Proportional Pathing Function - Kenwood TM-255A Product Manual

6 APRS IN ACTION
6.1.2 Decay Algorithm Function
The position beacon for a mobile station is usually transmitted at a fixed time interval to provide
consistency in vehicle tracking and station participation. However, when traffic is slow-moving, it
would be inefficient use of air-time to continue transmitting at this fixed interval. Therefore, while
parked or moving slowly, the beacon transmission interval gradually increases by using a decay
algorithm. This cleverly reduces air-time used by the station without reducing the quality of station
information being made available to the system.
Transmits the
position beacon
Decay Algorithm steps the transmission interval of the beacon down from 1 to 2 to 4 to 8 to 16 to
32 minutes when the mobile station is not moving. The speed at which Decay Algorithm decides
that the car is parked can be set using the Stopped dropdown list in the Page 7 tab of the APRS/
NAVITRA window of the MCP-2A programming software. If the mobile station's speed is less
than this setting, then Decay Algorithm will start.

6.1.3 Proportional Pathing Function

Proportional pathing recognizes that the value and timeliness of local information is more
important close to the sender and of less importance farther out in the network. To minimize
loading on the network from distant stations, the TM-D710A/E implements Proportional Pathing to
maintain a high update rate for local packets, but divides that rate by two at each additional hop
through the network as shown in the diagram below. If the transmit rate is set to 1 minute, then
locally, the packet will be seen every minute. But this will only be seen every 2 minutes via the
local digipeater. It will only be seen once every 4 minutes via surrounding two hops. This
algorithm drastically reduces the APRS netowrk load, while still providing good tracking to local
mobiles.
Transmits
every 1 minute
DIRECT WIDE1-1
(No hop) (1 hop)
Figure 6-2 Proportional Pathing adjusts Number of Relays (when using WIDE1-1,WIDE2-2)
To alleviate this problem, Proportional Pathing automatically changes the number of packet relay
hops every time the beacon is transmitted. The effect of this is that nearby stations will receive
updates frequently while more distant stations will receive half as many updates. The speed at
which Proportional Pathing decides that the car is moving can be set using the Moving dropdown
list in the Page 7 tab of the APRS/ NAVITRA window of the MCP-2A programming software. If
the mobile station's speed exceeds this setting, then Proportional Pathing will start.
20
CONTENTS
Traffic Jam!
1 minute 2 minutes
Figure 6-1 Decay Algorithm
Transmits Transmits
every 1 minute every 1 minute every 1 minute
DIRECT WIDE1-1,WIDE2-1
(No hop) (2 hops)
4 minutes 8 minutes 16 minutes
Transmits Transmits Transmits
every 1 minute every 1 minute
DIRECT WIDE1-1
(No hop) (1 hop)
32 minutes 32 minutes
Transmits
every 1 minute
DIRECT WIDE1-1,WIDE2-2
(No hop) (3 hops)
TM-D710A/E