IT 601 1

IT 601: Mobile Computing

Wireless LANs(most of the slides are borrowed from

Prof. Sridhar Iyer)

IT 601 2

Wireless LANs: Characteristics

• Advantages– Flexible deployment; Minimal wiring problems – More robust against disasters – Historic buildings, conferences, …

• Disadvantages– Low bandwidth compared to wired networks– Need to follow wireless spectrum regulations

IT 601 3

Infrastructure and Adhoc Networksinfrastructure network

ad-hoc network

APAP

AP

wired network

AP: Access Point

Source: Schiller

IT 601 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

IT 601 5

Difference Between Wired and Wireless

• If both A and C sense the channel to be idle at the same time, they send at the same time.

• Collision can be detected at sender in Ethernet.

A B CA

B

C

Ethernet LAN Wireless LAN

IT 601 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

IT 601 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)

IT 601 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”

IT 601 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

(CS fails)– Collision occurs at B.– A cannot receive the collision (CD fails).– A is “hidden” for C.

Hidden Terminal Problem

BA C

IT 601 10

Exposed Terminal Problem

– A starts sending to B.– C senses carrier, finds medium in use and has to

wait for A->B to end.– D is outside the range of A, therefore waiting is not

necessary.

A BCD

IT 601 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

A B CRTS

CTS CTSDATA

DRTS

IT 601 12

IEEE 802.11

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

IT 601 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

IT 601 14

802.11 architecture• The basic service set (BSS) is the basic building

block of an IEEE 802.11 LAN

ad-hoc network BSS2BSS1

IT 601 15

802.11 architecture (contd.)

• The ovals can be thought of as the coverage area within which member stations can directly communicate

• The Independent BSS (IBSS) is the simplest LAN. It may consist of as few as two stations

• IBSS is also called the ad hoc mode or DCF mode in 802.11

IT 601 16

802.11 - ad-hoc network

• Direct communication within a limited range– Station (STA):

terminal with access mechanisms to the wireless medium

– Basic Service Set (BSS):group of stations using the same radio frequency

802.11 LAN

BSS2

802.11 LAN

BSS1

STA1

STA4

STA5

STA2

STA3

Source: Schiller

IT 601 17

802.11 - infrastructure

Source: Schiller

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS

IT 601 18

PCF components

• Station (STA): terminal with access mechanisms to the wireless medium and radio contact to the access point

• Basic Service Set (BSS): group of stations using the same radio frequency

• Access Point: station integrated into the wireless LAN and the distribution system

• Portal: bridge to other (wired) networks• Distribution System: interconnection network to form one

logical network (ESS: Extended Service Set) based on several BSS

IT 601 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

IT 601 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)

IT 601 21

802.11- in the TCP/IP stack

mobile terminal

access point

server

fixed terminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructure network

LLC LLC

IT 601 22

802.11 - Layers and functions• PLCP Physical Layer Convergence

Protocol

– clear channel assessment signal (carrier sense)

• PMD Physical Medium Dependent

– modulation, coding• PHY Management

– channel selection, MIB• Station Management

– coordination of all management functions

PMD

PLCP

MAC

LLC

MAC Management

PHY Management

• MAC– access mechanisms,

fragmentation, encryption • MAC Management

– synchronization, roaming, MIB, power management

PH

YD

LC

Sta

tion

Man

agem

ent

7.8.1

IT 601 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

IT 601 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

IT 601 25

Spread-spectrum communications

Source: Intersil

IT 601 26

DSSS Barker Code modulation

Source: Intersil

IT 601 27

802.11 - MAC layer

• Traffic services– Asynchronous Data Service (mandatory) – DCF– Time-Bounded Service (optional) - PCF

• Access methods– DCF CSMA/CA (mandatory)

• collision avoidance via randomized back-off mechanism

• ACK packet for acknowledgements (not for broadcasts)

IT 601 28

802.11 access methods– DCF CSMA/CA (mandatory)

– DCF with RTS/CTS (optional)• avoids hidden terminal problem

– PCF (optional)• access point polls terminals according to a

list

IT 601 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

IT 601 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)

IT 601 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

collision of ACK)– Expected duration of transmission+ACK is included in

RTS/CTS packets

A B CRTS

CTS CTSDATA

DRTS

ACK

IT 601 32

802.11 - CSMA/CA

– station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

– if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

slot timedirect access if medium is free DIFS

IT 601 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)

IT 601 34

802.11 –CSMA/CA example

t

busy

boe

station1

station2

station3

station4

station5

packet arrival at MAC

DIFSboe

boe

boe

busy

elapsed backoff time

bor residual backoff time

busy medium not idle (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor

IT 601 35

802.11 - Collision Avoidance

• Collision avoidance: Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit– 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

IT 601 36

DCF Example

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31

B2 = 10

IT 601 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

IT 601 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• cw is doubled (up to a bound CWmax)

– Upon successful completion data transfer, restore cw to CWmin

IT 601 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)

IT 601 40

802.11 – Priorities (contd.)

• PIFS (PCF IFS)– medium priority, for time-bounded service using PCF– PIFSTime = SIFSTime + SlotTime

• DIFS (DCF IFS)– lowest priority, for asynchronous data service– DCF-IFS (DIFS): DIFSTime = SIFSTime +

2xSlotTime

IT 601 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

waiting time

otherstations

receiver

sender data

DIFS

contention

IT 601 42

802.11 –RTS/CTS

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

sender data

DIFS

contention

RTS

CTS

SIFS SIFS

NAV (RTS)

NAV (CTS)

IT 601 43

802.11 –RTS/CTS

• station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium)

• 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

IT 601 44

Fragmentation

t

SIFS

DIFS

data

ACK1

otherstations

receiver

senderfrag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

IT 601 45

802.11 - Point Coordination Function

IT 601 46

802.11 - PCF I

PIFS

stations‘NAV

wirelessstations

point coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

t0 = time when the superframe should have startedt1 = time when it actually started due to contention in the prev period

IT 601 47

802.11 - PCF II

tstations‘NAV

wirelessstations

point coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contentionperiod

contention free period

t2 t3 t4

t2 = time when CFP actually finishedt3 = initial planned CFP (but PCF finished polling earlier than expected)

IT 601 48

CFP structure and Timing

CFP is greater than beacon intervalDTIM – Delivery Traffic Indication Message

Source: 802.11 spec

IT 601 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

IT 601 50

PCF- Data transmission

Source: 802.11 standard

IT 601 51

Polling Mechanisms

• With DCF, there is no mechanism to guarantee minimum delay for time-bound services

• PCF wastes bandwidth (control overhead) when network load is light, but delays are bounded

• Implicit signaling mechanism for STAs to indicate when they have data to send improves performance

IT 601 52

Coexistence of PCF and DCF• PC controls frame transfers during a Contention

Free Period (CFP). – CF-Poll control frame is used by the PC to invite a

station to send data– 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

IT 601 53

PCF and DCF (contd.)

• The CFP alternates with a CP, when DCF controls frame transfers– The CP must be large enough to send at least one

maximum-sized MPDU including RTS/CTS/ACK• Superframe: One CFP + One CP. It repeats

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

IT 601 54

802.11 - Frame format

bytes

FrameControl

DurationID

Address1

Address2

Address3

SequenceControl

Address4 Data CRC

2 2 6 6 6 62 40-2312

version, type, fragmentation, security, ...

IT 601 55

802.11 - Frame format

• Types– control frames, management frames, data frames

• 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

IT 601 56

Frame Control Field

IT 601 57

Types of Frames

• Control Frames– RTS/CTS/ACK– CF-Poll/CF-End

• Data Frames

• Management Frames– Beacons– Probe Request– Probe Response– Association Request– Association Response– Dis/Reassociation– Authentication– Deauthentication– ATIM (Announcement

TIM)

IT 601 58

MAC address formatscenario to DS from

DSaddress 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP

0 1 DA BSSID SA -

infrastructurenetwork, to AP

1 0 BSSID SA DA -

infrastructurenetwork, within DS

1 1 RA TA DA SA

DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source AddressBSSID: Basic Service Set IdentifierRA: Receiver AddressTA: Transmitter Address

IT 601 59

802.11 - MAC management• Synchronization

– try to find a LAN, try to stay within a LAN; timer etc.• Power management

– sleep-mode without missing a message• Association/Reassociation

– scanning, i.e. active search for a network– roaming, i.e. change networks by changing APs

• MIB - Management Information Base– managing, read, write

IT 601 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)

IT 601 61

Synchronization using a Beacon (infrastructure)

beacon interval

tmedium

accesspoint

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

IT 601 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

IT 601 63

Synchronization using a Beacon (ad-hoc)

tmedium

station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

station2

B2 B2

random delay

IT 601 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

IT 601 65

Power saving with wake-up patterns (infrastructure)

• All stations (one station shown) wake up prior to TIM or DTIM– With every beacon the AP sends TIM (Traffic

Indication Map)• TIM contains a list of stations for which

unicast data frames are waiting• DTIM (Deliverry TIM) is for sending

broadcast frames– PS (Power Saving) poll is sent by STA in response to

TIM destined to the STA

IT 601 66

Power saving with wake-up patterns (infrastructure)

TIM interval

t

medium

accesspoint

busy

D

busy busy busy

T T D

T TIM D DTIM

DTIM interval

BB

B broadcast/multicast

station

awake

p PS poll

p

d

d

d data transmissionto/from the station

IT 601 67

Power saving with wake-up patterns (ad-hoc)

• PS in ad hoc mode is more complex (no centralized AP)

• All stations announce a list of buffered frames during a period when all of them are awake– Destinations are announced using ATIM (Adhoc TIM)

IT 601 68

Power saving with wake-up patterns (ad-hoc)

awake

A transmit ATIM D transmit datat

station1

B1 B1

B beacon frame

station2

B2 B2

random delay

A

a

D

d

ATIMwindow beacon interval

a acknowledge ATIM d acknowledge data

IT 601 69

802.11 - Roaming

• Scanning– scan the environment, i.e., – passive scanning

• listen into the medium for beacon signals (to detect other network)

– active scanning• send probes into the medium on each channel and wait for an

answer• 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

IT 601 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

IT 601 71

Hardware• Original WaveLAN card (NCR)

– 914 MHz Radio Frequency– Transmit power 281.8 mW– Transmission Range ~250 m (outdoors) at 2Mbps– SNRT 10 dB

• WaveLAN II (Lucent)– 2.4 GHz radio frequency range– Transmit Power 30mW– Transmission range 376 m (outdoors) at 2 Mbps (60m indoors)– Receive Threshold = –81dBm – Carrier Sense Threshold = -111dBm

IT 601 72

802.11 status

MAC

MIB

DSSS FH IRPHY

WEP

LLC

MAC Mgmt

802.11b5,11 Mbps

802.11g20+ Mbps

802.11a6,9,12,18,24

36,48,54 Mbps

OFDM

802.11isecurity

802.11fInter Access Point Protocol

802.11eQoS enhancements

IT 601 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

– Beacons in PCF– RTS-CTS in DCF

• Acknowledgements for reliability• Binary exponential backoff for congestion control• Power save mode for energy conservation

IT 601 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

IT 601 75

HIPERLAN2• Operates at 5 GHz• Data rates up to 54 Mbps• OFDM in the physical layer and a dynamic TDMA/TDD based MAC• QoS support

– Each connection has its QoS parameters (delay, jitter, bit error)• Connection oriented

– 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

IT 601 76

HIPERLAN2• Two modes of operation• Centralized Mode (CM)

– Like the infrastructure mode in 802.11, APs are connected to a core network and Mobile Stations (MS) are associated with APs.

• Direct Mode (DM)– 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

IT 601 77

HIPERLAN2

AP

MS MS

datacontrolcontrol

Centralized mode

AP/CC

MS MSdata

control

Direct mode

Different modes of operation of HiperLAN2

IT 601 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

IT 601 79

Bluetooth

M

S

S

S

SB

P

P SBM – MasterS - SlaveSB – StandbyP - Park

Bluetooth Piconet

IT 601 80

Bluetooth• Piconet

– A collection of bluetooth devices which are synchronized to the same hopping sequence

– 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– Parked devices

• Cannot participate in the piconet, but are known and can be activated within few msec

• Devices in standby do not participate in piconet.

IT 601 81

Bluetooth

• Piconet– Active members assigned a 3-bit active member

address (AMA)• Upto 8 devices can be active in a piconet

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

– Standby devices do not need address

IT 601 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