1. Wireless MAC

2. 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?

3. 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

4. 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

5. 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

6. 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

7. 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!

8. 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 packets/second, B gets packets/second

9. 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)

10. 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

11. 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

12. 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

13. 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

14. 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

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

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

17. 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 …

18. 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

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

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

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

22. 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!

23. 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?

24. 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?

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

26. 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

27. 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.

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

29. 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)

30. 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

31. 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

32. 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)

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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?