Here we show two locations of the same company, who may well have a circuit switched PBX network (e.g. Meridian, ISDX InDEX) linked by a 2Mbit/s Leased Line. A separate IP router network may well have developed using its own leased line - between the same places.
Why not combine the two?? But, the PBX needs 2Mbit/s all to its self - No it doesn't. It is rare for a PBX link to use more than 50% of its capacity - so the leased line is not fully used. The router network can dynamically use the (varying) spare bandwidth of the PBX link.
Metrodata have the FM4200 which maintains full signalling integrity of the PBX link while allowing the routers to share the leased line.
That saves the cost of the whole leased line. An FM4500 allows up to 4 router links to do similar.
For more information on the FM family.