# Clock signal summary

The following table summarizes the reference clocks that a clock master can drive:

Clock | Details
--- | ---
A_CLOCK | The set of primary bit clocks (CT8A) and framing signals (CTFrameA). The CT8A signal is an 8 MHz clocking reference for transferring data over the CT bus. The CTFrameA provides a low going pulse signal every 1024 (8 MHz) clock cycles.
B_CLOCK | The set of secondary bit clocks (CT8B) and framing signals (CTFrameB). The CT8B signal is an 8 MHz clocking reference for transferring data over the CT bus. The CTFrameB provides a low going pulse signal every 1024 (8 MHz) clock cycles.

The following table summarizes the timing references that a clock master can use:

Timing reference | Details
--- | ---
NETWORK | The timing signal from a digital trunk attached to the clock master board. Within the digital trunk interface, an 8 kHz reference is derived from the frequency of the incoming signal. The clock master is frequency-locked to this 8 kHz reference so that the long-term timing of the system matches that of the public telephone network. Note: No timing signal is available from an analog trunk.
NETREF / NETREF1 | The CTNETREF_1 signal. Can be 8 kHz, 1.544 MHz, or 8 MHz. NMS recommends using only 8 kHz signals for most boards.
NETREF2 | (H.110 only) The CTNETREF_2 signal. Can be 8 kHz, 1.544 MHz, or 8 MHz. NMS recommends using only 8 kHz signals for most boards.
OSC | Clock signal derived from an oscillator on the clock master board. Note: Use this timing reference source only if no network timing references are available.

Note: Not all boards support all signals. For more information about board models, refer to the board documentation.