192620010 Mobile & Wireless Networking Lecture 7: Wireless LAN - - PowerPoint PPT Presentation

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192620010 Mobile & Wireless Networking Lecture 7: Wireless LAN - - PowerPoint PPT Presentation

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192620010 Mobile & Wireless Networking Lecture 7: Wireless LAN [Schiller, Section 7.3] [Reader, Part 6] [ Optional: "IEEE 802.11n Development: History, Process, and Technology", Perahia, IEEE Communications Magazine, July 2008 ]

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192620010 Mobile & Wireless Networking Lecture 7: Wireless LAN [Schiller, Section 7.3] [Reader, Part 6] [Optional: "IEEE 802.11n Development: History, Process, and Technology",

Perahia, IEEE Communications Magazine, July 2008] Geert Heijenk

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Outline of Lecture 9

q Wireless LAN

q General Characteristics / IEEE 802.11 standard q IEEE 802.11 Physical Layers q Medium Access Control

l CSMA/CA l CSMA with RTS/CTS l MAC Quality of Service Enhancements

q MAC Frame Format q MAC Management q IEEE 802.11: Important Developments

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Design goals for wireless LANs

q global, seamless operation q low power for battery use q no special permissions or licenses needed to use the LAN q robust transmission technology q simplified spontaneous cooperation at meetings q easy to use for everyone, simple management q protection of investment in wired networks q security (no one should be able to read my data), privacy (no

  • ne should be able to collect user profiles), safety (low radiation)

q transparency concerning applications and higher layer

protocols, but also location awareness if necessary

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Comparison: infrastructure vs. ad-hoc networks

infrastructure network ad-hoc network

AP AP AP wired network AP: Access Point

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IEEE 802.11 - Architecture of an infrastructure network

Station (STA)

q terminal with access mechanisms

to the wireless medium and radio contact to the access point

Basic Service Set (BSS)

q group of stations using the same

radio frequency

Access Point

q station integrated into the wireless

LAN and the distribution system

Portal

q bridge to other (wired) networks

Distribution System

q interconnection network to form

  • ne logical network (ESS:

Extended Service Set) based

  • n several BSS

Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS2 802.11 LAN BSS1 Access Point STA1 STA2 STA3 ESS

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IEEE 802.11 - Architecture of an ad-hoc network

Direct communication within a limited range

q Station (STA):

terminal with access mechanisms to the wireless medium

q Independent Basic Service Set

(IBSS): group of stations using the same radio frequency Exception:

q IEEE 802.11p for Vehicular

Networks: communication without setting up a basic service set. 802.11 LAN IBSS2 802.11 LAN IBSS1 STA1 STA4 STA5 STA2 STA3

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IEEE standard 802.11

mobile terminal access point 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

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IEEE 802.11 - Layers and functions

PLCP Physical Layer Convergence Protocol

q clear channel assessment

signal (carrier sense)

PMD Physical Medium Dependent

q modulation, coding

PHY Management

q channel selection, MIB

Station Management

q coordination of all management

functions PMD PLCP MAC LLC MAC Management PHY Management

MAC

q access mechanisms,

fragmentation, encryption

MAC Management

q synchronization, roaming,

authentication, MIB, power management PHY DLC Station Management

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The Physical Layer

q IEEE 802.11

l Direct Sequence Spread Spectrum (DSSS) PHY

– 2.4 GHz : RF : 1 – 2 Mbps

l Frequency Hopping Spread Spectrum (FHSS) PHY

– 2.4 GHz : RF : 1- 2 Mbps

l Infrared (IR) PHY

– Indoor : IR : 1 and 2 Mbps

q IEEE 802.11b

l High Rate DSSS PHY

– 2.4 GHz : 5.5 Mbps – 11 Mbps

q IEEE 802.11a

l OFDM PHY

– 5 GHz : 6-54 Mbps

q IEEE 802.11g

l OFDM PHY

– 2.4 GHz : 6-54 Mbps

q IEEE 802.11n

l OFDM PHY + MIMO (up to 4x4) + 40 MHz channel (instead of 20 MHz)

– Extension of a/g: 6.5 – 600 Mbps

q IEEE 802.11ac

l OFDM PHY + MIMO (up to 8x8) + Multi-User MIMO (up to 4 simultaneous users)

+ 80 MHz + 256 QAM

– extension of n in 5GHz: up to 4 x 1.69 Gbit/s

q IEEE 802.11ad

l WiGig in 60 GHz, upto 7 Gbps

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IEEE 802.11a

Data rate

q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) q 6, 12, 24 Mbit/s mandatory

Transmission range

q 100m outdoor, 10m indoor

l E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40

m, 12 up to 60 m

Frequency

q Free 5 GHz ISM-band

Special Advantages/Disadvantages

q Advantage: fits into 802.x standards, free ISM-band, available, simple

system, uses less crowded 5 GHz band

q Disadvantage: stronger shading due to higher frequency

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IEEE 802.11g

Frequency

q Free 2.4 GHz ISM-band q But shared with many legacy WLANs and other systems, e.g. Bluetooth

Backward compatible with IEEE 802.11b Data rate (as 802.11a)

q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput significantly lower in presence of 802.11b stations

Transmission range

q Somewhat higher than 802.11a

Special Advantages/Disadvantages

q Advantage: backward compatibility, better propagation conditions q Disadvantage: crowded band, lower speed due to backward compatibility

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2400 [MHz] 2412 2483.5 2442 2472 channel 1 channel 7 channel 13 Europe (ETSI) US (FCC)/Canada (IC) 2400 [MHz] 2412 2483.5 2437 2462 channel 1 channel 6 channel 11 22 MHz 22 MHz

802.11 b/g Channel selection (non-overlapping)

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rate service payload variable bits 6 Mbit/s PLCP preamble signal data symbols 12 1 variable reserved length tail parity tail pad 6 16 6 1 12 1 4 variable 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s PLCP header

IEEE 802.11a/g – PHY frame format

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802.11 - MAC layer I

Access methods

q DCF CSMA/CA (mandatory)

l collision avoidance via randomized „back-off“ mechanism l minimum distance between consecutive packets l ACK packet for acknowledgements (not for broadcast)

q DCF w/ RTS/CTS (optional)

l reduces hidden terminal problem

q PCF (optional)

l access point polls terminals according to a list

q HCF EDCA (optional)

l CSMA/CA with priority levels

q HCF CCA (optional)

l Improved polling

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t medium busy SIFS PIFS DIFS DIFS next frame contention direct access if medium is free ≥ DIFS

802.11 - MAC layer II

Priorities

q defined through different inter frame spaces q no guaranteed, hard priorities q SIFS (Short Inter Frame Spacing)

l highest priority, for ACK, CTS, polling response

q PIFS (PCF IFS)

l medium priority, for time-bounded service using PCF

q DIFS (DCF, Distributed Coordination Function IFS)

l lowest priority, for asynchronous data service

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t medium busy DIFS DIFS next frame contention window (randomized back-off mechanism) slot time direct access if medium is free ≥ DIFS

802.11 - CSMA/CA access method (DCF) I

q station ready to send starts sensing the medium (Carrier Sense

based on CCA, Clear Channel Assessment)

q if the medium is free for the duration of an Inter-Frame Space (IFS),

the station can start sending (IFS depends on service type)

q 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)

q if another station occupies the medium during the back-off time of

the station, the back-off timer stops (fairness)

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t busy boe station1 station2 station3 station4 station5 packet arrival at MAC DIFS boe 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 DIFS boe busy boe boe bor bor

802.11 - competing stations - simple version

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802.11 - Binary Exponential Backoff

q Stations choose their backoff time randomly from contention

window

q Ideal contention window size is trade-of between acceptable load

and experienced delay

q Initial contention window size (CWmin) is 7 slots

(backoff time between 0 and 7)

q After collision (no ack), contention window is “doubled” until CWmax

= 255 is reached: 7 -> 15 -> 31 -> 63 -> 127 -> 255

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t SIFS DIFS data ACK waiting time

  • ther

stations receiver sender data DIFS contention

802.11 - CSMA/CA access method (DCF) II

q Sending unicast packets

q station has to wait for DIFS before sending data q receivers acknowledge at once (after waiting for SIFS) if the packet

was received correctly (CRC)

q automatic retransmission of data packets in case of transmission

errors

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t SIFS DIFS data ACK defer access

  • ther

stations receiver sender data DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS)

802.11 – CSMA/CA with RTS/CTS

q Sending unicast packets

q station can send RTS with reservation parameter after waiting for DIFS

(reservation determines amount of time the data packet needs the medium)

q acknowledgement via CTS after SIFS by receiver (if ready to receive) q sender can now send data at once, acknowledgement via ACK q other stations store medium reservations distributed via RTS and CTS

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t SIFS DIFS data ACK1

  • ther

stations receiver sender frag1 DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV (frag1) NAV (ACK1) SIFS ACK2 frag2 SIFS

Fragmentation

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802.11e - MAC services (QoS support)

  • Distributed Coordination Function
  • Point Coordination Function (not implemented)
  • + Hybrid Coordination Function (HCF)
  • backward compatible with DCF

HCF:

  • HCF Controlled Channel Access (HCCA)
  • Enhanced Distributed Channel Access (EDCA)
  • Differentiation priority levels
  • 4 access categories with separate queues in each STA

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EDCA

Multiple Access Categories with their own transmission queue Differentiation by means of:

  • Arbitray Inter Frame Spacing (AIFS)
  • Initial Contention Window (CWmin)
  • Maximum Contention Window (CWmax)
  • Transmission Opportunity Limit (TXOP Limit)

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DCF Summary

DIFS DIFS PIFS SIFS Busy Medium Backoff-Window Next Frame Slot Time Select Slot and Decrement Backoff as long as medium is idle Defer Access Contention Window Immediate access when medium is free >=DIFS

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EDCA Operation (AIFS)

DIFS/AIFS Busy Medium Backoff Slots Next Frame Slot Time Select Slot and Decrement Backoff as long as medium is idle Defer Access

SIFS PIFS DIFS AIFS[i] AIFS[j]

Contention Window Immediate access when Medium is free >= DIFS/AIFS[i]

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

802.11 – MAC Frame format

q Types

q control frames, management frames, data frames

q Duration /ID

q for NAV during RTS/CTS and fragmentation, and CFP

q Sequence control

q important against duplicated frames due to lost ACKs

q Addresses

q receiver, transmitter (physical), BSS identifier, sender (logical)

q Miscellaneous

q checksum, frame control, data

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MAC address format

scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network DA SA BSSID

  • infrastructure

network, from AP 1 DA BSSID SA

  • infrastructure

network, to AP 1 BSSID SA DA

  • infrastructure

network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

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Special Frames: ACK, RTS, CTS

Acknowledgement Request To Send Clear To Send

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 ACK RTS CTS

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802.11 - MAC management

Synchronization

q try to find a LAN, try to stay within a LAN q timer etc.

Power management

q sleep-mode without missing a message q periodic sleep, frame buffering, traffic measurements

Association/Reassociation

q integration into a LAN q roaming, i.e. change networks by changing access points q scanning, i.e. active search for a network

MIB - Management Information Base

q managing, read, write

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beacon interval t medium access point busy B busy busy busy B B B value of the timestamp B beacon frame

Synchronization using a Beacon (infrastructure)

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t medium station1 busy B1 beacon interval busy busy busy B1 value of the timestamp B beacon frame station2 B2 B2 random delay

Synchronization using a Beacon (ad-hoc)

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Power management

Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF)

q stations wake up at the same time

Infrastructure

q Traffic Indication Map (TIM)

l list of unicast receivers transmitted by AP

q Delivery Traffic Indication Map (DTIM)

l list of broadcast/multicast receivers transmitted by AP

Ad-hoc

q Ad-hoc Traffic Indication Map (ATIM)

l announcement of receivers by stations buffering frames l more complicated - no central AP l collision of ATIMs possible (scalability?)

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TIM interval t medium access point 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 to/from the station

Power saving with wake-up patterns (infrastructure)

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awake A transmit ATIM D transmit data t station1 B1 B1 B beacon frame station2 B2 B2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data

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

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802.11 - Roaming

No or bad connection? Then perform: Scanning

q scan the environment, i.e., listen into the medium for beacon

signals or send probes into the medium and wait for an answer

Reassociation Request

q station sends a request to one or several AP(s)

Reassociation Response

q success: AP has answered, station can now participate q failure: continue scanning

AP accepts Reassociation Request

q signal the new station to the distribution system q the distribution system updates its data base (i.e., location

information)

q typically, the distribution system now informs the old AP so it can

release resources

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IEEE 802.11 – standards and amendments

802.11e: MAC Enhancements – QoS 802.11h: Spectrum Managed 802.11a (DCS, TPC) – 802.11i: Enhanced Security Mechanisms IEEE 802.11-2007: new release of the standard that includes a, b, d, e, g, h, i & j. 802.11n: > 100 Mbps (MIMO) 802.11p: inter-vehicle communications 802.11s: Mesh Networking IEEE 802.11-2012: new release of the standard that includes k – z. 802.11ac: 1 Gbps in 5 GHz band (> bw, > mimo-channels, 256 QAM) 802.11ad: 7 Gbps in 60 GHz band (a.o. beamforming)