
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802.11 Network Infrastructure
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802.11 Srevices Staion Services
Authentication, Deauthentication Privacy MSDU delivery
Distribution System Services Association, Disassociation Distribution( route to 802.11) Integration( route to 802.x) Reassociation( hand-off, roaming)
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802.11 Protal
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802.11 Phy Operate within the 2.4 GHz
802.11-base products do not require user licensing or special training
FHSS(frequency hopping spread spectrum) DSSS(direct sequence spread spectrum)
FHSS and DSSS are fundamentally different signaling mechanisms and will not interoperate with one another
Spread-spectrum increase reliability, boost throughput, and allow many unrelated products to share the spectrum without explicit cooperation and with minimal interference
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802.11 FHSS More security and without interfere
2nd War 1600 hops/sec Limited speeds no higher than
2Mbps(hopping overhead)
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802.11 DSSS Divides the 2.4 GHz band into 14 twenty-two MHz channels Adjacent channels overlap on another partially, only 3 of 14 being completely nonoverlapping No hopping 11-bit chipping-Barker sequence
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802.11
BPSK-Binary Phase Shift Keying QPSK-Quadrature Phase Shift Keying Complementary Code Keying
Data Rate Code Length
Modulation
Symbol Rate
Bits/Symbol
1 Mbps 11(Barker Sequence)
BPSK 1 MSps 1
2 Mbps 11(Barker Sequence)
QPSK 1 MSps 2
5.5 Mbps 8(CCK) QPSK 1.375 MSps 411 Mbps 8(CCK) QPSK 1.375 MSps 8
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802.11 Infrastructure mode
The wireless network consists of at least one access point connected to the wired network infrastructure
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802.11 Ad hoc mode
It’s a simply set of 802.11 wireless stations that communicate directly with one another without using an access point or any connection to a wired network
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802.11LLC & MAC LLC-Logical Link Control
The same 802.2 and 48 bits addressing MAC is different
CSMA/CA(Carrier sense multiple access/collision avoidance) By using explicit packet ACK, which
means an ACK is sent by receiving station to confirm that the data packet arrived intact.
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802.11 Protocol Stack
Physical Layer
LLC
MAC
DataLinkLayer
LLCPCFDCFPLCPPMD
PLCP Preamble PLCP Header PSDU
Data Unit
Point Coordination Function( infrastructure)
MAC Header Frame Body ( 0~2312 bytes)
FCS
DSSS FHSS IR
2.4GHz 870~950nm
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802.11 DCF &PCF DCF:
A class of coordination function where the same coordination function logic is active in every station in the BSS
Both Ad Hoc and Infrastructure mode PCF
A class of possible coordination functions in which the coordination function logic is active in only one station in a BSS
Only Infrastructure mode
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802.11 CSMA/CA SIFS(Short interframe space)
Used for an ACK ,CTS, the second or subsequent MPDU of a fragment burst, and by a STA responding to any polling by the PCF. PIFS(PCF interframe space)
STAs operating under the PCF to gain priority access the medium DIFS( DCF interframe space)
STAs operating under the DCF to transmit data frames and management frames
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802.11 Access Priority
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802.11 Contention Window Backoff Time = Random * a SlotTime
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802.11 Backoff procedure
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802.11 RTS/CTS(Optional)
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802.11 MSDU/MPDU MSDU: Mac Service Data Unit MPDU: Mac Protocol Data Unit
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802.11 Mac Frame Format Mac frame format
Frame Control field
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802.11 Mac Frame Type Data
Handled via the MAC data service path Management
Handled via the MAC Management Service data path
Association request, Authentication… Control
RTS, CTS, ACK, Power Save-Poll…
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802.11 PCF Transfer Procedure
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802.11 FHSS Frequency地區 Frequency Hopping Channel
No.Frequency Range
( GHz )Minimum Channel Number
北美 79 2.400~2.4835 75歐洲 79 2.400~2.4835 20
西班牙 27 2.445~2.475 20法國 35 2.4465~2.4835 20
日本 23 2.471~2.497 --
註: 2.402GHz+(1MHz*78) = 2.480GHz ( 美 ) 2.473GHz+(1MHz*22) = 2.495GHZ( 日 )
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802.11 DSSS FrequencyChannel ID Frequency
( GHz ) 地區
美國 加拿大 歐洲 西班牙 法國 日本1 2.412 O O O2 2.417 O O O3 2.422 O O O4 2.427 O O O5 2.432 O O O6 2.437 O O O7 2.442 O O O8 2.447 O O O9 2.452 O O O
10 2.457 O O O O O11 2.462 O O O O O12 2.467 O O13 2.472 O O14 2.484 O
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802.11 DSSS vs. FHSSDSSS FHSS
Systems Collocation 3 26(15)
Noise and Interference Immunity
X O
Multipath Immunity X O
Throughput O X
Form Factor X O
Power Consumption X O
Cost X O
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802.11 DSSS with CDMALength of sequence
Number of available
sequences
Number of possible
collocated systems
15 2 2
31 6 6
63 6 6
255 16 16
1,023 60 60
For the collocation of 16 systems, 255 chip (bit) long sequences should be used. Every message bit should be represented by 255 bits! If message rate is 1 Mbps (minimum required in LANs), the rate of the transmitted signal would be 255 Mbps ! Expensive!
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802.11 Multipath Problem
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802.11 Collocated No.
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802.11 DSSS(Frequency usage example)
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Different Tech.(1)高
移動速度
低
室外
高速移動步行速度
靜止狀態
室內
固定狀態
Wired Line10Kbps 100Kbps 1Mbps 10Mbps 100Mbps
Cellular System
Wireless LAN1Mbps~10Mbps
PHS32~64Kbps
Wireless ATM
IMT-20002Mbps
註
註: Wireless Data: CDPD 、 GPS…
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Different Tech.(2)Peak Data rate
Relative cost
Data Network support
Voice Networksupport
Range in Home
Standby and currents
802.11FH
2 Mbps Medium/High
TCP/IP Via IP > 50m ~10mA,~400mA
HomeRF 1.6 Mbps Medium
TCP/IP Via IP & PSTN > 50m <1mA, ~300mA
Bluetooth
1 Mbps Medium
Via PPP Via IP & voice channel
< 10m <1mA, ~60mA
IrDA 1.6Mbps Low Via PPP Via IP < 2m line of sight and aimed
<10uA,~300mA