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The ADTRAN MX2820 Clock Card is a plug-in card, which generates a timing signal synchronized to either of two external BITS clock references. If both BITS clock references fail, the Clock Card meets Sonet minimum clock (SMC) holdover requirements to produce the clock reference. This signal is used by the MX2820 to synchronize its STS-1 transmit. Two Clock Cards provide 1-to-1 redundancy in the MX2820 system.