1. Wireless MAC

2. 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?

3. 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!

4. 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)

5. 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

6. 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

7. 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

8. Power Saving Mode (PS)
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

9. 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 ( octets)
FCS (4 octets)

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

11. 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 …

12. 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

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

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

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

16. 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!

17. 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?

18. 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?

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

20. 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

21. 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.

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

23. 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)

24. 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

25. 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

26. 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)

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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?