The Cisco 4000 Series Integrated Services Routers (ISRs) host the Cisco network interface modules (NIMs), which have evolved from the enhanced high-speed WAN interface card (EHWIC), increasing port density and module capability. Up to three integrated NIM slots on the 4000 Series allow for flexible configurations. The NIMs support online insertion and removal (OIR), reducing the downtime required for new or replacement modules.

The available modules include 2-port and 4-port foreign exchange station (FXS), direct inward dial (DID), foreign exchange office (FXO), a combination of 2-port FXS and 4-port FXO, 4-port ear and mouth (E/M), and 2-port and 4-port basic rate interface (BRI). The BRI cards support only voice features. The NIMs have a dedicated digital signal processor (DSP) that will be used for time-division multiplexing (TDM) analog voice services.