The phrase “PC load letter” is a printer error message on some mostly obsolete types of HP laserjet printers. Here, “PC” stands for “paper cassette” and the error indicates that no letter-sized paper is available for a print job with a letter size. The error message is instructing the user to load a letter-sized paper into the printer.
Time division duplex (TDD) refers to duplex communication links where uplink is separated from downlink by the allocation of different time slots in the same frequency band. It is a transmission scheme that allows asymmetric flow for uplink and downlink data transmission. Users are allocated time slots for uplink and downlink transmission.
Time division multiplexing separates uplink and downlink signals by matching full duplex communication over a half-duplex communication link. This method is highly advantageous in case there is an asymmetry of uplink and downlink data rates. TDD divides a data stream into frames and assigns different time slots to forward and reverse transmissions, thereby allowing both types of transmissions to share the same transmission medium.
When data over uplink increases, more communication capacity is allocated. This is taken away when the traffic load becomes lighter. It operates by toggling transmission directions over a time interval, which occurs rapidly and is barely visible to the user. TDD supports voice and symmetrical as well as asymmetric data services. It also handles a dynamic mix of both traffic types. The capacities of downlinks and uplinks are altered in favor of one direction over another by providing greater time allocation through time slots to downstream transmission intervals than to upstream ones.
Well known examples of TDD are:
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