The main task of the VoIP implementation in the LANCOM VoIP Router is to connect telephone calls from different local interfaces (LAN, WLAN, ISDN) to the WAN connections that can be accessed by the router. This enables switching between the local interfaces (local call) as well as between WAN interfaces.
The basis for the implementation and switching is the SIP protocol. The calls over all interfaces are converted into SIP by the interface converter (this mainly concerns the ISDN interfaces). The ISDN-ISDN bridge function is a special case that is activated when ISDN protocols cannot be mapped in SIP, which is why a bit-transparent connection is created between an ISDN-TE (external ISDN connection) and an ISDN-NT (internal ISDN connection).
Furthermore, the bit-transparent connection is usually used for calls between multiple local ISDN interfaces to achieve the highest possible compatibility and quality.