ĐIỆN tử VIỄN THÔNG l1 802 11 ACN2016 khotailieu

• Example CSMA/CD – Carrier Sense Multiple Access with Collision Detection – send as soon as the medium is free, listen into the medium if a collision occurs original method in IEEE 802.

ACN2016 1

Wireless LANs

AP wired network

AP: Access Point

Source: Schiller

ACN2016 4

Wireless LANs are different…

• Destination address does not equal destination location

• The media impact the design

– wireless LANs intended to cover reasonable geographic distances must be built from basic coverage blocks

• Impact of handling mobile (portable) stations

– Propagation effects

– Mobility management

– power management

ACN2016 6

Wireless PHY

– Medium has neither absolute nor readily

observable boundaries outside which stations are unable to receive frames

– Are unprotected from outside signals and are

significantly less reliable than wired PHYs

– Have time varying and asymmetric propagation properties

– Lack full connectivity

• the assumption that every station (STA) can hear every other STA in invalid

ACN2016 7

Wireless MAC: Motivation

• Can we apply media access methods from fixed

networks?

• Example CSMA/CD

– Carrier Sense Multiple Access with Collision Detection

– send as soon as the medium is free, listen into the

medium if a collision occurs (original method in IEEE 802.3)

ACN2016 8

Wireless MAC

– signal strength decreases inversely proportional to the square of the distance

– sender would apply CS and CD, but the collisions

happen at the receiver

– sender may not “hear” the collision, i.e., CD does not work

– CS might not work, e.g if a terminal is “hidden”

ACN2016 9

– A and C cannot hear each other

– A sends to B, C cannot receive A

– C wants to send to B, C senses a “free” medium

ACN2016 11

Solution for Hidden Terminals

• A first sends a Request-to-Send (RTS) to B

• On receiving RTS , B responds Clear-to-Send (CTS)

• Hidden node C overhears CTS and keeps quiet

– Transfer duration is included in both RTS and CTS

• Exposed node overhears a RTS but not the CTS

– D’s transmission cannot interfere at B

RTS

DATA D

RTS

ACN2016 12

IEEE 802.11

• Wireless LAN standard defined in the unlicensed spectrum (2.4 GHz and 5 GHz U-NII bands)

ACN2016 13

802.11 (contd.)

• Standards covers the MAC sublayer and PHY layers

• Three different physical layers in the 2.4 GHz band

– FHSS, DSSS and IR

• OFDM based PHY layer in the 5 GHz band

– Basic Service Set (BSS):

group of stations using the same radio frequency

ESS

ACN2016 18

PCF components

wireless medium and radio contact to the access point

radio frequency

the distribution system

logical network (ESS: Extended Service Set) based

on several BSS

ACN2016 19

Distribution System (DS) concepts

• The Distribution system interconnects multiple BSSs

• 802.11 standard logically separates the wireless medium from the distribution system – it does not preclude, nor demand, that the multiple

media be same or different

ACN2016 20

DS (contd.)

• An Access Point (AP) is a STA that provides access

to the DS by providing DS services in addition to

acting as a STA

• Data moves between BSS and the DS via an AP

• The DS and BSSs allow 802.11 to create a wireless network of arbitrary size and complexity called the Extended Service Set network (ESS)

application TCP

802.3 PHY 802.3 MAC

IP

802.11 MAC 802.11 PHY

LLC

infrastructure network

ACN2016 22

802.11 - Layers and functions

• PLCP Physical Layer Convergence Protocol

– clear channel assessment signal (carrier sense)

• PMD Physical Medium Dependent

PMD PLCP MAC

ACN2016 23

802.11 - Physical layer

• 3 versions: 2 radio (typically 2.4 GHz), 1 IR

– data rates 1, 2, 5.5, or 11 Mbit/s

• Infrared

– 850-950 nm, diffuse light, typ 10 m range

– carrier detection, energy detection, synchonization

• FHSS (Frequency Hopping Spread Spectrum)

– spreading, despreading, signal strength

– typically 1 Mbit/s (mandatory), 2Mbits/s (optional)– min 2.5 frequency hops/s (USA), two-level GFSK (Gaussian FSK) modulation

ACN2016 24

802.11 DSSS

• DSSS (Direct Sequence Spread Spectrum)

– DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying),

– DQPSK (differential quadrature PSK) for 2 Mbit/s, CCK

(complementary code keying) for 5.5 and 11 Mbits/s

– preamble and header of a frame is always transmitted with 1 Mbit/s

– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) (11 chip)

– max radiated power 1 W (USA), 100 mW (EU)

– min 1mW

ACN2016 25

Spread-spectrum communications

Source: Intersil

ACN2016 26

DSSS Barker Code modulation

Source: Intersil

ACN2016 27

802.11 - MAC layer

• Traffic services

– Asynchronous Data Service (mandatory) – DCF

– Time-Bounded Service (optional) - PCF

ACN2016 29

802.11 - Carrier Sensing

• In IEEE 802.11, carrier sensing is performed

– at the air interface (physical carrier sensing), and – at the MAC layer (virtual carrier sensing)

• Physical carrier sensing

– detects presence of other users by analyzing all detected packets

– Detects activity in the channel via relative signal strength from other sources

ACN2016 30

802.11 virtual carrier sensing

• Virtual carrier sensing is done by sending MPDU duration

information in the header of RTS/CTS and data frames

• Channel is busy if either mechanisms indicate it to be

– Duration field indicates the amount of time (in microseconds) required to complete frame transmission

– Stations in the BSS use the information in the duration field

to adjust their network allocation vector (NAV)

ACN2016 31

802.11 – Reliability: ACKs

– When B receives DATA from A, B sends an ACK

– If A fails to receive an ACK, A retransmits the DATA

– Both C and D remain quiet until ACK (to prevent

ACN2016 32

802.11 - CSMA/CA

– station ready to send starts sensing the medium

(Carrier Sense based on CCA, Clear Channel

DIFS DIFS

next frame

contention window (randomized back-off mechanism)

slot time direct access if

medium is free  DIFS

ACN2016 33

802.11 – CSMA/CA

– if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-

time)

– if another station occupies the medium during the

back-off time of the station, the back-off timer stops (fairness)

ACN2016 34

802.11 –CSMA/CA example

t busy

elapsed backoff time

bor residual backoff time busy medium not idle (frame, ack etc.)

– When transmitting a packet, choose a backoff interval

in the range [0,cw]; cw is contention window

– Count down the backoff interval when medium is idle– Count-down is suspended if medium becomes busy

– When backoff interval reaches 0, transmit RTS

B1 and B2 are backoff intervals

at nodes 1 and 2

cw = 31

B2 = 10

ACN2016 37

802.11 - Congestion Control

• Contention window ( cw ) in DCF: Congestion control achieved by dynamically choosing cw

• large cw leads to larger backoff intervals

• small cw leads to larger number of collisions

ACN2016 38

Congestion control (contd.)

• Binary Exponential Backoff in DCF:

– When a node fails to receive CTS in response to its

RTS, it increases the contention window

– Upon successful completion data transfer, restore cw

to CWmin

ACN2016 39

802.11 - Priorities

• defined through different inter frame spaces – mandatory idle time intervals between the transmission of frames

• SIFS (Short Inter Frame Spacing)

– highest priority, for ACK, CTS, polling response

– SIFSTime and SlotTime are fixed per PHY layer – (10 s and 20 s respectively in DSSS)

– lowest priority, for asynchronous data service

– DCF-IFS (DIFS): DIFSTime = SIFSTime +

2xSlotTime

ACN2016 41

802.11 - CSMA/CA II

• station has to wait for DIFS before sending data

• receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)

• automatic retransmission of data packets in case of transmission errors

t

SIFS DIFS

data ACK

ACN2016 42

802.11 –RTS/CTS

t

SIFS DIFS

data ACK

NAV (RTS)

NAV (CTS)

• acknowledgement via CTS after SIFS by receiver (if ready to receive)

• sender can now send data at once, acknowledgement via ACK

• other stations store medium reservations (NAV) distributed via RTS and CTS

ACN2016 44

Fragmentation

t

SIFS DIFS

SIFS

ACN2016 45

802.11 - Point Coordination Function

SIFS

D2

U2SIFS

CFend

contention period contention free period

t2 t3 t4

t2 = time when CFP actually finished t3 = initial planned CFP (but PCF finished polling earlier than expected)

ACN2016 48

CFP structure and Timing

CFP is greater than beacon interval DTIM – Delivery Traffic Indication Message

Source: 802.11 spec

ACN2016 49

CFP

• Then length of CFP is controlled by PC

– CFPMaxDuration field is used for this

• When CFP is more than beacon interval

– CFP_Dur_Remaining is included in beacons

– CFP_Dur_Remaining is set to 0 for beacons in CP

ACN2016 50

PCF- Data transmission

Source: 802.11 standard

– CF-End is used to signal the end of the CFP

• CFPs are generated at the CFP repetition rate and each CFP begins with a beacon frame

• Superframe: One CFP + One CP It repeats

according to the CFP repetition rate and each CFP begins with a beacon frame

Address 1

Address 2

Address 3

Sequence Control

ACN2016 56

Frame Control Field

– Association Request– Association Response– Dis/Reassociation

– Authentication

– Deauthentication

– ATIM (Announcement

TIM)

ACN2016 58

MAC address format

scenario to DS from

DS address 1 address 2 address 3 address 4

AP: Access Point

DA: Destination Address

SA: Source Address

BSSID: Basic Service Set Identifier

RA: Receiver Address

TA: Transmitter Address

– scanning, i.e active search for a network

– roaming, i.e change networks by changing APs

• MIB - Management Information Base

– managing, read, write

ACN2016 60

Synchronization using a Beacon (infrastructure)

• Synchronized clocks are needed for PCF, Power

saving and for frequency hopping

• Within a BSS timing is conveyed by a periodic

beacon

• STAs use the timestamp in beacon to adjust its

internal local clock

• AP always tries to send beacon at scheduled period (even if the prev beacon was delayed)

ACN2016 61

Synchronization using a Beacon (infrastructure)

beacon interval

t medium

ACN2016 62

Synchronization using a

Beacon (ad-hoc)

• Synchronization in ad hoc mode is more difficult,

since there is no AP for beacon transmission

• Each STA maintains its synchronization timer and

starts transmission of a beacon periodically

• Standard random back off is applied to beacon

frames so that only one STA wins transmitting beacon

ACN2016 63

Synchronization using a

Beacon (ad-hoc)

t medium

station1

busy

B1beacon interval

B1

value of the timestamp B beacon frame

random delay

ACN2016 64

Power saving in 802.11

• Basic idea is to switch off transceiver when there is

no communication

• Easy for sender since they know when to send data

• Receivers should wakeup periodically to check if it has to receive anything

• All stations announce a list of buffered frames during

a period when all of them are awake

– Destinations are announced using ATIM (Adhoc TIM)

a acknowledge ATIM d acknowledge data

• Station then selects the best AP (e.g based on signal strength)

– sends association Request to the AP

• association Response

– success: AP has answered, station is now associated with the new AP

– failure: continue scanning

ACN2016 70

Roaming (contd.)

• AP accepts Association Request

– signal the new station to the distribution system

– the distribution system updates its data base (i.e., location information)

– typically, the distribution system now informs the old

AP so it can release resources

ACN2016 73

IEEE 802.11 Summary

• Infrastructure (PCF) and adhoc (DCF) modes

• Signaling packets for collision avoidance

– Medium is reserved for the duration of the

transmission

• Acknowledgements for reliability

• Binary exponential backoff for congestion control

• Power save mode for energy conservation

ACN2016 74

HIPERLAN

• Wireless LAN ratified by ETSI

• HIPERLAN1

– First of the series of spec

– Supports five different priorities

– Data rate of 23.5 Mbps

– Forwards packets using several relays

• Extends communication beyond the radio range

– Power conservation by specific sleep and wakeup pattern – MSDU lifetime can be set to have time bound services

– MAC layer uses residual lifetime and user priority to choose the next MSDU to be transmitted

– Negotiation of QoS parameter during connection establishment

• Dynamic frequency selection

– Best frequency chosen based on interference level and usage of radio channels

• Power save

– Mobile devices can negotiate certain sleep and wakeup pattern for power save

• Access Points can have multiple transceivers

• APs can have sectorized antenna

– This is the optional ad hoc mode of HiperLAN2

– Data is directly exchanged between MS

• But the network is still controlled

• Done via an AP that has the central controller (CC) functionality

or via an MS that has CC functionality

• This ensures QoS support in ad hoc mode also

Centralized mode

AP/CC

data control

Direct mode

Different modes of operation of HiperLAN2

ACN2016 78

Bluetooth

• Design goal was to set up short range ad hoc

network (called piconets)

• 79 channels in the 2.4 GHz band with 1 MHz carrier spacing

• Devices perform frequency hopping at 1600 hops/s

• Maximum data rate of 1Mbps

• Range of about 10m

– One of the devices is the master, all others are slaves

– Master determines the hopping pattern in the piconet and the slaves have to synchronize to this pattern

– Each piconet has a unique hopping pattern

• Upto 8 devices can be active in a piconet

– Parked devices use 8-bit parked member address (PMA)

– Standby devices do not need address

ACN2016 82

Bluetooth

• Scatternet

– Only having 1 piconet within 80 MHz in total is not very efficient

– Many piconets with overlapping coverage can exist simulatenously

• A device may participate in two different piconets

– Bluetooth uses FH-CDMA for separation of piconet

– A slave first syncs to one piconet and communicates, then leaves that piconet and enters the other piconet (of scatternet) by syncing to its FH sequence – A master cannot be shared between two piconets of a scatternet

– Master can leave one piconet and enter the other as a slave

• All traffic in the former piconet is suspended until the master returns