There can be a mesh of providers in real cases. PLOP works as such. The reasons of the ways needed in different levels were also identified, for example, the way at IP device level is needed for handling location of IP devices and IP-based location services. An L2 example is given here. There are multiple access methods for the information.
Monitor calls the location center.
The center calls the PCS phone and the phone calls back with location information. Server needs to determine it's own location. IPsec is proposed for the purpose. Note that we can have multiple infrastructures involved. Some scenarios for an ISL arch has presented the following.