minstrel

Introduction
==============================================================================

This code is called minstrel, because we have taken a wandering minstrel
approach. Wander around the different rates, singing wherever you can. And
then, look at the performance, and make a choice. Note that the wandering
minstrel will always wander in directions where he/she feels he/she will get
paid the best for his/her work.

The minstrel autorate selection algorithm is an EWMA based algorithm and is
derived from

 1) An initial rate module we released in 2005,
  http://sourceforge.net/mailarchive/forum.php?forum_id=33966&max_rows=25&style=flat&viewmonth=200501&viewday=5

 2) the "sample" module in the madwifi-ng source tree.

The code released in 2005 had some algorithmic and implementation flaws (one
of which was that it was based on the old madwifi codebase) and was shown to
be unstable. Performance of the sample module is poor
(http://www.madwifi.org/ticket/989), and we have observed similar issues.

We noted:

 1) The rate chosen by sample did not alter to match changes in the radio
    environment.

 2) Higher throughput (between two nodes) could often be achieved by fixing
    the bitrate of both nodes to some value.

 3) After a long period of operation, "sample" appeared to be stuck in a low
    data rate, and would not move to a higher data rate.

We examined the code in sample, and decided the best approach was a rewrite
based on sample and the module we released in January 2005.

Theory of operation
==============================================================================

We defined the measure of successfulness (of packet transmission) as

                                  Mega bits transmitted
    Prob_success_transmission *  -----------------------
	     		              elapsed time

This measure of successfulness will therefore adjust the transmission speed to
get the maximum number of data bits through the radio interface. Further, it
means that the 1 Mbps rate (which has a very high probability of successful
transmission) will not be used in preference to the 11 Mbps rate.

We decided that the module should record the successfulness of all packets
that are transmitted. From this data, the module has sufficient information to
decide which packets are more successful than others. However, a variability
element was required. We had to force the module to examine bit rates other
than optimal. Consequently, some percent of the packets have to be sent at
rates regarded as non optimal.

10 times a second (this frequency is alterable by changing the driver code) a
timer fires, which evaluates the statistics table. EWMA calculations
(described below) are used to process the success history of each rate. On
completion of the calculation, a decision is made as to the rate which has the
best throughput, second best throughput, and highest probability of success.
This data is used for populating the retry chain during the next 100 ms.

As stated above, the minstrel algorithm collects statistics from all packet
attempts. Minstrel spends a particular percentage of frames, doing "look
around" i.e. randomly trying other rates, to gather statistics. The percentage
of "look around" frames, is set at boot time via the module parameter
"ath_lookaround_rate" and defaults to 10%. The distribution of lookaround
frames is also randomised somewhat to avoid any potential "strobing" of
lookaround between similar nodes.

TCP theory tells us that each packet sent must be delivered in under 26
ms. Any longer duration, and the TCP network layers will start to back off. A
delay of 26 ms implies that there is congestion in the network, and that fewer
packets should be injected to the device. Our conclusion was to adjust the
retry chain of each packet so the retry chain was guaranteed to be finished in
under 26 ms.

Retry Chain
==============================================================================

The HAL provides a multirate retry chain - which consists of four
segments. Each segment is an advisement to the HAL to try to send the current
packet at some rate, with a fixed number of retry attempts. Once the packet is
successfully transmitted, the remainder of the retry chain is
ignored. Selection of the number of retry attempts was based on the desire to
get the packet out in under 26 ms, or fail. We provided a module parameter,
ath_segment_size, which has units of microseconds, and specifies the maximum
duration one segment in the retry chain can last. This module parameter has a
default of 6000. Our view is that a segment size of between 4000 and 6000
seems to fit most situations.

There is some room for movement here - if the traffic is UDP then the limit of
26 ms for the retry chain length is "meaningless". However, one may argue that
if the packet was not transmitted after some time period, it should
fail. Further, one does expect UDP packets to fail in transmission. We leave
it as an area for future improvement.


The (re)try segment chain is calculated in two possible manners. If this
packet is a normal tranmission packet (90% of packets are this) then the retry
count is best throughput, next best throughput, best probability, lowest
baserate. If it is a sample packet (10% of packets are this), then the retry
chain is random lookaround, best throughput, best probability, lowest base
rate. In tabular format:

        Try | Lookaround rate    | Normal rate         
        ------------------------------------------------
         1  | Random lookaround  | Best throughput     
         2  | Best throughput    | Next best throughput
         3  | Best probability   | Best probability    
         4  | Lowest Baserate    | Lowest Baserate     

The retry count is adjusted so that the transmission time for that section of
the retry chain is less than 26 ms.

After some discussion, we have adjusted the code so that the lowest rate is
never used for the lookaround packet. Our view is that since this rate is used
for management packets, this rate must be working. Alternatively, the link is
set up with management packets, data packets are acknowledged with management
packets. Should the lowest rate stop working, the link is going to die
reasonably soon.

Analysis of information in the /proc/net/madwifi/athX/rate_info file showed
that the system was sampling too hard at some rates. For those rates that
never work (54mb, 500m range) there is no point in sending 10 sample packets
(< 6 ms time). Consequently, for the very very low probability rates, we
sample at most twice.

The retry chain above does "work", but performance is suboptimal. The key
problem being that when the link is good, too much time is spent sampling the
slower rates. Thus, for two nodes adjacent to each other, the throughput
between them was several Mbps below using a fixed rate. The view was that
minstrel should not sample at the slower rates if the link is doing
well. However, if the link deteriorates, minstrel should immediately sample at
the lower rates.

Some time later, we realised that the only way to code this reliably was to
use the retry chain as the method of determining if the slower rates are
sampled. The retry chain was modified as:

Try |         Lookaround rate              | Normal rate         
    | random < best    | random > best     |
--------------------------------------------------------------
 1  | Best throughput  | Random rate       | Best throughput     
 2  | Random rate      | Best throughput   | Next best throughput
 3  | Best probability | Best probability  | Best probability    
 4  | Lowest Baserate  | Lowest baserate   | Lowest baserate     

With this retry chain, if the randomly selected rate is slower than the
current best throughput, the randomly selected rate is placed second in the
chain. If the link is not good, then there will be data collected at the
randomly selected rate.  Thus, if the best throughput rate is currently 54
Mbps, the only time slower rates are sampled is when a packet fails in
transmission. Consequently, if the link is ideal, all packets will be sent at
the full rate of 54 Mbps. Which is good.

EWMA
==============================================================================

The EWMA calculation is carried out 10 times a second, and is run for each
rate. This calculation has a smoothing effect, so that new results have a
reasonable (but not large) influence on the selected rate. However, with time,
a series of new results in some particular direction will predominate. Given
this smoothing, we can use words like inertia to describe the EWMA.

By "new results", we mean the results collected in the just completed 100 ms
interval. Old results are the EWMA scaling values from before the just
completed 100 ms interval.

EWMA scaling is set by the module parameter ath_ewma_level, and defaults to
75%. A value of 0% means use only the new results, ignore the old results.  A
value of 99% means use the old results, with a tiny influence from the new
results.

The calculation (performed for each rate, at each timer interrupt) of the
probability of success is:

         Psucces_this_time_interval * (100 - ath_ewma_level) + (Pold * ath_ewma_level)
Pnew =  ------------------------------------------------------------------------------
                  100

                            number_packets_sent_successfully_this_rate_time_interval
Psuccess_this_time_interval=--------------------------------------------------------
                             number_packets_sent_this_rate_time_interval


If no packets have been sent for a particular rate in a time interval, no
calculation is carried out. The Psuccess value for this rate is not changed.

If the new time interval is the first time interval (the module has just been
inserted), then Pnew is calculated from above with Pold set to 0.

The appropriate update interval was selected on the basis of choosing a
compromise between

 * collecting enough success/failure information to be meaningful
 * minimising the amount of cpu time spent do the updates
 * providing a means to recover quickly enough from a bad rate selection.

The first two points are self explanatory. When there is a sudden change in
the radio environment, an update interval of 100 ms will mean that the rates
marked as optimal are very quickly marked as poor. Consequently, the sudden
change in radio environment will mean that minstrel will very quickly switch
to a better rate.

A sudden change in the transmission probabilities will happen when the
node has not transmitted any data for a while, and during that time
the environment has changed. On starting to transmit, the probability
of success at each rate will be quite different. The driver must adapt
as quickly as possible, so as to not upset the higher TCP network
layers.

Module Parameters
==============================================================================
The module has three parameters:

name              default value    purpose
ath_ewma_level      75%            rate of response to new data. A value of 100 is VERY responsive.
ath_lookaround_rate 10%            percent of packets sent at non optimal speed.
ath_segment_size   6000            maximum duration of a retry segment (microseconds)





Test Network
==============================================================================

We used three computers in our test network. The first two, equipped with
atheros cards running in adhoc mode. We used a program that sends a fixed
number of TCP packets between computers, and reports on the data rate. The
application reports on the data rate - at an application layer level, which is
considerably lower than the level used in transmitting the packets.

The third computer had an atheros card also, but was running network monitor
mode with ethereal. The monitor computer was used to see what data rates were
used on the air. This computer was a form of "logging of the connection"
without introducing any additional load on the first two computers.

It was from monitoring the results on the third computer that we started to
get some confidence in the correctness of the code. We observed TCP backoffs
(described above) on this box. There was much celebration when the throughput
increased simply because the retry chain was finished in under 26 ms.

Our view was that throughput between the two computers should be as close as
possible (or better than) what can be achieved by setting both ends to fixed
rates. Thus, if setting the both ends to fixed rates significantly increases
the throughput, a reasonable conclusion is that a fault exists in the adaptive
rate rate.

We recorded throughputs (with minstrel) that are within 10% of what is
achieved with the experimentally determined optimum fixed rate.


Notes on Timing
==============================================================================

As noted above, minstrel calculates the throughput for each rate. This
calculation (using a packet of size 1200 bytes) determines the transmission
time on the radio medium. In these calculations, we assume a contention window
min and max value of 4 and 10 microseconds respectively.

Further, calculation of the transmission time is required so that we can
guarantee a packet is transmitted (or dropped) in a minimum time period.  The
transmission time is used in determining how many times a packet is
transmitted in each segment of the retry chain.

Indeed, the card will supply the cwmin/cwmax values directly
 iwpriv if_name get_cwmin <0|1|2|3> <0|1>

We have not made direct calls to determine cwmin/cwmax - this is an area for
future work. Indeed, the cwmin/cwmax determination code could check to see if
the user has altered these values with the appropriate iwpriv.

The contention window size does vary with traffic class. For example, video
and voice have a contention window min of 3 and 2 microseconds
respectively. Currently, minstrel does not check traffic class.

Calculating the throughputs based on traffic class and bit rate and variable
packet size will significantly complicate the code and require many more
sample packets. More sample packets will lower the throughput achieved. Thus,
our view is that for this release, we should take a simple (but reasonable)
approach that works stably and gives good throughputs.


Values of cwin/cwmax of 4 and 10 microseconds are from 
IEEE Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer
(PHY) specifications, Amendment : Medium Access Control (MAC) Enhancements for
Quality of Service (QoS) P802.11e/D12.0, November 2004.


Internal variable reporting
==============================================================================

The minstrel algorithm reports to the proc file system its internal
statistics, which can be viewed as text. A sample output is below:

cat /proc/net/madwifi/ath0/rate_info


rate data for node:: 00:02:6f:43:8c:7a
rate     throughput  ewma prob   this prob  this succ/attempt   success    attempts
     1         0.0        0.0        0.0          0(  0)          0           0
     2         1.6       92.4      100.0          0(  0)         11          11
     5.5       3.9       89.9      100.0          0(  0)         11          11
    11         6.5       86.6      100.0          0(  0)         10          11
     6         4.8       92.4      100.0          0(  0)         11          11
     9         6.9       92.4      100.0          0(  0)         11          11
    12         9.0       92.4      100.0          0(  0)         11          11
    18        12.1       89.9      100.0          0(  0)         11          11
    24        15.5       92.4      100.0          0(  0)         11          11
    36        20.6       92.4      100.0          0(  0)         11          11
 t  48        23.6       89.9      100.0          0(  0)         12          12
T P 54        27.1       96.2       96.2          0(  0)        996        1029


Total packet count::    ideal 2344      lookaround 261

There is a separate table for each node in the neighbor table, which will
appear similar to above.

The possible datarates for this node are listed in the column at the left. The
calculated throughput and "ewma prob" are listed next, from which the rates
used in retry chain are selected. The rates with the maximum throughput,
second maximum throughput and maximum probability are indicated by the letters
T, t, and P respectively.

The statistics gathered in the last 100 ms time period are displayed in the
"this prob" and "this succ/attempt" columns.

Finally, the number of packets transmitted at each rate, since module loading
are listed in the last two columns. When interpreting the last four columns,
note that we use the words "succ" or "success" to mean packets successfully
sent from this node to the remote node. The driver determines success by
analysing reports from the hal. The word "attempt" or "attempts" means the
count of packets that we transmitted. Thus, the number in the success column
will always be lower than the number in the attempts column.

When the two nodes are brought closer together, the statistics starts
changing, and you see more successful attempts at the higher rates. The ewma
prob at the higher rates increases and then most packets are conveyed at the
higher rates.

When the rate is not on auto, but fixed, this table is still available, and
will report the throughput etc for the current bit rate. Changing the rate
from auto to fixed to auto will completely reset this table, and the operation
of the minstrel module.
