10.1.4.3. Call Between an Access Point and the Central System 
Figure 171. Signaling for an access point to a central system call
Signaling from and to the connected telephone in the access point
is always transmitted over the IP network to the call processing infrastructure
in the OpenScape 4000 central system, and from there back again.
Figure 172. Voice link (payload) for an access point to a central system call
The central call processing infrastructure establishes the voice link
and makes all features available. The voice link from the access point
is switched to the IP network with the aid of the HG 3575 module. The
HG 3500 module functions as a gateway and converts the IP data stream
back into a PCM data stream that can be processed by the central system.
This data stream is transferred to the connected subscriber within the
central system. The call is therefore switched in both the IP network
and in the OpenScape 4000 central system. The access point and the gateway
module each occupy one channel of the available transport capacity to
the IP network.