Wireless LAN

Wireless LANs

• Evolution and Technology

• IEEE 802.11

• Bluetooth

• Zigbee and IEEE 802.15

Evolution

• Early experiences (1970-72): IBM, HP, Motorola– Abandoned due to limited performance and unavailability of

frequency bands• Early challenges:

– Complexity and cost– Bandwidth– Coverage– Interference– Frequency administration

• Emergence of unlicensed bands– Release of Industrial, Scientific and Medical (ISM) bands in 1985

• Applications: military, home and enterprise networks, mobile networks, teetherless access

Media Access

• Media in wireless networks is shared and is scarce – access must be controlled

• Observations:– Contention is at the receiver, not at the sender –

makes the carrier sense approach inappropriate– Unlike Ethernet, congestion is location-dependent– The media access protocol should propagate

congestion information explicitly rather than having each device learn about congestion independently

– Media access protocol should propagate synchronization information about contention periods, so that all devices can contend effectively

IEEE 802.11

• Standardization group formed in 1990, first standards completed in 1997

• IEEE 802.11 is the first WLAN standard; only one to secure a market

• 802.11b: PHY layer supports 11 Mbps using CKK (complementary code keying) technology

• 802.11a: PHY layer supports 54 Mbps using OFDM

• Uses CSMA/CA for contention data• Supports both infrastructure as well as ad hoc

modes

Requirements

• Single MAC to support multiple PHY layers

• Mechanism to support multiple overlapping network

• Provisions to handle interference

• Mechanism to handle hidden terminals

• Privacy and access control

Logical link control

Point coordination function (PCF)

Distributed coordination function (DCF)

2.4-Ghz frequency-

hopping spread

spectrum 1Mbps 2Mbps

2.4-Ghz direct

sequence spread

spectrum 1Mbps 2Mbps

Infrared 1Mbps 2Mbps

5-Ghz orthogonal FDM 6, 9. 12. 18, 24, 36, 48, 54

Mbps

2.4-Ghz direct

sequence spread

spectrum 5.5 Mbps 11 Mbps

Contention-free service Contention

service

MAC layer

IEEE 802.11 Protocol Architecture

IEEE 802.11 IEEE 802.11a IEEE 802.11b

Topology

Basic Service Set (BSS)

BSS

BSS

An Extended Service Set

(ESS)

Infrastructure Network

Ad hoc Network

Layered Protocol Architecture

• MAC sublayer is responsible for access mechanisms and fragmentation/reassembly

• MAC management is responsible for roaming in Extended Service Set (ESS), power management, association/dissociation/reassociation/ process for registration connection management

• PHY management: decides on channel tuning– Physical Layer convergence protocol (PLCP): carrier sensing

and forming packets– Physical Medium Dependent (PMD): modulation and coding

techniques for signaling

• Station management: coordination of interaction between MAC and PHY layers

Low Layer Protocol Stack

MAC Management

Data Link Layer

Sta

tion

Man

agem

ent

PHY Management

LLC

MAC

PLCP

PMD

Physical Layer

PLCP: Physical Layer Convergence ProtocolPMD: Physical Medium Dependent

PHY Layer

• When the MAC protocol data unit (MPDU) arrive at the PLCP layer, a header is attached that is designed specifically for the PMD

• The PLCP packet is then transmitted by the PMD according to specification of the signaling techniques

• IEEE 802.11 defines three PLCP packet formats:– FHSS (frequency hopping spread spectrum)– DSSS (direct sequence spread spectrum)– DFIR (diffused infrared)

FHSS

• PMD hops over 78 channels of 1 MHz each in the center of 2.44 GHz ISM bands

• Each BSS can select one of the three patterns of 26 hops:– (0, 3, 6, 9, …, 75)– (1, 4, 7, 10, …, 76)– (2, 5, 8, 11, …, 77)

• IEEE 802.11 specifies specific random hopping pattern for each of these frequency groups that facilitates multivendor interpretability

• Multiple Basic Service Set (BSS) can co-exist in the same area by up to three APs using different frequency groups

DSSS

• DSSS communicates using non-overlapping pulses at 11 Mcps

• The ISM band at 2.4 GHz is divided into 11 overlapping channels spaced at 5 MHz

• A PHY layer management sublayer of AP covering a BSS can select one of the choices

• Because of wider bandwidth, DSSS provides a better coverage and a more stable signal

Carrier Sense Multiple Access (CSMA appropriateness?)

• Carrier sense provides information about potential collision at the sender, but not at the receiver

• Since the receiver and sender are not co-located, carrier sense does not provide adequate information for collision avoidance – interference at the sender does not imply interference at the receiver

Carrier Sensing

• Carrier sensing in IEEE 802.11 is performed physically or virtually

• PHY sensing is through the clear channel assignment (CCA) signal produced by PLCP

• CCA is generated by sensing detected bits or by checking the RSS

• Virtual carrier sensing is done based on a network allocation vector (NAV) – more later

MAC Layer

• MAC Sublayer:– Defines the access mechanisms and packet

formats

• MAC Management:– Defines roaming support in the ESS, power

management and security

MAC Sublayer

• Reliable data delivery

• Access mechanisms– Contention-based

• CSMA/CA

– Contention-free• RTS/CTS• Point Coordination Function (PCF)

Reliable Data Delivery

• High degree of unreliability and large timers for retransmissions used in higher layers motivates to deal with errors at the MAC layer

• Each transmission is followed by an ACK as an atomic unit. Retransmission is done if the ACK is not received

• RTS/CTS exchange

Hidden Terminal Problem

A

B

XNode X finds that the mediumis free, and transmits a packet

No carrier ≠OK to transmit

A is transmitting a packet to B

Exposed Terminal Problem

A is transmitting a packet to B

X can not transmit to Y, eventhough it will not interfere at B

A

B

XY

Presence of carrier ≠ holds off transmission

Busy Tone

A

B

XA

B

XY

X OK to transmit X not OK to transmit

1. Receiver transmits busy tone when receiving data2. All nodes hearing busy tone keep silent3. Requires a separate channel for busy tone

B is receiving a packet from A

RTS/CTS dialog

RTS = Request to Send

RTS

Any node that hears this RTS will defer medium access.

Defer

RTS/CTS Dialog

CTS = Clear to Send

CTS

Any node that hears this CTS will defer medium access.

Defer

Defer

RTS

RTS/CTS Dialog

ACK

Defer

Defer

Data

Access Control

• Distributed Coordination Function (DCF)

• Point Coordinated Function (PCF) Centralized

Distributed Coordination Function (DCF)

• DCF sublayer makes use of a simple CSMA algorithm

• Collision detection (CD) is not included because of its impracticability in wireless networks

• DCF includes a set of delays called interframe space (IFS) to provision priority

Wait for frame to transmit

Medium idle?

Wait IFS

Still idle?

Transmit frame

Wait until current transmission ends

Wait IFS

Still idle?

Exponential backoff while medium idle

Transmit frame

Yes

No

No

Yes

No

Yes

IEEE 802.11 Medium Access Control Logic

IEEE 802.11 DCF

• Uses RTS-CTS exchange to avoid hidden terminal problem– Any node overhearing a CTS cannot transmit for the

duration of the transfer– Any node receiving the RTS cannot transmit for the

duration of the transfer• To prevent collision with ACK when it arrives at the sender

• Uses ACK to achieve reliability

IEEE 802.11 DCF

• CSMA/CA– Contention-based random access– Collision detection not possible while a node is

transmitting

• Carrier sense in 802.11• Physical carrier sense• Virtual carrier sense using Network Allocation Vector (NAV)

– NAV is updated based on overheard RTS/CTS packets, each of which specified duration of a pending Data/Ack transmission

• Collision avoidance• Nodes stay silent when carrier sensed busy (physical/virtual)• Backoff intervals used to reduce collision probability

Backoff Interval

• When the channel is busy, choose a back-off interval in the range [0,cw]– cw is contention window

• Count down the back-off interval when medium is idle– Count-down is suspended if medium becomes

busy

• When back-off interval reaches 0, transmit RTS

Dynamic Contention Window

• Binary Exponential Back-off in 802.11 DCF – When a node fails to receive CTS in response

to its RTS, it increases the contention window• cw is doubled (up to an upper bound)

– When a node successfully completes a data transfer, it restores cw to cwmin

Priority-based Access Provisioning

• Using different values of inter frame space (IFS)• SIFS (short IFS): used for immediate response

actions• PIFS (Point coordination function IFS): used by

the centralized controller while issuing polls• DIFS (Distributed coordination function IFS):

minimum delay for asynchronous frames contending for access

DIFS > PIFS > SIFS

802.11 CSMA/CA

S2

S1

R

S2 S1 R X

X

Channel B

usy

DIFS

Channel Id

le

DIFS: DCF Inter-Frame Space

RTS

SIFS: Short Inter-Frame Space

CTS

SIFS

NAV

NAV

SIFS

DATASIFS

ACK

B2=9

B1=5

cw = 15

RTS

B2=4

B1=7

DIFSC

hannel Id

le

Point Coordination Function (PCF)

• PCF is implemented on top of DCF• The time sensitive traffic are controlled by the PCF and

the remaining traffic contend for access using CSMA/CA• The centralized polling master (point coordinator) issues

polls using PIFS • The poll responses use SIFS• The point coordinator could issue polls in a round robin

fashion• Seizing of the medium by the PCF is avoided by using

superframes where the point coordinator is allowed to poll for a fixed duration and then idle for the rest of the superframe period to allow the asynchronous traffic to contend for the medium.

MAC Frame Format

• Frame Control (FC): Indicated type of frame, provides control information

• Duration/connection ID (D/I): If used as a duration field -indicates time (in s) for which the channel will be allocated for transmission of a MAC frame. In some control frames, it contains an association, or connection identifier

• Addresses: Context dependent. Types include source, destination, transmitting station, receiving station

• Sequence Control: Used for fragmentation/reassembly. • Frame Body: Contains an MPDU or its fragment• Cyclic Redundancy Check (CRC): 32-bit frame check sequence

FC D/I Address Address Address SC Address Frame Body CRC2 2 6 6 6 2 6 0-2312 4

Frame Control Field

• Protocol Version (PV): 802.11 version, currently version 0• Type: Identifies the frame as control, management, or data• Subtype: Identifies the function of frame• To DS: The MAC coordination sets this bit to 1 in a frame destined to the

distribution system• From DS: The MAC coordination sets this bit to 1 in a frame leaving the

distribution system• More Fragments (MF): Set to 1 if more fragments follow• Retry (RT): Set to 1 if retransmission• Power Management (PM): Set to 1 if transmitting station is in sleep mode• More Data (MD): Indicates that a station has additional data to send• Wired Equivalent Privacy (WEP): WEP implemented• Order (O): The frames must be processed in order if set to 1.

PV Type SubType TO

DSFROM

DSMF RT PM MD W O

422 1 1 1 1 1 1 1 1

IEEE 802.11 Management Sublayer

• Registration

• Handoff

• Power Management

• Security

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

• Received Signal Strength (RSS) measurements are done on the beacon message

• Association: process by which an MS registers with an AP

Handoff

• Mobility Types:– No transition – MS is static or moving within a

BSA– BSS transition – MS moves from one BSS to

another within the same ESS– ESS transition – MS moves from one BSS to

another BSS which belong to a different ESS

• Reassociation service is used when an MS moves from one BSS to another within the same ESS

Handoff procedure in IEEE 802.11

Beacon Periodically AP1

AP2

AP3

1. Strong Signal

2. Weak Signal; start scanning for handoff

4. Pro

be

Resp

onse

5. Choose AP with strongest response

8. IAPP indicates

reassociation to old AP

7. Reassociation Response

6. Reassociation

Request3. Probe Request

Power Management

• How to power-off during idle periods?• IEEE 802.11 buffers data at the AP, and sends

the data when the MS is awakened• Using TSF, all MSs are synchronized – they

wake up at the same time to listen to beacon• With every beacon a 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

Security

• There are provisions for authentication and privacy in IEEE 802.11

• Open system authentication (default)– Request frame sends the authentication algorithm id – the response frame sends the result

• Shared key authentication– Request frame sends the authentication frame id for

the shared key that is shared between itself and the AP

– The second station sends a challenge text– The first station sends the encrypted challenge as the

response– The second station sends the authentication result