NAME

rssadapt, ieee80211_rssadapt_choose, ieee80211_rssadapt_input, ieee80211_rssadapt_lower_rate, ieee80211_rssadapt_raise_rate, ieee80211_rssadapt_updatestats — rate adaptation based on received signal strength

SYNOPSIS

#include <net80211/ieee80211_var.h>
#include <net80211/ieee80211_rssadapt.h>

void
ieee80211_rssadapt_input(struct ieee80211com *ic, struct ieee80211_node *ni, struct ieee80211_rssadapt *ra, int rssi);

void
ieee80211_rssadapt_lower_rate(struct ieee80211com *ic, struct ieee80211_node *ni, struct ieee80211_rssadapt *ra, struct ieee80211_rssdesc *id);

void
ieee80211_rssadapt_raise_rate(struct ieee80211com *ic, struct ieee80211_rssadapt *ra, struct ieee80211_rssdesc *id);

void
ieee80211_rssadapt_updatestats(struct ieee80211_rssadapt *ra);

int
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);

DESCRIPTION

The rssadapt module provides rapid adaptation of transmission data rate to 802.11 device drivers based on received-signal strength (RSS). A driver needs only to provide rssadapt with indications of RSS and failure/success of transmissions for each 802.11 client or peer. For each transmit packet, rssadapt chooses the transmission data rate that offers the best expected throughput, given the packet's length and destination.

rssadapt models an 802.11 channel very simply (see also the BUGS section). It assumes that the packet-error rate (PER) is determined by the signal-to-noise ratio (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.

FUNCTIONS

ieee80211_rssadapt_choose(ra, rs, wh, len, fixed_rate, dvname, do_not_adapt)

Choose the transmission data rate for a packet. The return value of ieee80211_rssadapt_choose() is an index into rs, indicating its choice of transmit data rate.

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 ieee80211_rssadapt_input(), whose arguments are:

ieee80211_rssadapt_lower_rate(ic, ni, ra, id)

ieee80211_rssadapt_raise_rate(ic, ra, id)

Drivers call ieee80211_rssadapt_raise_rate() and ieee80211_rssadapt_lower_rate() to indicate transmit successes and failures, respectively.

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 rssadapt in tracking the exponential-average packet rate by calling ieee80211_rssadapt_updatestats() every 1/10th second for each node's ieee80211_rssadapt object.

ALGORITHM

rssadapt monitors the RSS from neighboring 802.11 nodes, recording the exponential average RSS in each neighbor's ieee80211_rssadapt structure. rssadapt uses transmit success/failure feedback from the device driver to fill a table of RSS thresholds. The table is indexed by packet size, L, and a data rate, R, to find out the minimum exponential-average RSS that a node must show before rssadapt will indicate that a packet L bytes long can be transmitted R bits per second with optimal expected throughput. When the driver indicates a unicast packet is transmitted unsuccessfully (that is, the NIC received no ACK for the packet), rssadapt will move the corresponding RSS threshold toward the exponential average RSSI at the time of transmission. Thus several consecutive transmit failures for the same <L, R> tuple will ensure that the RSS threshold rises high enough that rate R is abandoned for packets 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 “decay”. This ensures that occasionally rssadapt indicates the driver should try the next higher data rate, just in case conditions at the receiver have changed (for example, noise levels have fallen) and a higher data rate can be supported at the same RSS level.

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.

DATA STRUCTURES

An ieee80211_rssdesc describes a transmission attempt.

struct ieee80211_rssdesc { 
        u_int                    id_len; 
        u_int                    id_rateidx; 
        struct ieee80211_node   *id_node; 
        u_int8_t                 id_rssi; 
};

id_len is the length, in bytes, of the transmitted packet. id_node points to the neighbor's ieee8021_node, and id_rssi is the exponential-average RSS at the time the packet was transmitted. id_rateidx is an index into the destination-neighbor's rate-set, id_node->ni_rates, indicating the transmit data rate for the packet.

An ieee80211_rssadapt contains the rate-adaptation state for a neighboring 802.11 node. Ordinarily a driver will “subclass” ieee80211_node. The ieee80211_rssadapt structure will be a subclass member. In this way, every node's rssadapt condition is independently tracked and stored in its node object.

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; 
};

ra_avg_rssi is the exponential-average RSS, shifted left 8 bits. ra_nfail tells the number of transmit failures in the current update interval. ra_nok tells the number of transmit successes in the current update interval. ra_pktrate tells the exponential average number of transmit failure/success indications over past update intervals. This approximates the rate of packet-transmission. ra_rate_thresh contains RSS thresholds that are indexed by <packet length, data rate> tuples. When this node's exponential-average RSS exceeds 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. ra_last_raise and ra_raise_interval are used to control the rate that RSS thresholds “decay”. ra_last_raise indicates when ieee80211_rssadapt_raise_rate() was last called. ra_raise_interval tells the minimum period between consecutive calls to ieee80211_rssadapt_raise_rate(). If ieee80211_rssadapt_raise_rate() is called more than once in any period, the second and subsequent calls are ignored.

CODE REFERENCES

The code for rssadapt is in the file sys/net80211/ieee80211_rssadapt.c.

wi(4) contains a reference implementation. See sys/dev/ic/wi.c.

HISTORY

rssadapt first appeared in NetBSD 3.0.

AUTHORS

David Young <dyoung@NetBSD.org>

BUGS

To cope with interference from microwave ovens, frequency-hopping radios, and other sources of RF pulse-trains and bursts, rssadapt should adapt the fragmentation threshold as well as the data rate.

For improved throughput, rssadapt should indicate to drivers when they should use the 802.11b short-preamble.

The constants in ieee80211_rssadapt_updatestats() should be configurable.