The POSIP-OC3-50 provides a single 155.520-Mbps, Packet OC-3 network interface for Cisco 7000 series and 7500 series routers. Network interfaces reside on the POSIP-OC3-50 and provide a direct connection between the high-speed Cisco Extended Bus (CxBus or CyBus) and the external networks. The physical layer interface is OC-3. The POSIP-OC3-50 uses an R5000 central processing unit (CPU), running at 200 Mhz, and comes configured with 32 MB of synchronous dynamic random-access memory (SDRAM) and 4 MB of static random-access memory (SRAM) as the factory default memory configuration. The SDRAM and SRAM on the POSIP-OC3-50 models can be upgraded as your system configuration requires and SRAM and SDRAM memory upgrade options are made available by Cisco Systems.The Packet-Over-SONET specification for optical-fiber transmission defines two types of fiber: single mode and multimode. Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode fiber allows only one mode of light to propagate through the fiber, while multimode fiber allows multiple modes of light to propagate through the fiber. Multiple modes of light propagating through the fiber travel different distances depending on the entry angles, which causes them to arrive at the destination at different times (a phenomenon called modal dispersion); therefore, single-mode fiber is capable of higher bandwidth and greater cable run distances than multimode fiber.