Medium Access Control Enhancements for Quality of Service

1. Medium Access Control Enhancements for Quality of Service IEEE Std 802.11eTM-2005 November 2005

2. 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

3. 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)

4. MAC Architecture IEEE 802.11 MAC Architecture IEEE 802.11e 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.

5. 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

6. 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.

7. 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.

8. 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).

9. 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.

10. EDCF – Access Category (2/5)

11. EDCA * from WMM (Wi-Fi Multimedia)

12. EDCF – Access Category (3/5) • The parameter set is specified in the “EDCA parameter set element” of beacon frames. • Default QoS parameter set:

13. EDCF – Access Category (4/5) • An QSTA has four ACs. • Collision between ACs within a QSTA is called internal collision.

14. 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:

15. 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.

16. 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.

17. 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

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

19. 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:

20. HCF Controlled Access – QoS Control Field (2/3) • TID:

21. 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:

22. 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.

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 and an action details. • for QSTA to request a traffic stream see next page

25. Action Management Frame –QoS Management Actions • For QoS management, the “Action Details” field contains following values:

26. 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)

27. 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.

28. (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.

29. 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.

30. 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.

31. (cont.)Group Acknowledgement (2/6) • Message sequence:

32. (cont.) Group Acknowledgement (3/6) • An example of immediate group ack:

33. (cont.)Group Acknowledgement (4/6) • An example of delayed group ack:

34. (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.

35. (cont.)Group Acknowledgement (6/6) • A receiver or sender can transmit DELGA frame to terminate a group acknowledgment.

36. Scheduling Algorithms for IEEE802.11e Networks

37. 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

38. TSPEC Element Format

39. 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.

40. 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.

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

42. A simple scheduler (2/3) • Calculate the number of MSDUs Ni of station i during one SI (based on the Mean Data Rate): • Calculate the TXOPi duration of station i: • Ri: physical transmission rate • O: overhead • M: maximum MSDU size

43. 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)

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