Medium Access Control Enhancements for Quality of Service

Medium Access Control Enhancements for Quality of Service IEEE Std 802. 11 e. TM-2005 November 2005 1

Outline (we will cover only the RED parts) • • • MAC Architecture Hybrid Coordination Function (HCF) Enhanced Distributed Coordination Function (EDCF) Transmission Opportunity (TXOP) HCF Controlled Access Requests for Network Services Group ACK Action Management Frame Scheduling Algorithms 2

Characteristics of IEEE 802. 11 e • The major enhancement of 802. 11 e – – – Traffic differentiation Concept of Transmission Opportunity (TXOP) Enhanced DCF (contention-based) HCP controlled channel access (contention free) Burst ACK (optional) Direct link protocol (DLP) 3

MAC Architecture IEEE 802. 11 MAC Architecture • • • IEEE 802. 11 e 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 Qo. S for real-time applications. 4

HCF - Introduction • HCF combines functions from the DCF and PCF with enhanced Qo. S-specific mechanisms. • HCF consists of – Enhance DCF (EDCF) for contention-based access – Controlled Access (HCCA) for contention-free access 5

EDCA 6

EDCF – Traffic Category • The EDCF provides differentiated access to the WM for 8 priorities, identical to IEEE 802. 1 D 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). 7

EDCF – Access Category (1/5) • EDCF defines access category (AC) mechanism to support the priority mechanism at 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. 8

EDCF – Access Category (2/5) IFS (Inter Frame Spacing) 9

EDCA 10

EDCF – Access Category (3/5) • The parameter set is specified in the “EDCA parameter set element” of beacon frames. • Default Qo. S parameter set: video voice 11

EDCF – Access Category (4/5) Queues • An QSTA has four ACs. • Collision between ACs within a QSTA is called internal collision. Collisions will be resolved internally (giving to higher-priority queues). 12

EDCF – Access Category (5/5) • The mapping from 8 priories to 4 ACs is: 13

TXOP 14

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 Qo. S(+)CF-Poll from HC, and the length of TXOP is specified in the Qo. S(+)CF-Poll. 15

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. 16

HCF Controlled Access – Frame Formats • General frame format for 802. 11 and 802. 11 e: =>IEEE 802. 11 => IEEE 802. 11 e • Frame Control Field: see next page 17

HCF Controlled Access - New Data/Management/Control Frames 18

HCF Controlled Access – Qo. S Control Field (1/3) • Symbols: – Qo. S+CF-Poll: including 3 control messages, Qo. S Data+CFPoll, Qo. S Data+CF-Ack+CF-Poll, Qo. S CF-Ack+CF-Poll – Qo. S(+)CF-Poll: Qo. S+CF-Poll and Qo. S CF-Poll (4 control messages) • The Qo. S control field: 19

HCF Controlled Access – Qo. S Control Field (2/3) – TID: 20

HCF Controlled Access – Qo. S Control Field (3/3) – EOSP (End of Service Period): – Ack Policy: • Normal Ack: An Ack or Qo. S 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. , Qo. S CF-Poll, or Qo. S CF-Ack+CF-Poll). • Block Ack: 21

Request for Network Services Request for TXOP Request for TSPEC 22

Requests for Network Services • Request for polled TXOP: – Non-AP QSTAs may request a polled TXOP by sending a Qo. S Data frame, Qo. S Null frame, or Qo. S CF -ACK frames with Qo. S control field to HC. • Request for a traffic stream: – Non-AP QSTAs may request a traffic stream by sending an Action managements frame (described latter) to HC. 23

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) 24

Action Management Frame – Introduction • An action management frame (refer to subtype 1101 in frame control field) contains a category field an action details. – for QSTA to request a traffic stream see next page 25

Action Management Frame – Qo. S Management Actions • For Qo. S management, the “Action Details” field contains following values: 26

Action Management Frame – Qo. S 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) 27

Action Management Frame – Qo. S Management Actions : Traffic Stream Management (2/3) • After HC receives an ADDTS request frame, it responds with an ADDTS respond frame. 28

(cont. ) Traffic Stream Management (3/3) • Service Schedule: – HC aggregates admitted TSPECs for a single QSTA and establishes a Service Schedule, in the schedule element field, for the QSTA. • The schedule Qo. S action frame is used by the QSTA for power management, internal scheduling, etc. • Use DELTS frame to delete a traffic stream. 29

Action Management Frame – Qo. S Management Actions : Schedule • The HC can update the Service Schedule at any time by sending a Schedule Qo. S Action management frame which contains a Schedule element. 30

Group ACK 31

Action Management Frame – Qo. S Management Actions : Group Acknowledgement (1/4) • The Group Acknowledgement mechanism improves the channel efficiency by allowing a group of Qo. S 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. 32

(cont. ) Group Acknowledgement (2/4) • Message sequence: 33

(cont. ) Group Acknowledgement (3/4) • An example of immediate group ack: 34

(cont. ) Group Acknowledgement (4/4) • An example of delayed group ack: 35

Scheduling Algorithms for IEEE 802. 11 e Networks 36

TSPEC • Information element of management frames • Define the characteristics and Qo. S expectation of a traffic stream – Negotiated between QSTA and HC • TSPEC setup & delete – Use management frame with new subtype Action containing TSPEC element 37

TSPEC Element Format 38

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. 39

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. 40

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) 41

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 – 42

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) 43

References 1. Y. Xiao, “An Analysis for Differentiated Services in IEEE 802. 11 and IEEE 802. 11 e 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 IEEE 802. 11 E NETWORKS”, IEEE Wireless Communications, June 2003. 44