The basic transport frame of an SDH network, operating at a minimum of 155Mbps (STM-1). The payload of an STM-1 frame can vary, supporting potentially 63 E1 trunks, a combination of E1 to E3 bearers, a single E4 trunk or alternatively IP data. The STM-1 frame is comprised of Overhead blocks, Payload blocks (Virtual Containers) and Pointers, the ratio of each depending on the initial payload to be transported. STM-1 can be multiplexed to create higher order STM, for example an STM-4 would be 4 x STM-1 and an STM-16 would be 4 x STM-4. SDH (Synchronous Digital Hierarchy) uses the following STM (Synchronous Transport Modules) and rates: STM-1 (155Mbps), STM-4 (622Mbps), STM-16 (2.5Gbps), and STM-64 (10Gbps).