Internetworking Terms

The interface between the telco and the user, also known as the local digital subscriber line (DSL) loop.

unnumbered acknowledgement.

user agent client. A client application that initiates the SIP request.

Universal Asynchronous Receiver/Transmitter. Integrated circuit, attached to the parallel bus of a computer, used for serial communications. The UART translates between serial and parallel signals, provides transmission clocking, and buffers data sent to or from the computer.

1. unavailable seconds. The PM parameter that measures the duration in seconds for which the path is unavailable; the time interval in seconds, starting with the first of 10 or more consecutive Severely Errored Seconds (SESs) and ending at the beginning of 10 consecutive non-SESs.

2. user agent server. A server application that contacts the user when a SIP request is received, and then returns a response on behalf of the user. The response accepts, rejects, or redirects the request.

user authentication.

Ungermann-Bass Net/One. Routing protocol, developed by UB Networks, that uses hello packets and a path-delay metric, with end nodes communicating using the XNS protocol. There are a number of differences between the manner in which Net/One uses the XNS protocol and the usage common among other XNS nodes.

Universal Broadband Router. The uBR7246 and uBR7223 are DOCSIS-compliant cable modem termination systems (CMTSs). The uBR900, uBR904, and uBR924 are DOCSIS-certified cable modems.

unspecified bit rate. QoS class defined by the ATM Forum for ATM networks. UBR allows any amount of data up to a specified maximum to be sent across the network but there are no guarantees in terms of cell loss rate and delay. Compare with ABR, CBR, and VBR.

unspecified bit rate plus. UBR service complemented by ATM switches that use intelligent packet discard mechanisms, such as EPD or TPD. See also EPD and TPD.

universal call model. Used interchangeably with LCM.

User Control Point. Cisco UCP is a carrier-class service policy administration system that enables personalized IP services. The Cisco UCP distributed, fault-tolerant architecture integrates authentication, authorization, and accounting (AAA); roaming; and address management services into operations support systems of a service provider.

UniDirectional Link Protocol. Protocol used by inexpensive, receive-only antennas to receive data via satellite.

User Datagram Protocol. Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.

Unnumbered frame. One of three SDLC frame formats. See also I-frame and S-frame.

unnumbered information.

Unnumbered information frame. See also I-frame, S-frame, and U-frame.

Universal I/O serial port (Cisco router).

UK Education and Research Networking Association.

Underwriters Laboratories. Independent agency within the United States that tests product safety.

Companding technique commonly used in North America. U-law is standardized as a 64-kbps CODEC in ITU-T G.711.

upper-layer protocol. Protocol that operates at a higher layer in the OSI reference model, relative to other layers. ULP is sometimes used to refer to the next-highest protocol (relative to a particular protocol) in a protocol stack.

Universal Mobile Telephone Service. A 3G mobile wireless telecommunications system whose standards are being developed by the Third Generation Partnership Project (3GPP).

HDLC configuration with one primary station and multiple secondary stations.

User-Network Interface. ATM Forum specification that defines an interoperability standard for the interface between ATM-based products (a router or an ATM switch) located in a private network and the ATM switches located within the public carrier networks. Also used to describe similar connections in Frame Relay networks. See also NNI, Q.920/Q.921, and SNI.

Message sent to a single network destination. Compare with broadcast and multicast.

Address specifying a single network device. Compare with broadcast address and multicast address. See also unicast.

Unicast Reverse Path Forwarding is an input function and is applied only on the input interface of a router at the upstream end of a connection.

Unlicensed National Information Infrastructure. Term coined by federal regulators to describe the access of information to citizens and business. Equivalent to the term "information superhighway," it does not describe system architecture or topology.

Traffic within the excess rate (the difference between the insured rate and the maximum rate) for an ATM VCC. This traffic can be dropped by the network if congestion occurs. See also CLP, insured rate, and maximum rate.

Feature that allows the authentication of outgoing Voice over IP (VoIP) telephone connections, using the Open Settlement Protocol (OSP).

Literally meaning one polarity, the fundamental electrical characteristic of internal signals in digital communications equipment. Contrast with bipolar.

In broadband networks, the balance between signal loss and signal gain through amplifiers.

Operating system developed in 1969 at Bell Laboratories. UNIX has gone through several iterations since its inception. These include UNIX 4.3 BSD (Berkeley Standard Distribution), developed at the University of California at Berkeley, and UNIX System V, Release 4.0, developed by AT&T.

See UUCP.

HDLC frames used for various control and management purposes, including link startup and shutdown and mode specification.

System process that cannot affect the state of system security through incorrect or malicious operation, usually because its operation is confined by a security kernel.

usage parameter control. See traffic policing.

See U-law.

unidirectional path switched ring. Path switched SONET rings that employ redundant, fiber-optic transmission facilities in a pair configuration. One fiber transmits in one direction and the backup fiber transmits in the other. If the primary ring fails, the backup takes over.

Set of frequencies used to send data from a subscriber to the headend.

User Registrar. One of the suite of software products included in the Cisco Subscriber Registration Center (CSRC) product. UR enables cable network subscribers to self-provision account registration, and to activate their cable modem and PC over the cable network using a Web user interface. User Registrar activates subscriber devices with account-appropriate privileges through updates to an LDAP directory.

A story, which might start with a grain of truth, that has been retold and ends up on the Internet. Some legends that periodically make their rounds include "The Infamous Modem Tax," "Craig Shergold/Brain Tumor/Get Well Cards," and "The $250 Cookie Recipe." Urban legends are conceptually similar to space junk that stays in orbit for years.

uniform resource identifier. Type of formatted identifier that encapsulates the name of an Internet object, and labels it with an identification of the name space, thus producing a member of the universal set of names in registered name spaces and of addresses referring to registered protocols or name spaces. [RFC 1630]

uniform resource locator. Type of formatted identifier that describes the access method and the location of an information resource object on the Internet.
[RFC 1738] See also browser.

See traffic policing.

Initiated in 1979, one of the oldest and largest cooperative networks, with more than 10,000 hosts and a quarter of a million users. Its primary service is a distributed conferencing service called news.

See uauth.

In the context of wavelength routing, a user port is a port that originates or terminates on a node; in other words, it is a port on the NE that points to a non-wavelength router NE.

Coordinated Universal Time. Time zone at zero degrees longitude. Formerly called Greenwich Mean Time (GMT) and Zulu time.

unshielded twisted-pair. Four-pair wire medium used in a variety of networks. UTP does not require the fixed spacing between connections that is necessary with coaxial-type connections. Five types of UTP cabling are commonly used: Category 1 cabling, Category 2 cabling, Category 3 cabling, Category 4 cabling, and Category 5 cabling. Compare with STP. See also EIA/TIA-586 and twisted pair.

Universal Terminal Support. A data link layer protocol (P1024C) that runs in full-duplex mode over synchronous serial (V.24) lines and uses the ASCII character set.

UNIX-to-UNIX Copy Program. Protocol stack used for point-to-point communication between UNIX systems.

UNIX-to-UNIX decode. Method of decoding ASCII files that were encoded using uuencode. See also uuencode.

UNIX-to-UNIX encoding. Method of converting binary files to ASCII so they can be sent over the Internet via e-mail. The name comes from its use by the UNIX operating system's uuencode command. See also uudecode.

Universal Voice Module.

Universal Voice Module-Channelized.

Universal Voice Module-Unchannelized.