Carrier sense is high when transmitting, receiving, or the medium is otherwise sensed as being in use. If a collision is detected, COL also goes high while the collision persists. In addition, the MAC may weakly pull-up the COL signal, allowing the combination of COL high with CRS low (which a PHY will never produce) to serve as indication of an absent/disconnected PHY. Management signals MDC and MDIO constitute a synchronous serial data interface similar to I²C. As with I²C, the interface is a multidrop bus so MDC and MDIO can be shared among multiple PHYs. Limitations The interface requires 18 signals, out of which only two (MDIO and MDC) can be shared among multiple PHYs. This presents a problem, especially for multiport devices; for example, an eight-port switch using MII would need 8 × 16 + 2 = 130 signals. Reduced media-independent interface Reduced media-independent interface (RMII) is a standard which was developed to reduce the number of signals required to connect a PHY to a MAC. Reducing pin count reduces cost and complexity for network hardware especially in the context of microcontrollers with built-in MAC, FPGAs, multiport switches or repeaters, and PC motherboard chipsets. Four things were changed compared to the MII standard to achieve this. These changes mean that RMII uses about half the number of signals compared to MII. The two clocks TXCLK and RXCLK are replaced by a single clock. This clock is an input to the PHY rather than an output, which allows the clock signal to be shared among all PHYs in a multiport device, such as a switch. The clock frequency is doubled from 25 MHz to 50 MHz, while the data paths are narrowed from 4 bits to 2 bits. RXDV and CRS signals are multiplexed into one signal. The COL signal is removed. MDC and MDIO can be shared among multiple PHYs. The receiver signals are referenced to the REF_CLK, same as the transmitter signals. This interface requires 9 signals, versus MII's 18. Of those 9, on multiport devices, MDIO, MDC, and REF_CLK may be shared leaving 6 or 7 pins per port. RMII requires a clock where MII requires a clock and data is clocked out two bits at a time vs 4 bits at a time for MII or 1 bit at a time for SNI (10 Mbit/s only). Data is sampled on the rising edge only (i.e.