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Mobile and Wireless Networking 2013 / 2014

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


2

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


3

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

one should be able to collect user profiles), safety (low radiation) q  transparency concerning applications and higher layer

protocols, but also location awareness if necessary


4

Comparison: infrastructure vs. ad-hoc networks

infrastructure network

ad-hoc network

AP AP

AP

wired network

AP: Access Point


5

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

one logical network (ESS: Extended Service Set) based on several BSS

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS


6

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


7

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


8

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

PH

Y D

LC

Sta

tion

Man

agem

ent


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


10

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


11

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


12

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)


13

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


14

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


15

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


16

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)


17

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


18

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


19

t

SIFS

DIFS

data

ACK

waiting time

other 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


20

t

SIFS

DIFS

data

ACK

defer access

other 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


21

t

SIFS

DIFS

data

ACK1

other stations

receiver

sender frag1

DIFS

contention

RTS

CTS SIFS SIFS

NAV (RTS) NAV (CTS)

NAV (frag1) NAV (ACK1)

SIFS ACK2

frag2

SIFS

Fragmentation


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

22


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)

23


DCF Summary

frame at the beginning of the immediately following slot. Otherwise, the station defers its frame transmissionaccording to the binary exponential backo! algorithm: The station waits until the medium has become idle foran interval of DIFS and then sets its backo! timer. The backo! timer gives the additional time it should waitbefore the next transmission attempt. It is set as the product of aStotTime and a pseudorandom integer that isdrawn from a uniform distribution over the interval [0,CW]. The contention window CW is set to its minimumvalue CWmin (CWmin = 31) for the first time a frame is backed o!. Every time the frame incurs a collision, CWis increased to (CW + 1) · 2 ! 1, with the restriction that CW cannot exceed its maximum possible valueCWmax (CWmax = 1023). The backo! timer is decreased by one time slot for every consecutive idle slot afterthe medium has been sensed idle for an interval of DIFS, and is suspended whenever the medium becomesbusy. The backo! process resumes when the wireless medium has been detected to be idle for an intervalof DIFS. When the backo! timer reaches zero, the station transmits the frame immediately.

If the destination station receives the frame correctly, it sends a positive acknowledgment (ACK) frameafter a time interval of Short InterFrame Space (SIFS). Note that SIFS is shorter than DIFS. When the sourcestation does not receive an ACK within an ACKTimeout interval, it assumes the frame has encountered a col-lision, updates the contention window CW according to the binary exponential backo! algorithm describedabove, and sets its backo! timer according to a newly selected random value within [0,CW]. A maximumof 7 retransmissions (4 retransmissions) for short frames (long frames) are allowed before the frame isdropped. The basic access procedure is depicted in Fig. 1(a). To further decrease the overhead incurred inframe collision and hidden terminal e!ects, Request-To-Send (RTS) and Clear-To-Send (CTS) frames mayalso be exchanged before the data transmission.

It is worth noting that collision avoidance is achieved by a virtual carrier sense mechanism. Whether thechannel is idle or busy is not solely determined by the physical carrier sense result, but also by the value ofthe NAV timer maintained by the MAC. A duration value is included in each frame that is transmitted bya station and indicates how long the transmission will last, including any subsequent acknowledgments andfragments. Each station in the vicinity of the transmitting station receives the frame and uses the durationvalue to update its NAV. Therefore the NAV value indicates how long another station has access to the wire-less medium no matter what is the real activity of that station.

2.2. IEEE 802.11E enhanced distributed channel access

As mentioned in Section 1, IEEE 802.11e defines a new HCF coordination function that includes both thecontention-based channel access method, EDCA, and the controlled channel access method, HCCA. AsEDCA is essentially an extension of the legacy DCF mechanism for which several analytical models existand can be leveraged, we focus on EDCA in this paper.

In EDCA, several parameters that control how and when a node gains access to the medium (e.g., the con-tention window, the inter-frame space, and the transmission opportunity) di!er among di!erent priority levels,so as to favor/disfavor data transmission from high-priority/low-priority flows. A total of eight user prioritylevels are available, and mapped to four access categories (ACs). Each AC corresponds to one of the fourtransmit queues that implement the EDCA contention algorithm with di!erent parameters. These parameters

DIFS

DIFS

PIFS

SIFSBusy Medium Backoff-Window Next Frame

Slot Time

Select Slot and Decrement Backoff as longas medium is idle

Defer Access

Contention Window

Immediate access whenmedium is free >=DIFS

DIFS/AIFS

Busy Medium Backoff Slots Next Frame

Slot Time


Defer Access

SIFS

PIFS

DIFS

AIFS[i]

AIFS[j]

Contention Window

Immediate access whenMedium is free >= DIFS/AIFS[i]

Fig. 1. Access methods in IEEE 802.11 DCF and IEEE 802.11e EDCA: (a) basic access in DCF and (b) basic access in EDCA.

1958 Y. Ge et al. / Computer Networks 51 (2007) 1955–1980

24


EDCA Operation (AIFS)

frame at the beginning of the immediately following slot. Otherwise, the station defers its frame transmissionaccording to the binary exponential backo! algorithm: The station waits until the medium has become idle foran interval of DIFS and then sets its backo! timer. The backo! timer gives the additional time it should waitbefore the next transmission attempt. It is set as the product of aStotTime and a pseudorandom integer that isdrawn from a uniform distribution over the interval [0,CW]. The contention window CW is set to its minimumvalue CWmin (CWmin = 31) for the first time a frame is backed o!. Every time the frame incurs a collision, CWis increased to (CW + 1) · 2 ! 1, with the restriction that CW cannot exceed its maximum possible valueCWmax (CWmax = 1023). The backo! timer is decreased by one time slot for every consecutive idle slot afterthe medium has been sensed idle for an interval of DIFS, and is suspended whenever the medium becomesbusy. The backo! process resumes when the wireless medium has been detected to be idle for an intervalof DIFS. When the backo! timer reaches zero, the station transmits the frame immediately.

If the destination station receives the frame correctly, it sends a positive acknowledgment (ACK) frameafter a time interval of Short InterFrame Space (SIFS). Note that SIFS is shorter than DIFS. When the sourcestation does not receive an ACK within an ACKTimeout interval, it assumes the frame has encountered a col-lision, updates the contention window CW according to the binary exponential backo! algorithm describedabove, and sets its backo! timer according to a newly selected random value within [0,CW]. A maximumof 7 retransmissions (4 retransmissions) for short frames (long frames) are allowed before the frame isdropped. The basic access procedure is depicted in Fig. 1(a). To further decrease the overhead incurred inframe collision and hidden terminal e!ects, Request-To-Send (RTS) and Clear-To-Send (CTS) frames mayalso be exchanged before the data transmission.

It is worth noting that collision avoidance is achieved by a virtual carrier sense mechanism. Whether thechannel is idle or busy is not solely determined by the physical carrier sense result, but also by the value ofthe NAV timer maintained by the MAC. A duration value is included in each frame that is transmitted bya station and indicates how long the transmission will last, including any subsequent acknowledgments andfragments. Each station in the vicinity of the transmitting station receives the frame and uses the durationvalue to update its NAV. Therefore the NAV value indicates how long another station has access to the wire-less medium no matter what is the real activity of that station.

2.2. IEEE 802.11E enhanced distributed channel access

As mentioned in Section 1, IEEE 802.11e defines a new HCF coordination function that includes both thecontention-based channel access method, EDCA, and the controlled channel access method, HCCA. AsEDCA is essentially an extension of the legacy DCF mechanism for which several analytical models existand can be leveraged, we focus on EDCA in this paper.

In EDCA, several parameters that control how and when a node gains access to the medium (e.g., the con-tention window, the inter-frame space, and the transmission opportunity) di!er among di!erent priority levels,so as to favor/disfavor data transmission from high-priority/low-priority flows. A total of eight user prioritylevels are available, and mapped to four access categories (ACs). Each AC corresponds to one of the fourtransmit queues that implement the EDCA contention algorithm with di!erent parameters. These parameters

DIFS

DIFS

PIFS

SIFSBusy Medium Backoff-Window Next Frame

Slot Time


Defer Access

Contention Window

Immediate access whenmedium is free >=DIFS

DIFS/AIFS

Busy Medium Backoff Slots Next Frame

Slot Time


Defer Access

SIFS

PIFS

DIFS

AIFS[i]

AIFS[j]

Contention Window

Immediate access whenMedium is free >= DIFS/AIFS[i]

Fig. 1. Access methods in IEEE 802.11 DCF and IEEE 802.11e EDCA: (a) basic access in DCF and (b) basic access in EDCA.

1958 Y. Ge et al. / Computer Networks 51 (2007) 1955–1980

25


26

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


27

MAC address format

scenario to DS fromDS

address 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP

0 1 DA BSSID SA -

infrastructurenetwork, to AP

1 0 BSSID SA DA -

infrastructurenetwork, 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


28

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


Address CRC

2 2 6 4 bytes


Address CRC

2 2 6 4 bytes

ACK

RTS

CTS


29

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


30

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)


31

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)


32

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?)


33

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)


34

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)


35

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


36

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)

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