Usually "slow" refers to signals with a risetime being short compared to the physical length of the interconnect. For gigabit data rates there is nothing like a "slow" signal as the interconnect length is always short relative to the risetime. However, it also depends on the topology that is used. If you have a point-multipoint topology, e..g. the CA bus in a memory subsystem, reflections and crosstalk reaches a critical level much faster than the risetime rule would indicate.
That's where an analog simulation is required to verify already in the design phase if a system will work before it is built. ADS allows to use a schematic-based approach with a very broad library of e.g. transmission line models. An example simulation schematics looks like this:
Time Domain Simulation Schematic :: 2Slot Motherboard Schematic Time Domain Simulation :: 2Rank DIMM Schematic
All kind of different models can be used for this simulation:
ADS library models (e. g. transmission lines) BSIM4 transistor based models Spice lumped elements or netlists IBIS models S-parameter models (e. g. generated by 2.5D and 3D Modeling)
Depending on the simulation target, different simulations can be performed. The most common approach is to simulate a long PRBS (Pseudo Random Bit Stream) pattern on a victim signal and different bitpatterns on the neighboring aggressors. Resulting data is post-processed and a data eye is generated. In ADS this is done in the DataDisplay server.
Time Domain Simulation :: CA data eye with timing evaluation according DRAM spec
Frequency domain simulations can be performed in this environment too. These simulations allow the characterization and comparison of parasitics for passive components over a wide frequency range. The resulting S-parameter set shows the impact of Insertion Loss, Transmit loss and Cross-Talk.