IEEE 802.11 based WLANs

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S-72.1130 Telecommunication

Systems

Wireless Local Area Networks

Outline

LAN basics

Structure/properties of LANs

WLANs

Link layer services

Media access layer

 frames and headers

CSMA/CA

Physical layer

 frames

 modulation

Direct sequence

Frequency hopping

Infrared

Installation

Security

2

LAN Basics

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

Typical Wired LAN

Transmission Medium

Network Interface Card

(NIC)

Unique MAC “physical” address

Serial format in 10BASE5

~ 10 Mb/s

– baseband - 500 m

RAM

RAM

ROM

Ethernet

Processor

Reference: A. Leon-Garcia, I. Widjaja, Communication

Networks , Instructor's Slide Set

NIC implements MAC protocol & physical port. Parallel interface to PC

4

Example: How Ring Networks Work

A node functions as a repeater

Only destination station copies the frame, all other nodes discard the frame

C

A

A

B transmits frame addressed to A

Unidirectional link

Signal propagates encoded by line codes

Example: 802.5

C

A

A

Reliability: link failure (FDDI applies double ring)

A copies the frame at the reception

B

B

C

A

C

A

C ignores the frame

A

A

B absorbs the returning frame

B

B

5

Token Ring

A ring consists of a single or dual (FDDI) cable in the shape of a loop. Ring reservation supervised by the rotating token.

Each station is physically connected to each of its two nearest neighbors. Data in the form of packets passes around the ring from one station to another in uni-directional way.

Advantages :

(1) Access method supports heavy load without statistical multiplexing degradation of performance because the medium is shared for pair-wise stations

(2) In practice several packets can simultaneous circulate between different pairs of stations.

Disadvantages:

(1) Complex management - especially for several rings

(2) Re-initialization of the ring whenever a failure occurs

6

Example: Bus Network

In a bus network, one node’s transmission traverses the entire network and is received and examined by every node. The access method can be :

(1) Contention scheme : multiple nodes attempt to access bus; only one node succeeds at a time (e.g. CSMA/CD in Ethernet

802.3

)

(2) Round robin scheme : a token is passed between nodes; node holding the token can use the bus (e.g.

Token bus 802.4

)

Advantages:

(1) Simple access method

(2) Easy to add or remove stations

Disadvantages: term

A

(1) Poor efficiency with high network load in contention schemes

(2) Security taken care by upper network levels

D

B C

- Line coded, serial data

D

- Twisted pair or coaxial cable term term: terminator impedance

7

Wireless Local Area networks

(WLANs) - basics

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

Wireless LANs (WLANs) - features

High date rates

IEEE 802.11b supports rates up to 11 MBps (in practice 6

Mb/s), and 802.11g reaches up to 54 Mb/s, need to have the bandwidth

No new wiring and installation on difficult-to-wire areas

Offices, public places, and homes

Factories, vehicles, roads, and railroads

Mobility

Increases working efficiency and productivity

Roaming support: extended on-line times

-> universal access & seamless services

Reduced installation time

No cabling time

Easy setup

Standard enables interoperability between different vendors

Roaming with GSM and UMTS is a research issue

9

WLAN Technology Challenges

Flexible error control: in physical, MAC and/or in upper levels

Physical level takes care of physical transmission of packets over a medium (modulation, line coding, channel coding)

Interference & noise

Working in ISM band means sharing the frequency bands with microwave oven, cordless telephones, Bluetooth etc.

-> Modulation and MAC design challenge:

Pros: Freedom from spectral regulatory constraints at ISM Band (Industrial, Science and Medical)

Multi-path propagation

Remedies: channel coding / rake-reception

Dynamic network management

Stations movable and may be operated while moved

 addressing and association procedures

 interconnections (roaming)

10

Challenges …

MAC protocol takes care of optimizing throughput for the expected services

Wireless channel is also the reason why access method for

802.11 is CSMA/CA and not CSMA/CD

Security

Difficult to detect collisions in wireless environment

-> Hidden terminal problem (see PSTN lecture)

Traditional WEP (Wired Equivalent Privacy) now replaced by

WPA (Wi-Fi Protected Access) and 802.11i (WPA2)

AAA (Authentication, Authorization, Accounting) can be taken care by a dedicated server as RADIUS (Remote

Authentication Dial In User Service )

CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance

CSMA/CD: Carrier Sense Multiple Access/Collision Detection

11

IEEE 802 LAN Standards

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

IEEE 802-series of LAN Standards

802 standards free to download from http://standards.ieee.org

/getieee802 hub stations hub stations router hub stations hub

WiMAX server

Demand priority: A round-robin (token rings) method to provide LAN access based on message priority level

DQDB: Distributed queue dual buss, see PSTN lecture

13

The IEEE 802 LAN Standards

( http://www.ieee802.org/ )

OSI Layer 3

Network

IEEE 802.2

Logical Link Control (LLC)

IEEE 802.3

Carrier

Sense

Ethernet

IEEE 802.4

Token

Bus

IEEE 802.5

Token

Ring b: Wi-Fi

IEEE 802.11

Wireless a b g

Physical Layers

- options: twisted pair, coaxial, optical, radio paths;

(not for all MACs above!)

LLC OSI Layer 2

(data link)

MAC

OSI Layer 1

(physical)

Bus (802.3…) Star (802.3u…) Ring (802.5…)

14

IEEE 802.11 Wireless Local

Area Networks (WLANs):

Service Sets

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

IEEE 802.11 Architecture

802.11 networks can work in

Basic service set (BSS)

Extended service set (ESS)

BSS can also be used in ad-hoc networking

Propagation boundary

Network

LLC

MAC

FHSS DSSS IR

Internet

PHY

LLC: Logical Link Control Layer

MAC: Medium Access Control Layer

PHY: Physical Layer

FHSS: Frequency hopping SS

DSSS: Direct sequence SS

SS: Spread spectrum

IR: Infrared light

BSS: Basic Service Set

ESS: Extended Service Set

Station B

Distribution system

Station A

BSS 1

BSS 2

Basic (independent) service set ( BSS )

Access Point

Extended service set ( ESS )

( infrastructure-mode )

Portal: gateway access to other networks/Internet

16

Basic and Extended Service Sets

Basic Service Set (BSS) – indoor radius of tens of meters with a single AP

Operates in Basic Service Area (BSA) that is much like the area of a cell in cellular mobile communications

BSSs may geographically overlap, be physically disjoint, or they may be collocated (one BSS may use several antennas)

Ad-hoc or Infrastructure (nomadic) mode: Access coordinated by the MAC protocols

Extended Service Set (ESS)

Multiple BSSs interconnected by a Distribution System (DS)

Each BSS is like a cell and stations in BSS communicate via an Access Point (AP) with the DS

Portals attached to DS provide gateways as access to

Internet or other ESS

17

Distribution system (DS) services

DS provides distribution services :

Transfer MAC SDUs between APs in ESS (I)

Transfer MSDUs between portals & BSSs in ESS (II)

Transfer MSDUs between stations in same BSS (III)

Multicast, broadcast, or stations’s preference

ESS looks like a single BSS to LLC layer

Propagation boundary

III

III

SDU: Service Data Unit

(inter-layer data)

LLC: Logical Link Control Layer

MAC: Medium Access Control Layer

MSDU: MAC Service Data Unit

PHY: Physical Layer

FHSS: Frequency hopping SS

DSSS: Direct sequence SS

SS: Spread spectrum

IR: Infrared light

BSS: Basic Service Set

ESS: Extended Service Set

AP: Access Point

IIIb

Station A

Basic (independent) service set (BSS)

Access Point

Station B

Internet

BSS 1

II

Distribution system

I

BSS 2

Extended service set (ESS)

Portal: gateway access to other networks/Internet

18

IEEE 802.11 Mobility (b/g)

Standard defines the following mobility types:

No-transition: no movement or moving within a local BSS

BSS-transition: station movies from one BSS in one ESS to another

BSS within the same ESS

ESS-transition: station moves from a BSS in one ESS to a BSS in a different ESS (continuos roaming not supported)

Especially: 802.11 don’t support roaming with GSM!

For fast, seamless roaming

802.11r

- Address to destination mapping

- seamless integration of multiple BSS

ESS 2

ESS 1

19

IEEE 802 LAN Standard:

Logical Link Layer (LLC)

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

802.11 WLAN Architecture

Logical Link Control (LLC)

LLC provides addressing and data link control

– common to all 802 LANs

Utilizes services of HDLC

(High-level Data Link Control)

Therefore, LLC SAPs separate upper layer data exchanges =>

NIC applies different buffer segments for each SAP (port)

LLC provides means to exchange frames between

LANs using different MACs

IEEE 802.2

Logical Link Control (LLC) b: Wi-Fi

IEEE 802.3

Carrier

Sense

Ethernet

IEEE 802.4

Token

Bus

IEEE 802.5

Token

Ring

IEEE 802.11

Wireless a b g

Physical layer: DSSS, FHSS, IR

CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance

LLC: Logical Link Control Layer

MAC: Medium Access Control Layer

SS: Spread Spectrum

FHSS: Frequency hopping SS

DSSS: Direct sequence SS

IR: Infrared light

NAV: Network Allocation Vector

SAP: Service Access Point

DCF: Distributed Coordination Function

PCF: Point Coordination Function

NIC: Network Interface Card

LLC

MAC

PHY

21

Logical Link Control Layer (LLC)

Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2)

Objective: exchange data between users across LAN using 802-based

MAC controlled link

Provides addressing and data link control ( routing )

Independent of topology, medium, and chosen MAC access method

Data to higher level protocols

Info: carries user data

Supervisory: carries flow/error control

Unnumbered: carries protocol control data

Source

SAP

LLC’s Protocol Data Unit (PDU)

(SAP: Service Access Point)

22

SAP Addressing

IEE802.11 (CDMA)...

IEE802.11 (CSMA/CA)...

ATM...

Reference: W.

Stallings: Data and Computer

Communications,

7th ed

23

A TCP/IP Packet

Encapsulation

Control header

MAC frame with new control fields

Traffic to the target BSS / ESS

*Protocol data unit

TCP makes logical connection to deliver the packet

LLC constructs PDU* by adding a control header

SAP (service access point)

MAC lines up packets by using a MAC protocol

PHY layer transmits packet using a modulation method

(DSSS, OFDM, IR, FHSS)

24

Encapsulation …

Reference: W.

Stallings: Data and Computer

Communications,

7th ed

25

LLC Services

A Unacknowledged connectionless service

Point-to-point, multicast (assigned users), broadcast (group of users) addressing no error or flow control - no ack-signal higher levels take care or reliability - thus fast

Often referred as ‘Unnumbered frame mode of HDLC*’

B Connection oriented service

 connection phases: Connection setup, data exchange, and release supports unicast only error/flow control (cyclic redundancy check (CRC)), sequencing

‘Asynchronous mode of HDLC’

C Acknowledged connectionless service

Can handle several logical connections, distinguished by their SAPs ack-signal used error and flow control by stop-and-wait ARQ faster setup than for B *High-Level Data Link Control 26

IEEE 802.11 Wireless Local

Area Networks (WLANs):

Media Access Protocol

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

Selecting a Medium Access Control

Environment: Wired / Wireless?

Applications:

What type of traffic?

Voice streams? Steady traffic, low delay/jitter

Data? Short messages? Web page downloads?

Enterprise or consumer market? Reliability, cost

Scale:

How much traffic can be carried?

How many users can be supported?

Examples :

Design MAC to provide wireless DSL-equivalent access for rural communities

Design MAC to provide Wireless-LAN-equivalent access to mobile users (user in a car travelling at 130 km/h)

28

MAC techniques - examples

Contention

Medium is free for all, packet collisions do happen

A node senses the free medium and occupies it as long as data packet requires it

Example: Ethernet (IEEE 802.3 CSMA/CD)

Reservation (short term statistical access)

Gives everybody a turn

Reservation time depends on token holding time (set by network operator)

For heavy loaded networks

Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI

Reservation (long term)

Link reservation for multiple packets (whole session)

Example: scheduling a time slot: GSM using TDMA. FDMA applied for uplink/dowlink separation.

Hybrid… (example: contention+reservation)

Flexible compromise: 802.11 WLANs

29

Media Access Control (MAC):

Ways to Share a Medium

Medium sharing techniques

Static channelization

Dynamic medium access control

Medium sharing required for multiple users to access the channel

Communications by

 unicasting

 multicasting broadcasting

FDMA,TDMA, CDMA

Uses partition medium

Dedicated allocation to users

Examples:

Satellite transmission

Cellular

Telephone

Scheduling Random access

(contention)

Polling (take turns):

Token ring 802.5

Reservation systems:

Request for slot in transmission schedule

802.4

Loose coordination

Send, wait, retry if necessary

Aloha

CSMA/CD (Ethernet)

CSMA/CA (802.11

WLAN)

30

Example 802.3: MAC of Ethernet (CSMA/CD*)

CSMA/CD:

1. If the medium is idle, transmit; otherwise, go to step 2

2. If the medium is busy, continue listening ( CS : carrier sensing ) until the channel is idle, then transmit immediately

3. If a collision is detected ( CD ) during transmission know about collision and then cease transmission

, transmit brief jamming signal to assure all stations

4. After transmitting the jamming signal, wait a random time (back-off time), then attempt to transmit again

*Carrier sense multiple access/collision detection

31

Throughput Performance of CSMA/CD

 a

 t prop

/ a : normalized

delay-bandwidth product r

: normalized load

: aggregated rate [frames/second] r

(Load) ~ throughput

We can see that in Ethernet transfer delays grow very fast as the load increases for the given value of delay-bw product a .

Note: Large value of parameter a scales results for propagation delay and/or signaling rate – if their product becomes larger, throughput (in terms of transfer delay) gets smaller.

t prop

: one-way delay, R: signaling rate,

L: frame length

Reference: A. Leon-Garcia, I. Widjaja,

Communication Networks, 2nd ed 32

802.11 WLAN Architecture

Medium Access Control (MAC) - Summary

802.11 MAC

Services

Station service :

Authentication, privacy, MSDU* delivery

Distributed system:

Association **, participates to data distribution

Transmits frames based on MAC addresses (in NIC)

Connectionless/Connection oriented frame transfer service

Coordinates access to medium

Joining the network (NAV,

 addressing)

MAC scheme CSMA/CA:

Contention-free access (PCF)

Contention access (DCF)

* MSDU: MAC service data unit

** with an access point in extended or basic service set (ESS,BSS)

IEEE 802.2

Logical Link Control (LLC) b: Wi-Fi

IEEE 802.3

Carrier

Sense

Ethernet

IEEE 802.4

Token

Bus

IEEE 802.5

Token

Ring

IEEE 802.11

Wireless a b g

Physical layer: DSSS, FHSS, IR

LLC

MAC

PHY

CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance

LLC: Logical Link Control Layer

MAC: Medium Access Control Layer

SS: Spread Spectrum

FHSS: Frequency hopping SS

DSSS: Direct sequence SS

IR: Infrared light

NAV: Network Allocation Vector

SAP: Service Access Point

DCF: Distributed Coordination Function

PCF: Point Coordination Function

NIC: Network Interface Card 33

IEEE 802.11 Coordination Functions

Reference: W.

Stallings: Data and Computer

Communications,

7th ed

34

Media Access Control in 802.11 WLANs

Distributed Wireless Foundation MAC (DWFMAC):

Distributed access control mechanism (CSMA/CA)

Optional centralized control on top (PCF)

MAC flavours provided by coordination functions:

Distributed coordination function (DCF) – CSMA/CA

Contention algorithm to provide access to all traffic

Asynchronous, best effort-type traffic

Application: bursty traffic, add-hoc networks

Point coordination function (PCF) – polling principle

(rarely applied in practical devices)

Centralized MAC algorithm

Connection oriented

Contention free

Built on top of DCF

Application: timing sensitive, high-priority data

35

IEEE 802.11 MAC (DWFMAC):

Timing in Basic Access

duration depends on MAC load type duration depends on network condition

MAC frame: Control, management , data + headers

(size depends on frame load and type)

Reference: W.

Stallings: Data and Computer

Communications,

7th ed

PCF: Point Coordination Function (asynchronous, connectionless access)

DCF: Distributed Coordination Function (connection oriented access)

DIFS: DCF Inter Frame Space (minimum delay for asynchronous frame access)

PIFS: PCF Inter Frame Space (minimum poll timing interval)

SIFS: Short IFS (minimum timing for high priority frame access as ACK,

CTS, MSDU…)

MSDU: MAC Service Data Unit

36

IEEE 802.11

MAC Logic

(DWFMAC)

IFS: Inter Frame Space (= DIFS, SIFS, or

PIFS)

DWFMAC: Distributed Wireless Foundation

MAC

Reference: W. Stallings: Data and Computer Communications,

7th ed duration depends on MAC load type

37

DWFMAC summarized

Collision Avoidance

When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots)

Station transmits frame when backoff timer expires

If collision occurs, recompute backoff over interval

Receiving stations of error-free frames send ACK

Sending station interprets non-arrival of ACK as loss

Executes backoff and then retransmits

Receiving stations use sequence numbers to identify duplicate frames

38

Carrier Sensing in 802.11 MAC - Summary

Physical Carrier Sensing

Analyze all detected frames for errors

Monitor relative signal strength from other sources

Virtual Carrier Sensing at MAC sublayer (avoids hiddenterminal problem)

Source stations inform other stations of transmission time (in m sec) for an MPDU (MAC Protocol Data Unit)

Carried in Duration

CTS (Clear to send) field of RTS (Request to send) &

Stations adjust their Network Allocation Vector to indicate when the channel will become idle

(NAV)

Channel busy if either sensing is busy

Reference: A. Leon-Garcia, I. Widjaja, Communication

Networks , Instructor's Slide Set

39

DIFS

Transmission of MPDU without RTS/CTS

Source

Data

SIFS

ACK

NAV: Network allocation vector

DIFS: DCF Inter Frame Space (async)

SIFS:

SIFS: Short IFS (ack, CTS…)

RTS: Request to send

CTS: Clear to send

MPDU: MAC Protocol Data Unit

DCF: Distributed Coordination Function

PCF: Point Coordination Function

Destination

DIFS

Other

NAV

Defer (postpone) access for other stations

Reference: A. Leon-Garcia, I. Widjaja, Communication

Networks , Instructor's Slide Set

Wait for

Reattempt Time

40

Transmission of MPDU with RTS/CTS (DCF)

Hidden terminal solution

DIFS

RTS Data

Source

SIFS

CTS

SIFS

Destination

NAV: Network allocation vector

DIFS: DCF Inter Frame Space (async)

SIFS:

SIFS: Short IFS (ack, CTS…)

RTS: Request to send

CTS: Clear to send

MPDU: MAC Protocol Data Unit

DCF: Distributed Coordination Function

PCF: Point Coordination Function

SIFS

Ack

DIFS

Other

NAV (RTS)

NAV (CTS)

NAV (Data)

Reference: A. Leon-Garcia, I. Widjaja,

Communication Networks , Instructor's Slide Set

Defer access

RTS: Request to Send

CTS: Clear to Send

41

PCF Frame Transfer

TBTT Fixed super-frame interval

Contention-free repetition interval (PCF)

B

SIFS

D1 +

Poll

SIFS SIFS

D2+Ac k+Poll

U 1 +

ACK

PIFS

SIFS SIFS

CF

End

U 2 +

ACK

Contention period (DCF)

Reset NAV

NAV

CF_Max_duration

D1, D2 = frame sent by point coordinator

U1, U2 = frame sent by polled station

TBTT = target beacon transmission time

B = beacon frame (initiation)

NAV: Network allocation vector

PIFS: PCF Inter Frame Space

DIFS: DCF Inter Frame Space (async)

SIFS: SIFS: Short IFS (ack, CTS…)

RTS: Request to send

CTS: Clear to send

MPDU: MAC Protocol Data Unit

DCF: Distributed Coordination Function

PCF: Point Coordination Function

42

Point Coordination Function

PCF provides connection-oriented, contention-free service through polling

Point coordinator (PC) in AP performs PCF

Polling table up to implementer

Contention free period (CFP) repetition interval

Determines frequency with which contention free period occurs

Initiated by beacon frame

Coordinator (PC) in AP transmitted by Point

During CFP stations may only transmit to respond to a poll from PC or to send ACK

All stations adjust Network Allocation Vector indicate when channel will becomes idle

(NAV) to

Reference: A. Leon-Garcia, I. Widjaja, Communication

Networks , Instructor's Slide Set

43

MAC Frame Types

Management frames

Station association & disassociation with AP (this establishes formally BSS)

Timing & synchronization

Authentication & de-authentication (option for identifying other stations)

Control frames

Handshaking

ACKs during data transfer

Data frames

Data transfer

Reference: A. Leon-Garcia, I. Widjaja, Communication

Networks , Instructor's Slide Set

44

MAC Frame

NOTE: This frame structure is common for all data send by a 802.11 station control info (WEP, data type as management, control, data ...) next frame duration frame ordering info for RX

-Basic service identification BSSID*

-source/destination address

-transmitting station

-receiving station frame specific, variable length

*BSSID: a six-byte address typical for a particular access point

(network administrator sets)

CRC: Cyclic Redundancy Check

WEP: Wired Equivalent Privacy frame check sequence

(CRC)

45

IEEE 802.11 Wireless Local

Area Networks (WLANs):

Physical Level

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

802.11 WLAN bands and technologies - summary

IEEE 802.11 standards and rates

IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) [FH, DS]

IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi [QPSK]

IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) [OFDM]

IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward compatible to 802.11b [OFDM]

IEEE 802.11 networks work on license free Industrial, Science,

Medicine (ISM) bands:

26 MHz 83.5 MHz 200 MHz 255 MHz

EIRP power in Finland

902 928 2400 2484 5150 5350 5470 5725 f/MHz

100 mW

200 mW indoors only

1 W

EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna

Equipment technical requirements for radio frequency usage defined in ETS 300 328

47

802.11-wireless LANs, Dec. ’07

Ref: http://en.wikipedia.org/wiki/802.11n

48

802.11 WLAN Architecture

Physical Level (PHY)

802 Physical level specifies

Star, bus or ring topology

Cabling and electrical interfaces: Twisted pair, coaxial, fiber…

Line coding (wired

LANs) or modulation (WLANs)

Three physical layers for

802.11

FHSS : Frequency Hopping

Spread Spectrum (SS)

DSSS : Direct Sequence SS

IR : Infrared transmission

IEEE 802.2

Logical Link Control (LLC) b: Wi-Fi

IEEE 802.3

Carrier

Sense

Ethernet

IEEE 802.4

Token

Bus

IEEE 802.5

Token

Ring

IEEE 802.11

Wireless a b g

Physical layers

LLC

MAC

PHY

CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance

LLC: Logical Link Control Layer

MAC: Medium Access Control Layer

SS: Spread Spectrum

FHSS: Frequency hopping SS

DSSS: Direct sequence SS

IR: Infrared light

NAV: Network Allocation Vector

SAP: Service Access Point

DCF: Distributed Coordination Function

PCF: Point Coordination Function

NIC: Network Interface Card 49

Physical Level of 802.11: DSSS

DSSS-transmitter

802.11 supports 1 and 2 Mbps data transmission, uses BPSK and QPSK modulation (802.11b,a,g apply higher rates)

802.11 applies 11 chips Barker code for spreading - 10.4 dB processing gain

Defines 14 overlapping channels, each having 22 MHz channel bandwidth, from

2.401 to 2.483 GHz

Power limits 1000mW in US, 100mW in EU, 200mW in Japan

Immune to narrow-band interference, cheaper hardware

PPDU:Baseband Data Frame Unit, BPSK: Binary Phase Shift Keying, QPSK: Quadrature PSK

DSSS: Direct Sequence Spread Spectrum, PN:Pseudo Noise

50

Physical Level of 802.11: FHSS

Supports 1 and 2 Mbps data transport and applies two level - GFSK modulation* (Gaussian Frequency Shift Keying)

79 channels from 2.402 to 2.480 GHz ( in U.S. and most of EU countries) with 1 MHz channel space

78 hopping sequences with minimum 6 MHz hopping space, each sequence uses every 79 frequency elements once

Minimum hopping rate

2.5 hops/second

Tolerance to multi-path, narrow band interference, security

Low speed, small range due to FCC TX power regulation (10mW)

* f

 f c f , f nom

160 kHz

51

26 MHz 83.5 MHz 200 MHz 255 MHz

902 928 2400 2484 5150 5350 5470 5725 f/MHz

Operates at 5 GHz band

Supports multi-rate 6 Mbps, 9 Mbps,… up to 54 Mbps

Uses Orthogonal Frequency Division Multiplexing (OFDM) with 52 subcarriers, 4 us symbols (0.8 us guard interval)

Applies inverse discrete Fourier transform (IFFT) to combine multicarrier signals to single time domain symbol

52

Review questions

LAN

Basics

WLAN

Basics

802

LANs

802.11

Service Sets

802.11

LLC

802.11

MAC

802.11

PHY

Review questions

Logical link control (LLC) services in 802.11

What is the role of Distributed Coordination Function

(DCF) and Point Coordination Function (PFC) in 802.11

MAC?

Describe 802.3 MAC Scheme

What is the basic difference between CSMA/CD and

CSMA/CA? Which one is applied in 802.11 and why?

Discuss factors than should be considered while choosing a medium access technique

Carrier sensing in 802.11 MAC

Mobility support in 802.11b/g

MAC frame types

54

References and Supplementary Material

- A. Leon-Garcia, I. Widjaja: Communication Networks (2th ed.)

- W. Stallings: Data and Computer Communications, 7th ed

- Kurose, Ross: Computer Networking (2th ed.)

- Jim Geier: Wireless LANs, SAMS publishing

- 802 Standards, IEEE

Supplementary Material (distributed by Edita):

HDLC: A. Leon-Garcia, I. Widjaja: Communication

Networks, 2th ed.: pp. 333-340

WLANs: W. Stallings: Data and Computer

Communications, 7th ed, pp. 544-568

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