# Configuring the clocks

The boards in a CT bus (MVIP-95, H.100, H.110) system are synchronized using clocks
. In a system, one board drives the bus clock signals. All other boards reference their clocks from the bus.

On the MVIP-95 bus, H.100 bus, and H.110 bus, the board that drives the bus clocks is the MVIP clock master. In a digital system, the MVIP clock master derives the bus clock signals from the digital trunk's high-quality timing references. In an analog system with no digital telephone network interfaces, a board is configured as the MVIP bus clock master using the on-board oscillator to drive the clock signals.

The H.100 bus has a secondary clock signal that can be configured as a backup clock reference. On the H.100 bus, this clock signal is CT_NETREF. The H.110 bus has two secondary clock signals that can be configured as a backup clock reference: CT_NETREF_1 and CT_NETREF_2.

Note: CT_NETREF and CT_NETREF_1 are the same physical signal.

The H.100 bus and the H.110 bus can be configured to have a backup secondary clock master to which a clock fallback can occur. For more information, refer to swiConfigBoardClock. For further information about configuring clocks on the H.100 bus and H.110 bus, refer to the ECTF H.100 Hardware Compatibility Specification: CT Bus R1.0 and the ECTF H.110 Hardware Compatibility Specification: CT Bus R1.0.

In a multi-chassis MVIP system, all the boards in the PC chassis must be driven by the same clock source. Configure one H bus (H.100 or H.110) in the system as the H bus clock master, driving the H bus clock signals. Then configure the H bus in each PC chassis to drive the MVIP bus in their chassis.

The Switching service provides functions to control the clocks on the underlying MVIP switching device. It provides functions for configuring the:

- Board's clock source (H bus or network).

- Secondary clock signal on the bus.

The following table lists when to use the clock configuration functions available in the Switching service:

To... | Use...
--- | ---
Establish the clock source for an MVIP board | swiConfigBoardClock
Define the source of the NETREF clocks on the H.100/H.110 bus | swiConfigNetrefClock
Retrieve information regarding the configuration of the board clocking and current status of the clocks | swiGetBoardClock
Retrieve the status of a potential TDM bus clock timing reference | swiGetTimingReference

Refer to the H.110 clock configuration example for a sample clock configuration using swiConfigBoardClock and swiConfigNetrefClock.