A main distribution frame (MDF) is the primary point at which permanent outside plant facilities terminate within a telephone exchange. At this termination point, the outside line is interconnected to the specific telecommunications equipment that is used to provide individual subscribers with their appropriate telecommunications service, such as a voice switch. In larger central offices (COs), intermediate distribution frames (IDFs) are often required for distributing pairs within the CO and for providing isolation points for troubleshooting.

Figure 1. Distribution frames are used to terminate outside plant facilities within a telephone exchange and interconnect subscribers to termination equipment.

Distribution frames are essential to the delivery of telecommunications services. They provide a scalable means by which services can be delivered to individual subscribers without requiring major cabling installations or changes. In addition, by serving as a termination point for all copper pairs going to subscriber locations, they are an ideal location for test access into the last mile, allowing service providers to monitor and manage their entire physical copper infrastructure. Finally, distribution frames often serve as a demarcation point between different carriers' domains. As a result, they provide a simple means by which multiple service providers can work together in a deregulated environment while maintaining independent network infrastructures.