- ARF
  - Original WaveLAN-II (Lucent) algorithm, circa 1996
  - def: packet is new data, may have several transmissions.
  - state: current bit rate, # consecutive successful transmissions 
  - start at highest
  - "Okay" packet is one that required just one transmission.  Maintain okay_transmit_count
  - step down if no ack after num_xmits
  - Step up if 10 consecutive "okay" transmissions occur (this is
    called a probe)
  - what would ARF do on the steep links?    ++ (start at 11, fail,
    pick 5.5)
  - ..gradual links?  -- (get stuck at high rate sometimes, don't step down
    until complete failure)
  - ..lossy links?  -- (Once you step down, don't step up - get stuck
    at low rate sometimes)

  - Adaptive ARF: even on steep, sends probes.  Adapt step-up
    parameter, double every time increase bitrate and subsequent
    packet fails.
    - Will AARF do better on gradual? (no)  Lossy? (no)

- ONOE
  - Atsushi Onoe (engineer at Sony/Japan)
  - Highest bitrate with < 50% loss
    - makes sense in 802.11b: 11/2=5.5; 5.5/2 ~= 2; 2/2=1, less sense
      in 802.11a (36,24,18,12,9,6)
    - Maintain num_tx, num_tx_ok
    - Start at highest bit rate like ARF.
    
    - Each second on the wall clock:
      - if #1: if no successful transmits, step down
      - if #2: if less than 50% success rate, step down
      - if #3: if less than 10% success rate, lose credit
      - if #4: if greater than 90% success rate, gain a credit, step up
        if you have more than 10 credits.

  - what would ONOE do on the steep links? +
    1st sec. 11
    2nd sec. 5.5
    3rd sec. 5.5
    ...
    10th sec. 11
    11th sec 5.5
    12th sec 5.5

  - what would ONOE do on the gradual links? +
    1st sec. 11
    2nd sec. 5.5 (line 3)
    3rd sec. 5.5
    NO probe up to 11 periodically every 10 sec. (credits don't accum)

  - what would ONOE do on the lossy links?  --
    1st sec 11
    2nd sec 5.5
    3rd sec 2
    4th sec 1

- Measuring time to send a packet
  - Any problems with this?
    - What about backoff time before transmissions apart from the last
      one?
    - What about intervening transmissions from other nodes?
    - Not counted in this calculation, but impacts all bit rates about
      the same.

- ARF mostly chooses correct bit rate
  - x-axis: static-best throughput
  - y-axis: **hypothetical** -- take the bitrate that ARF chose most
    often, plot throughput of static-best for that bitrate
  - conclusion: ARF chose right bitrate most of the time!

  - Probing overhead of ARF:
    - (flip back to "Measring time to send a packet")
    
    Baseline rates
    ==============
    - 1 Mbit pkt time = 1500 bytes * 8 bits/byte / 1 Mbits/s = 12 ms
    - 2 Mbit pkt time = ... = 6 ms
    - 48 Mbit pkt time = ... = 250 us
    - 54 Mbit pkt time = ... = 125 us
    
    ARF
    ===
    - assume: move up every 10, fail at next bitrate, with 4 retxmits

    - backoff time for 4 retxmits = 155+315+635+1275 us ~= 2.4 ms

    - 1 Mbit: packet time for 4 retxmits > 6 ms * 4 = 24 ms
              amortized --"-- = 2.4 ms / packet
              resulting time = 12 ms + 2.4 ms ~= 15 ms/packet (20% slowdown)

    - 48 Mbit: packet time for 4 retxmits @ 54 Mbits = 125 us = 0.5 ms
                  backoff: 2.4 ms
               amortized --"-- = 290 us/packet
               resulting time = 540 us/packet (half rate)

- 802.11a throughput distribution
  - Onoe gap: Bicket attributes to lossy links

- Explaining the Onoe gap
  - Nice example of an operating principle of systems research: ask
    why, then dig deeper in the data and present to your audience why.
  - Thruput vs bit rate chosen
  - each line = one link in the network
  - Left: Onoe chooses the best bitrate
  - Right: Onoe chooses lowest bitrate, can see what it should have
    chosen

- The price of delivery confirmation
  - broadcast throughput: no acks
  - attributes it to 24 mbits/s acks on 36 mbits/s links