The Cisco 3900 Series Integrated Services Routers offer the highest performance and services density among the ISR portfolio. The series comprises the Cisco 3945E, Cisco 3925E, Cisco 3945, and Cisco 3925 routers, which differ primarily in performance capabilities and service-module slot count. The Cisco 3900 Series ISRs offer a field-replaceable Cisco Services Performance Engine (SPE), which hosts the system route processor, enhanced high-speed WAN interface card (EHWIC) slots, and onboard Ethernet and digital-signal-processor (DSP) slots - an option available uniquely on the Cisco 3900 Series. The SPE is a modular motherboard that can be upgraded with higher-performance engines developed in the future. The Cisco 3945E ships with the Cisco SPE250, the Cisco 3925E ships with the SPE200, the Cisco 3945 ships with the SPE150, and the Cisco 3925 ships with the SPE100. When ordering SPE spares or upgrading, it is important to note that the SPE250 and SPE150 are supported only on Cisco 3945 platforms, whereas the SPE200 and SPE100 are supported on the Cisco 3925 platforms