Avaya ESS User Manual page 42

ESS Overview
A network problem occurs where Port Network 4 is isolated from the CSS and the DS1C link
(see Figure 14).
Note:
If only the DS1C link was lost, Port Network 4 would not transition to the ESS
Note:
server. Communication between the DS1C location and the main location would
be disrupted since there is no bearer path.
After the no service time out interval expires, the IPSI in Port Network 4 requests service from
the S8500 ESS server. The Public Switched Telephone Network resources enable local and
long distance calling.
42 Avaya Enterprise Survivable Servers (ESS) Users Guide