Wireless MAC

Puzzle Doors numbered 1-100 All doors initially open Toggle switch outside every door If switch is pressed, door will close if it is currently open, and open if it is currently closed For i=1 to 100, you press switches of doors that are multiples of i Which doors are closed at the end of the process?

Interframe Spacing 802.11 uses 4 different interframe spacings Interframe spacing plays a large role in coordinating access to the transmission medium Varying interframe spacings create different priority levels for different types of traffic!

Types of IFS SIFS DIFS Short interframe space Used for highest priority transmissions – RTS/CTS frames and ACKs DIFS DCF interframe space Minimum idle time for contention-based services (> SIFS)

Types (contd.) PIFS EIFS PCF interframe space Minimum idle time for contention-free service (>SIFS, <DIFS) EIFS Extended interframe space Used when there is an error in transmission

Point Coordination Function Phase during which AP controls all transmissions explicitly AP can take control of the medium by transmitting a special beacon Beacon has information about how long the AP wants to operate in PCF mode Stations set their NAV accordingly

PCF (contd.) AP issues a poll message to stations in a particular order Stations can respond with data in response to the poll message AP can also send data piggybacked with the poll message

Power Saving Mode (PS) 802.11 stations can maximize battery life by shutting down the radio transceiver and sleeping periodically During sleeping periods, access points buffer any data for sleeping stations The data is announced by subsequent beacon frames To retrieve buffered frames, newly awakened stations use PS-poll frames Access point can choose to respond immediately with data or promise to delivery it later

IEEE 802.11 MAC Frame Format Overall structure: Frame control (2 octets) Duration/ID (2 octets) Address 1 (6 octets) Address 2 (6 octets) Address 3 (6 octets) Sequence control (2 octets) Address 4 (6 octets) Frame body (0-2312 octets) FCS (4 octets)

Wireless Fair Queuing Wireless channel capacities are scarce Fair sharing of bandwidth becomes critical Both short-term and long-term fairness important

Wireless FQ & Wireless Environment Location dependent and bursty errors For the same wireless channel, a mobile station might experience a clean channel while another might experience high error rates. Why? In wireline fair queuing, the channel is either usable by all flows or unusable by any of the flows …

Wireless Channel Model Base station performs arbitration Schedules both uplink and downlink traffic Neighboring cells use different channels Every mobile host has access to base-station

Wireless Channel Characteristics Dynamically varying capacity Location dependent channel errors and bursty errors Contention No global state Scarce resources (battery & processing power)

Service Model Short term fairness Long term fairness Short term throughput bounds Long term throughput bounds Delay bounds for packets

Some terminology … Error free service Leading flows Lagging flows In sync flows

Impact of Location Dependent Errors Example 1 3 flows f1, f2, f3 Period 1: f3 experiences lossy channel Flows f1 and f2 receive ½ of channel Period 2: f3 experiences clear channel Wireline fair queuing would give a net service of 5/6 to f1 and f2, and 1/3 to f3 – UNFAIR! Wireline fair queuing does not distinguish between flows that are not backlogged and flows that are backlogged but cannot transmit!

Impact (Contd.) Example 2 Same scenario Flow f1 has only 1/3 offered service Hence, for period 1 f2 receives 2/3 service If some compensation is given to f3 during period 2, should f1 be penalized for compensating f3?

Issues addressed by Wireless Fair Scheduling Is it acceptable to compromise on separation for f1? How soon should f3 get its share back? Should f2 give up service and over what period of time?

Generic Wireless FS Model Error free service Lead/lag/in-sync Compensation model Channel monitoring and prediction

Error Free Service Reference for how much service a flow should receive in an ideal error free channel Example: WFQ Each packet stamped with a finish tag based upon the packet’s arrival time and the weight of the flow Packet with the minimum finish tag transmitted

Lead and lag model Lag Lead Two approaches Lag of flow incremented as long as the flow is backlogged and is unable to transmit. Such a flow will be compensated at a later time. Lag of flow incremented only if the slot given up by the flow is taken up by another flow (which will have its lead incremented). At a later time, compensation will be given at the expense of a flow with lead.

Compensation Model No explicit compensation Flow with maximum lag is given preference Leading and lagging flows swap slots Bandwidth is reserved for compensation

Instantiations Channel state dependent packet scheduling (CSDPS) Idealized wireless fair queuing (IWFQ) Wireless packet scheduling (WPS) Channel-condition independent fair queuing (CIFQ) CBQ-CSDPS Server based fairness approach (SBFA) Wireless fair service (WFS)

CSDPS CSDPS allows for the use of any error-free scheduling discipline – e.g. WRR with WFQ spread When a flow is allocated a slot and is not able to use it, CSDPS skips that flow and serves the next flow No measurement of lag or lead No explicit compensation model

CSDPS (Contd.) Lagging flows can thus make up lags only when leading flows cease to become backlogged or experience lossy channels sometime No long-term or short-term fairness guarantees

IWFQ WFQ is used for the error free service Packets tagged as in WFQ. Of the flows observing a clean channel, the flow with the minimum service tag packet is served Tags implicitly capture the service differences between flows (lagging flows will have a smaller service and hence will be scheduled earlier)

IWFQ (Contd.) Channel capture by lagging flows possible resulting in short term unfairness and starvation Even in-sync flows can become lagging during such capture periods Coarse short-term fairness guarantees because of possible starvation Provides long-term fairness

WPS WRR with WFQ spread used for error free service A frame of slot allocations generated by WPS based on WRR (with WFQ spread) Intra frame swapping attempted when a flow is unable to use a slot If intra-frame swapping is not possible lag incremented as long as another flow can use the slot

WPS (Contd.) At the beginning of next frame, weights for calculating spread readjusted to accommodate lag and lead If intra-frame swapping succeeds most of the time, in-sync flows not affected Complete channel capture prevented as each flow has a non-zero weight when frame spread is calculated No short-term fairness guarantees, but provides long-term fairness

CIFQ STFQ (Start time fair queuing) used for the error free service Lag or lead computed as the difference between the actual service and the error free service A backlogged leading flow relinquishes slot with a probability p, a system parameter A relinquished slot is allocated to the lagging flow with the maximum normalized lag

CIFQ (Contd.) In-sync flows not affected since lagging flows use slots given up by leading flows Lagging flows can still starve leading flows under pathological scenarios Provides both short-term and long-term fairness

CBQ-CSDPS Same as IWFQ except that no explicit error free service is maintained Rather, lead/lag is measured based on the actual number of bytes s transmitting during each time window A flow with normalized rate r is leading if it has received channel allocation in excess of s*r, and lagging if it has received channel allocation less than s*r Lagging flows are allowed precedence

CBQ-CSDPS Same problem as in IWFQ – lagging flows given precedence, and hence can capture channel Short term fairness is thus not guaranteed Additionally, leads and lags are computed not based on error-free service, but based on a time window of measurement … performance sensitive to the time window

SBFA Any error free service model can be used SBFA reserves a fraction of the channel bandwidth statically for compensation by specifying a virtual compensation flow When a flow is unable to use a slot, it queues a slot-request to the compensation flow Scheduler serves compensation flow just as other flows When the compensation flow gets a slot, it turns the slot over to the flow represented by the head-of-line slot-request

SBFA (Contd.) Scheduled to Tx F1 Cannot transmit because of error Slot queued into compensation flow Cannot transmit because of error Compensation Flow of weight w Slot scheduled for Tx and handed over to F1

SBFA (Contd.) No concept of a leading flow All bounds supported by SBFA are only with respect to the remaining fraction of the channel bandwidth Performance of SBFA is sensitive to the statically reserved fraction No short-term fairness Long-term fairness dependent upon the reserved fraction

Wireless Fair Service Uses an enhanced version of WFQ in order to support delay-bandwidth decoupling Lag of a flow incremented only if there is a flow that can use the slot Both lead and lag are bounded by per-flow parameters A leading flow with a lead of L and a lead bound of Lmax relinquishes a fraction L/Lmax of the slots allocated to it by the error-free service This results in an exponential reduction in the number of slots relinquished

WFS (Contd.) Service degradation is graceful for leading flows In-sync flows are not affected Tightest short-term fairness among all algorithms discussed Compensation for lagging flows can take up more time than other algorithms

Recap Wireless Fair Scheduling Why wireline algorithms cannot be used Key components of a a wireless fair scheduling algorithm Different approaches for wireless fair scheduling

Puzzle You have a deck of 52 cards You draw out 5 cards randomly and look at the cards You can now show 4 of the cards to a friend, and the friend should identify the 5th card How do you do this?