Glossary - U

UART

Universal Asynchronous Receiver/Transmitter (hardware)
An integrated circuit used for serial communications, containing a transmitter (parallel-to-serial converter) and a receiver (serial-to-parallel converter), each clocked separately.
The parallel side of a UART is usually connected to the bus of a computer. When the computer writes a byte to the UART's transmit data register (TDR), the UART will start to transmit it on the serial line. The UART's status register contains a flag bit which the computer can read to see if the UART is ready to transmit another byte. Another status register bit says whether the UART has received a byte from the serial line, in which case the computer should read it from the receive data register (RDR). If another byte is received before the previous one is read, the UART will signal an "overrun" error via another status bit.
The UART may be set up to interrupt the computer when data is received or when ready to transmit more data.
The UART's serial connections usually go via separate line driver and line receiver integrated circuits which provide the power and voltages required to drive the serial line and give some protection against noise on the line.
Data on the serial line is formatted by the UART according to the setting of the UART's control register. This may also determine the transmit and recieve baud rates if the UART contains its own clock circuits or "baud rate generators". If incorrectly formated data is received the UART may signal a "framing error" or "parity error.
Often the clock will run at 16 times the baud rate (bits per second) to allow the receiver to do centre sampling - i.e. to read each bit in the middle of its allotted time period. This makes the UART more tolerant to variations in the clock rate ("jitter") of the incoming data.
An example of a late 1980s UART was the Intel 8450. In the 1990s, newer UARTs were developed with on-chip buffer space for data. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the Intel 16550 has a 16 byte FIFO.

UAS

Unavailable Seconds (transmission, error)

UCD

Uniform Call Distributor (telephone)

UBR

Unspecified Bit Rate (ATM)
One of the ATM Forum best effort service types (the other one is ABR). Realistically, no traffic parameters are specified by the source, so, no actual quality commitment is made by the network management.

UDP

User Datagram Protocol (network, Internet)
A connectionlesstransport protocol without any guarantee of packet sequence or delivery. It functions directly on top of IP.

UME

UNI Management Entity (ATM)
Software at the UNIs for providing the ILMI functions. The code residing in the ATM devices at each end of a UNI circuit that implements the management interface to the ATM network. See LMI, SNMP.

UNI

User Network Interface (ATM)
The protocol adopted by the ATM Forum to define connections between ATM user (CPE) and ATM network (switch). Several versions exists, the pricipal ones are UNI 3.0, UNI 3.1, and the last one is UNI 4.0.
UNI 3.0 published in 1993, specifies the complete range of ATM traffic characteristics, including cell structure, addressing, signaling, adaptation layers, and traffic management. The ATM Forum specifications refer to two standards being developed, one between a user and a public ATM network, called public UNI and one between a user and a private ATM network called P-UNI.

UNI 2.0

ATM Forum UNI specification for the physical (PHY) and the ATM layers, the ILMI, OAM (traffic control), PVC support.

UNI 3.0

An upgrade of UNI 2.0 with traffic control for PCR and the operation over current transmission systems as some of the additional features.

UNI 3.1

A corrected version of UNI 3.0, this specification also includes SSCOP standards.

UNI 4.0

This UNI specification refers to signaling issues in ABR and VP, and QoS negotiation.

UPC

Usage Parameter Control (ATM)
A traffic policing function which ensures the equipment accessing the network at the UNI adheres to the traffic contract" and the QoS parameters expected by the network. Prevents congestion by not admitting excess traffic onto the network when all resources are in use. UPC changes the CLP bit of cells that exceed traffic parameters so they are dropped.

URL

Uniform Resource Locator (network, world wide web)
(Previously "Universal"). A draft standard for specifying an object on the Internet, such as a file or newsgroup. URLs are used extensively on the World-Wide Web. They are used in HTML documents to specify the target of a hyperlink.
Here are some example URLs:

• ftp://wuarchive.wustl.edu/mirrors/msdos/graphics/gifkit.zip
• http://www.w3.org/default.html
• news:alt.hypertext
• mailto:dbh@doc.ic.ac.uk

The part before the first colon specifies the access scheme or protocol. The part after the colon is interpreted according to the access scheme. In general, two slashes after the colon introduce a hostname (host:port is also valid, or for FTP user:passwd@host or user@host). Schemes include: ftp, http (World-Wide Web), gopher or WAIS. The "file" scheme should only be used to refer to a file on the same host but is often used incorrectly as a synonym for ftp. Other less commonly used schemes include News, telnet or mailto (e-mail). The port number can generally be omitted from the URL and will default to port 80. The last (optional) part of the URL may be a query string preceded by "?" or a "fragment identifier" preceded by "#". The later indicates a particular position within the specified document.
Only alphanumerics, reserved characters (:/?#"<>%+) used for their reserved purposes and "$", "-", "_", ".", "&", "+" are safe and may be transmitted unencoded. Other characters are encoded as a "%" followed by two hexadecimal digits. Space may also be encoded as "+".
The authoritative URL specification from CERN.

Usenet

Users' Network (network, Internet)
A collection of thousands of topically named newsgroups, the computers which run the protocols NNTP, and the people who read and submit Usenet news. Not all Internet hosts subscribe to Usenet and not all Usenet hosts are on the Internet. See also Network News Transfer Protocol (NNTP), UNIX-to-UNIX CoPy (UUCP).
Usenet is a distributed bulletin board(BBS) system supported mainly by Unix machines and the people who post and read articles thereon. Originally implemented in 1979-1980 by Steve Bellovin, Jim Ellis, Tom Truscott, and Steve Daniel at Duke University, it has swiftly grown to become international in scope and is now probably the largest decentralised information utility in existence.
Usenet encompasses government agencies, universities, high schools, businesses of all sizes and home computers of all descriptions. As of early 1993, it hosts well over 1200 newsgroups ("groups" for short) and an average of 40 megabytes (the equivalent of several thousand paper pages) of new technical articles, news, discussion, chatter, and flamage every day. To join in you need a specific client called a news reader.
Not all Internet hosts subscribe to Usenet and not all Usenet hosts are on the Internet but there is a large overlap.
Network News Transfer Protocol is a protocol used to transfer news articles between a news server and a news reader. The uucp protocol is sometimes used to transfer articles between servers, though this is probably less common now that most backbone sites are on the Internet.
Stanford University runs a service to send news articles by electronic mail. See http://woodstock.stanford.edu:2000/ or send electronic mail to netnews@db.stanford.edu with "help" in the message body.
MORE
Notes on news by Lars Magne Ingebrigtsen larsi@ifi.uio.no.
Usenet FAQ: http://www.cis.ohio-state.edu/hypertext/faq/usenet/top.html

USI

User Service Interface
Interface described in the CBDS service between a user and the network. It allows to define a quality of service for the final user (what is not available with SMDS).

UTOPIA

Universal Test & Operation Physical Interface for ATM
A ATM Forum physical layer specification for local connectivity between ATM devices.

UTP

Unshielded Twisted Pair
Twisted Pair Cable is the most common form of cable today, it is used to connect telephone subscribers to exchanges (switching centres) and wire buildings. Two insulated wires are twisted around each other, and combined with others into a cable. Twisted pair is starting to be a favourite choice for interconnecting PCs on a Local Area Network (LAN). In general, each twisted pair supports a single voice channel. Twisted pair used in Local Area Networks has several ratings. Category 3 has a speed rating of 10 million bits per second (the speed of ethernet), whereas category 5 has a speed rating of 100 million bits per second.
UTP (Unshielded twisted pair) is cable which has no ground shield. Cables are often provided with a ground shield which helps to reduce signal interference from external sources, thus making the signal travelling down the cable less prone to alteration. Twisted pair cable is provided in two forms, UTP and STP.

UUI

User to User Information (ATM)
Field of header of the AAL2 SAR-PDU cell.

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