High speed data leased line services commonly use a BNC or RJ45 coaxial copper type of physical presentation to the Customer Premise Equipment (CPE) - this is the physical presentation. Electrically the signal structure may well conform to the International G703 standard.
The Customer Premise Equipment (CPE) generally uses a data terminal equipment (DTE) interface that is wildly different from the network presentation.
Interface modules may well be available to connect the CPE (e.g. Router) directly to the G703 BNC/RJ45 presentation - but generally they're very expensive. The best solution is for the service provider (SP) to provide the required DTE interface as part of his service.
This way the customer gets the service he wants from a single SP and does not need to source the interface converter in his own right.
This solution adds more reliability in that this converter is inherently reliable, and diagnostic and are provided to ensure maximum uptime. The need for the usher to buy the converter separately is avoided.
The most typical use is where routers with X21 DTE interfaces, need to connect to RJ45 or BNC - presented 2Mbit/s high speed leased lines.