I hope I understood your question correctly -
There is no limitation on the distance between two nodes but as you know cabling is a far bigger and costly problem then the node itself so each operator minimize the cost of operation but keeping the nodes at correct places.
If underlying IP network is secure i.e. same vendor then there is no need to IPSec (or similar) between two nodes if underlying network is not secure and rented (which is mostly the case) the connectivity is secured using IPSec or something similar.
I could not understand your last question, please make it clear...