Glossary V

UART: The Universal Asynchronous Receiver/Transmitter: A chip that governs asynchronous communications. Its primary function is to buffer incoming data, but it also buffers outbound bits.
UDP: User Datagram Protocol: A connectionless Transport layer protocol in the TCP/IP protocol stack that simply enables datagrams to be exchanged without acknowledgements or delivery guarantees, requiring other protocols to handle error processing and retransmission. UDP is defined in RFC 768.
U reference point: Reference point between a TE1 and an ISDN network. The U reference point understands ISDN signaling techniques and uses a two-wire connection.
unicast: Used for direct host-to-host communication. Communication is directed to only one destination and is originated from only one source.
unidirectional shared tree: A method of shared tree multicast forwarding. This method allows only multicast data to be forwarded from the RP.
uniquely: A host is uniquely identified by the MAC address, because this address is not repeated on any other host anywhere.
unnumbered frames: HDLC frames used for control-management purposes, such as link startup and shutdown or mode specification.
UplinkFast: Enables a switch to immediately begin forwarding frames on blocked ports when a failure is detected on the root port.
upstream: Routers nearer to the transmitting multicast server are said to be upstream from the receiving client.
UTP: unshielded twisted-pair: Copper wiring used in small-to-large networks to connect host devices to hubs and switches. Also used to connect switch to switch or hub to hub.

U