18.2.5 Trace support

Tracefiles using satellite nodes and links are very similar to conventional ns tracing described in Chapter . Special SatTrace objects (class SatTrace derives from class Trace) are used to log the geographic latitude and longitude of the node logging the trace (in the case of a satellite node, the latitude and longitude correspond to the nadir point of the satellite).

For example, a packet on a link from node 66 to node 26 might normally be logged as:

+ 1.0000 66 26 cbr 210 ------- 0 66.0 67.0 0 0

but in the satellite simulation, the position information is appended:

+ 1.0000 66 26 cbr 210 ------- 0 66.0 67.0 0 0 37.90 -122.30 48.90 -120.94

In this case, node 66 is at latitude 37.90 degrees, longitude -122.30 degrees, while node 26 is a LEO satellite whose subsatellite point is at 48.90 degrees latitude, -120.94 degrees longitude (negative latitude corresponds to south, while negative longitude corresponds to west).

One addition is the Class Trace/Sat/Error, which traces any packets that are errored by an error model. The error trace logs packets dropped due to errors as follows, for example:

e 1.2404 12 13 cbr 210 ------- 0 12.0 13.0 0 0 -0.00 10.20 -0.00 -10.00

It may happen that a satellite node generates a packet that it cannot forward (such as in sat-mixed.tcl). This will show up as a drop in the tracefile with a destination field set to -2, and the coordinates set to -999.00:

d 848.0000 14 -2 cbr 210 ------- 1 14.0 15.0 6 21 0.00 10.00 -999.00 -999.00

This indicates that node 14, in trying to send a packet to node 15, could not find an available route.

To enable tracing of all satellite links in the simulator, use the following commands before instantiating nodes and links:

set f [open out.tr w]
$ns trace-all $f

Then use the following line after all node and link creation (and all error model insertion, if any) to enable tracing of all satellite links:

$ns trace-all-satlinks $f

Specifically, this will put tracing around the link layer queues in all satellite links, and will put a receive trace between the mac and the link layer for received packets. To enable tracing only on a specific link on a specific node, one may use the command:

$node trace-inlink-queue $f $i
$node trace-outlink-queue $f $i

where $i$ is the index of the interface to be traced.

The implementations of the satellite trace objects can be found in ~ns/tcl/lib/ns-sat.tcl and ~ns/sattrace.{cc,h}.