Wireless MAC

Puzzle Two twins A & B A always speaks the truth, and believes all true propositions (say 2+2=4) to be true, and all false propositions (say 2+2=3) to be false B always lies, and believes all true propositions to be false, and all false propositions to be true You meet one of the twins. How many questions do you need to identify which twin he is?

Puzzle Two twins A & B A always speaks the truth, and believes all true propositions (say 2+2=4) to be true, and all false propositions (say 2+2=3) to be false B always lies, and believes all true propositions to be false, and all false propositions to be true You meet one of the twins. How many questions do you need to identify which twin he is? Are you the truth speaker

MACA Recap No carrier sensing Request-to-send (RTS), Clear-to-send (CTS) exchange to solve hidden terminal problem RTS-CTS-DATA exchange for every transmission

MACAW Based on MACA Design based on 4 key observations: Contention is at receiver, not the sender Congestion is location dependent To allocate media fairly, learning about congestion levels should be a collective enterprise Media access protocol should propagate synchronization information about contention periods, so that all devices can contend effectively

Back-off Algorithm MACA uses binary exponential back-off (BEB) BEB: back-off counter doubles after every collision and reset to minimum value after successful transmission Unfair channel allocation! Example simulation result: 2 stations A & B communicating with base-station Both have enough packets to occupy entire channel capacity A gets 48.5 packets/second, B gets 0 packets/second

BEB Unfairness Since successful transmitters reset back-off counter to minimum value Hence, it is more likely that successful transmitters continue to be successful Theoretically, if there is no maximum back-off, one station can get the entire channel bandwidth Ideally, the back-off counter should reflect the ambient congestion level which is the same for all stations involved!

BEB with Copy MACAW uses BEB with Copy Packet header includes the BEB value used by transmitter When a station overhears a packet, it copies the BEB value in the packet to its BEB counter Thus, after each successful transmission, all stations will have the same backoff counter Example simulation result (same setting as before: A gets 23.82 packets/second, B gets 23.32 packets/second

MILD adaptation Original back-off scheme uses BEB upon collision, and resetting back-off to minimum value upon success Large fluctuations in back-off value Why is this bad? MACAW uses a multiplicative increase and linear decrease (MILD) scheme for back-off adaptation (with factors of 1.5 and 1 respectively)

Accommodating Multiple Streams If A has only one queue for all streams (default case), bandwidth will be split as AB:1/4, AC:1/4, DA:1/2 Is this fair? Maintain multiple queues at A, and contend as if there are two co-located nodes at A A B C D

Other modifications (ACK) ACK packet exchange included in addition to RTS-CTS-DATA Handle wireless (or collision) errors at the MAC layer instead of waiting for coarse grained transport (TCP) layer retransmission timeouts For a loss rate of 1%, 100% improvement in throughput demonstrated over MACA

Other modifications (DS) In the exposed terminal scenario (ABCD with B talking to A), C cannot talk to D (because of the ACK packet introduced) What if the RTS/CTS exchange was a failure? How does C know this information? A new packet DS (data send) included in the dialogue: RTS-CTS-DS-DATA-ACK DS informs other stations that RTS-CTS exchange was successful

Other modifications (RRTS) Request to Request to Send Consider a scenario: A – B – C – D D is talking to C A sends RTS to B. However, B does not respond as it is deferring to the D-C transmission A backs-off (no reply to RTS) and tries later In the meantime if another D-C transmission begins, A will have to backoff again

RRTS (contd.) The only way A will get access to channel is if it comes back from a back-off and exactly at that time C-D is inactive (synchronization constraint!) Note that B can hear the RTS from A! When B detects the end of current D-C transmission (ACK packet from C to D), it sends an RRTS to A, and A sends RTS

MACAW Recap Backoff scheme New control packets BEB with Copy MILD Multiple streams New control packets ACK DS RRTS Other changes (see paper)

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 Prison with 62 prisoners. Warden gives prisoners the following option: Prisoners will all be isolated in cells (they cannot talk to each other) Warden will pick a prisoner at random and walk them to a room with a switch Switch is initially in the OFF position (and not connected to anything) Prisoner can choose to toggle the switch or leave it in its current state Prisoner can also choose to make the statement “All 62 prisoners have visited the room” If the prisoner is correct, all prisoners go free. Otherwise, they spend the rest of their lives in prison Prisoners can strategize before being isolated What is the strategy?