17.2.9 Integration with wired and wireless code

Recently (November 2001), support has been added to connect traditional OTcl-based wired nodes with the satellite nodes. This section describes the capabilities and limitations of that code.

The satellite code (and the wireless code) normally performs all routing in C++, while the traditional ns code uses a mix of OTcl and C++ code. For backward compatibility reasons, it is difficult to fully integrate both the wired and wireless code. The strategy for integrating wireless and wired code has been to define a special gateway node (called a "basestation"), to use hierarchial routing, and to locate a single basestation node in the wireless network with a network stack located in both the wireless and the wired subnet. Because routing is not fully integrated, the topology of the simulation is limited to only one gateway node per wireless subnet (i.e., a packet cannot enter the wireless network from one wired gateway and leave via another).

The satellite/wired code integration takes a different strategy. By selecting the node configuration $ns node-config -wiredRouting ON option, the C++ routing in the satellite code is turned off, and instead, all satellite topology changes lead to upcalls into the OTcl code. As a result, the link_ array in OTcl is manipulated according to all topology changes, and OTcl-based routing can occur. The penalty for doing this is a much longer execution time for larger simulations (such as Teledesic), but for smaller simulations, the difference is not as noticeable.

An example script detailing the use of this new option is shown in ~ns/tcl/ex/sat-wired.tcl, and a similar test in the satellite test suite exercises this code. Additionally, all of the satellite example scripts in ~ns/tcl/ex directory can be converted to OTcl routing by using the $ns node-config -wiredRouting ON option. However, there are a few caveats:

• The wired routing option for satellite has only been tested with (the default) static routing: $ns rtProto Static. The code triggers a global routing table update upon any satellite topology change.
• The option data_driven_computation_ can not be set to ``true'' when wiredRouting is ON. Note that the enabling or disabling of data_driven_computation_ can give subtle differences in simulation output since routes are computed at different times (while propagation delays are continuously changing). This effect can be seen by toggling this parameter in the Iridium example script ~ns/tcl/ex/sat-iridium.tcl.
• In the trace file, when a packet is dropped due to ``no route to host'' (such as when there is a topology change), the trace looks a bit different depending on whether wiredRouting is turned OFF or ON. In the former case, there is one line per drop, with the destination labelled as ``-2''. In the latter case, there are three events (enque ``+'', deque ``-'', and drop ``d'') corresponding to the same packet, and the destination is shown as ``-1''.
• In rare cases, there may be warning messages during the execution indicating ``node out of range.'' This can occur if a node becomes disconnected in the topology and then another node tries to send a packet to it. For example, try enabling wiredRouting in the file ~ns/tcl/ex/sat-mixed.tcl. This occurs because the routing table is dynamically sized upon topology change, and if a node becomes disconnected it may not have any entries inserted in the routing table (and hence the routing table is not grown to accommodate its node number). This warning should not affect actual trace output.
• There has been no attempt to interoperate with wireless or mobile-IP code.