802.11 WLAN

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IEEE 802.11 Overview

• Adopted in 1997Defines:• MAC sublayer • MAC management protocols and services• Physical (PHY) layers– IR – FHSS– DSSS

1-25-06 Lecture 4: 802.11 and more 2

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Wifi and WLAN

• Wi-Fi (Wireless Fidelity) is a generic term that refers to the IEEE 802.11 communications standard for Wireless Local Area Networks (WLANs).

• Wi-Fi Network connect computers to each other, to the internet and to the wired network.

• Maximum radiated power 1 W in US and 100 mW in EU countries (India commonly follows EU regulations but exact figures are not known to me right now)

802.11 particulars

• 802.11b (WiFi)– Frequency: 2.4 - 2.4835 Ghz DSSS– Modulation: DBPSK (1Mbps) / DQPSK (11 Mbps)– Orthogonal channels: 3• There are others, but they interfere. (!)

– Rates: 1, 2, 5.5, 11 Mbps• 802.11a: Faster, 5Ghz OFDM. Up to 54Mbps• 802.11g: Faster, 2.4Ghz, up to 54Mbps

IEEE 802.11b

• Appeared in late 1999• Operates at 2.4GHz radio spectrum• 11 Mbps (theoretical speed) - within 30 m Range• 4-6 Mbps (actual speed)• 100 -150 feet range• Most popular, Least Expensive• Interference from mobile phones and Bluetooth

devices which can reduce the transmission speed.

IEEE 802.11a

• Introduced in 2001 (less popular) • Operates at 5 GHz• 54 Mbps (theoretical speed)• 15-20 Mbps (Actual speed)• 50-75 feet range• More expensive• Not compatible with 802.11b

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IEEE 802.11g

• Introduced in 2003• Combine the feature of both standards (a and

b)• 100-150 feet range• 54 Mbps Speed• 2.4 GHz radio frequencies• Compatible with IEEE 802.11 b

802.11 Physical Layer

There are three sublayers in physical layer:

• Direct Sequence Spread Spectrum (DSSS)• Frequency Hoping Spread Spectrum (FHSS)• Diffused Infrared (DFIR) - Wide angle

Lecture 4: 802.11 and more

DSSS• Direct sequence signaling technique divides the 2.4 GHz band into 11 22-

MHz channels. Adjacent channels overlap one another partially, with three of the 11 being completely non-overlapping. Data is sent across one of these 22 MHz channels without hopping to other channels.

802.11 details• Three types of frames (see the WLAN tutorial manual for

details): – data frame– control frame– management frame

• Fragmentation– 802.11 can fragment large packets (this is separate from IP

fragmentation).• Preamble (check this carefully)

– 72 bits @ 1Mbps, 48 bits @ 2Mbps– Note the relatively high per-packet overhead.

• Control frames– RTS/CTS/ACK/etc.

• Management frames– Association request, beacons, authentication, etc.

Overview, 802.11 Architecture

STASTA

STA STA

STASTASTA STA

APAP

ESS

BSS

BSSBSS

BSS

Existing Wired LAN

Infrastructure Network

Ad Hoc Network

Ad Hoc Network

BSS: Basic Service SetESS: Extended Service Set

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1Access Point

802.11 - Architecture of infrastructure network •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 (EES: Extended Service Set) based on several BSS

STA1

STA2 STA3

ESS

802.11 - Architecture of an 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

802.11 modes• Infrastructure mode– All packets go through a base station– Cards associate with a BSS (basic service set)– Multiple BSSs can be linked into an Extended Service Set

(ESS)• Handoff to new BSS in ESS is pretty quick

– Wandering around CMU• Moving to new ESS is slower, may require re-addressing

– Wandering from CMU to Pitt• Ad Hoc mode– Cards (network interface card or NIC) communicate

directly– Perform some, but not all, of the AP functions

Services:• Station services (activity within a single cell):

– authentication, – de-authentication, – privacy, – delivery of data

• Distribution services (manage cell membership and interact with stations outside the cell):– association– disassociation– Re-association– distribution– Integration

A station maintain two variables:

• authentication state (≥ 1) i.e., can authenticate with more than one AP-s

• association state (≤ 1) i.e., can associate with one or zero APs

IEEE standard 802.11

mobile terminal

access point

server

fixed terminal

applicationTCP

802.11 PHY802.11 MAC

IP

802.3 MAC802.3 PHY

applicationTCP

802.3 PHY802.3 MAC

IP

802.11 MAC802.11 PHY

LLC

infrastructure network

LLC LLC

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

PMDPLCPMACLLC

MAC Management

PHY Management

•MAC– access mechanisms,

fragmentation, encryption •MAC Management

– synchronization, roaming, MIB, power management

PHY

DLC

Stat

ion

Man

agem

ent

MIB: management information base

MAC Management Functions• The MAC Management is responsible for the following:– maintaining time synchronization between stations• transmitting beacons

– channel scanning– forming, joining or leaving a BSS or IBSS– power management– association and re-association

• The last four functions above are carried out in response to requests from the Station Management Entity (SME)

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MAC Management Structure

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Time Synchronization and Beacons

Time Synchronization• All stations maintain a Time Synchronization

Function (TSF)• The TSF is a timer, counting in microseconds with

a modulus (i.e. length) of 264 (210)6 = 1018

• Stations within a BSS or IBSS synchronize their TSF by transmitting or receiving Beacons. Each beacon contains the value of the TSF of the transmitting station

• All TSF values are adjusted to take account of propagation and processing delays

Beacon:• Beacons are management frames and contain all network

information• Beacons are transmitted periodically to announce the

presence of a BSS (or IBSS)• In an infrastructure network, only the AP transmits the

beacon• In an IBSS beacon generation is distributed amongst the

participating STA-s• The Beacon frame consists of MAC header (to be discussed

later), frame body and FCS

Beacon (contd):• Important parameters in a beacon are:

– Time interval: beacons are transmitted periodically. This field provides the period of beacon transmission

– Time stamp: Timestamp is the time at which an event is recorded by the computer- and not necessarily the time when the event occurred. The beacon timestamp enables the receiving STA to update its local clock and synchronise with the transmitting STA

– Service set identifier (SSID): the SSID identifies a specific ESS. An AP transmits the SSID in the beacon frame and an STA is usually required to configure itself with the same SSID. Noted afterwards that it poses security problems and it is possible to disable this field

– Supported rate: a beacon may inform that only 1 and 5.5 Mbps rates are supported, or 11 Mbps is not supported etc (for example)

– Parameter sets: includes information about modulation, hopping parameters (for frequency hopped system) etc

Beacons: – Capability set: the requirements that the STA-s in the BSS should adhere

to; for example, if they must use wired equivalent privacy (WEP) – Traffic indication map (TIM): is sent by an AP to indicate if an STA in a

power saving mode would have data to receive (from the AP)

• An 802.11 probe –response frames are quite close to beacon frames except that they do not contain the TIM field

• A beacon interval is a configurable parameter and expressed in time unit (TU). A time unit is 1024 micro-sec (note the difference from a mili-sec). The beacon interval is commonly set to 100 TU.

• In an IBSS, an initiating STA sends a beacon; if an STA does not hear a beacon within a random time interval it may send its own beacon frame to instantiate an IBSS

Beacon Frame

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Synchronization in a BSS• In a BSS, the AP is the only station to transmit

beacons.• Beacons indicate – that it is an Access Point– the TSF value of the AP– the beacon period (i.e. time between beacons)

• All other stations shall update their TSF to the value in the beacons sent by the AP

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Beacon Transmission in a BSS• Beacons are supposed to be transmitted at regular

intervals• However, If a beacon transmission is delayed due to the

medium being busy, subsequent beacons shall be transmitted according to the original schedule

Authentication Process IEEE 802.11 defines two different MAC layer authentications:

- Open System Authentication - Shared Key AuthenticationThese authentication mechanisms are defined with respect to infrastructure network only. Pre-authentication:Pre-authentication is typically done by a STA while it is already associated with an AP. If the authentication is left until re-association time, this may impact the speed with which a STA can reassociate between APs, limiting BSS-transition mobility performance.

Authentication will be discussed afterwards

Synchronization in an IBSS• The station that instantiates the IBSS sets the beacon

period• All other stations shall wait until they have received a

beacon before sending any beacons• A station shall update its TSF to the value in the received

beacon if the received value is greater than the station’s TSF

• Given the time the beacon was received, and the beacon period, stations can calculate Target Beacon Transmission Time (TBTT) for all following beacons

802.11 Management Operations• Scanning• Association/Reassociation• Time synchronization (using Time synchronization function (TSF))

• Power management

Scanning & Joining• Goal: find networks in the area

• Passive scanning– No require transmission saves power– Move to each channel, and listen for Beacon frames

• Active scanning– Requires transmission saves time– Move to each channel, and send Probe Request frames to solicit

Probe Responses from a network

• Joining a BSS: listen to the beacon and get the following done:– Synchronize in TSF and frequency (using beacon time stamp): Adopt

PHY parameters : authenticate

I wish to discuss authentication separately

A glimpse of the authentication process

Data protection

802.1X authentication

802.1X key management RADIUS-based key distribution

Security capabilities discovery

Authentication Server

Access Point

Station

Association in 802.11

AP

1: Association request

2: Association response

3: Data trafficClient

Reassociation in 802.11

New AP

1: Reassociation request

3: Reassociation response

5: Send buffered frames

Old AP

2: verifypreviousassociation

Client 6: Data traffic

Time Synchronization in 802.11

• Timing synchronization function (TSF)– AP controls timing in infrastructure networks– All stations maintain a local timer– TSF keeps timer from all stations in sync

• Periodic Beacons convey timing– Beacons are sent at well known intervals– Timestamp from Beacons used to calibrate local

clocks– Local TSF timer mitigates loss of Beacons

Power Management in 802.11• A station is in one of the three states– Transmitter on– Receiver on– Both transmitter and receiver off (dozing)

• AP buffers packets for dozing stations• AP announces which stations have frames buffered

in its Beacon frames• Dozing stations wake up to listen to the beacons• If there is data buffered for it, it sends a poll frame to

get the buffered data

1-25-06 Lecture 4: 802.11 and more 36

1-25-06 Lecture 4: 802.11 and more 37