Higher speed routers use the high speed serial interface (HSSI) - X21 is not ideal for 8 Mbit/s and above. HSSI is ok up to 52 Mbit/s and so can connect to leased line rates of 8 (E2), 34 (E3), and 45 (DS3) Mbit/s.
These high speed data leased line services can use a BNC coaxial copper type of physical presentation. Electrically the signal structure may well conform to the International G703 standard.
The (CPE) generally uses a data terminal equipment (DTE) interface that is widely different from the network presentation, in this case HSSI.
Interface modules may be available to connect the CPE (e.g. Routers directly to the High-speed coaxial presentation - but generally they're very expensive. The solution is to provide the required DTE interface, by the user sourcing the INTERFACE CONVERTER from a Metrodata Channel.
This solution adds more reliability in that the converter is inherently reliable, and diagnostics are provided to ensure maximum uptime. The need for the user to buy the converter separately is avoided.
The FM models are also manageable via an SNMP agent (LM1100).