communication technology laboratory wireless communication group ieee 802.11 - wireless local area...

Communication Technology LaboratoryWireless Communication Group

IEEE 802.11 - Wireless Local Area Networks

2Communication Technology LaboratoryWireless Communication Group

Updated Schedule: 8:15-9:00 9:15-10:00 10:15-11:00 11:15-12:00

L01:16.09. Fundamentals of wireless

communications. 1 Fundamentals of wireless


communications. 1

L02:23.09.Introduction –First Exercise

Fundamentals of wireless communications. 2



L03:30.09. Fundamentals of wireless

communications. 3Fundamentals of wireless


communications. 3

L04: 07.10. Presentation of Ex 1/ 1 Presentation of Ex 1/2 WLAN - 1 WLAN - 1

L05: 14.10.optional: wrap up of

simulation basicsrevised solutions of Ex 1/1 and

EX 1/2 (rest of WLAN - 1) WLAN - 2 WLAN - 2

L06: 21.10.Introduction-

Second ExercisePresentation of Ex 1 -

Combination step Vehicular Networks Vehicular Networks

L07: 28.10. UWB 1 UWB 1

L08: 04.11. UWB 2 UWB 2

L09: 11.11. Presentation of Ex 2/1 Presentation of Ex 2/2 WBAN WBAN

L10: 18.11.Introduction –

Third ExercisePresentation of Ex 2 -

Combination step WPAN WPAN

L11: 25.11. RFID 1 RFID 1

L12: 02.12. Presentation of Ex 3/1 Presentation of Ex 3/2 RFID 2 RFID 2

L13: 09.12. Presentation of Ex 3 -

Combination step RFID 3 RFID 3

*


Wireless Communication Technology according to IEEE

Local wireless networksWLAN 802.11

802.11a

802.11b802.11i/e/…/w

802.11gWiFi

802.11h

Personal wireless nwWPAN 802.15

802.15.4

802.15.1 802.15.2

Bluetooth

802.15.4a/bZigBee

802.15.3

Wireless distribution networks/ Wireless metropolitan area nwWMAN 802.16 (Broadband Wireless Access)

802.20 (Mobile Broadband Wireless Access), 802.16e (WiMAX mobile)

+ Mobility

WiMAX

802.15.3a/b

802.15.5

Wireless Networks

802.11n


Wireless Access Technologies:

Wireless Local Area Networks (WLAN) 802.11

Wireless Networks, 802.11

Structure:

1.Introduction

2.Network architecture

3.Reference model

4.Physical layer

5.MAC sublayer

6.MAC sublayer management

5

Prologue (1)


Prologue (2)

“Faster Wi-Fi Will Grow Rapidly.” [In-Stat, 2011]

“The emerging 802.11ac standard, which is aimed at gigabit-speed wireless LANs, will be quickly adopted over the next four years…“

“In-Stat estimates that nearly 350 million routers, client devices and attached modems with 11ac will ship annually by 2015 …”

“… 1.5 billion products equipped with 11n will be sold that year [2015], more than double the estimated 700 million in 2011.”

“800 mln WiFi households globally by 2016” [IT Facts, 2012]

“Already used in some 439 mln households worldwide, equivalent to 25% of all households, Wi-Fi home network penetration will expand to 42%, reaching nearly 800 mln by 2016.”

“Hotspot Usage to Reach 120 Billion Connects by 2015” [In-Stat, 2012]

“Worldwide hotspot venues will increase to over 1 million in 2013.”


Prologue (3)

WLAN: fast growing market Expanding year after year (along with the rapid spread of broadband

infrastructure) Hot Spots: WLANs at airports, railway stations, universities, cafes, etc. Already in 2006: “82% of US hotels offer wireless Internet” Wifi chipsets in PCs, notebooks, smartphones, tablets

Dominant standards: IEEE 802.11g with 54 Mbit/s @ 2,4 GHz (successor of 802.11 b with11 Mbit/s @ 2,4

GHz, the former dominant standard)

The high throughput 802.11n MIMO standard

WiFi Alliance (1999/ 2000), WiFi Certification

Topics of this lecture:

• Standardization• OSI reference model• PHY• MAC

Introduction to IEEE 802

IEEE 802.11: WLAN – standard (’97/ ‘99) Purpose: provide wireless connectivity to

automatic machinery, equipment, or stations, which may be portable or hand-held, or which may be mounted on moving vehicles within a local area

Differences: wired LAN / WLAN destination location < >dest. Address channel: wired < > wireless portable < > mobile CSMA/CD <> CSMA/CA security issues, power saving ...

Other IEEE standards: 802.2: Logical Link Control (LLC) 802.3: CSMA / CD, Ethernet 802.4: Token Bus, 802.5 Token Ring

IEEE802.11(3,4,5) specifies MAC and PHY layer

Physical Layer (PHY): Layer 1 of OSI basic reference model

Medium Access Control (MAC): lower half of layer 2 in OSI reference model (layer 2a)

Logical Link Control (LLC): upper half of layer 2 (layer 2b) can provide connection-oriented and

connectionless services flow control, sliding window, error

checking, confirmation of received data

LLC standardized for all 802 MACs




IEEE 802.11 – Seamless Integration

CSMA/CDEthernet

Token Bus

Token Ring


Internet TCP/IP layered architecture


Network structure (1)

10


Quelle: MR

Ethernet

LLC

Network structure (2)

Thanks to Maximilian Riegel, Siemens Mobile;some pictures of this lecture are taken from his presentations (formerly at http://www.max.franken.de).


http://www.max.franken.de/

12

Characteristics of wireless LANs

Advantages very flexible within the reception area mobile communications Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings) more robust against disasters like, e.g., earthquakes, fire - or users

pulling a plug...

Disadvantages typically very low data rate per user compared to wired networks

due to shared medium products have to follow many national restrictions if working wireless,

it takes a vary long time to establish global solutions low coverage range


13

Design goals for wireless LANs

Low power for battery use No special permissions or licenses needed (ISM band) Robust transmission technology (for the wireless channel) Simplified spontaneous cooperation at meetings (ad hoc) Easy to use for everyone, simple management Security (no one should be able to read my data), privacy (no one

should be able to collect user profiles), safety (e.g. low radiation) Transparency concerning applications and higher layer protocols,

but also location awareness if necessary

Using existing LAN infrastructure (global, seamless operation)



802.11F: Inter Access Point Protocol


WLAN standardization

DFS: dynamic frequency selectionTPC: transmit power control

WLAN IEEE 802.11

Structure:

1.Introduction


3.Reference model

4.Physical layer

5.MAC sublayer





802.11 Network architectures

Quelle: MR


Station (STA): Any device that contains an IEEE 802.11 conformant MAC and PHY interface to the wireless medium (WM).

Basic Service Set (BSS): Set of stations controlled by a single coordination function (CF).

CF: logical function, determines when a STA operating within a BSS is permitted to transmit and may be able to receive protocol data units (PDUs) via the WM.

Station Services (SS): set of services that support transport of MAC service data units (MSDUs) between STAs within a BSS.

Independent Basic Service Set (IBSS): BSS that forms a self-contained network, and in which no access to a distribution system (DS) is available (=> Ad Hoc network)

Ad Hoc Mode: 802.11 IBSS

Network composed solely of stations within mutual communication range of each other via the wireless medium (WM); typically created in a spontaneous manner.

Principal distinguishing characteristic: limited temporal and spatial extent.


Quelle: MR



Quelle: MR

transport layer

networklayer

LLC(layer 2b)

Infrastructure Mode (1)


Access Point (AP): Any entity that has STA functionality and provides access to the DS via the wireless medium (WM) for associated stations => AP implements both the 802.11 MAC and the DS MAC protocols.



Quelle: MR

Access Point (AP) A

AP B


STA 1

STA 2

Several connected BSSs may form (together with integrated LANs) an Extended Service Set (ESS)

The architectural component used to interconnect BSSs is the distribution system (DS).

802.11: Distribution System Services (DSS) are specified (not the DS itself) The medium used by the DS is called Distribution System Medium (DSM),

and is not specified. Examples are a Wireless Medium, a cable, or a fibre-optic cable, ...


Portal: The logical point at which MAC service data units (MSDUs) from a non-IEEE 802.11 local area network (LAN) enter the DS of an ESS.





WLAN IEEE 802.11

Structure:

1.Introduction


3.Reference model

4.Physical layer

5.MAC sublayer


OSI basic reference model:

802.11 specifies the layers 1 and 2a

Coexistence with other 802 LANs (Bridge on LLC layer) several 802.11 WLANs

Compatibility to other (802) LANs: Mobility of STAs handled in the MAC layer; so,

for upper protocol layers, 802.11 shows no differences to other 802 networks

One MAC for all 802.11 PHYs Enhancements: 802.11e, 802.11n, …



IEEE 802.11


MAC Sublayer Entitiy

Multiple access protocol, fragmentation, encryption

Physical Layer Convergence Protocol (PLCP)

Carrier sense

Physical Medium Dependent Sub-layer (PMD)

Modulation and coding

MAC Sublayer Management Entity

Synchronisation, power management, roaming, MAC MIB

PHY Layer Management

Channel tuning, PHY MIB (Management Information Base)

Reference model: Protocol Entities

MAC

Sublayer

PLCP Sublayer

PMD Sublayer

MAC Sublayer

Management

PHY LayerManage-ment

Sta

tio

nM

anag

emen

t

LLC(Logical Link Control)

DA

TA

Lin

kP

HY




WLAN IEEE 802.11

Structure:

1.Introduction


3.Reference model

4.Physical layer

5.MAC sublayer


5 different Physical layer technologies

FHSS (Frequency Hopping Spread Spectrum) 2.4 GHz band: 1 and 2 Mbit/s, 2GFSK, 4GFSK frequency hopping: 79 frequencies

DSSS (Direct Sequence Spread Spectrum) 2.4 GHz band: 1, 2, 5.5 and 11 Mbit/s DBPSK, DQPSK, 11-chip Barker Sequence, CCK

OFDM (Orthogonal Frequency Division Multiplexing) 802.11a: 5 GHz band, 6, 9, 12, 18, 24, 36, 48 and 54 Mbit/s BPSK, QPSK, 16-QAM and 64-QAM (each with 2 different

coding rates) 52 sub-carriers 802.11g: OFDM in 2.4 GHz with 54 Mbit/s

Baseband IR 1 and 2 Mbit/s, 4-PPM and 16-PPM

MIMO (Multiple Input – Multiple Output) – OFDM 802.11n: upto 600 Mbit/s (details see 802.11n chapter)


Quelle: MR

Physical Layer – OFDM


IEEE 802.11 a: (in CH) OFDM @ 5.15 – 5.35 GHz (Indoor) and @ 5.47 – 5.725 GHz (Indoor&Outdoor) 802.11 g: OFDM @ 2,4 - 2,4835 GHz


, 802.11 g and 802.11n

and 802.11 g

Physical Layer – 802.11a. OFDM


Transmitter and receiver block diagram for the OFDM PHY

802.11a PHY Data Format

http://www.ewh.ieee.org/r6/scv/comsoc/0205.pdf

28


Physical Layer – OFDM (802.11a)

29

Subcarrier frequency allocation

Bandwidth: 20 MHzFFT block length : 64Number of used carriers: 52 (including 4 pilots)Guard interval: 0.8 µs


802.11a OFDM Parameters

30

802.11a Subcarrier Assignment


31


Subcarrier Modulation Schemes


Correction

32

modulation


802.11a PHY Data Rates

33


Comparison of mostly used 802.11 PHYs

Data rates

802.11a: OFDM, up to 54 Mbit/s

802.11b: DSSS, up to 11 Mbit/s

802.11g: DSSS (downwardly compatible to 802.11b) / OFDM (up to 54 Mbit/s)

Channels

802.11a: 8 (non-overlapping) channels @ 5.15 – 5.35 GHz (20 MHz each) => up to 8 APs in the same area

802.11b/ g: only 3 non-overlapping (out of 13) channels (25 MHz each)=> only up to 3 APs in the same area


Channels: 802.11b

European channel selection—non-overlapping

European channel selection—overlapping



Channels: 802.11a

OFDM PHY frequency channel plan for the United States



WLAN IEEE 802.11

Structure:

1.Introduction


3.Reference model

4.Physical layer

5.MAC sublayer



Overview: MAC Sublayer


Two multiple access schemes -> two Coordination Functions:

(1) Distributed Coordination Function (DCF): CSMA / CA (contention

based)

or optional

(2) Point Coordination Function (PCF): Polling (central allocation)

Different frame formats

Fragmentation / defragmentation

Encryption

Multiple access schemes

39


Point Coordination Function (optional):

Only in Infrastructure Mode (ESS) Only for systems using an AP as central point of BSS AP gives transmit right to the STAs; STAs are polled one after another

(Polling) Higher priority than DCF (see Interframe Spacing)

Distributed Coordination Function:

For IBSS and Infrastructure mode (ESS) Based on Carrier Sense Function in PHY, called Clear Channel

Assessment (CCA) CSMA / CA for broadcast frames CSMA / CA + ACK otherwise Optional: (parameterised) RTS / CTS – handshake for Virtual Carrier

Sense (protection against „Hidden Nodes“)


MAC Sublayer: DCF (1)

DIFS Contention Window

Slot time

Defer access

Backoff-Window

Next Frame

Decrement Backoff Timer as long as medium idle

SIFS

PIFSDIFS

Medium busy

transmit, if medium is free >= DIFS

CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance):


Channel access: If WM seems to be free for a time >= DIFS, STA transmits immediately If WM busy, STA waits until the end of the ongoing transmission and

starts Backoff Procedure After this the status of the channel is checked again




Backoff Procedure (for a STA willing to transmit): STA sets its Backoff Timer to a random backoff time. In this time STA

waits and uses carrier-sense mechanism. Only if the WM seems to be idle, the STA decrements the Backoff

Timer. Backoff procedure starts also, if a collision is detected.

Backoff procedure reduces the probability of collisions without such a procedure this probability would be high after a

successful transmission, because then all the STAs prepared to transmit would start their transmissions at the same time.

42



MAC Sublayer: DCF (4) - CSMA / CA + ACK – Protocol

Ack

Data

Next MPDU

Src

Dest

Other

Contention Window

Defer transmission Backoff Procedure

DIFS

SIFS

DIFS


The (physical) Carrier-Sense mechanism is provided by the PHY (CCA).

After unsuccessful transmissions the max. backoff time increases exponentially up to a limit.

In a direct transmission between 2 STAs successful transmissions (CRC correct) are acknowledged immediately (positive acknowledgement) using ACK Frames.

If no ACK is received the frame is repeated.

MAC Sublayer: DCF (5) - RTS-CTS Handshake (optional)

Control frames are exchanged:

“Ready To Send“: RTSSTA wants to start transmission

“Clear To Send”: CTSreceiver is ready for transmission

These frames contain a field indicating the length of the upcoming transmission.

All STAs receiving at least one of the two control frames are now informed about the length of the upcoming transmission (Virtual Carrier Sense)

STAs store this information in their Net Allocation Vector (NAV)

Carrier-Sense mechanism: CCA + NAV


RTS

CTS Ack

Data

NAV Next MPDU

Src

Dest

Other

CW

Defer transmission Backoff Procedure

NAV

(RTS)

(CTS)

DIFS

MAC Sublayer: DCF (6) - Hidden Nodes

The „Hidden Nodes“ problem can be eased by the RTS / CTS mechanism.

STAs can be configured, to use the RTS / CTS mechanism always, never or from a given threshold upwards (for short frames the overhead may be too high).


AP

STA

CTS RangeRTS Range

STA APRTS

CTS

Data

Ack

STAs can’t hear each other

but the AP.

STA

Exposed Nodes

STA S1/S2 does not generate interference at STA R2/R1

S1 and S2 on the other hand are in communication range of each other

They are „exposed nodes“

Thus S1 and S2 could transmit simultaneously CSMA/CA prevents this, as e.g. S2

senses the channel busy if S1 transmits

RTS/CTS can help here: if e.g. S2 detects the RTS message of S1 but does not receive the CTS answer from R1 it can conclude, that it is an exposed node and transmit concurrently with S1

46


MAC Sublayer: Fragmentation Partitioning MSDUs (MAC Service Data Units) or MMPDUs (MAC Management Protocol

Data Units) into smaller MAC level frames Purpose: increase reliability, by increasing the probability of successful transmission Fragmentation: MSDU or MMPDU are sent as independent transmissions, each of which

is separately acknowledged (not for Broadcast frames) Backoff procedure and retransmission if no ACK received Information about duration of transm. included in fragments and in ACKs => NAV is set Defragmentation at receiver Can be combined with RTS - CTS

Src

Dest

SIFS

RTS

NAV (RTS)

NAV (CTS)

Other

PIFS

DIFS

BackoffNAV (Fragment 0)

NAV (ACK 0)

SIFS

CTS ACK 0 ACK 1

Fragment 0 Fragment 1


MAC Sublayer: PCF (1)

CFP CP

CFP repetition interval

Variable length

PCF waits when medium is busy: delay

CF burst (PCF)WM busyPCFDCF

Async traffic waits

"Reset NAV"

NAV

CFP repetition interval

AP of a BSS can become Point Controller (PC) -> Polling Master

PCF gains control of the WM by using Beacon Management Frames to set the NAV in STAs

Contention Period (CP): DCF has control in this period (CSMA/CA: contention based multiple access)

Contention Free Period (CFP): PCF has control (Polling: central allocation, no contention)

CP and CFP alternate under PC control

Length and repetition interval of the CFPs are controlled by the PC

MAC Sublayer: PCF (2)

STA immediately responses to a CF-Poll_Frame

Responses have variable length

“Reset NAV“ by last frame of AP

No RTS / CTS under PCF (Polling) PCF better suited for time critical services



However, legacy PCF was rarely (or even never) used; => But, it is the basis for a CF in 802.11e

MAC Sublayer: Interframe Spacing

SIFS (Short Interframe Space):time between ACK frames, CTS frames, fragmented data frames and PCF polls. E.g.: Frequency Hopping PHY - 28ms (802.11a: 16µs)

PIFS (PCF Interframe Space):PCF has higher priority than DCF=> PIFS < DIFS; PIFS = SIFS + Slot TimeE.g.: Frequency Hopping PHY - 78ms (802.11a: 25µs)

DIFS (DCF Interframe Space):= SIFS + 2 Slot TimeE.g.: Frequency Hopping PHY - 128ms (802.11a: 34µs)

EIFS (Extended Interframe Space):used if in previous transmission an error occurred

IFS independent of the STA bit rate




WLAN IEEE 802.11

Structure:

1.Introduction


3.Reference model

4.Physical layer

5.MAC sublayer


Overview: MAC Sublayer Management

Synchronization needed for Detection of WLANs Staying in a WLAN Synchronization functions:

TSF (Time Synchronisation Function) Timer, Beacon generation

Power management Sleep mode (without missing a message) Power management functions

Periodic sleep, frame buffering, Traffic Indication Map

Association and Reassociation Connection to a network Roaming Scanning

Management Information Base



MAC Sublayer Management: Synchronisation

All STAs in a BSS are synchronized to a common clock important for Hop Timing in FH PHY for Point Coordination Timing

for Power Management …

Timing Synchronization Function (TSF) keeps timers for all STAs in a BSS synchronized Each STA has a local TSF Timer

Beacons contain timestamps Beacons contain also further management information (e.g. for power management, roaming)

The timestamps calibrate the local clocks of the STAs

In an IBSS (ad hoc) network all STAs transmit Beacons

In an ESS the AP controls the timing

Beacons are generated periodically, but they can be delayed due to CSMA/CA


Time axis

Beacon Interval

X X X X

"Actual time" in Beacon

Beacon Medium busy



MAC Sublayer Manag.: Power Management


Power Save (PS) / Sleep Mode for STAs, to save energy

ESS (infrastructure mode):AP buffers frames for STAs in PS mode and transmits them at “known times” STAs that currently have buffered MSDUs within the AP are identified in a Traffic

Indication Map (TIM) TIM is included in all Beacons of the AP STAs in PS mode wake up in periodical intervals to receive Beacons (also because

of the TSF)

=> Contention Period, under control of DCFIf there is a MSDU for a STA buffered in the AP, the STA transmits PS-Poll to

the AP,which shall respond with the corresponding buffered MSDU immediately

=> CFP, PCF No PS-Poll, STA remains active until MSDU is received or CFP ends

Broadcast, multicast frames: also buffered by AP, delivered after Delivery TIM (DTIM)

IBSS (ad hoc mode): If destination is in PS mode, source notifies using ATIM (Announcement Traffic

Indication Message, or ad hoc TIM)

MAC Sublayer Management: Scanning

Scanning necessary for: Finding and joining a new network Setting up an IBSS (ad hoc network) Finding a new AP while performing handover or roaming

MAC used by different PHYs, most of them using more than one channel

Scanning: Active:

STA sends Probe on each channel waits for an answer (Probe Response)

Passive: listening for Beacons

Beacon as well as Probe Response contain all information needed to join the network

Seamless handover not defined in 802.11




SuccessfulAssociation orReassociation

MAC Sublayer Management: Association

Authentication: “Shared Key” (using WEP/WPA encryption) or “Open System” Goal: provide access control equal to a

wired LAN

Authentication service: provides a mechanism for one STA to identify another STA

Association establishing a logical connection between

STA and AP

each STA must become associated with an AP before it is allowed to send data through the AP onto the DS

connection is necessary for the DS to know where and how to deliver data to the STA



Active Scanning, Association, Handover

Quelle: MR

Handover: Scanning, (Re) association request



Outline

• IEEE 802.11e– MAC Quality of Service Enhancements

• IEEE 802.11n: – Enhancements for higher throughput

802.11e

59

WLAN IEEE 802.11e

Introduction MAC Architecture

DCF HCF: EDCA, HCCA

Main Differences from legacy IEEE 802.11 Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation

802.11e

Introduction

60

802.11e

IEEE802.11e: MAC Quality of Service Enhancements

nQSTA: HCF not present QSTA: both DCF and HCF are present.

PCF is optional in all STAs.

Finalized in Nov. 2005

Defines MAC procedures to support LAN applications with QoS requirements Transport of voice, audio, and

Video over 802.11 WLANs.

QoS enhancements available to QoS stations (QSTAs) associated

with a QoS access point (QAP) in a QoS basic service set (QBSS).


Main QoS Problems with legacy 802.11

TBTT: Target Beacon Transmission Time

802.11e

Unknown transmission durations of the polled stations No prioritization Unpredictable beacon delays

Unpredictable throughput per STA

62

WLAN IEEE 802.11e

802.11e


DCF HCF: EDCA, HCCA


63

HCF

802.11e

The HCF uses both methods contention-based: enhanced distributed channel access (EDCA), and contention-free: HCF controlled channel access (HCCA)

New feature of 802.11e MAC: Transmission Opportunity (TXOP) interval of time when a STA has the right to initiate transmissions defined by a starting time and a maximum duration allocated via contention (EDCA-TXOP) or granted through HCF (polled-TXOP) duration of an EDCA-TXOP is limited by a QBSS-wide TXOP limit

distributed in beacon frames duration of a polled TXOP specified by duration field inside the poll frame

Although the poll frame is a new frame as part of 802.11e, also the legacy stations set their NAVs upon receiving this frame.

64

Traffic Specification (TSPEC) 1/2

TSPEC:

Quality of service (QoS) characteristics of a data flow to and from a non-access point (non-AP) QSTA

Contains the set of parameters that define the characteristics and QoS expectations of a traffic flow (like data rate, burst size, and service interval).

Parameterized QoS:

Treatment of MAC protocol data units (MPDUs) depends on the parameters associated with the MPDU

Primarily provided through HCCA mechanism

Also provided by EDCA when used with a traffic specification (TSPEC) for admission control.

Traffic Specification (TSPEC) 2/2

65

Traffic stream (TS):

Set of MAC service data units (MSDUs) to be delivered subject to the QoS parameter values provided to the MAC in a particular TSPEC.

TSs are meaningful only to MAC entities of QSTAs.

These MAC entities determine the TSPEC applicable for delivery of MSDUs belonging to a particular TS using the TS identifier (TSID)

66

Overview of MAC services (1)

By default, asynchronous MSDU transport is performed on a best-effort connectionless basis.

However, the QoS facility uses a traffic identifier (TID) to specify differentiated services on a per-MSDU basis.

The QoS facility also permits more synchronous behavior to be supported on a connection-oriented basis using TSPECs.

No guarantees that the submitted MSDU will be delivered successfully.

Asynchronous data service provided by the MAC: Broadcast and multicast transport available.

67

Overview of MAC services (2)

QSTAs in a QBSS differentiate their MSDU delivery according to the designated traffic category or traffic stream (TS) of individual MSDUs

QSTA: MAC uses a set of rules that tends to cause higher UP (user priority) MSDUs in a BSS to be sent before lower UP MSDUs.

MAC sublayer entities determine the UPs for MSDUs based on the TID values provided with MSDUs.

a TSPEC has been provided for a TS

=> MAC attempts to deliver MSDUs belonging to that TS in accordance with the QoS parameter values contained in the TSPEC.

68

WLAN IEEE 802.11e

802.11e


DCF HCF: EDCA, HCCA



EDCA (Enhanced Distributed Channel Access)

802.11e

Up to 8 UPs (or TCs)

EDCA EDCA EDCA EDCA

transmission attempt

802.11e

DCF access to the medium depending on Traffic Categories (TCs) 8 different priorities (UP: User Priority)

4 different buffers with different priority of access to the medium: Access Categories (ACs); Average time to wait depends on priority

Per-queue channel access: internal collision resolution (EDCAF)


EDCA(2)

UP-to-AC mappings

BK: Background

BE: Best effort

VI: Video

VO: Voice

For each AC: an enhanced distributed channel access function (EDCAF) contends for TXOPs using a set of EDCA parameters

EDCAF: enhanced variant of the DCF


EDCA (3)

802.11e

EDCA delivers traffic according to different ACs by varying the following quantities:

Amount of time a QSTA senses the channel to be idle before backoff or transmission (AIFS), or

length of the contention window to be used for the backoff, or duration a STA may transmit after it acquires the channel (length of TXOP).


EDCA (4)

802.11e

with 802.11a:

Slot: 9 μs

SIFS: 6 μs

PIFS: 25 μs

DIFS: 34 μs

AIFS: ≥ 34 μs

EDCA: priority according to AC by varying: AIFS[AC] length of the contention window to be used for the backoff length of TXOP

AIFS: Arbitration Inter frame Space - shall be used by QSTAs to transmit: all Data type frames (MPDUs), all Management type frames (MMPDUs), the following

control frames: PS-Poll, RTS


EDCA parameters (lengths of CW, IFS, TXOP)

802.11e

AIFS number- AIFSN: number of slots after a SIFS duration a QSTA should defer before either invoking a backoff or starting a transmission (minimum value for a QSTA is 2, for a QAP it is 1).

BK: Background

BE: Best effort

VI: Video

VO: Voice

0 indicates that a single MSDU or MMPDU

AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime

802.11a: aCWmin = 15, aCWmax 1023

OFDMDSS


Wireless Station

Backoff

AIFSAccess Point

Backoff

AIFS

Frame 1

Ack

SIFS

Frame 2

Ack

SIFS

Busy

Beacon Beacon

t

t

t < EDCA TXOP limit

TXOP Bursting – Multiple Frame Tx

(Reduces Backoff Overhead)

TXOP: Transmit Opportunity • given to the TC with highest priority of the colliding TCs, • all Management type frames (MMPDUs), and the following control frames: PS-Poll, RTS, CTS (when not transmitted as a response to the RTS), BlockAckReq and BlockAck

EDCA TXOP

802.11e

75

WLAN IEEE 802.11e

802.11e


DCF HCF: EDCA, HCCA


76

HCCA (1)

802.11e

The HCCA mechanism uses a QoS-aware centralized coordinator, called a hybrid coordinator (HC).

HC: collocated with the QAP higher medium access priority than non-AP STAs allocates TXOPs to itself and other QSTAs in order to provide limited-

duration controlled access phase (CAP) for contention-free transfer of QoS data.

HCCA TXOP allocation may be scheduled during the CP and CFP.

77

HCCA (2)

HCF Controlled Channel Access (HCCA): Like PCF mechanism, HCCA controls channel access through AP-directed

polling. AP's Hybrid Controller takes QoS into consideration when scheduling STA

transmission times and durations, giving some traffic a bigger share of the channel.

STAs using HCCA submit reservation requests AP then assigns transmit opportunities based on 8 possible Traffic Stream

Identifiers (TSIDs). - TSIDs are themselves based upon Transmission Specification (TSPEC) parameters

like data rate, burst size, and service interval. This parameterized QoS mechanism is arguably more complex than prioritized

QoS. - E.g., HCCA requires STAs to know what they'll want to send in advance. - However, in WLANs used primarily for voice or video streams, HCCA with well-tuned

QoS parameters can enable more efficient channel utilization by eliminating "wasted" backoff time.

78

Controlled Access Phase (CAP) Generation

802.11e

(controlled access phase)

HC shall sense the WM. If WM is determined idle for one PIFS period, HC transmits first frame of any

permitted frame exchange sequence. Duration value is set to cover the CFP or the TXOP. First permitted frame in a CFP after a TBTT is the Beacon frame.

HCCA TXOP

79

Access Point

Data + Ack

AckBusy

Beacon

t

t

PIFS

SIFS

Polled TXOP limit

Data Data

Downlink TXOP limit

Poll + Data

SIFS

Data

SIFS

UPLINK TXOP DOWNLINK TXOP

Ack

SIFS PIFS

Ack

SIFS

SIFS

Piggybacking(Reduces Poll and Ack

Overheads)

Wireless Station

802.11e

80

WLAN IEEE 802.11e

802.11e


DCF HCF: EDCA, HCCA


81

Admission Control

Important for the provision of guaranteed QoS parameters

Goal: Limit amount of traffic admitted into a service class so that QoS of existing flows will not degrade, while the medium resources can be maximally utilized

An IEEE 802.11 network may use admission control to administer policy or regulate the available bandwidth resources.

Admission control is also required when a QSTA desires guarantee on the amount of time that it can access the channel.

HC administers admission control in the network.

Admission control, in general, depends on vendors’ implementation of the scheduler, available channel capacity, link conditions, retransmission limits, and the scheduling requirements of a given stream.

802.11e

82

Block Acknowledgment

The Block Ack mechanism improves channel efficiency by aggregating several acknowledgments into one frame.

There are two types of Block Ack mechanisms: immediate and delayed.

The Block Ack mechanism is initialized by an exchange of ADDBA (add Block Acknowledgment) Request/Response frames.

The number of frames in the block is limited, and the amount of state that is to be kept by the recipient is bounded.

Acknowledgments of frames belonging to the same TID (Traffic Identifier assigned by higher layers), but transmitted during multiple TXOPs, may be combined into a single BlockAck frame.

802.11e

83

No Acknowledgment

ACK does not need to be used in a QBSS in case of time-critical services when a retransmission is not reasonable.

There is no MAC-level recovery, and the reliability of this traffic is reduced, due to the increased probability of lost frames from interference, collisions, or time-varying channel parameters.

A protective mechanism (such as transmitting using HCCA, RTS/CTS, should be used to reduce the probability of other STAs transmitting during the TXOP.

802.11e

84

DLS (direct-link setup) Operation (1)

DCF: In general, STAs are not allowed to transmit frames directly to other STAs in a BSS (exception: IBSS, i.e. Ad-hoc network) They should always rely on the AP for the delivery of the frames

However, STAs with QoS facility (i.e., QSTAs) may transmit frames directly to another QSTA by setting up such data transfer using DLS. Need for this protocol: motivated by the fact that the intended recipient may be in

PS mode, in which case it can be awakened only by the QAP.

Second feature of DLS: exchange of rate set and other information between the sender and the receiver

DLS prohibits STAs going into PS mode for the duration of the direct stream as long as there is an active DLS between the two STAs.

DLS does not apply in a QIBSS, where frames are always sent directly from one STA to another.

802.11e

85

DLS (direct-link setup) Operation (2)

The handshake involves four steps QSTA-1 intending to exchange frames directly with another non-AP STA (QSTA-2)

sends a DLS Request frame to the QAP (step 1a); request contains the rate set, capabilities of QSTA-1, and the MAC

addresses of QSTA-1 and QSTA-2.

If QSTA-2 is associated in the BSS, and direct streams are allowed in the policy of the BSS, and QSTA-2 is indeed a QSTA => QAP forwards the DLS Request frame to QSTA-2 (step 1b).

If QSTA-2 accepts the direct stream, it sends a DLS Response frame to the QAP (step 2a) containing rate set, capabilities of QSTA-2, and the MAC addresses of QSTA-1 and QSTA-2.

QAP forwards the DLS Response frame to QSTA-1 (step 2b)

=> direct link becomes active and frames can be sent from QSTA-1 to QSTA-2 and from QSTA-2 to QSTA-1

802.11e

86

WLAN IEEE 802.11e

802.11e


DCF HCF: EDCA, HCCA



Outline

• IEEE 802.11e– MAC Quality of Service Enhancements

• IEEE 802.11n: – Enhancements for higher throughput

802.11e


WLAN 802.11n: High Throughput WLAN

802.11n

MIMO- OFDM WLAN standard

Goals: Using MIMO PHY for higher data rates

higher spectral efficiency

higher diversity gains (i.e. increased link reliability)

extended communication range

Several MAC enhancements (e.g. MSDU aggregation to reduce overhead, RIFS – Reduced IFS, Block ACK, …)


IEEE WLAN 802.11n: MIMO-OFDM


IEEE 802.11n standard specifies MAC and PHY for a high throughput WLAN PHY is based on MIMO OFDM in the 2.4 GHz and 5 GHz band

- operating in 20 MHz bandwidth

- operation in 40 MHz bandwidth is optional.

Mandatory in 802.11n for an AP: support of 1 and 2 spatial streams for 20 MHz bandwidth (MIMO)

for an 802.11n STA: one spatial stream

Optional features include transmit beamforming

space-time block codes (STBC) or hybrid STBC/ Spatial Multiplexing (SM)

support of 3–4 spatial streams in 20 MHz mode and of 1–4 spatial streams in 40 MHz mode is optional.


WLAN 802.11n: PHY

802.11n

OFDM in 2.4 and 5 GHz (ISM) band

Mandatory payload communication capabilities of up to 135 Mbit/s

Optional modes capable of supporting data rates up to 540 Mbit/s (600 Mbit/s for reduced OFDM Guard Interval (GI))

1 TX, 2 TX, 3 TX, and 4 TX (transmit antenna modes)

MIMO Spatial Multiplexing (SpaMuX) gain: 1, 2, 3, or 4 (spatial sub-channels)

540 Mbit/s mode: 64-QAM, code rate = 5/6, # spatial sub-channels = 4 (600 Mbit/s: optional 400 ns GI instead of 800ns)

NTX-STBC (Space Time Block Coding) Modes: more TX antennas than # of used spatial sub-channels

Low Density Parity Check (LDPC) codes: optional error correction codes

Extended communication range (Tx beamforming, STBC, LDPC)

Mandatory and Optional PHY Features

Eldad Perahia: “IEEE 802.11n Development: History, Process, and Technology,” IEEE Communications Magazine, July 2008.

91802.11n

Up to 4 spatial streams: 4x4 MIMO (up to factor 4 in data rate)

40 MHz bandwidth (factor 2)

802.11a/g encoder rate ¾:for .11n increased to 5/6 (11% increase in data rate)

Four extra OFDM subcarriers squeezed into the spectral mask (48 -> 52: 8% increase)

Optional OFDM guard interval of 400 ns (11% increase)

Mixed format: backward compatible to .11a/g OFDM

but additional training fields in preamble for MIMO training (20μs in .11a to 48 μs in .11n with 4 streams)

91

Summary of 802.11n MAC Enhancements

92802.11n

Frame aggregation as key method to increase efficiency on MAC Increases the length of the data

portion of the frame relative to PHY and MAC overhead

Block Ack: -> 802.11e Enhanced by reduced interframe

spaces (RIFS) Reverse direction protocol: e.g. FTP

over TCP, TCP Ack in the same TXOP

TxBF: PHY, but MAC for exchange of beamforming weights, CSI, channel sounding, …

92

WLAN 802.11n: Code Rates (1)

• IEEE 802.11n, October 2009

93802.11n

93



94802.11n

94



95802.11n

95


96802.11n

96

WLAN 802.11n: Transmitter


• CSD: Cyclic Shift Diversity

97802.11n

97

A Glimpse on 802.11ac

Expected Throughput: per single link at least 500 Mbit/s, for 2 users simultaneously (i.e. 1 Gbit/s sum throughput) Up to 1.75 Gbit/s expected per single link (as a first step)

5 GHz band mandatory (2.4 GHz band still supported => backward compatible to 802.11n)

Bandwidth: 80 and 160 MHz @ 5 GHz MIMO: 8 antennas, multi-user MIMO Symbol alphabet: 256-QAM

Beamforming MAC modifications Approval of .11ac standard: expected not before early 2014


Multi-User MIMO

Uplink: Multiple Access Channel(MIMO-MAC)

Downlink: Broadcast Channel (MIMO-BC)

99

Diversity, MIMO

AP STA 1

STA 2

MU-MIMO - Downlink

MIMO transmit processing at AP (to separate both STAs)

No joint decoding at STAs on receive side

The Tx MIMO signals for different STAs interfere with each other

AP to use Tx signal processing to cancel (or reduce) this interference

Rates to the STAs are bounded by the achievable rate region (capacity region) of the MIMO broadcast channel

See, for instance: D. Tse, P. Viswanath, "Fundamentals of Wireless Communication", Cambridge University Press, 2005. [Tse, 2005]

100

LTE Advanced

MU-MIMO - Uplink

MIMO receive processing at AP

STAs transmit simultaneously to AP

Extension to legacy MAC necessary (!)

No joint Tx processing at STAs

However, each STA may use CSITfor beamforming

Rates of STAs are bounded by achievable rate region (capacity region)given by the MIMO multiple access channel

See, for instance: D. Tse, P. Viswanath, "Fundamentals of Wireless Communication", Cambridge University Press, 2005. [Tse, 2005]

101

LTE Advanced

A Glimpse on 802.11ad [Wikipedia, 21.06.2013]

Wireless Gigabit Alliance (WiGig): Organization promoting the adoption of multi-gigabit speed wireless communications technology operating over the unlicensed 60 GHz frequency band.

Creation of WiGig (IEEE 802.11 ad) was announced on May 7, 2009 The completed version 1.0 WiGig specification was announced in December

2009. In May 2010, WiGig announced the publication of its specification, the opening of

its Adopter Program, and the liaison agreement with the Wi-Fi Alliance to cooperate on the expansion of Wi-Fi technologies.

In June 2011, WiGig announced the release of its certification-ready version 1.1 specification.

WiGig specification will allow devices to communicate at multi-gigabit speeds.

Enables high performance wireless data, display and audio applications that supplement the capabilities of today’s wireless LAN devices.


A Glimpse on 802.11ad [Wikipedia, 21.06.2013]

WiGig tri-band enabled devices, which operate in the 2.4, 5 and 60 GHz bands, will deliver data transfer rates up to 7 Gbit/s, about as fast as an 8 antenna

802.11ac transmission, and nearly 50 times faster than the highest 802.11n rate, while maintaining compatibility with existing Wi-Fi devices. However, the promised 7 Gbit/s rate makes use of the 60 GHz band which

cannot go through walls; it is a line-of-sight technology. When roaming away from the main room the protocol will switch to make use of

the other lower bands at a much lower rate, but which propagate through walls.


Appendix

The Steps to an Amendment of the Standard in the 802.11 Working Group

Discussion of new ideas in the Wireless Next Generation Standing Committee

Development of the purpose and scope of the amendment in a Study Group

Drafting of the amendment in a Task Group

Letter ballot votes in Working Group for iterative improvement of draft

Approval of the draft by the Working Group

Review by a sponsor ballot pool

Approval and ratification by the IEEE Standards Association Board

Overview of 802.11 Task Groups and Study Groups (as of October 2010) source http://www.ieee802.org/11/QuickGuide_IEEE_802_WG_and_Activities.htm

http://www.ieee802.org/11/QuickGuide_IEEE_802_WG_and_Activities.htm

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