Glossary - U
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
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.
- Unavailable Seconds (transmission, error)
- Uniform Call Distributor (telephone)
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.
User Datagram Protocol (network, Internet)
A connectionlesstransport protocol without any guarantee
of packet sequence or delivery. It functions directly on top of IP.
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
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),
- 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
- UNI 4.0
This UNI specification refers to signaling issues in ABR
and VP, and QoS negotiation.
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.
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:
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.
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.
or send electronic mail to email@example.com
with "help" in the message body.
Notes on news by Lars Magne Ingebrigtsen firstname.lastname@example.org.
Usenet FAQ: http://www.cis.ohio-state.edu/hypertext/faq/usenet/top.html
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
Universal Test & Operation Physical Interface for ATM
A ATM Forum physical layer
specification for local connectivity between ATM devices.
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.
User to User Information (ATM)
Field of header of the AAL2 SAR-PDU cell.