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

UA: Unnumbered Acknowledgement
Unnumbered Acknowledgement (UA) is a data link command or encoding used as a response that acknowledges the receipt and acceptance of the Set Asynchronous Balance Mode (SABM), SABME, and DISC command protocol data units. Delivery of these messages is not ACKed or NACKed by the receiver and do not contain a sequence number.

UAA: Universally Administered Address
Universally Administered Address (UAA), also known as burned-in addresses, is one of two types of the MAC addresses. The other type is Locally Administrated Address (LAA). UAA is uniquely assigned to a device by its manufacturer. The first three octets (in transmission order) identify the organization which issued the identifier, and are known as the Organizationally Unique Identifier (OUI). The following three (MAC-48 and EUI-48) or five (EUI-64) octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness.

UAC: User Agent Client
User Agent Client(UAC), a term in the Session Initiation Protocol (SIP) based VOIP system, is a client application that initiates the SIP request.

UART: Universal Asynchronous Receiver/Transmitter
Universal Asynchronous Receiver/Transmitter(UART) is an 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.

UAS: Unavailable Seconds
Unavailable Seconds(UAS), an error message used in DS1 circuits, are calculated by counting the number of seconds that the interface is unavailable, starting with the first of 10 or more consecutive Severely Errored Seconds (SESs) and ending at the beginning of 10 consecutive non-SESs.

UAS: User Agent Server
User Agent Server(UAS), a term in the Session Initiation Protocol (SIP) based VOIP system, is 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.

UBR: Unspecified Bit Rate
Unspecified Bit Rate(UBR) is a 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.

UBR+ : Unspecified Bit Rate Plus
Unspecified Bit Rate Plus(UBR+) is UBR service complemented by ATM switches that use intelligent packet discard mechanisms, such as early packet discard (EPD) or tail packet discard (TPD).

UCAID: University Corporation for Advanced Internet Development
University Corporation for Advanced Internet Development(UCAID), now called Internet2, is a non-profit consortium which develops and deploys advanced network applications and technologies, mostly for high-speed data transfer. It is led by 207 US universities and partners from the networking and technology vendors. Some of the technologies it has developed include IPv6, IP multicasting and quality of service.

U-Frame: Unnumbered Frame
Unnumbered Frame (U-Frame) is one of three SDLC and then HDLC frame formats. The other two frames are information and supervisory. Unnumbered frame is used for various control and management purposes, including link startup and shutdown and mode specification.

UI: Unnumbered Information
Unnumbered Information(UI), a message in HDLC protocol, is used to send information to a secondary station.

UI-Frame: Unnumbered Information Frame
Unnumbered Information Frame (UI-Frame) is a frame used to transfer unacknowledged information between two link layer entities. The format and encoding are the same as specified in ITU-T Recommendation 0.921/1.441. The cyclic redundancy check (CRC) is derived over the entire frame.

UKERNA: UK Education and Research Networking Association
UK Education and Research Networking Association (UKERNA) is government-funded, with the primary aim of providing and developing a network infrastructure that meets the needs of the education and research communities. UKERNA manages the operation and development of JANET on behalf of JISC (Joint Information Systems Committee) for the UK Further and Higher Education Funding Councils. JISC also works in partnership with the Research Councils.

UL: Underwriters Laboratories
Underwriters Laboratories (UL) is an independent and non-profit organization that tests product safety for the US markets.

ULP: Upper-layer Protocol
Upper-layer Protocol(ULP) typically refers to the protocols that operate at a higher layer in the OSI reference model, such as layers 4-7. ULP is sometimes used to refer to the next-highest protocol (relative to a particular protocol) in a protocol stack.

Unbalanced Configuration
Unbalanced Configuration is an HDLC configuration with one primary station and multiple secondary stations.

UNC: Universal Naming Conversation
Universal Naming Conversation (UNC) provides a naming convention for identifying network resources. UNC names consist of three parts, a server name, a share name, and an optional file path, that are combined using backslashes as follows \\server\share\file_path The server portion of a UNC path refers to names maintained by a network naming service such as DNS or WINS. Share names can be defined by a system administrator or, in some cases, exist automatically within the local operating system.

Unclassified but Sensitive Internet Protocol Router Network
Unclassified but Sensitive Internet Protocol Router Network, previously known as Non-secure Internet Protocol Router Network (NIPRNET), is a network of Internet protocol routers owned by the United States Department of Defense (DOD). Created by the Defense Information Systems Agency (DISA), NIPRNET is used to exchange unclassified but sensitive information between "internal" users as well as providing users access to the Internet.

Unicast
Unicast is the process of forwarding unicasted traffic from a source to a destination on network. Unicasted traffic is destined for a unique address. In this case, there is just one sender and one receiver. The term exists in contra-distinction to multicast, communications between a single sender and a group of selected receivers, and anycast, communications between any sender and a group of receivers near the sender in a network. An earlier term, point-to-point communication, is similar in meaning to unicast.

Unicast Address
Unicast Address refers to the address specifying a single network device that the unicasted traffic is destined for. In IPv6, the unicast address consists of n bits for the prefix, and (128-n) bits for the interface ID.

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

U-NII: Unlicensed National Information Infrastructure
Unlicensed National Information Infrastructure(U-NII), equivalent to the term "information superhighway, is a US government used term that describes the access of information to citizens and business.

UNINETT
UNINETT is a Norwegian government-owned company responsible for deploying and maintaining a national research-based computer network (often referred to as "Uninett"). They also conduct network research and pilot projects related to high-speed connectivity. All of Norway's colleges and universities are connected, and any non-commercial research or educational institutions such as libraries, archives and schools may be connected for a yearly fee.

Uninsured Traffic
Uninsured Traffic refers to the traffic in an ATM network 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.

Unity Gain
Unity Gain, in networking, refers to the balance between signal loss and signal gain through amplifiers. A unity gain of factor 1 is equivalent to 0 dB where both input and output are at the same voltage level.

Unnumbered Frames
Unnumbered Frames is a type of HDLC frames used for various control and management purposes, including link startup and shutdown and mode specification.

Untagged Frame
Untagged Frame, a term used in the virtual LAN (VLAN) refers to the framse without an 802.1Q VLAN identifier.

Update Timer
Update Timer refers to the time interval that regulates how often a routing protocol sends its next periodic routing updates. Distance vector routing protocols send full routing updates every update interval.

Upload
Upload is the process of sending a copy of a file to a remote network location. For example, when you send an email with attachments through an email client software, the email client needs to upload the files to the email server first. The opposite is download.

UPnP: Universal Plug and Play
Universal Plug and Play (UPnP) is an architecture for pervasive peer-to-peer network connectivity of intelligent appliances, wireless devices, and PCs of all form factors. The scope of UPnP is large enough to encompass many existing, as well as new and exciting scenarios including home automation, printing and imaging, audio/video entertainment, kitchen appliances, automobile networks, and proximity networks in public venues. UPnP is a distributed, open networking architecture that leverages TCP/IP and the Web technologies to enable seamless proximity networking in addition to control and data transfer among networked devices. Under UPnP, there are no device drivers; common protocols are used instead. UPnP networking is media-independent. UPnP devices can be implemented using any programming language, and on any operating system.

UPSR: Unidirectional Path Switched Ring
Unidirectional Path Switched Ring (UPSR) is the 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.

Upstream
Upstream refers to the data flow direction from an individual user's station to the core network. For example, from the user's perspective, upstream network traffic flows away from the local computer toward the remote destination. Conversely, downstream traffic flows to the user's computer. Traffic on most networks flows in both upstream and downstream directions simultaneously.

URI: Uniform Resource Identifier
Uniform Resource Identifier (URI), is a short string of characters used to identify or name a network resource for the interaction with representations of the resource over a network, typically the World Wide Web. URIs are defined in schemes defining a specific syntax and associated protocols, the same general syntax as Uniform Resource Locator (URL); in fact, URLs are one type of URI. Whereas URLs always refer to network addresses (including a protocol specification, host name or address, and local path), a URI does not necessarily refer to a remote resource.

URN: Uniform Resource Name
Uniform Resource Name (URN) is an Internet resource with a name that, unlike a URL, has persistent significance -- that is, the owner of the URN can expect that someone else (or a program) will always be able to find the resource. A frequent problem in using the Web is that Web content is sometimes moved to a new site or a new page on the same site. Since links are made using Uniform Resource Locators (URLs), they no longer work when content is moved. A URN has the format like a UTL. For example urn:def://blue_laser where "def://" might indicate an agency or an accessible directory of all dictionaries, glossaries, and encyclopedias on the internet and "blue laser" was the name of a term. The result of using the agency could be the "best definition," the "longest definition," or even all definitions that the agency could find of "blue laser".

USENET
Usenet is a distributed Internet discussion system initiated in 1979 that evolved from a general purpose UUCP network of the same name. The medium is distributed among a large number of servers, which store and forward messages to one another. Individual users download and post messages to a single server, usually operated by their ISP or university, and the servers exchange the messages between each other. Today, almost all Usenet traffic is carried over the Internet. The current format and transmission of Usenet articles is very similar to that of Internet email messages. However, whereas email is usually used for one-to-one communication, Usenet is a one-to-many medium.

User Port
A user port is a port in a network device such as a router that originates or terminates on a node.

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

UUCP: UNIX-to-UNIX Copy Program
UNIX-to-UNIX Copy Program (UUCP) is a protocol stack used for point-to-point communication between UNIX systems. UUCP protocols allow remote execution of commands and transfer of files, email and netnews between computers. Specifically, UUCP is one of the programs in the suite; it provides a user interface for requesting file copy operations. The UUCP suite also includes uuxqt (user interface for remote command execution), uucico (communication program), uustat (reports statistics on recent activity), and uuname (reports the UUCP name of the local system). Although UUCP was originally developed on and is most closely associated with Unix, UUCP implementations exist for several other operating systems, including Microsoft's MS-DOS and Digital's VAX/VMS.

UUdecode: UNIX-to-UNIX decode
UNIX-to-UNIX decode(Uudecode) is a method of decoding ASCII files that were encoded using UNIX-to-UNIX encoding (Uuencode).

UUencode: UNIX-to-UNIX encoding
UNIX-to-UNIX encoding (Uuencode) is a method of converting binary files to ASCII so they can be sent over the Internet over the UUCP mail system. Since UUCP converted characters between various computer's character sets, uuencode was used to convert the data to fairly common characters that were unlikely to be "translated" and thereby destroy the file. The program uudecode reverses the effect of uuencode, recreating the original binary file exactly. uuencode/decode became popular for sending binary files by e-mail and posting to usenet newsgroups etc. It has now been largely replaced by MIME.