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

2001/3/22 802.11 by Yu-Chun Pan 2

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

2001/3/22 802.11 by Yu-Chun Pan 3

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

2001/3/22 802.11 by Yu-Chun Pan 10

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

2001/3/22 802.11 by Yu-Chun Pan 14

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

2001/3/22 802.11 by Yu-Chun Pan 29

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