PSTN : Local Loop

Local Loop
The local loop is the connection (wireless or wired) between a customer’s telephone or data equipment and a LEC or other telephone service provider. Traditionally, the local loop (also called “outside plant”) has been composed of copper wires that extend from the EO switch. The EO is the last switching office in the telephone network that connects customers to the telephone network.

The EO switch cables meet the copper (or other types of lines) at the main distribution frame (MDF). The MDF is a wiring rack that allows technicians to splice the local loop lines with the lines from the switching system. Local loop lines leave the MDF in bundles (possibly thousands of wires in each bundle) and arrive in other junction points such as local distribution frames (LDF). The LDF allows the connection of the final connection (the “drop”) to the business or residence. At the entry to the customer’s location, there is often a network termination (NT) device that isolates the telephone network from the wiring inside the customers building.

Figure 1 depicts a traditional local loop distribution system. This diagram shows a central office (CO) building that contains an EO switch. The EO switch is connected to the MDF splice box. The MDF connects the switch to bundles of cables in the “outside plant” distribution network. These bundles of cables periodically are connected to local distribution frames (LDFs). The LDFs allow connection of the final cable (called the “drop”) that connects to the house or building. A NT block isolates the inside wiring from the telephone system. Twisted pair wiring is usually looped through the home or building to provide several telephone connection points, or jacks, so telephones can connect to the telephone system


Figure 2: Local Loop