.\"     $NetBSD: rssadapt.9,v 1.5 2006/02/25 01:04:26 wiz Exp $
.\"
.\" Copyright (c) 2004 David Young.  All rights reserved.
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.\" This code is by David Young.
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.Dd March 23, 2004
.Dt RSSADAPT 9
.Os
.Sh NAME
.Nm rssadapt ,
.Nm ieee80211_rssadapt_choose ,
.Nm ieee80211_rssadapt_input ,
.Nm ieee80211_rssadapt_lower_rate ,
.Nm ieee80211_rssadapt_raise_rate ,
.Nm ieee80211_rssadapt_updatestats
.Nd rate adaptation based on received signal strength
.Sh SYNOPSIS
.In net80211/ieee80211_var.h
.In net80211/ieee80211_rssadapt.h
.Ft void
.Fn ieee80211_rssadapt_input "struct ieee80211com *ic" \
"struct ieee80211_node *ni" "struct ieee80211_rssadapt *ra" "int rssi"
.Ft void
.Fn ieee80211_rssadapt_lower_rate "struct ieee80211com *ic" \
"struct ieee80211_node *ni" "struct ieee80211_rssadapt *ra" \
"struct ieee80211_rssdesc *id"
.Ft void
.Fn ieee80211_rssadapt_raise_rate "struct ieee80211com *ic" \
"struct ieee80211_rssadapt *ra" "struct ieee80211_rssdesc *id"
.Ft void
.Fn ieee80211_rssadapt_updatestats "struct ieee80211_rssadapt *ra"
.Ft int
.Fn ieee80211_rssadapt_choose "struct ieee80211_rssadapt *ra" \
"struct ieee80211_rateset *rs" "struct ieee80211_frame *wh" "u_int len" \
"int fixed_rate" "const char *dvname" "int do_not_adapt"
.Sh DESCRIPTION
The
.Nm
module provides rapid adaptation of transmission data rate to 802.11
device drivers based on received-signal strength
.Pq RSS .
A driver needs only to provide
.Nm
with indications of RSS and failure/success of transmissions for
each 802.11 client or peer.
For each transmit packet,
.Nm
chooses the transmission data rate that offers the best expected
throughput, given the packet's length and destination.
.Pp
.Nm
models an 802.11 channel very simply
.Po
see also the
.Sx BUGS
section
.Pc .
It assumes that the packet-error rate
.Pq PER
is determined by the signal-to-noise ratio
.Pq S/N
at the receiver, the transmission data rate, and the packet length.
The S/N determines the choice of data rate that yields the lowest
PER for all packets of a certain length.
.Sh FUNCTIONS
.Bl -tag -width 18n -compact
.It Fn ieee80211_rssadapt_choose "ra" "rs" "wh" "len" "fixed_rate" "dvname" \
"do_not_adapt"
Choose the transmission data rate for a packet.
.Bl -tag -width "do_not_adapt" -compact
.It Fa ra
Ordinarily, the
.Nm
state object belonging to the node which is the packet destination.
However, if the destination is a broadcast/multicast address, then
.Fa ra
belongs to the BSS node,
.Va ic-\*[Gt]ic_bss .
.It Fa rs
A list of eligible data rates for the node; for example, the
rates negotiated when the node associated with the network.
.It Fa len
The packet length in bytes, including the 802.11 header and
frame check sequence
.Pq FCS .
.It Fa fixed_rate
If the operator has set the data rate using, for example,
.Ic "ifconfig wi0 media ds1" ,
then
.Fa fixed_rate
tells the index of that rate in
.Fa rs .
.Nm
obeys a fixed data rate whenever the 802.11 standard allows it:
sometimes the standard requires multicast/broadcast packets to be
transmitted at a so-called
.Dq basic rate .
.It Fa dvname
The device driver uses
.Fa dvname
to indicate the name of the
interface for the purpose of diagnostic and debug messages.
The driver sets
.Fa dvname
to
.Dv NULL
when no messages are desired.
.It Fa do_not_adapt
If
.Fa do_not_adapt
is non-zero, then
.Fn ieee80211_rssadapt_choose
will choose the highest rate in
.Fa rs
that suits the destination, regardless of the RSS.
.El
The return value of
.Fn ieee80211_rssadapt_choose
is an index into
.Fa rs ,
indicating its choice of transmit data rate.
.It Fn ieee80211_rssadapt_input "ic" "ni" "ra" "rssi"
The RSS serves as a rough estimate of the S/N at each node.
A driver provides RSS updates using
.Fn ieee80211_rssadapt_input ,
whose arguments are:
.Bl -tag -width "rssi" -compact
.It Fa ic
The wireless interface's 802.11 state object.
.It Fa ni
The 802.11 node whose RSS the driver is updating.
.It Fa ra
The node's
.Nm
state object.
.It Fa rssi
The node's received signal strength indication.
The range of
.Fa rssi
is from 0 to 255.
.El
.It Fn ieee80211_rssadapt_lower_rate "ic" "ni" "ra" "id"
.It Fn ieee80211_rssadapt_raise_rate "ic" "ra" "id"
Drivers call
.Fn ieee80211_rssadapt_raise_rate
and
.Fn ieee80211_rssadapt_lower_rate
to indicate transmit successes and failures, respectively.
.Bl -tag -width "ni" -compact
.It Fa ic
The 802.11 state object.
.It Fa ni
The neighbor to whom the driver transmitted.
.It Fa ra
The neighbor's
.Nm
state object.
.It Fa id
DIsplays statistics on the transmission attempt.
.El
.It Fn ieee80211_rssadapt_updatestats "ra"
An 802.11 node is eligible for its RSS thresholds to decay every
1/10 to 10 seconds.
It is eligible more often (every 1/10 second) at high packet rates,
and less often (every 10 seconds) at low packet rates.
A driver assists
.Nm
in tracking the exponential-average packet rate by calling
.Fn ieee80211_rssadapt_updatestats
every 1/10th second for each node's
.Vt ieee80211_rssadapt
object.
.Bl -tag -width "ra" -compact
.It Fa ra
The neighbor's
.Nm
state object.
.El
.El
.Sh ALGORITHM
.Nm
monitors the RSS from neighboring 802.11 nodes, recording the
exponential average RSS in each neighbor's
.Vt ieee80211_rssadapt
structure.
.Nm
uses transmit success/failure feedback from the
device driver to fill a table of RSS thresholds.
The table is indexed by packet size,
.Va L ,
and a data rate,
.Va R ,
to find out the minimum exponential-average RSS that a node must show before
.Nm
will indicate that a packet
.Va L
bytes long can be transmitted R bits per second with optimal expected
throughput.
When the driver indicates a unicast packet is transmitted unsuccessfully
.Po
that is, the NIC received no ACK for the packet
.Pc ,
.Nm
will move the corresponding RSS threshold toward the exponential
average RSSI at the time of transmission.
Thus several consecutive transmit failures for the same
.Ao
.Va L ,
.Va R
.Ac
tuple will ensure that the RSS threshold rises high enough that
rate
.Va R
is abandoned for packets
.Va L
bytes long.
When the driver indicates a successful transmission,
the RSS threshold corresponding to the same packet length, but the
next higher data rate, is lowered slightly.
The RSS threshold is said to
.Dq decay .
This ensures that occasionally
.Nm
indicates the driver should try the next higher data rate,
just in case conditions at the receiver have changed
.Po
for example, noise levels have fallen
.Pc
and a higher data rate can be supported at the same RSS level.
.Pp
The rate of decay is controlled.
In an interval of 1/10th second
to 10 seconds, only one RSS threshold per neighbor may decay.
The interval is connected to the exponential-average rate that packets
are being transmitted.
At high packet rates, the interval is shortest.
It is longest at low packet rates.
The rationale for this is that RSS thresholds should not decay
rapidly if there is no information from packet transmissions to
counteract their decay.
.Sh DATA STRUCTURES
An
.Vt ieee80211_rssdesc
describes a transmission attempt.
.Pp
.Bd -literal -offset indent
struct ieee80211_rssdesc {
        u_int                    id_len;
        u_int                    id_rateidx;
        struct ieee80211_node   *id_node;
        u_int8_t                 id_rssi;
};
.Ed
.Pp
.Fa id_len
is the length, in bytes, of the transmitted packet.
.Fa id_node
points to the neighbor's
.Vt ieee8021_node ,
and
.Fa id_rssi
is the exponential-average RSS at the time the packet was
transmitted.
.Fa id_rateidx
is an index into the destination-neighbor's rate-set,
.Fa id_node-\*[Gt]ni_rates ,
indicating the transmit data rate for the packet.
.Pp
An
.Vt ieee80211_rssadapt
contains the rate-adaptation state for a neighboring 802.11 node.
Ordinarily a driver will
.Dq subclass
.Vt ieee80211_node .
The
.Vt ieee80211_rssadapt
structure will be a subclass member.
In this way, every node's
.Nm
condition is independently tracked and stored in its node object.
.Pp
.Bd -literal -offset indent
struct ieee80211_rssadapt {
        u_int16_t               ra_avg_rssi;
        u_int32_t               ra_nfail;
        u_int32_t               ra_nok;
        u_int32_t               ra_pktrate;
        u_int16_t               ra_rate_thresh[IEEE80211_RSSADAPT_BKTS]
                                              [IEEE80211_RATE_SIZE];
        struct timeval          ra_last_raise;
        struct timeval          ra_raise_interval;
};
.Ed
.Pp
.Fa ra_avg_rssi
is the exponential-average RSS, shifted left 8 bits.
.Fa ra_nfail
tells the number of transmit failures in the current update interval.
.Fa ra_nok
tells the number of transmit successes in the current update interval.
.Fa ra_pktrate
tells the exponential average number of transmit failure/success
indications over past update intervals.
This approximates the rate of packet-transmission.
.Fa ra_rate_thresh
contains RSS thresholds that are indexed by
.Aq "packet length, data rate"
tuples.
When this node's exponential-average RSS exceeds
.Fa ra_rate_thresh[i][j] ,
then packets at most 128 x 8^i bytes long are eligible to be
transmitted at the rate indexed by j.
.Fa ra_last_raise
and
.Fa ra_raise_interval
are used to control the rate that RSS thresholds
.Dq decay .
.Fa ra_last_raise
indicates when
.Fn ieee80211_rssadapt_raise_rate
was last called.
.Fa ra_raise_interval
tells the minimum period between consecutive calls to
.Fn ieee80211_rssadapt_raise_rate .
If
.Fn ieee80211_rssadapt_raise_rate
is called more than once in any period, the second and subsequent
calls are ignored.
.Sh CODE REFERENCES
This section describes places within the
.Nx
source tree where actual code implementing or using
.Nm
can be found.
All pathnames are relative to
.Pa /usr/src .
.Pp
The code for
.Nm
is in the file
.Pa sys/net80211/ieee80211_rssadapt.c .
.Pp
.Xr wi 4
contains a reference implementation.
See
.Pa sys/dev/ic/wi.c .
.Sh SEE ALSO
.Xr wi 4
.Rs
.%A Javier del Prado Pavon
.%A Sunghyun Choi
.%T "Link Adaptation Strategy for IEEE 802.11 WLAN via Received Signal \
Strength Measurement"
.%J "ICC'03"
.%P pp. 1108-1113
.%C Anchorage, Alaska
.%D May 2003
.Re
.Sh HISTORY
.Nm
first appeared in
.Nx 3.0 .
.Sh AUTHORS
.An David Young Aq dyoung@NetBSD.org
.Sh BUGS
To cope with interference from microwave ovens, frequency-hopping
radios, and other sources of RF pulse-trains and bursts,
.Nm
should adapt the fragmentation threshold as well as the data rate.
.Pp
For improved throughput,
.Nm
should indicate to drivers when they should use the 802.11b
short-preamble.
.Pp
The constants in
.Fn ieee80211_rssadapt_updatestats
should be configurable.