Sequence diagrams are good when you're already inside the system. Showing my age here but IMO, for sketching a high-level view of the whole system and how its c

Sequence diagrams are good when you're already inside the system. Showing my age here but IMO, for sketching a high-level view of the whole system and how its components interact, nothing beats SSADM's data flow diagrams. You can go as detailed as you like. Break up parts into a new component: draw a ring around them, network traffic: note where the flows cross the rings and document the message sizes and rates. Sigh Happy days ...