Crosstalk is everywhere

Crosstalk is everywhere.  Really, in a more general sense, noise coupling is everywhere.  Usually the method of noise coupling is traditional “crosstalk” – the unwanted transfer of noise from one place to another through coupled electric fields.  This most often occurs on PCB designs with dense routing, and on wide parallel busses.  Even on newer SERDES busses, however, it is still an issue, as many such busses have multiple lanes, such as PCI Express.  And crosstalk is also an issue on SERDES busses when they are routed close to slower, much higher voltage signals such as 3.3V and 5V signals.  Crosstalk can also occur in a similar fashion between higher-voltage switching power supplies and sensitive lines like resets.

But it is not only crosstalk that causes noise coupling.  Shared return paths are another common method of noise coupling.  This occurs most often in connectors without sufficient ground pins.  Since ground pins act as the return paths in connectors, an insufficient number of ground pins will cause shared return paths and hence coupling between signals travelling through the connectors.  A similar type of situation can occur in boards without enough stitching vias near signal layer transitions.

And coupling can also occur through the PDN.  An inadequately designed PDN can directly result in simultaneous switching noise, or SSN.

All of these coupling problems can be identified and resolved through simulation in HyperLynx.
 
To learn more about how to control this noise, take a look at this article:
http://pcdandf.com/cms/component/content/article/171-current-issue/8973-designers-notebook