The PA-E3 is a single-width, one-port or two-port module that integrates data service unit (DSU) functionality into the Cisco router. The module provides one or two high-speed serial E3 interfaces. The one-port PA-E3 provides two network interfaces per VIP2 or VIP4 for Cisco 7000 series and Cisco 7500 series routers and the Catalyst RSM/VIP2 for Catalyst 5000 family switches, and one high-speed interface on the Cisco 7100 series, Cisco 7200 series, and Cisco uBR7200 series routers. The two-port PA-2E3 provides four network interfaces per VIP2 or VIP4 for Cisco 7000 series and Cisco 7500 series routers and the Catalyst RSM/VIP2 for Catalyst 5000 family switches, and two high-speed interfaces on the Cisco 7100 series, Cisco 7200 series, and Cisco uBR7200 series routers. Serial network interfaces reside on modular port adapters, which provide a direct connection between the high-speed bus in the router and the external networks. The PA-E3 provides a full-duplex synchronous serial E3 interface for transmitting and receiving data at rates of up to 34 megabits per second (Mbps). The port adapter supports both 16- and 32-bit cyclic redundancy checks (CRCs). The default is 16-bit CRCs; to enable 32-bit CRCs, you use a configuration command.