From the Summary: "The Constitution states that 'a Majority of each [House] shall constitute a quorum to do business.' The Senate presumes that it is complying with this requirement and that a quorum is always present unless and until the absence of a quorum is suggested or demonstrated. This presumption allows the Senate to conduct its business on the floor with fewer than 51 Senators present until a Senator 'suggests the absence of a quorum.' Except when the Senate has invoked cloture, the presiding officer may not count to determine if a quorum is present. When the absence of a quorum is suggested, therefore, the presiding officer directs the Clerk to call the roll. The Senate cannot resume its business until a majority of Senators respond to the quorum call or unless, by unanimous consent, 'further proceedings under the quorum call are dispensed with' before the last Senator's name has been called. If a quorum fails to respond, the Senate may adjourn or take steps necessary to secure the attendance of enough Senators to constitute a quorum. It usually takes the latter course by agreeing to a motion that instructs the Sergeant at Arms to request the attendance of absent Senators."
CRS Report for Congress, 96-452
Congressional Research Service: https://crsreports.congress.gov/