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Medium Access Control Enhancements for Quality of Service

IEEE Std 802.11eTM-2005

November 2005

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Outline

• MAC Architecture

• MAC Control-Plane Architecture

• Hybrid Coordination Function (HCF)

• Enhanced Distributed Coordination Function (EDCF)

• Transmission Opportunity (TXOP)

• HCF Controlled Access

• Requests for Network Services

• Traffic Streams

• Action Management Frame

• Scheduling Algorithms

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Characteristics of IEEE 802.11e

• The major enhancement of 802.11e– Traffic differentiation

– Concept of Transmission Opportunity (TXOP)

– Enhanced DCF (contention-based)

– HCP controlled channel access (contention free)

– Burst ACK (optional)

– Direct link protocol (DLP)

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MAC Architecture

• DCF : A contention-base access for 802.11. • PCF : An option to support contention-free access in 802.11.• Hybrid Coordination Function (HCF): IEEE 802.11 Task Group E (TGe)

proposes HCF to provide QoS for real-time applications.

IEEE 802.11 MAC Architecture IEEE 802.11e MAC Architecture

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HCF - Introduction

• HCF combines functions from the DCF and PCF with enhanced QoS-specific mechanisms.

• HCF consists of – Enhance DCF (EDCF) for contention-based access

– Controlled Access (HCCA) for contention-free access

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HCF – Definitions (1/2)

• Definition:– Hybrid coordinator (HC): the point coordinator for

HCF.

– QoS access point (QAP): An access point (AP) that implements the access point functions specified in the IEEE 802.11e standard.

– QoS facility: The set of enhanced functions, frame exchange sequences, and management objects.

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HCF – Definitions (2/2)

– QoS station (QSTA): An IEEE 802.11 station which implements QoS facility and HCF.

– QoS basic service set (QBSS): A basic service set that supports QoS facility specified in the IEEE 802.11e.

– QoS independent basic service set (QIBSS): An independent basic service set in which one or more of its stations support the QoS facility.

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EDCF – Traffic Category

• The EDCF provides differentiated access to the WM for 8 priorities, identical to IEEE 802.1D priority tag, for non-AP STAs.– Priorities are numbered from 0 (the lowest priority) to 7

(the highest priority).

• The set of MSDUs with the same priority is refer to a Traffic Category (TC).

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EDCF – Access Category (1/5)

• EDCF defines access category (AC) mechanism to support the priority mechanism at the non-AP QSTAs.

• An AC is an enhanced variant of the DCF which contends for transmission opportunity (TXOP) using the set of parameters such as CWmin[AC], CWmax[AC], AIFS[AC], etc.

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EDCF – Access Category (2/5)

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EDCAS

IFS

2 slots

2 slots

3 slots

7 slots

0 ~ 3 slots

0 ~ 7 slots

0 ~ 15 slots

0 ~ 15 slots

Voice

Video

BestEffort

Back-ground

Random BackoffWait

MinimumWait

Assumptions: WME Default Parameters Backoff values shown are for

initial CW equal to CWmin = 15

AIFSN

AIFSN

AIFSN

AIFSN

SIF

SS

IFS

SIF

S

* from WMM (Wi-Fi Multimedia)

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EDCF – Access Category (3/5)

• The parameter set is specified in the “EDCA parameter set element” of beacon frames.

• Default QoS parameter set:

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EDCF – Access Category (4/5)

• An QSTA has four ACs. • Collision between ACs within a QSTA is called

internal collision.

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EDCF – Access Category (5/5)

• Internal collision resolution: High priority AC wins the right of transmission, but low priority AC back off as if it experiences a collision.

• The mapping from 8 priories to 4 ACs is:

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TXOP

• A TXOP is defined by a starting time and a maximum duration.

• Two types of TXOP: EDCF TXOP and Polled TXOP.– An EDCF TXOP begins when the wireless medium is

determined to be available under the EDCF rules, and the length of TXOP is specified in beacon frames.

– An Polled TXOP begins when a QSTA receives a QoS(+)CF-Poll from HC, and the length of TXOP is specified in the QoS(+)CF-Poll.

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HCF Controlled Access – Introduction

• Differences between hybrid coordinator (HC) and point coordinator (PC):– HC can poll QSTAs in both

CP and CFP

– HC grants a polled TXOP to one QSTA, which restricts the duration of the QSTA’s access to the medium.

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HCF Controlled Access – Frame Formats

• General frame format for 802.11 and 802.11e:

• Frame Control Field:

=>IEEE 802.11

=> IEEE 802.11e

see next page

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HCF Controlled Access - New Data/Management/Control Frames

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HCF Controlled Access – QoS Control Field (1/3)

• Symbols:– QoS+CF-Poll: including 3 control messages, QoS Data+CF-Poll, QoS

Data+CF-Ack+CF-Poll, QoS CF-Ack+CF-Poll

– QoS(+)CF-Poll: QoS+CF-Poll and QoS CF-Poll (4 control messages)

• The QoS control field:

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HCF Controlled Access – QoS Control Field (2/3)

– TID:

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HCF Controlled Access – QoS Control Field (1/3)

– EOSP (End of Service Period):

– Ack Policy: • Normal Ack: An Ack or QoS CF-Ack is required after a SIFS.

• No Ack:

• No Explicit Ack: There may be a response frame, but it is neither the Ack nor any Data frame of subtype +CF-Ack. (e.g., QoS CF-Poll, or QoS CF-Ack+CF-Poll).

• Block Ack:

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Requests for Network Services

• Request for polled TXOP: – Non-AP QSTAs may request a polled TXOP by

sending a QoS Data frame, QoS Null frame, or QoS CF-ACK frames with QoS control field to HC.

• Request for a traffic stream:– Non-AP QSTAs may request a traffic stream by send

an Action managements frame (described latter) to HC.

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Traffic Stream

• A traffic stream is the set of MSDUs which is delivered with the same traffic specification.– Traffic specification (TSPEC) includes the information

of mean/max/min data rate, delay bound, etc.

– (more later about definition of TSPEC)

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Action Management Frame –Introduction

• An action management frame (refer to subtype 1101 in frame control field) contains a category field and an action details.

– for QSTA to request a traffic stream

see next page

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Action Management Frame –QoS Management Actions

• For QoS management, the “Action Details” field contains following values:

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Action Management Frame –QoS Management Actions : Traffic

Stream Management (1/3)• A QSTA can request a traffic stream by sending

an ADDTS request frame to HC.

TSPEC(information

element)

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Action Management Frame –QoS Management Actions : Traffic

Stream Management (2/3)• After HC receives an ADDTS request frame, it

responds with an ADDTS respond frame.

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(cont.) Traffic Stream Management (3/3)– HC aggregates admitted TSPECs for a single QSTA

and establishes a Service Schedule, in the schedule element field, for the QSTA.

• The schedule QoS action frame is used by the QSTA for power management, internal scheduling, etc.

• Use DELTS frame to delete a traffic stream.

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Action Management Frame –QoS Management Actions :

Schedule• The HC can update the Service Schedule at any

time by sending a Schedule QoS Action management frame which contains a Schedule element.

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Action Management Frame –QoS Management Actions : Group

Acknowledgement (1/6)• The Group Acknowledgement mechanism

improves the channel efficiency by allowing a group of QoS Data MPDUs to be transmitted, each separated by a SIFS period, and aggregating several acknowledgements into ONE frame.

• Two types of Group ACK mechanisms:– immediate: for high-bandwidth, low latency traffic

– delayed: for applications that can tolerate moderate latency.

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(cont.)Group Acknowledgement (2/6)

• Message sequence:

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(cont.) Group Acknowledgement (3/6)

• An example of immediate group ack:

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(cont.)Group Acknowledgement (4/6)

• An example of delayed group ack:

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(cont.)Group Acknowledgement (5/6)

• Use ADDGA request frame to initiate a group acknowledgement.

• After receiving ADDGA request frame, the receiver responds with ADDGA response frame.

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(cont.)Group Acknowledgement (6/6)

• A receiver or sender can transmit DELGA frame to terminate a group acknowledgment.

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Scheduling Algorithms for IEEE802.11e Networks

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TSPEC

• Information element of management frames• Define the characteristics and QoS expectation of

a traffic stream– Negotiated between QSTA and HC

• TSPEC setup & delete– Use management frame with new subtype Action

containing TSPEC element

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TSPEC Element Format

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Main Parameters of TSPEC

• User priority (UP): priority to be used for the transport of packets in cases where relative prioritization is required (e.g., it can be used for admission control). It goes from 0 (lowest) to 7 (highest).

• Maximum MSDU size (M): maximum size of the packets, in octets.• Maximum Burst Size (MBS): maximum size of the data burst that

can be transmitted at the peak data rate, in octets• Minimum PHY rate (R): physical bit rate assumed by the scheduler

for transmit time and admission control calculations, in units of bits per second.

• Peak data rate (PR): maximum bit rate allowed for transfer of the packets.

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Main Parameters of TSPEC

• Mean data rate (ρ): average bit rate for transfer of the packets, in units of bits per second.

• Delay bound (D): maximum delay allowed to transport a packet across the wireless interface (including queuing delay), in milliseconds.

• Nominal MSDU size (L): nominal size of the packets, in octets.

• Maximum Service Interval (MSI): interval required by TS in this TSPEC between the start of two successive TXOPs.

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A Simple Scheduler (1/3)

• Use some of TSPEC parameters to generate a schedule– Mean date rate

– Nominal MSDU size

– Maximum Service Interval or Delay Bound

• The schedule for an admitted stream i is calculated in three steps

1. find an n which satisfies the following inequality, and then calculate the scheduled Service Interval (SI)

all stations i(beacon interval) / min { }iSI n MSI

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A simple scheduler (2/3)

2. Calculate the number of MSDUs Ni of station i during one SI (based on the Mean Data Rate):

3. Calculate the TXOPi duration of station i:– Ri: physical transmission rate– O: overhead– M: maximum MSDU size–

ii

i

SIN

L

max ,i ii

i i

N L MTXOP O O

R R

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A Simple Scheduler (3/3)

• When a new stream is admitted– If the current SI is changed, all admitted streams need to

recalculate their TXOP durations

• When a stream is dropped– Announce the new schedule to all QSTAs

• Admission control (referenced design)

1

1

kcpk i

i

T TTXOP TXOP

SI SI T

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References

1. Y. Xiao, “An Analysis for Differentiated Services in IEEE 802.11 and IEEE 802.11e Wireless LANs”, Int’l Conf. on Distributed Computing Systems, 2004, pp. 32-39.

2. A. GRILO, M. MACEDO, and M. NUNES, “A SCHEDULING ALGORITHM FOR QOS SUPPORT IN IEEE802.11E NETWORKS”, IEEE Wireless Communications, June 2003.