Holter unit is a mobile monitor for collecting data that is later uploaded to another system for analysis. Unlike a telemetry unit, these monitors do not require a central receiving station and can be used at home, outdoors, or just about anywhere. Five leads are frequently the maximum for a Holter ECG monitor, since being ambulatory is difficult with a full 12-lead ECG. Data is most commonly retrieved from the monitor by removing the memory card; however, USB and other methods are also used. Most patients need only to be monitored for a day or two.