802.11 (e) – Enhanced MAC for QoS (Proposed) & Efficiency

1. 802.11 (e) – Enhanced MAC forQoS (Proposed) & Efficiency 1

2. Contents • What is QoS and why do we need it? • Overview of 802.11 (e) • Differentiated Traffic Classes • Enhanced Distributed Channel Access protocol (EDCA) • Allocated Transmission Opportunities (TXOPs) • Burst ACKs • Direct Link Protocol 2

3. What is QoS and why do we need it? 3

4. What is QoS? • QoS is short for Quality of Service • It describes the service given to a particular frame or sequence of frames • QoS can be specified using, among others, the following parameters: • Delay/Latency • Available Bandwidth • Error Correction used • Acknowledgement Scheme 4

5. Why do we need QoS? • All 802 MAC schemes are Best Effort i.e. there is no guarantee that packets will be delivered at all (and hence, within any QoS requirements) • All frames are treated equally – there is no priority given to frames with different requirements • Different applications have different needs e.g.: • Voice traffic has very tight delay requirements. There is often little gained from retransmitting lost frames or frames with errors, since they will arrive too late • Data traffic (e.g. using ftp) requires no bit errors, but has less stringent delay constraints 5

6. How do we provide QoS? • In order to provide different applications with the appropriate QoS, we need to take two steps: • 1. Categorize the traffic. • For each category, define the QoS parameters that apply to it • Each category may contain frames from many sources, or from a single ‘connection’ • 2. Define a scheme to take account of the requirements of each category • This may be as simple as giving priority to one category over another, or may be more complex, allocating specific transmission ‘slots’ to each category 6

7. Overview of 802.11 (e) 7

8. Features of 802.11 (e) • Fully backwards-compatible • Stations without 802.11(e) implemented will still be able to operate in an 802.11(e) environment • Provides two means of QoS provisioning: • Prioritizing traffic • Allocating specific transmission times for traffic • Provides three optional means for increasing the efficiency (throughput) of the network: • Burst Acknowledgements • Direct Link Protocol • No Acknowledgement 8

9. Hybrid Coordination Function (HCF) • The function that implements these features is called the Hybrid Coordination Function (HCF). It consists of two parts: • a distributed function that operates at every station using 802.11(e) • in an Infrastructure BSS, a centralized scheduling function called the Hybrid Coordinator (HC) that operates at the Access Point • It can coexist with both DCF and PCF • Note: we shall assume in the following slides that all stations are implementing the HCF 9

10. Differentiated Traffic Classes 10

11. Traffic Classification • Each packet passed to an 802.11(e) MAC is allocated either to one of the Traffic Streams(TS) or to one of the Traffic Categories (TC) • Each station may have up to 8 Traffic Streams and 8 Traffic Categories (in each direction) in use simultaneously • Layers above the MAC specify through the MAC SAP the TS or TC that each frame belongs to 11

12. Traffic Categories • Traffic categories allow higher layers to specify different priorities for delivery (relative to other frames sent by the same station) • (The 8 ‘User Priorities’ as specified by 802.1D map to the 8 Traffic Categories in 802.11(e)) 12

13. Stream-type Traffic 13

14. Traffic Streams • For traffic that is periodic, a station may set up a Traffic Stream for that traffic • Traffic streams are allocated Transmission Opportunities (TXOPs) on a regular basis by the HC • In setting up a Traffic Stream, a station may specify any of the following: • Acknowledgement policy (no Acknowledgement, normal Acknowledgement, or Burst Acknowledgement) • Priority • Interarrival time of MSDUs, • Minimum and Mean data rate, Maximum burst size • Delay and Jitter (Delay variation) bounds 14

15. Traffic Streams (2) • Traffic Streams are set up by negotiation: • the Station sends a request to the HC to set up the Traffic Stream • the HC may accept, refuse or modify the specifications • The specifications of the Traffic Stream (called the TSPEC) are not guarantees; the HC does not guarantee that it will be able to meet the requirements, even of a TSPEC it has accepted 15

16. Prioritized Traffic - Enhanced Distributed Channel Access (EDCA) 16

17. Prioritized Traffic • Packets that do not belong to a Traffic Stream are transmitted using a protocol similar to DCF, called Enhanced Distributed Channel Access (EDCA) • We have already seen how different Inter-Frame Spaces can be used to give prioritized access (e.g. for Acknowledgements) • EDCA extends this principle to give different classes of data different priorities 17

18. EDCA (1) • 802.11 (e) defines different Access Categories. Each Access Category has a different Arbitration Inter Frame Space (AIFS), CWmin and CWmax, depending on the priority of traffic in that Access Category • Each Access Category runs the DCF protocol independently, using its appropriate parameters • A station may run up to four Access Categories • The mapping from Traffic Category to Access Category is specified in the proposed standard 18

19. EDCA (2) • Each AC acts independently as if it were a separate station • Collisions are resolved within the station (highest priority wins) • ‘Internal’ collisions have the same effect as collisions on the wireless medium 19

20. Default EDCA Parameter Set for Non-AP Stations (Based on IEEE P802.11e/D8.0, February 2004) 20

21. Default EDCA Parameter Setfor the Access Point (AP) (Based on IEEE P802.11e/D8.0, February 2004) 21

22. Default EDCA Parameter Setsfor 802.11a and 802.11b (Based on IEEE P802.11e/D8.0, February 2004) 22

23. TransmissionOpportunities (TXOP) 23

24. Transmission Opportunities • A Transmission Opportunity (TXOP) can be acquired in two ways: • A station can be allocated a Polled TXOP by the HC • An Access Category can successfully contend on the medium • In both cases, a TXOP allows a station or AC to transmit for a specified period of time • A station may transmit multiple MPDUs within a TXOP • Furthermore, it may fragment MSDU or MMPDUs as necessary in order to use up any remaining time in a TXOP • All frames within a TXOP are separated by SIFS 24

25. Polled TXOPs • Polled TXOPs are scheduled by the HC in order to satisfy the requirements of the TSPECs which it has accepted • The HC initiates a polled TXOP by transmitting a ‘QoS-Poll’ frame to a station • The Duration field in a ‘QoS-poll’ and subsequent frames is set to the remaining length of the TXOP + one slot time. • This means that the TXOP is ‘protected’ by the NAV. At the end of a polled TXOP, the HC can regain control of the channel by waiting for only a PIFS • TXOPs can start during either the CFP or CP, but must finish within that period 25

26. Controlled Access • In order to satisfy TSPECs and to deliver data it has queued, the HC may initiate a Controlled Access Period (CAP) • Similar to a CFP, a CAP may be used by the HC to transmit data or to allocate TXOPs to other stations • The HC gains control of the medium by waiting for an idle period equal to PIFS • A CAP may be started at any point, including immediately after a beacon. (Recall that a CFP can only follow a DTIM beacon) 26

27. Block Acknowledgements 27

28. Block ACKs • A Block Acknowledgement is used to acknowledge multiple MPDUs from a single originator • This allows the transmitter to send multiple fragments separated by SIFS (where an ACK would otherwise be sent) • This reduces the overhead on the Wireless medium 28

29. Block ACK Procedure • Block Acknowledgements can only be used if agreed on in advance by both endpoints • Subject to the agreement to use Burst ACKs, the procedure is as shown below: 29

30. Block ACK details • The originator may transmit using multiple TXOPs before requesting a Block ACK subject to the following restrictions: • the buffer size of the receiver, • A Block ACK can only be used to ACK MPDUs from the same Traffic Stream or Traffic Category • Two forms of Burst ACK are used: • Immediate Burst ACK (as shown in previous slide): Recipient replies to Burst ACK request immediately with Burst ACK • Delayed Burst ACK: Recipient replies to Burst ACK request with normal ACK frame; when Burst ACK is ready, it is transmitted at the next available opportunity. 30

31. Block ACK details (2) • 802.11(e) stations must maintain a sequence number counter on a per-Traffic Category/Traffic Stream, per-destination basis • The Block Ack scheme uses the Sequence Control value (the concatenation of the Sequence number and Fragment) to identify MPDUs • Block Ack requests indicate the starting Sequence number (Fragment 0) value for which acknowledgement is requested • A Block ACK indicates (using a bitmap) all subsequent MPDUs that have been received correctly 31

32. Direct Link Protocol 32

33. Direct Link Protocol – Overview • Normally, within an Infrastructure BSS, all transmissions are made via an AP • If the destination is in the same BSS as, within transmitting range of the source and not in power save mode, the wireless medium can be more efficiently by transmitting frames directly from source to destination • In order for this to occur, however, the source must be sure that the above conditions are met 33

34. Direct Link Protocol • Before direct transmissions between source and destination can begin, a handshake protocol, via the AP is used • This ensures that the appropriate conditions are met and allows stations to exchange capability (and, it is intended, security information) • (Recall that only the AP is capable of waking a station in PS-mode) • A similar handshake (again via the AP) is used to tear down the DLP 34