Wireless Ethernet. Wireless LANs: Design Goals. Structure of a WLAN. Infrastructure Network

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Chapter 3.2: WLAN

Wireless Ethernet

• Wireless equivalent to Ethernet: “Wireless LAN” (WLAN) • Exclusively data-oriented, wide-band Internet access solution • Standardized by the IEEE as IEEE 802.11

IEEE 802.11 (data rate of 2 MBit/s), standardised in 1997 IEEE 802.11a with 54 MBit/s, use of a 5 GHz frequency band IEEE 802.11b with 11 MBit/s in a 2.4 GHz frequency range IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s

IEEE 802.11n: data rates up to several hundreds of MBit/s (not finished) … 802.11 • 1 or 2 MBit/s • 2.4 GHz • FHSS, DSSS 802.11a • 54 MBit/s • 5 GHz • OFDM 802.11b • 11 MBit/s • 2.4 GHz • DSSS 802.11g • 54 MBit/s • 2.4 GHz • OFDM, DSSS

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Chapter 3.2: WLAN

Wireless LANs: Design Goals

• Global, seamless operation • Low power for battery use

• No special permissions or licenses needed to use the LAN • Robust transmission technology

• Simplified spontaneous cooperation at meetings • Easy to use for everyone, simple management • Protection of investment in wired networks

• Security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation)

• Transparency concerning applications and higher layer protocols, but also location awareness if necessary

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Kommunikation und verteilte Systeme

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Chapter 3.2: WLAN

Structure of a WLAN

1. Infrastructure network

• Access Points (APs)are attached to an existing fixed network (Ethernet, Satellites, …)

• Each AP manages all communication in its reception range

• APs using the same frequency range must have enough distance to avoid disturbances

• Control functionality (medium access, mobility management, authentication, …) are realized within the infrastructure, wireless devices only need a minimum of functionality

2. Ad-hoc Network

• If no AP is available, stations also can build up an own LAN

• The transmission now takes place directly between the stations

• Higher complexity needed within the stations (control functionality)

Fixed network L a p to p AP AP AP L a p to p L a p to p L a p to p L a p to p Laptop Laptop Laptop

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Chapter 3.2: WLAN

Infrastructure Network

Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS₂ 802.11 LAN BSS1 Access Point STA1 STA₂ STA₃ ESS • Station (STA)

Computer with access mechanism to the wireless medium and by this radio connection to the AP

• Access Point (AP)

Station which is integrated both in the radio and the wired network

(distribution system) • Basic Service Set (BSS)

Group of stations incl. the AP within an AP transmission range

• Portal

Gateway to another fixed network • Distribution system

Connection of different AP areas to one logical network (EES: Extended service set). Simplest principle: switch

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Chapter 3.2: WLAN

Ad-hoc Network

802.11 LAN IBSS2 802.11 LAN IBSS₁ STA1 STA4 STA₅ STA2 STA₃

Direct communication within limited range • Station (STA)

Computer with access mechanism to the wireless medium

• Independent Basic Service Set (IBSS) Group of stations which use the same carrier frequency within a transmission range

Different IBSS are possible by spatial separation or by using different carrier frequencies

No designated stations for the forwarding of data, routing,… …

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Chapter 3.2: WLAN

802.11 Protocols

Medium Access Control

• Access mechanism, fragmenting, encryption

• MAC management: synchronization, roaming between APs, power management

Physical layer

• Channel selection, modulation, coding Applications should not

be aware of the existence of the wireless network (except capacity, longer access times)

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IEEE 802.11 Variants

Improved measurement/evaluation/management of radio parameters (e.g. signal strength), e.g. for enabling location based services

802.11k

Japanese variant of 802.11a for the frequency range of 4,9 GHz - 5 GHz 802.11j

Authentication/encryption for 802.11a/b/g, e.g. WPA 802.11i

54 MBit/s WLAN in the 5 GHz band with dynamic adaptation of channel and frequency choice as well as automatic adaptation of transmission power (enhancement of IEEE 802.11a for Europe)

802.11h

54 MBit/s WLAN in the 2,4 GHz band 802.11g

Inter Access Point Protocol (IAPP), allows communication between Access Points of different vendors, e.g. for exchanging roaming information

802.11f

QoS und streaming enhancement for 802.11a/g/h 802.11e

"World Mode", Adaptation to regional regulations (e.g. used frequency ranges) 802.11d

Wireless Bridging between Access Points 802.11c

11 MBit/s WLAN in the 2,4 GHz band 802.11b

54 MBit/s WLAN in the 5 GHz band 802.11a

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IEEE 802.11 Variants

Support of Virtual WLANs 802.11q

3650-3700 MHz Operation in the U.S. 802.11y

Protection of Management Frames 802.11w

Wireless network management 802.11v

Interworking with non-802 networks (for example, cellular) 802.11u

Wireless Performance Prediction (WPP) - test methods and metrics 802.11t

ESS Mesh Networking 802.11s

Fast roaming between APs to avoid gaps in Voice over WLAN audio 802.11r

WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars)

802.11p

Enhancement for a future, faster WLAN with data rate of 100 - 600 MBit/s 802.11n

Summary of earlier enhancements, correction of errors in former specifications (maintenance)

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Chapter 3.2: WLAN

802.11 – Physical Layer

Variants for transmission: 2 using radio (in the 2.4 GHz band), 1 using infrared • FHSS (Frequency Hopping Spread Spectrum)

– 79 different channels with 1 MHz bandwidth each

– Hopping between 2 channels for 1 MBit/s, between 4 channels for 2 MBit/s – Min. 2.5 hops/sec

– GFSK modulation

– Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW • DSSS (Direct Sequence Spread Spectrum)

– DBPSK modulation for 1 MBit/s (Differential Binary Phase Shift Keying), DQPSK for 2 MBit/s (Differential Quadrature PSK)

– Chipping sequence: (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1), a Barker-Code – Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW

• Infrared

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

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Chapter 3.2: WLAN

IEEE 802.11b

• Data rate – 1, 2, 5.5, 11 MBit/s, depending on SNR

– User throughput max. approx. 6 MBit/s

• Transmission range

– 100m outdoor, 30m indoor (directed links: several km) – Max. data rate ~ 10m (indoor) • Frequency range

– Unlicensed 2.4 GHz ISM band • Security

– SSID, WPA2 • Connection setup time

– Connectionless, „always on“

• QoS

– Best effort, no guarantees (some defined in “bad” way, later on much better standardized in 802.11e) • Manageability

– Limited (no automatic key distribution, symmetrical encryption)

• Special advantages/disadvantages – Advantages: free ISM band, many

vendors, simple system

– Disadvantage: heavy interferences on the ISM band, no QoS, relatively low data rates

• Usage

– Preferred version in Europe

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Chapter 3.2: WLAN

Channels in IEEE 802.11b

2400 [MHz] 2412 2437 2462 2483.5

Channel 1 Channel 6 Channel 11

22 MHz

• Two APs using the same frequency would have interferences in the overlapping area – thus: divide the whole frequency range in channels

• Each channel in IEEE 802.11b has a bandwidth of 22 MHz

• 13 channels in Germany (2412, 2417, 2422, …, 2472 MHz), 11 in USA/Canada • Channels overlap! Non-overlapping choice of channels:

• Ideal case: only use channels 1, 6 und 11:

11 6 1 6 11 1

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Chapter 3.2: WLAN

Channels in IEEE 802.11b

Available in the ISM band (most of Europe): 2400 – 2483,5 MHz

MHz 2400 2410 2420 2430 2440 2450 2460 2470 2480 Channel 1 2401 2412 2423 Channel 1 2401 2412 2423 Carrier frequency Channel 6 2426 2437 2448 Channel 11 2451 2462 2473 Channel 2 2406 2417 2428 Channel 7 2431 2442 2453 Channel 12 2456 2467 2478 Channel 3 2411 2422 2433 Channel 8 2436 2447 2458 Channel 13 2461 2472 2483 Channel 4 2416 2427 2438 Channel 9 2441 2452 2463 Channel 14 2473 2484 2495 Channel 5 2421 2432 2443 Channel 10 2446 2457 2468 Japan ( 1 – 14) USA/Canada: channel 1 - 11

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Chapter 3.2: WLAN

Dynamic Rate Shifting

Bits/Symbol Used Symbol Rate

Modulation Code length Data Rate 8 11 Mbit/s 4 1,375 MS/s Modified DSSS/QPSK 8 (CCK) 5,5 Mbit/s 2 DSSS/QPSK 2 Mbit/s 1 1 MS/s DSSS/PSK 11 (barker code) 1 Mbit/s

Adjustment of the data rate to the transmission quality:

CCK: Complementary Code Keying

• Use of an 8-chip spreading sequence where each chip is modulated with QPSK • QPSK has 4 states, chipping sequence has length 8 →48_{resulting states}

• Select 64 (for 11 Mbit/s) resp. 4 (for 5,5 Mbit/s) of the states which have as good cross correlation characteristics as possible (i.e. are as different as possible) • That means: make use of 4 resp. 16 code words which can be transferred instead

of only 1 as with the barker code (i.e. skip some robustness)

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Chapter 3.2: WLAN

Channels

The whole 2.4GHz ISM band is divided into 11 resp. 13 overlapping channels. On each channel, DSSS is used for signal spreading:

→One sub-band has a bandwidth of 22 MHz. The sent data are spread to those bandwidth to avoid environmental disturbances

→The chips of the barker code resp. CCK are sent in sequence – this increases the number of symbols per second compared with “pure” sending of the data, thus a larger bandwidth is needed

→Purpose: even if the frequency range is disturbed partly, enough of the signal power reaches the receiver on the rest of the channel; if a non-spread transmission would take place, the whole data would be lost in case of narrowband interference

→If CCK is used, we use “several codes” instead of the same chipping sequence everytime - the transmission becomes more susceptible for disturbances than with use of the barker code, if we have a distortion (maybe caused by an overlapping channel)!

Channel n

22 MHz

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Range of IEEE 802.11b

10 30 60 100 m 0 2 4 6 8 10 Data rate Mbit/s Distance 802.11 802.11b

Due to “abused” spreading in case of CCK, the higher data transmission rates are more susceptible for disturbances. Thus, a smaller range results:

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Range of 802.11b

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Chapter 3.2: WLAN

IEEE 802.11a

• Data rates – 6, 9, 12, 18, 24, 36, 48, 54 MBit/s, depending on SNR

– User Throughput: max. 32 MBit/s – 6, 12, 24 MBit/s mandatory • Transmission range

– 100m outdoor, 10m indoor (e.g. 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30 m, 18 up to 40 m, 12 up to 60 m) • Frequency range – Free 5.15-5.35 + 5.725-5.825 GHz ISM band • Security – SSID, WPA2

• Connection setup time

– Connectionless, „always on“ • QoS

– Best effort, no guarantees (same as for 802.11b) • Manageability

– Limited (same as for 802.11b) • Special advantages/disadvantages

– Advantages: uses less crowded free ISM band, available worldwide, simple system, many vendors

– Disadvantages: strong shading due to high frequencies, no QoS • Usage

– Preferred version in USA

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Chapter 3.2: WLAN

Channels in IEEE 802.11a

5150 5180 5200 5350 _[MHz] 36 44 16,6 MHz center frequency = 5000 + 5·channel-no. [MHz] channel-no. 40 48 52 56 60 64 149 153 157 161 5220 5240 5260 5280 5300 5320 5725 5745 5765 5825 _[MHz] 16,6 MHz channel-no. 5785 5805

Channels are also overlapping, as in 802.11b:

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Chapter 3.2: WLAN

subcarrier number

Modulation in 802.11a: OFDM

• OFDM with 52 subcarriers (64 in total, 6 as guard space on each side) • Subcarriers overlap with 312,5 kHz spacing, but orthogonality of chosen

frequencies allows for clear separation

• 48 data subchannels + 4 subchannels for phase reference (pilot) • Pilots are used by the receiver to deal with multipath propagation: phase

references for the whole band are sent here, the receiver can interpolate phase shifts for the data carriers

1 7 21 26

-26 -21 -7 -1

channel center frequency

312,5 kHz phase reference (pilot)

And: IEEE 802.11g simply is introducing OFDM on the existing 802.11b system, i.e. replacing of DSSS by OFDM for higher data rates (while keeping the ability to switch to DSSS for interworking with 802.11b)

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Chapter 3.2: WLAN

Medium Access Control

We can assign one channel with an AP – but then we have to coordinate all mobile stations in their communication with the AP. Chosen for IEEE 802.11a/b/g/…: „Wireless Ethernet“ – MAC protocol is oriented at CSMA/CD

• Hidden Station Problem • Exposed Station Problem

Solution of the problems, especially Hidden Station

CSMA/CA – CSMA with Collision Avoidance

Types of traffic

• Asynchronous data service (standard) Exchange of data by „best effort“ Support of broadcast and multicast • Time-bound services (optional)

Implementation of some degree of QoS Only for infrastructure networks

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Chapter 3.2: WLAN

802.11 – MAC Layer: DFWMAC

Access strategies

• DFWMAC-DCF CSMA/CA (standard)

DFWMAC: Distributed Foundation Wireless MAC DCF: Distributed Coordination Function

collision avoidance by random access with backoff mechanism Minimum time between two frames

ACKs for acknowledging correct receipt (not for broadcast) • DFWMAC-DCF with RTS/CTS (optional)

Avoidance of Hidden Stations

MACA variant (Multiple Access with Collision Avoidance) • DFWMAC-PCF (optional)

PCF: Point Coordination Function

Collision-free, centralized Polling strategy where the AP has a list of all connected stations

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Chapter 3.2: WLAN

802.11 – MAC Layer

Prioritiesfor medium access

• defined through different timing intervals • no guaranteed priorities

• SIFS (Short Inter Frame Spacing) – 10µs

– highest priority, used for ACK, CTS, polling response • PIFS (PCF IFS) – 30µs

– medium priority, for time-bounded services using PCF • DIFS (DCF IFS) – 50µs

– lowest priority, für asynchronous data service

t Medium busy SIFS

PIFS DIFS DIFS next frame contention direct access, if

time the medium is free ≥DIFS

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t Medium busy SIFS

PIFS DIFS DIFS next frame contention window (randomized backoff mechanism)

802.11 - CSMA/CA Method

time slot (20 µs) waiting time

• Mandatory for all implementations

• Before sending, a station performs carrier sense

• If the medium is free for at least the duration of a DIFS, the station may send • If the medium is occupied, when becoming free the station waits for one DIFS and

then randomly chooses a backoff time (collision avoidance, in multiples of a slot time). The station continues to listen to the medium

• If the medium is occupied by another station during the backoff time, the backoff timer stops. In the next try, no new backoff time is chosen randomly, but the old timer is gone on with.

• Also usable for broadcast

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Example - Backoff

data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 data wait

B1 and B2 are backoff intervals at nodes 1 and 2

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Chapter 3.2: WLAN

Competing Stations

bo_e boe bo_e t busy Station1 Station₂ Station3 Station₄ Station5 DIFS bo_e boe boe busy bor bo_r DIFS boe boe bo_ebo_r DIFS busy busy DIFS bo_e busy boe boe bor bor boe Sending request

elapsed backoff time bor remaining backoff time busy Medium busy (Frame, ACK, etc.)

The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 7 and is doubled with each collision up to CWmax= 255

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Chapter 3.2: WLAN

802.11 - CSMA/CA Method

Unicast transmission: the receipt is additionally confirmed, since collisions possibly are not detected by the transmitter

• Data can be sent after waiting for DIFS

• Receivers answer immediately (after SIFS, without additional backoff time), if the frame arrived correctly (CRC)

• In case of an error the frame is repeated automatically. No special treatment of a transmission repetition, same access mechanism as before

t SIFS DIFS Data ACK waiting time other stations receiver sender Data DIFS contention

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Chapter 3.2: WLAN

Competing Stations (with ACK)

t busy boij Station1 Station₂ Station3 Station4 Station5 Sending request SIFS bo11 bo21 bo51 busy

jth_{backoff time of station i} busy Medium occupied (Frame, ACK, etc.)

DIFS bo₄₁ bo51 bo11 DIFS busy busy DIFS bo11 busy bo42 bo52

The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 8 and is doubled with each collision up to CWmax= 256

ACK DIFS

ACK Acknowledgement

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Chapter 3.2: WLAN

802.11 – DFWMAC with RTS/CTS

Optional extension for the avoidance of the hidden station problem: • RTS with holding time as parameter can be sent after waiting for DIFS

(plus backoff time)

• Confirmation of the receiver by CTS after SIFS (also containing holding time) • Immediate sending of the data is possible, confirmation by ACK

• Other stations store the holding time, which were sent in the RTS and CTS, in their NAV (Network Allocation Vector)

• Collisions are only possible with RTS/CTS messages, but substantial overhead through RTS/CTS messages t waiting time other stations receiver sender contention SIFS DIFS data ACK data DIFS RTS CTS SIFS _SIFS NAV (RTS) NAV (CTS)

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Chapter 3.2: WLAN

802.11 – DFWMAC with RTS/CTS

t SIFS DIFS data ACK1 frag1 DIFS contention RTS CTS SIFS _SIFS NAV (RTS) NAV (CTS) NAV (frag₁) NAV (ACK1) SIFS ACK2 frag2 SIFS other stations receiver sender

• Fragmenting data can decrease the damage caused by transfer errors • Special mechanism: adapt size of the fragments to current error rate of the

medium

• First: normal reservation with RTS/CTS

• Fragments and ACKs (except the last for each case) contain reservation durations

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Chapter 3.2: WLAN

DFWMAC-PCF

PIFS D1 U₁ SIFS NAV SIFS D2 U₂ SIFS SIFS super-frame t₀ t₁

• PCF for guarantees concerning bandwidth and access delay • AP controls medium access and cyclic queries all stations (Polling)

• Super-frames with competition-free period and competition period (like before) • If the medium gets free (t1) after the begin of the super-frame (t0), the coordinator

cyclic asks all stations x (Dx) for sending needs. If necessary, they answer with Ux

(the data to be sent)

• If the phase is ended earlier than planned (t₂instead of t₃), more time remains for the competition phase (end is announced by a control frame CF_end)

t D3 PIFS D4 U₄ SIFS SIFS CFend contention contention-free period t₂ t₃ t4

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What is implemented?

Any vendor has to implement the standard CSMA/CA variant, the other two are optional

• RTS/CTS very often is implemented by AP manufacturers, but: disabled! • Usual method:

A frame size threshold is defined, and only frames longer than the threshold are sent with RTS/CTS (to avoid overhead for small frames)

The threshold value in basic configuration is sent to maximum allowed frame length…

Changing the threshold value allows you to enable the RTS/CTS Only possibility to really avoid collisions

• PCF mechanism usually is not implemented

Not needed in many cases, and not possible in ad-hoc networks Would allow for real-time data transmission, but is not good in it, thus it

doesn’t became prominent – instead, a QoS enhancement for real-time transmission was defined (IEEE 802.11e)

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Frame Format

• Types

Control frames, administrative frames, data frames • Sequence numbers

For detecting duplicated frames due to lost ACKs • Addresses

Receiver, transmitter (physical), sender (logical), BSS identifier • Misc

Duration of transmission, data

Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC 2 2 6 6 6 2 6 0-2312 4 bytes Protocol

version Type Subtype To DS More Frag Retry Power Mgmt More Data WEP 2 2 4 1 From DS 1 Order bits 1 1 1 1 1 1

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Chapter 3.2: WLAN

Frame Format

Frame Control

• Protocol version, frame type (administration, control, data), fragmenting, encryption information, meaning of the following address fields

Duration ID

• Sent along with RTC, CTS for setting the NAV Sequence Control

• Recognition of duplicated frames by sequence numbers CRC

• Checksum for detecting transmission errors Addresses

• Each field contains a 48-Bit MAC address. MAC frames can be transferred between two stations, between station and AP or between two APs within the distribution system. In the field Frame Control, two bits are determining the current meaning of the addresses. Addresses can be: Final destination, source address, BSS Identifier, intermediate sender address, intermediate receiver address

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Chapter 3.2: WLAN

MAC Address Format

DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address

BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

Scenario to DS from DS address 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -

infrastructure network, from AP 0 1 DA BSSID SA - infrastructure network, to AP 1 0 BSSID SA DA - infrastructure network, within DS 1 1 RA TA DA SA

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Chapter 3.2: WLAN

Special Frames

Frame Control Duration Receiver Address Transmitter Address CRC 2 2 6 6 4 bytes Frame Control Duration Receiver Address CRC 2 2 6 4 bytes Frame Control Duration Receiver Address CRC 2 2 6 4 bytes Acknowledgement, ACK Request to Send, RTS Clear to Send, CTS

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Chapter 3.2: WLAN

FHSS Frame Format (PHY)

Synchronization SFD PLW PSF HEC Payload

Preamble Header

80 16 12 4 16 variable Bits

• Synchronization

– Synchronization of receivers by the pattern 010101... • SFD (Start Frame Delimiter)

– 0000110010111101 to announce start of frame • PLW (PLCP_PDU Length Word)

– Length of payload including the 32 Bit CRC (at the end of the payload). Allowed values are between 0 and 4095

• PSF (PLCP Signaling Field)

– Data rate of payload (1 or 2 Mbit/s) • HEC (Header Error Check)

– CRC with x16_+x12_+x5₊₁

transmission with 1 Mbit/s

transmission with 1 or 2 Mbit/s

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Chapter 3.2: WLAN

DSSS Frame Format (PHY)

Synchronization SFD Signal Service HEC Payload

Preamble Header

Length 16 • Synchronization

– Synchronization, gain setting, energy detection, frequency offset compensation • SFD (Start Frame Delimiter)

– 1111001110100000 as start pattern • Signal

– Data rate of payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) • Service

– Reserved for future use, standard: 00 for 802.11 frames • Length (length of payload) and HEC (CRC) as for FHSS

transmission with 1 Mbit/s transmission with 1 or 2 Mbit/s

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Chapter 3.2: WLAN

IEEE 802.11b – Frame Format (PHY)

synchronization SFD signal service HEC payload

Preamble Header

length 16

192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s

short synch. SFD signal service HEC Payload

Preamble (1 Mbit/s, DBPSK) Header (2 Mbit/s, DQPSK) 56 16 8 8 16 variable Bits length 16 96 µs 2, 5.5 or 11 Mbit/s Long frame format:

Short frame format, optional:

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IEEE 802.11a – Frame Format (PHY)

rate service payload

variable Bits

6 Mbit/s

Preamble, SFD Signal Data

Symbols

12 1 variable

reserved length parity tail tail pad 6 16 6 1 12 1 4 variable 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s PLCP-Header

Kommunikation und verteilte Systeme

802.11 - MAC Management

• Synchronization

Find a LAN, try to remain in the LAN

Synchronization of internal clocks (e.g. FHSS, PCF, power saving mechanisms)

Timer etc. • Power management

Sleep mode without missing a message

Periodic sleeping, frame buffering, traffic monitoring • Association/Re-association

Integration into a LAN

Roaming, i.e. moving between networks from one Access Point to another Scanning, i.e. active search for a network

• MIB - Management Information Base

Managing, read and write of management attributed and state variables inside APs, the distribution system, etc

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Chapter 3.2: WLAN

t Medium AP busy B

busy busy busy

B B B

value of the timestamp B beacon frame

Synchronization using a Beacon

• Beacon frame contains time stamps and administrative information for power saving mechanisms and roaming

• Varying times between beacon frames, since the medium can be occupied • In infrastructure networks: AP takes over the sending of the beacons

Interval of the periodic radio signal (beacon): 20ms - 1s

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Chapter 3.2: WLAN

Synchronization using a Beacon (Ad-hoc)

t Medium Station₁ busy B1 beacon interval

busy busy busy

B1

value of the timestamp B beacon frame Station2

B2 B2

random backoff • All stations try to send a Beacon frame in fixed intervals

• Standard access procedure with backoff

• One station wins and sends a beacon frame at first. All other stations synchronize to this frame.

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Chapter 3.2: WLAN

Power Management

• Idea: Switch off the sending/receiving device when not needed • Timing Synchronization Function

Regular activation of all stations. Transmissions for sleeping stations are buffered; when waking up, the stations receive the transmission

• Infrastructure:

AP can store all pending frameworks for sleeping stations

With each beacon frame, a Traffic Indication Map (TIM) is sent along which indicates, for which stations frames are buffered.

Additionally: List for broadcast/multicast receivers (Delivery Traffic Indication Map, DTIM)

• Ad-hoc

Similar to the infrastructure mod, an aA-hoc Traffic Indication Map (ATIM) is defined

Stations, which have data to send, announce the receivers of stored packages More complex, no central AP: all stations have to temporarily store frames Collisions of ATIMs possible (scalability?)

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Chapter 3.2: WLAN

Power Management with Wake-up Patterns

(Infrastructure)

TIM interval t Medium AP busy D

busy busy busy

T T D T TIM D DTIM DTIM interval B B B _{Broadcast/Multicast} Station awake p PS Poll p d d d Data transmission from/to the station

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Chapter 3.2: WLAN

Power Management with Wake-up Patterns

(Ad-hoc)

awake

A ATIM transmission D data transmission t Station1 B1 B1 B _{beacon frame} Station₂ B2 B2 random backoff A a D d ATIM

window beacon interval

a ACK for ATIM d ACK for data

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Chapter 3.2: WLAN

802.11 - Roaming

Bad or even no connection? • Scanning

– Scanning of environment (listen for beacons of APs or send a probe and wait for a response)

• Reassociation Request

– Station requests joining the network to AP(s) • Reassociation Response

– If an AP responds, the station takes part in the network – Otherwise, go on scanning

• AP accepts Reassociation Request

– Announce new station to the Distribution System

– Distribution System updates its databases (location information) – The old AP is informed by the Distribution System

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Quality of Service – IEEE 802.11e

The PCF variant of CSMA/CA should allow some quality in data transmission: • By polling at certain times, allow for deterministic delay of information • Also, guarantee a certain data rate to each participant

• But…frames in polling can be between 0 and 2304 bytes… and the data rate on physical layer can change due to channel conditions…

→ no way to calculate transmission time of a frame in advance, thus the above quality cannot be given

Solution: define additional CSMA/CA variants which can give priority to real-time data (defined in IEEE 802.11e)

• Only an add-on the IEEE 802.11a/b/g, not a stand-alone WLAN standard • Definition of

Extended Distributed Channel Access (EDCA)as better version of DCF using

several classes of access priority by refining the inter-frame gapsand introducing so-called Transmission Opportunities (TXOP)

Hybrid Coordination Function Controlled Channel Access (HCCA)as better

version of PCF also using TXOP

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Extended Distributed Channel Access

The scheme from before (all stations use the DIFS time interval) is refined: • Assign different priorities to different data streams (traffic classes, TC)

• As before, priority is given by waiting times: the Arbitration Inter-Frame space (AIFS)

t busy SIFS PIFS DIFS = AIFS[TC7] RTS contention window AIFS[TC₆] AIFS[TC0]

• Classify all data streams in traffic classes regarding their QoS • 8 priority classes, TC 7 has highest priority

• Give longer waiting times to lower priority – thus higher priority streams can start sending earlier Best Effort Background Background Video Probe Video Video Voice Voice 0 1 1 2 2 2 3 3 0 1 2 3 4 5 6 7 Purpose Access Category (AC)

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Chapter 3.2: WLAN

EDCF Implementation

With EDCF, each station has to handle up to 8 queues performing the same access procedure as “plain” DCF with backoff counter (BC) and contention window (CW):

One more enhancement: each class also a TXOP is assigned, which is a maximum sending duration – after getting medium access, for time of TXOP several frames can be sent (Contention Free Burst)

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Chapter 3.2: WLAN

HCCA

As in PCF, HCCA is a combination of a contention-free period and a contention period

• In the contention-free period the AP polls the stations

Difference to PCF: stations can place reservations for the polling phase The AP polls stations by granting a TXOP oriented at reservation wishes and

current traffic load

• In the contention period, EDCF is used

Question: why giving QoS? Why not overprovisioning, i.e. only increase the data rate?

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Chapter 3.2: WLAN

Faster!

Not an end with 802.11a/g – go on with 802.11n • up to 600 MBit/s!

• over 70 – 250m!

How to achieve such a data rate while keeping compatibility to 802.11a/b/g? • Applied to 2.4 as well as 5 GHz ISM band to only have a single variant for the

future

• Modify OFDM with increasing symbol rate and slightly enlarge the bandwidth: →increase data rate from 54 MBit/s to 65 MBit/s

• Optional: Greenfield mode, i.e. skip support for 802.11a/b/g (an increasing number of legacy devices reduces the average throughput in the whole network)

• Optional: increase a channel’s bandwidth to 40 MHz (dynamic adaptation to other WLANs in the environment necessary!)

• Use MIMO– multiple input multiple output

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Chapter 3.2: WLAN

MIMO

MIMO means: use several antennas in parallel to send data to one receiver • Apply Space Division Multiplexing (SDM) – i.e. split the data stream into multiple

parts (called spatial stream) and transmit each part with a separate antenna (for up to 4 antennas)

• Necessary: power control – only use MIMO if necessary, otherwise lots of power is consumed

• Apply beam-forming to focus the sender’s antennas to the receiver’s antennas • By antenna diversity, a receiver can find out the angle of incidence of certain

spatial streams and thus distinguish between several streams

• Optional: apply diversity on improving signal strength, i.e. improve signal by receiving the same stream with several antennas and combine the outputs (for up to 4 antennas, but only if the number of receiver antennas is larger than the number of spatial streams)

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Chapter 3.2: WLAN

802.11n – MAC Layer

Many improvements on PHY layer, only a few on the MAC layer:

• Introduce Reduced Inter-Frame Space (RIFS) to shorten the waiting time after detecting the medium to be idle

• Use frame aggregation, i.e. pack together several frames of one station and remove redundant header information

Availability of 802.11n?

• Draft version 2 finished this year

• Lot of products of several vendors (compliance to a non-finished standard?) • Potential problems with a patent?

• Planned release date – varies between September 2008 and March 2009…

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Chapter 3.2: WLAN

802.11s – WLAN Mesh Networking

Other WLAN variant: mesh networks • Classical WLAN: wired

infrastructure between APs • Sometimes called “Wireless

Paradox”

Let APs interconnect in wireless manner, also using WLAN (lower costs, simple installation, resilient, …)

Figures from: IEEE 802.11s tutorial

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Mesh Topology

Figures from: IEEE 802.11s tutorial

Mesh Point

Special component, establishes peer links with neighbors

Mesh AP

As mesh point, but additionally implements AP functionallity

Mesh Portal

As mesh point, but additionally connects to some other network

Changes in the 802.11 standard regarding: • Addresses

• MAC scheme (oriented at 802.11e) • Synchronization / power modes

• Security

• And: routing (layer 3!)

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Secure or not Secure…

Within a WLAN „data are flying free through the air“.

Within WLAN everybody in transmission range can share your Access Point. Thus: security!

WEP: Wired Equivalent Privacy

• Authentication at the Access Point, encryption of data before transmission • Connection is only possible if knowing the WEP key

• But: no key management, short keys

• Thus: WPA/WPA2(Wi-Fi Protected Access) today give much better security ... but many users are overtaxed with configuring an Access Point – even if today a good user guide to install security functions is implemented on APs, there is a lot of Registration of allowed MAC addresses

• But: MAC addresses can be faked, large effort for large networks Hiding of SSID

• Broadcast of SSID in beacons can be switched of, thus only someone knowing the SSID can join the network (but: intuitive names? Default names?)

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Chapter 3.2: WLAN

Wardriving

New kind of sports: search for open WLANs. Just take:

• A notebook with WLAN card and a connector for a GPS device • A software for detcting Access Points,

e.g. Network Stumbler

• A GPS receiver • Time for driving around

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Chapter 3.2: WLAN

Warchalking

What can be found at walls after a wardiver has passed...

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Chapter 3.2: WLAN

• Bluetooth may act like a rogue member of a 802.11 network – does not know anything about gaps, IFS etc.

• IEEE 802.15-2 discusses these problems

– Proposal: Adaptive Frequency Hopping (only co-existence, no collaboration) • Real effects? Many different opinions, tests, formulae, …

– Results from complete breakdown to almost no effect

– Bluetooth (FHSS) seems to be more robust than 802.11b (DSSS) – Maybe Bluetooth adaptive frequency hopping has better effect

802.11 vs. 802.15/Bluetooth

t f [MHz] 2402 2480 802.11b 3 channles (separated by installation) AC K DI F S DI F S SIFS 1000 byte SIFS DIF S 500 byte AC K DI F S 500 byte SIFS AC K DI F S 500 byte DI F S ₁₀₀ byte SIFS AC K DI F S ₁₀₀ byte SIFS AC K DI F S ₁₀₀ byte SIFS AC K DI F S ₁₀₀ byte SIFS AC K DI F S ₁₀₀ byte SIFS AC K 802.15 79 channels (separated by hopping pattern)