S to Z and numbers¶

S¶

S-Tag¶

Service VLAN tag. In IEEE 802.1ad double tagging (Q-in-Q), the outer VLAN tag added by the service provider, with TPID 0x88A8. It sits outside the customer’s own C-Tag.

S-GW¶

Serving Gateway. The LTE core gateway that forwards users’ packets between the eNodeBs and the PGW. It anchors the user plane during handovers and buffers data for idle devices while the MME pages them.

SAAL¶

Signalling ATM Adaptation Layer. The ATM adaptation layer that carries signalling messages reliably between ATM switches.

SaaS¶

Software as a Service. Software that a provider hosts and runs, and customers use over the internet, usually in a browser, on a subscription.

SAE¶

System Architecture Evolution. The 3GPP work that defined the LTE core network, the Evolved Packet Core: a simpler, all-IP core that also supports other access networks, such as Wi-Fi.

SAFe¶

Scaled Agile Framework. A framework for running agile development across many teams in a large organisation, with set roles, planning events and portfolio management.

SBC¶

Session Border Controller. A device at the edge of a VoIP network that controls the signalling, and usually the media, of calls crossing it. It provides security, NAT traversal, interworking and policing.

SC-FDMA¶

Single-Carrier FDMA. A multiple access scheme, also called DFT-spread OFDMA, with a lower peak-to-average power ratio than OFDMA. That saves battery power, so LTE uses it for the uplink.

Scala¶

A statically typed language for the Java virtual machine that combines object-oriented and functional programming, and works with Java libraries.

Scaled Agile¶

Agile methods applied to large projects and programmes, with many teams working on one product.

SCCP¶

Signalling Connection Control Part. The SS7 layer above MTP that adds richer addressing (global title translation), connection-oriented services and segmentation. MAP and other application parts use it.

Scenario¶

A narrative of an activity, told as a story, often a concrete path through a use case.

Schedule¶

A plan that sets the timing, resources and dependencies of the tasks needed to finish a project or part of one.

Scheduler¶

The part of an operating system, or a network device, that decides which task or packet gets a resource next, such as a processor or a link. Its aims include keeping resources busy, sharing them fairly and meeting deadlines or quality of service targets.

See also: Round Robin, Preemption, RTOS.

SCM¶

Software Configuration Management. Tracking and controlling changes to software: version control, baselines, builds and releases. It is the software part of configuration management.

Scratch¶

A visual, block-based programming language and online community from the MIT Media Lab, designed to teach children to code.

Scribe¶

The person who records issues in an inspection or other formal meeting.

Scrum¶

An agile framework in which a small team works in fixed-length sprints towards a sprint goal. It has three roles: product owner, Scrum Master and developers. Its events are sprint planning, the daily scrum, the sprint review and the retrospective.

See also: Sprints, Product Owner, Scrum Master.

Scrum Master¶

The Scrum role that helps the team follow Scrum and work well, by coaching, removing obstacles and protecting the team’s focus.

Scrum of Scrums¶

A way to coordinate several Scrum teams: each team sends a representative to a regular meeting to raise dependencies and problems between teams.

SCTP¶

Stream Control Transmission Protocol. A transport protocol (RFC 9260) that delivers messages reliably like TCP, keeps message boundaries like UDP, and supports multiple streams and multi-homing. It carries SS7 signalling over IP (SIGTRAN) and LTE control traffic.

SDES¶

SDP Security Descriptions. A way to exchange the keys for SRTP (secure RTP) inside the SDP of a SIP call. It relies on the signalling itself being encrypted.

SDH¶

Synchronous Digital Hierarchy. The ITU-T standard for synchronous multiplexing of digital signals over fibre, from STM-1 (155 Mbit/s) upwards, with strong management and fast protection switching. It replaced PDH; SONET is the North American equivalent.

SDK¶

Software Development Kit. The libraries, tools, documentation and examples needed to build software for a particular platform or service.

SDLC¶

Systems Development Life Cycle. A structured process for developing a system, from feasibility study through to maintenance. Waterfall, spiral and iterative models are all versions of it.

SDMA¶

Space-Division Multiple Access. Using smart antennas to point beams at different users, so that users in different places can share the same frequency at the same time.

SDN¶

Software-Defined Networking. Separating a network’s control plane from its data plane, and putting control in software controllers that configure the forwarding devices through open interfaces. The network is then managed centrally and programmatically.

See also: Control Plane, Data Plane, NaaS.

SDP¶

Session Description Protocol. A text format (RFC 8866) for describing multimedia sessions: media types, codecs, addresses and ports. SIP uses it in an offer and answer exchange to agree the media of a call.

SDS¶

Software Design Specification. A document that sets out a system’s detailed design: its modules, interfaces, data models, state machines, timing and memory layout. Ideally it is kept up to date alongside the code.

SDSL¶

Symmetric Digital Subscriber Line. DSL with the same speed in both directions, aimed at businesses. HDSL and G.SHDSL are examples.

Sed¶

Stream editor. A Unix tool, written at Bell Labs in the 1970s, that transforms text line by line using a small command language, most often for substitutions with regular expressions.

Secure Element¶

SE. A tamper-resistant chip that stores keys and runs sensitive code in isolation, with protection against physical and side-channel attacks. SIM cards, payment cards and many IoT devices use one.

See also: Root of Trust, TrustZone, SIM.

SE¶

See Secure Element.

Semaphore¶

A synchronisation primitive: a counter that threads decrement to take one unit of a resource, waiting if none is left, and increment to release it. A binary semaphore works like a lock.

Semantic Versioning¶

SemVer. A version numbering scheme, MAJOR.MINOR.PATCH. Increase the major number for incompatible API changes, the minor number for compatible new features and the patch number for compatible bug fixes. Pre-release and build labels can follow, as in 2.1.0-rc.1+build.5.

Sequence Model¶

A model, such as a UML sequence diagram, that shows the messages exchanged between objects or roles in time order.

SF¶

Superframe. The T1 framing format that groups 12 frames and uses their framing bits for alignment. Extended Superframe (ESF), with 24 frames, replaced it.

SFTP¶

SSH File Transfer Protocol. A protocol for transferring and managing files over an SSH connection. It is unrelated to FTP and FTPS, apart from its purpose.

SGML¶

Standard Generalized Markup Language. The ISO standard (ISO 8879) for defining markup languages that describe a document’s structure rather than its appearance. HTML and XML derive from it.

SGSN¶

Serving GPRS Support Node. The GPRS and UMTS core node that serves devices in its area. It tracks their location, authenticates them and routes their packets to and from the GGSN.

SGW¶

Signalling Gateway. A gateway that carries SS7 signalling between circuit-switched networks and IP networks, for example passing ISUP from an MGCF over SCTP (SIGTRAN) to the SS7 network over MTP.

Sh¶

The Bourne shell, the standard shell of Version 7 Unix, written by Stephen Bourne. /bin/sh on Unix-like systems today is a shell compatible with it, as POSIX defines.

See also: Bash, Ksh, Shell Scripting, POSIX.

SHA¶

Secure Hash Algorithm. A family of cryptographic hash functions published by NIST. SHA-1 (160-bit) is broken and should not be used for security. SHA-2 (SHA-256, SHA-512) and SHA-3 are current.

Shell Scripting¶

Writing scripts for a command-line shell, such as Bash, to automate tasks like handling files and running other tools.

Sidetone¶

The small part of a speaker’s own voice fed back into their earpiece, so that a phone line sounds alive and people do not shout.

SIM¶

Subscriber Identity Module. The smart card, or embedded chip (eSIM), that holds a mobile subscriber’s IMSI and secret key. The network uses them to identify and authenticate the subscriber.

SIMPLE¶

SIP for Instant Messaging and Presence Leveraging Extensions. The IETF’s SIP-based protocols for instant messaging and presence. XMPP is the main alternative.

Simple Design¶

The agile practice of keeping the design as simple as the current needs allow, guided by rules such as Kent Beck’s.

Simplex¶

Communication in one direction only, such as broadcast radio. ITU usage also applies the term to links that work one way at a time, which most people call half duplex.

Single Step Test¶

A test in a debugger, stepping through the source code line by line as it runs.

SIP¶

Session Initiation Protocol. The IETF’s text-based signalling protocol (RFC 3261) for setting up, changing and ending voice, video and messaging sessions. It is modelled on HTTP and carries SDP to agree the media. SIP is the basis of most VoIP and of IMS.

SIP Trunk¶

A VoIP connection, using SIP, between a business’s phone system and a service provider, replacing ISDN lines for external calls.

SISO¶

Single-Input, Single-Output. A system with one input and one output; in radio, a link with one antenna at each end.

Skew¶

Clock skew. The difference in the arrival time of the same clock signal at different parts of a circuit, or between the readings of two clocks.

SLA¶

Service Level Agreement. An agreement between a provider and a customer on the service’s measurable levels, such as availability, response and repair times, with remedies if they are missed.

SLIP¶

Serial Line Internet Protocol. A very simple way of sending IP packets over a serial line (RFC 1055), with no negotiation or error checking. PPP replaced it, though it is still used on small microcontrollers.

SME¶

Subject Matter Expert. A person with deep knowledge of a particular area, often a business domain that the software serves. SME also commonly means small and medium-sized enterprise.

Smoke Test¶

A quick test that a new build works at all, run before deeper testing.

Smoke testing¶

Quick, shallow testing of a system’s most important functions, to decide whether it is stable enough for further testing.

See also: Smoke Test, Regression testing.

SMP¶

Symmetric Multiprocessing. A design in which two or more identical processors, or cores, share one memory and are run by one operating system that treats them all equally. Most multi-core computers work this way.

SMPF¶

Shared Metallic Path Facility. An Openreach product that shares one copper line between two providers. Typically one provides voice over the low frequencies and the other broadband over the high frequencies.

SMSC¶

Short Message Service Centre. The network element that stores SMS messages and forwards them to their destination, retrying until the recipient is reachable.

Snake Case¶

Writing compound names in lower case with underscores between the words, as in snake_case. Python uses it for functions and variables.

SNMP¶

Simple Network Management Protocol. The IETF protocol for monitoring and managing network devices. Managers read and set variables in each device’s MIB, and devices send traps when events happen. SNMPv3 adds proper security.

SNTP¶

Simple Network Time Protocol. A simpler form of NTP, without NTP’s long-term state and filtering, for devices that need only modest clock accuracy.

SOA¶

Service-Oriented Architecture. An architecture style in which applications are built from services. Each service provides a business function through a network interface, can be used without knowing how it works, and can be combined with others.

SOAP¶

An XML-based messaging protocol for web services, usually over HTTP. A SOAP envelope holds a header and a body, and WSDL describes the services. REST has largely replaced it for new APIs.

Softswitch¶

A call control system implemented in software on general-purpose servers, which routes calls in VoIP and next-generation networks. It controls media gateways rather than switching media itself.

Software Change Request¶

See Change Request.

Software Configuration Management¶

See SCM.

Software Development Plan¶

SDP. On software projects, another name for the Project Plan.

Software Engineering Body of Knowledge¶

SWEBOK. The IEEE Computer Society’s guide to the generally accepted knowledge of software engineering, organised into knowledge areas.

Software Engineering Lab¶

SEL. The Software Engineering Laboratory at NASA Goddard, which studied and improved software processes from 1976 into the 2000s.

Software Engineering Process Group¶

SEPG. A group that leads software process improvement in an organisation, and gives projects independent technical oversight.

Software Requirements Specification¶

SRS. A document that captures the requirements for a software system, often alongside models and prototypes.

SOLID Principles¶

Five object-oriented design principles promoted by Robert C. Martin: single responsibility, open-closed, Liskov substitution, interface segregation and dependency inversion. Together they aim for code that is modular and easy to change.

SoGEA¶

Single Order Generic Ethernet Access. An Openreach broadband product over fibre to the cabinet with no phone service on the line, ordered as one product. It is the standalone broadband line used as the copper phone network closes.

SONET¶

Synchronous Optical Network. The North American standard for synchronous multiplexing over fibre, equivalent to SDH, with rates starting at OC-1 (51.84 Mbit/s).

Source Code¶

The human-readable instructions of software, written in a programming language, that a compiler or interpreter turns into something a computer can run.

See also: Machine code, Version Control.

SPEEX¶

Speex. An open, patent-free speech codec from the Xiph.Org Foundation, based on CELP. Opus has replaced it.

SPI¶

Serial Peripheral Interface. A synchronous, full-duplex serial bus for short-distance links between a microcontroller and peripherals such as sensors, flash memory and displays. It uses a clock, two data lines and a chip select for each device.

Spiral Lifecycle¶

A lifecycle made of a series of small projects, each tackling the biggest remaining risks, until the system is delivered.

Spiral Model¶

Barry Boehm’s risk-driven process model. Each loop of the spiral sets objectives, identifies and resolves risks, develops and tests, and plans the next loop. It can borrow from other models, such as waterfall or prototyping, as the risks suggest.

Sprint Planning¶

The Scrum event at the start of a sprint, in which the team agrees the sprint goal and chooses the backlog items to work on.

Sprint retrospective¶

The Scrum event at the end of a sprint, in which the team reviews how it worked and agrees improvements.

See also: Heartbeat Retrospective, Scrum.

Sprints¶

Fixed-length iterations, usually two to four weeks, in which a Scrum team produces a usable increment.

SQL¶

Structured Query Language. The standard language for defining, querying and changing data in relational databases.

SS7¶

Signalling System No. 7. The ITU-T set of protocols that telephone networks use to set up and clear calls and to provide services such as number translation, number portability, prepaid and SMS. In the UK it is also called C7. Its layers include MTP, SCCP, TCAP, ISUP and MAP.

See also: MTP, SCCP, ISUP, TCAP, MAP, CCS.

SSADM¶

Structured Systems Analysis and Design Method. A waterfall method for analysing and designing information systems, developed for and widely used in UK government from the 1980s.

SSH¶

Secure Shell. A cryptographic protocol for logging in to and running commands on remote machines securely over an untrusted network. It also tunnels other traffic and carries SFTP.

SSID¶

Service Set Identifier. The name of a Wi-Fi network.

SSIS¶

SQL Server Integration Services. The Microsoft SQL Server component for data integration and ETL workflows.

SSL¶

Secure Sockets Layer. The predecessor of TLS. All SSL versions are broken and must not be used, though “SSL” is still often said when TLS is meant. See TLS.

Stack¶

A protocol stack: the layered set of protocols that together provide communication, such as Ethernet, IP, TCP and HTTP, or the software that implements them.

mermaid diagram

A typical protocol stack, from application to physical layer.

See also: Internet Protocol Suite, L1, L2.

Staff Days¶

A measure of effort: how much work an activity needs, in days of one person’s time, as distinct from calendar days.

Stage¶

Often a synonym for phase. More precisely, a phase can be divided into stages.

Staged Delivery Lifecycle¶

A lifecycle that delivers a system in stages, with the most important functions first.

Stakeholder¶

A person or organisation with an interest in a project’s outcome, such as its customers, users and sponsors, and any group it affects.

Standard¶

An agreed, documented set of rules, conventions or requirements that work must follow.

Statistical Estimation¶

Estimating with a statistical model built from historical data, usually predicting effort and time from size. It works best when calibrated on similar past projects.

STB¶

Set-Top Box. A device that receives television signals, by aerial, satellite, cable or IP, and turns them into pictures and sound on a TV.

Story Mapping¶

Arranging user stories in two dimensions: along the user’s journey from left to right, and by priority from top to bottom. The map helps plan releases.

Story Splitting¶

Breaking a user story into smaller stories, each of which still has value of its own.

Storyboard¶

A sequence of pictures showing a scenario or use case.

See also: Scenario, Low Fidelity Prototype.

STRIDE Threat Model¶

Microsoft’s mnemonic for six kinds of security threat: Spoofing, Tampering, Repudiation, Information disclosure, Denial of service and Elevation of privilege. Each one threatens a security property: authenticity, integrity, non-repudiation, confidentiality, availability and authorisation.

STP¶

Spanning Tree Protocol. The IEEE 802.1D protocol that stops loops in Ethernet networks by blocking redundant links so that one loop-free path remains, and unblocking them if the active path fails. Radia Perlman invented the algorithm.

Strategy¶

The overall approach taken to a problem or goal.

STUN¶

Session Traversal Utilities for NAT. A protocol (RFC 8489) with which a device behind a NAT discovers the public address and port the NAT has given it. ICE, SIP and WebRTC use it.

Subversion¶

SVN. Apache Subversion, a centralised version control system designed as a successor to CVS.

Sustainable Pace¶

Working at a pace the team could keep up indefinitely, without overtime and burnout.

SVC¶

Switched Virtual Circuit. A virtual circuit set up on demand for a call or transfer and cleared when it ends.

Swift¶

Apple’s compiled, multi-paradigm language for its platforms, released in 2014 as the successor to Objective-C. It is open source and also runs on Linux.

Switch¶

A network device that connects devices on a LAN and forwards each frame only to the port where its destination MAC address is. It learns those addresses from the traffic it sees. A layer-3 switch can also route.

System¶

A set of hardware, software and processes that work together. In a project, the system is what the project was set up to build.

System Test¶

A test of the whole system’s behaviour, using a project build in the system test environment.

System Test Environment¶

An environment that copies the live environment and its conditions as closely as possible, for system testing.

SystemX¶

System X. The UK’s national digital telephone exchange system, developed by the Post Office (later BT) with GEC, Plessey and STC, and introduced in the 1980s. It includes concentrators, digital local exchanges (DLEs) and trunk exchanges (DMSUs).

T¶

T-Shirt Sizing¶

Estimating by sorting items into rough relative sizes, such as extra small, small, medium, large and extra large, rather than giving false precision.

Task¶

A small, distinct unit of work in a project.

Task Board¶

A board with columns such as “To do”, “In progress” and “Done”, on which cards for tasks move as work progresses.

Task Estimate¶

An estimate for a single task, usually made during planning and tracking and fed into work plans and detailed schedules.

TCAP¶

Transaction Capabilities Application Part. The SS7 layer that manages dialogues between applications, such as MAP, INAP and CAP, using transaction IDs to run many at once. It encodes messages with ASN.1 BER.

Tcl/tk¶

Tcl is a dynamic scripting language, and Tk is the cross-platform GUI toolkit that grew up with it. Together they offer a quick way to build desktop interfaces, and other languages, such as Python’s Tkinter, also use Tk.

TCP¶

Transmission Control Protocol. The internet’s main transport protocol (RFC 9293). It gives applications a reliable, ordered, error-checked byte stream over IP, with flow and congestion control. The web, email and file transfer use it.

Tcsh¶

An enhanced C shell that adds command-line editing and completion. It was long the default shell for the root user on FreeBSD.

TDD¶

Test-Driven Development. Writing a failing unit test first, then the simplest code that passes it, then refactoring, in short repeated cycles.

See also: Unit Testing, Refactoring, BDD.

TDM¶

Time-Division Multiplexing. Sharing a channel by giving each signal its own repeating time slot. Digital telephony carries each call in a 64 kbit/s slot, for example 30 calls on an E1.

Team¶

In agile terms, a small group of people working on the same product or goal, nearly all of them full time.

Team Room¶

A space set aside for a team for the length of a project, so that its members, including the product owner, can work together closely.

Technical Debt¶

The future cost of rework caused by choosing a quick solution now over a better one that would take longer. Like money, it gathers interest: left unpaid, it makes every later change harder.

Template¶

An outline or framework for creating an artefact.

Test¶

A single test case, or testing in general.

Test Activity¶

A specific test procedure, combining one or more test techniques and test types.

Test Case¶

TC. The inputs, steps and expected results that check whether a feature works or a requirement is met.

Test Case Specification¶

TCS. A document listing the test cases that verify one or more features.

Test Design¶

Working out how to test a product: linking test cases to its requirements and design, when testing needs real analysis and structure.

Test Design Specification¶

TDS. A document recording the test design for a group of test cases.

Test Level¶

See Test Type.

Test Plan¶

TP. A document setting out the scope, approach, resources, test items and schedule of testing.

Test Technique¶

The method used in a test activity, such as boundary value analysis or equivalence partitioning.

Test Type¶

A category of testing, such as unit, integration, system or acceptance testing, defined by what it tests, how, why and when.

Testing¶

Running software to find defects and to show that it meets its requirements.

Three Amigos¶

A meeting of the three main perspectives on a piece of work, business, development and testing, to agree what it should do before, during and after development.

Three C’s¶

Card, Conversation, Confirmation. The three parts of a user story: a short note on a card, the conversation that fills in the detail, and the acceptance tests that confirm it is done.

Three Questions¶

The traditional structure of the daily stand-up: what did I complete, what will I do next, and what is in my way?

TIA¶

Telecommunications Industry Association. The US trade body, accredited by ANSI, that develops standards such as TIA-568 for structured cabling.

Timebox¶

A fixed, agreed period in which a person or team works towards a goal, and which ends on time whether or not everything is done.

TKIP¶

Temporal Key Integrity Protocol. The stop-gap Wi-Fi security protocol, from 802.11i, that replaced WEP on existing hardware. It is no longer secure and was deprecated in 2012.

TLS¶

Transport Layer Security. The protocol that encrypts and authenticates connections between applications, such as HTTPS. A handshake authenticates the server, usually with an X.509 certificate, and agrees keys. Symmetric encryption then protects the data. TLS 1.3 (RFC 8446) is current, and it replaced SSL.

mermaid diagram

A TLS 1.3 handshake, simplified.

TLV¶

Type-Length-Value. An encoding in which each element has a type code, a length and then the value. Receivers can skip elements they do not understand, so formats can grow. BER, IS-IS and many other protocols use it.

TOGAF¶

The Open Group Architecture Framework. A widely used enterprise architecture framework. Its Architecture Development Method (ADM) guides architecture work across the business, data, application and technology domains.

Token Ring¶

A LAN technology from IBM, standardised as IEEE 802.5. A token passes round a ring of stations, and only the station holding it may transmit. There are therefore no collisions. Ethernet replaced it.

TOML¶

Tom’s Obvious, Minimal Language. A configuration file format, created by Tom Preston-Werner, designed to be easy to read and to map clearly to a dictionary. Python’s pyproject.toml and Rust’s Cargo.toml use it.

Top Level Schedule¶

See Business Schedule.

Top-Down Estimation¶

Estimating a whole system first, then dividing the total among its parts.

Triple DES¶

3DES, or TDEA. DES applied three times to each block with two or three keys, to extend its life after DES’s 56-bit key became too short. It is slow and now deprecated in favour of AES.

TTY¶

Teletypewriter. Originally an electromechanical typewriter used as a terminal. On Unix the name now means any terminal device, including a terminal emulator window.

TrustZone¶

Arm’s hardware security extension that splits a processor into a secure world and a normal world. The secure world holds keys, secure boot and trusted applications, isolated from the main operating system. It exists on Cortex-A and, as Armv8-M, on Cortex-M.

See also: Root of Trust, Secure Element.

TUP¶

Telephone User Part. The first SS7 call control protocol, for ordinary telephone calls without ISDN features. ISUP has largely replaced it.

Twisted pair¶

Cable in which the two wires of each circuit are twisted together, which cuts interference and crosstalk. It is used for telephone lines and Ethernet.

U¶

UA¶

User Agent. Software acting for a user. In HTTP, the browser or client, which names itself in the User-Agent header; in SIP, an endpoint that makes or receives calls.

Ubiquitous Language¶

In domain-driven design, a shared vocabulary drawn from the business domain, used consistently by everyone, from conversations and requirements through to the code.

UDP¶

User Datagram Protocol. A simple, connectionless transport protocol (RFC 768) that sends datagrams with port numbers and a checksum, but no guarantee of delivery or order. DNS, VoIP media and streaming use it, along with protocols that add their own reliability.

Bits 0 to 15	Bits 16 to 31
Source port	Destination port
Length	Checksum
Data	Data

The UDP header: four 16-bit fields, then the data.

UER¶

Universal Edge Router. Juniper’s name for its broadband edge routers, which do the job of a BRAS.

UML¶

Unified Modeling Language. The standard graphical language for modelling software systems, maintained by the Object Management Group. It was created by Grady Booch, Ivar Jacobson and James Rumbaugh in the mid-1990s. Its diagrams include class, sequence, activity, state and use case diagrams.

See also: Class Model, Sequence Model, Use Case.

UMTS¶

Universal Mobile Telecommunications System. The 3GPP third-generation (3G) mobile system, which evolved from GSM. It uses wideband CDMA radio, and HSPA later speeded up its data.

UNI¶

User-Network Interface. The demarcation between a service provider’s network and a customer’s equipment, where the provider’s responsibility ends.

Unicast¶

Sending data from one sender to one receiver, each identified by its own address. Most internet traffic is unicast.

See also: Multicast, Broadcast, Anycast.

Unicode¶

The standard that gives a unique number, or code point, to every character in the world’s writing systems, plus symbols and emoji. It is kept in step with ISO/IEC 10646. UTF-8 and UTF-16 are ways of encoding it.

Unified Communications¶

UC. Bringing an organisation’s communication services, such as voice, video, messaging, presence, conferencing and voicemail, together behind one consistent user experience across devices.

Unit Test¶

The lowest level of component test: a test of one small piece of code on its own.

Unit Testing¶

Writing and running small automated tests, written by the developers, that each check a narrow piece of the code.

Unix¶

A family of multi-user, multitasking operating systems descended from the original developed at Bell Labs by Ken Thompson, Dennis Ritchie and others from 1969. BSD, Solaris, AIX and macOS are Unix systems; Linux is Unix-like.

Update¶

Planned work to bring an artefact up to date with the project’s current needs.

Upstream¶

In a project, the early activities and artefacts, such as chartering, planning, requirements and architecture. In networking, the direction from the customer towards the network.

URI¶

Uniform Resource Identifier. A string that identifies a resource (RFC 3986). URLs, which say where to find it, and URNs, which name it, are both kinds of URI.

URL¶

Uniform Resource Locator. A URI that says how and where to get a resource: a scheme, such as https, a host, a path and optional query and fragment, as in https://example.com/page?id=1#top.

URN¶

Uniform Resource Name. A URI that names a resource permanently, without saying where it is, as in urn:isbn:0451450523.

Usability Testing¶

Watching real users try to do realistic tasks with a product, to find where it is hard to use.

Use Case¶

A description of how a user (actor) interacts with a system to achieve a goal, including the main path and alternatives.

Use Case Model¶

The set of use cases and actors that describes a system’s behaviour from the outside.

Use Case Specification¶

The document that describes one use case in full.

User Experience¶

UX. The whole of a person’s experience of using a product, including but going beyond its user interface: how useful, easy and pleasant it is.

User Interface¶

UI. The part of a system through which people interact with it, such as screens, controls and messages.

User Interface Design¶

Designing how people interact with a system: layout, controls, flows and feedback.

User Interface Prototype¶

A prototype built to explore a user interface’s behaviour or look and feel.

User Plane¶

The part of a network that carries users’ traffic, as opposed to the control plane, which carries signalling. In routers it is also called the data plane or forwarding plane. In mobile networks, separating the two planes lets each scale on its own.

User Stories¶

Short descriptions of functionality from the user’s point of view, agreed with the customer or product owner, that the team uses to divide up the work.

UTC¶

Coordinated Universal Time. The world’s primary time standard, kept by atomic clocks and adjusted with leap seconds to stay close to the Earth’s rotation. It has no daylight saving time, and for everyday purposes it is the same as GMT.

UTF-16¶

A Unicode encoding that uses one 16-bit unit for characters in the Basic Multilingual Plane and two units, a surrogate pair, for the rest. Windows, Java and JavaScript use it internally. UTF-32, by contrast, uses a fixed 32 bits for every code point.

UTF-8¶

The most widely used Unicode encoding, designed by Ken Thompson and Rob Pike. It uses one to four bytes per character, and the first 128 characters are the same as ASCII, so ASCII text is valid UTF-8. It is the default encoding of the web.

UTRAN¶

UMTS Terrestrial Radio Access Network. The 3G radio access network: the Node Bs and the RNCs that control them.

V¶

V Model¶

A development model that extends waterfall by pairing each specification and design stage with the test stage that checks it. Requirements pair with acceptance testing, and so on down to unit tests. Drawn out, the stages form a V.

mermaid diagram

The V model: each design stage is verified by a matching test stage.

V.90¶

The ITU-T standard, from 1998, for 56 kbit/s dial-up modems. It achieves 56 kbit/s downstream and 33.6 kbit/s upstream by using the digital connection at the network end.

V.92¶

The ITU-T successor to V.90, which raised upstream speed to 48 kbit/s and added quicker connection and modem-on-hold.

VAD¶

Voice Activity Detection. Detecting whether speech is present in an audio signal. VoIP and mobile systems use it to stop sending during silence, which saves bandwidth, and generate comfort noise at the other end.

VB¶

Visual Basic. Microsoft’s event-driven language and IDE, first released in 1991, for building Windows applications quickly from forms and components. Classic VB became legacy in 2008; VB.NET is its .NET successor.

VBS¶

VBScript. Microsoft’s scripting language based on Visual Basic, used for Windows administration and old web pages. Microsoft is removing it from Windows.

VC¶

Virtual Circuit. A connection across a packet network that behaves like a dedicated circuit between two endpoints, set up before data flows. X.25, Frame Relay and ATM use virtual circuits.

VCI¶

Virtual Channel Identifier. The 16-bit field in an ATM cell header that, with the VPI, identifies the virtual channel the cell belongs to.

VCID¶

Virtual Circuit Identifier. The identifier that both ends of a pseudowire use to match up the same circuit, for example in an MPLS VPWS.

VDSL¶

Very-high-bit-rate DSL. DSL using frequencies up to 30 MHz on short lines. VDSL2 (G.993.2) gives tens of megabits per second, and up to around 80 Mbit/s downstream from a street cabinet. It is the technology of fibre-to-the-cabinet broadband.

Velocity¶

The amount of work, usually in story points, that a team completes in an iteration. Teams use it to forecast what they can take on next.

Verifier¶

In an inspection, the person who checks that the rework has been done properly.

Version¶

A formally released revision of an artefact, recognised by configuration management.

See also: Revision, Semantic Versioning.

Version Control¶

Recording every change to files, such as source code, so that any earlier version can be recovered and several people can work in parallel and merge their work. Git is the most widely used tool.

VHDL¶

VHSIC Hardware Description Language. A language, standardised as IEEE 1076, for describing and simulating digital hardware, from systems down to logic gates. It came out of the US Very High-Speed Integrated Circuits programme in the 1980s. Verilog is its main alternative.

VISP¶

Virtual ISP. An internet service provider that sells another ISP’s service under its own brand.

VLAN¶

Virtual LAN. A separate broadcast domain created on shared switches, usually by tagging frames with an 802.1Q VLAN ID, so that one physical network carries several isolated LANs.

See also: Dot1Q, Dot1Q trunking, Switch.

VLR¶

Visitor Location Register. The database, usually built into each MSC, of the subscribers currently in its area. It copies their details from their HLR.

VM¶

Virtual Machine. A software emulation of a computer, on which an operating system and applications run as if on real hardware. A hypervisor shares the physical machine between several VMs.

VOD¶

Video on Demand. Letting viewers choose and watch video when they want, rather than at a broadcast time, by streaming or downloading.

VoIP¶

Voice over IP. Carrying voice calls over IP networks, with signalling protocols such as SIP to set up calls and RTP to carry the media.

VoLTE¶

Voice over LTE. Voice calls over an LTE network as IP packets, using IMS, following the GSMA IR.92 profile. It needs no circuit-switched network, connects calls faster and supports HD voice.

Vorbis¶

An open, royalty-free lossy audio codec from the Xiph.Org Foundation, usually used in the Ogg container (Ogg Vorbis). Opus has largely replaced it.

VPI¶

Virtual Path Identifier. The field in an ATM cell header that identifies a virtual path, a bundle of virtual channels. See VCI.

VPLS¶

Virtual Private LAN Service. A provider service that joins several customer sites into one Ethernet broadcast domain over an MPLS network, using a mesh of pseudowires. It is defined in RFCs 4761 and 4762.

VPN¶

Virtual Private Network. A private network built over a shared or public one, such as the internet or a provider’s MPLS core. It uses tunnelling and often encryption, so that remote sites or users act as if directly connected.

VPWS¶

Virtual Private Wire Service. A point-to-point layer-2 service, such as Ethernet, carried over an MPLS network by a pseudowire. It is also called a virtual leased line or Ethernet over MPLS (EoMPLS).

VRF¶

Virtual Routing and Forwarding. Several separate routing tables in one router, so that different customers or networks, even with overlapping addresses, stay apart. It is the building block of MPLS layer-3 VPNs.

W¶

W-CDMA¶

Wideband CDMA. The UMTS air interface, which uses direct-sequence CDMA over pairs of 5 MHz channels.

W3C¶

World Wide Web Consortium. The international body, founded by Tim Berners-Lee, that develops standards for the web, such as HTML, CSS and XML.

Walkthrough¶

An informal review in which the author takes one or more people through an artefact to find defects.

WAN¶

Wide Area Network. A network that spans a large area, linking sites in different cities or countries, usually over carriers’ circuits. The internet is the largest WAN.

WANET¶

Wireless Ad hoc Network. A decentralised wireless network with no routers or access points, in which every node forwards traffic for the others. A MANET is the mobile kind.

WAP¶

Wireless Access Point. A device that lets Wi-Fi devices connect to a wired network. WAP also stood for Wireless Application Protocol, an early way of browsing on mobile phones.

Waterfall¶

A sequential development model in which work flows through requirements, design, construction, testing and deployment, each finished before the next starts. Winston Royce’s 1970 paper, often cited as its origin, described it as risky.

Waterfall Lifecycle¶

A lifecycle that moves through ordered phases, from concept to delivery, with a review at the end of each.

WBC¶

Wholesale Broadband Connect. BT Wholesale’s broadband access product on the 21CN network. ISPs connect at regional aggregation points to collect their customers’ traffic.

WBCC¶

Wholesale Broadband Connect Converge. A BT Wholesale package, based on WBC, that combined voice and broadband on one bill for the ISP.

WBMC¶

Wholesale Broadband Managed Connect. A BT Wholesale product, built on WBC, in which BT carries the traffic end to end to a single handover point. It replaced IPStream.

Weekly Individual Plan¶

A plan in which each person sets their miniature milestones for the week.

WEP¶

Wired Equivalent Privacy. The original Wi-Fi security from 1997, which is easily broken. WPA and WPA2 replaced it, and it was deprecated in 2004.

What Requirement¶

See Functional Requirement.

Why Requirement¶

See Business Requirement.

Wi-Fi¶

Wireless LAN technology based on the IEEE 802.11 standards. Wi-Fi is the Wi-Fi Alliance’s trademark for certified products. Recent generations are numbered, such as Wi-Fi 6 (802.11ax) and Wi-Fi 7 (802.11be).

Wiki¶

A website that its users edit together in the browser, usually with simple markup.

WiMAX¶

Worldwide Interoperability for Microwave Access. Wireless broadband based on the IEEE 802.16 standards, promoted as an alternative to DSL and as a 4G candidate. LTE won out.

WIN-T¶

Warfighter Information Network-Tactical. The US Army’s mobile tactical communications network, giving units secure voice, data and video on the move without fixed infrastructure.

Wireless Mesh Network¶

A wireless network in which nodes connect in a mesh and relay traffic for each other, with some nodes acting as gateways to a wired network. The nodes rarely move, so routes stay stable.

See also: WANET, MANET, Network Topologies.

Wireless Repeater¶

A device that receives a Wi-Fi signal and rebroadcasts it to extend coverage. Also called a range extender.

Wireshark¶

A free, open-source packet analyser, formerly called Ethereal, used for troubleshooting networks and developing protocols.

WISP¶

Wireless ISP. An internet service provider whose access network is wireless, such as fixed wireless or Wi-Fi.

WLR¶

Wholesale Line Rental. A UK product that let a provider rent a phone line from Openreach and bill the customer for line and calls. It is being withdrawn as the copper phone network closes.

WMA¶

Windows Media Audio. Microsoft’s family of audio codecs, including lossy, professional, lossless and voice versions.

Work Breakdown Structure¶

WBS. A hierarchical breakdown of all a project’s work into work packages. Larger projects keep it as a separate document; smaller ones hold it in the work plan or schedule.

Work Item¶

A unit of tracked work, such as a task, story or defect. Also used for the artefact the work produces.

Work Package¶

The lowest level of a work breakdown structure, often a tangible deliverable, which is broken down further into activities and tasks for estimating and scheduling.

Work Plan¶

A plan of the work packages and their task estimates, used to plan, track and control detailed work.

Work Product¶

See Artefact.

WPA2¶

Wi-Fi Protected Access 2. The Wi-Fi security certification, from 2004, based on the full IEEE 802.11i standard with AES encryption. It replaced WEP and WPA, and WPA3 now succeeds it.

WPS¶

Wi-Fi Protected Setup. A Wi-Fi Alliance scheme, from 2006, for adding devices to a network with a button or PIN rather than a passphrase. The PIN method is insecure and should be disabled.

Write-only code¶

Code so hard to read that nobody, often not even its author, can understand it later.

WS-BPEL¶

Web Services Business Process Execution Language. The full name of BPEL.

WSDL¶

Web Services Description Language. An XML language for describing a web service’s interface: its operations, messages, data types and endpoints. It is mostly used with SOAP.

X¶

X.25¶

An ITU-T packet-switching protocol suite for WANs, from 1976, using virtual circuits with error correction at every hop. It was widely used in the 1980s, for example for cash machines, and IP has replaced it.

X.509¶

The ITU-T standard format for public key certificates, which bind an identity, such as a host name, to a public key under a CA’s signature. TLS and many other protocols use them.

XML¶

Extensible Markup Language. A W3C text format for structured documents and data, with user-defined, nested elements. It is the basis of many formats and protocols, such as SOAP and SVG.

XML Schema¶

A description of the allowed structure and content of a type of XML document, used to validate documents. DTD, XSD and RELAX NG are schema languages.

XML-RPC¶

A simple remote procedure call protocol that encodes calls in XML and sends them over HTTP. SOAP grew out of it.

XMPP¶

Extensible Messaging and Presence Protocol. An open, XML-based protocol for instant messaging and presence, originally called Jabber. It has been extended for many other real-time uses.

XP¶

Extreme Programming. An agile method that focuses on engineering practices, such as pair programming, test-driven development, continuous integration, refactoring and small releases.

Xsd¶

XML Schema Definition. The W3C’s XML schema language. It describes the elements and attributes an XML document may contain, with data types, so documents can be validated.

XSS¶

Cross-Site Scripting. A web vulnerability in which an attacker gets a site to include their script in pages other users view. The script then runs with the site’s privileges in the victims’ browsers. Encoding output and a content security policy prevent it.

Y¶

YAML¶

YAML Ain’t Markup Language. A human-readable data format that uses indentation for nesting, widely used for configuration files. YAML 1.2 is a superset of JSON.

Z¶

ZigBee¶

A low-power, low-rate wireless mesh protocol based on IEEE 802.15.4, used in home automation and sensors. It is simpler and uses less power than Bluetooth or Wi-Fi.

See also: Bluetooth, Wireless Mesh Network.

Numbers¶

.NET¶

Microsoft’s software platform for building and running applications. Code compiles to an intermediate language that the Common Language Runtime runs, with a large class library. The original .NET Framework was Windows only; modern .NET is open source and cross-platform.

100 Mbps (Fast Ethernet)¶

Fast Ethernet. Ethernet at 100 Mbit/s, introduced in 1995 as IEEE 802.3u. 100BASE-TX over Cat 5 cable is by far the most common form.

5ESS¶

A digital telephone switch developed by Western Electric for AT&T, first in service in 1982. It became one of the most widely used local exchanges in the United States.

5G¶

The fifth generation of mobile networks, standardised by 3GPP from Release 15 (2018) to meet the ITU’s IMT-2020 targets. It uses the New Radio (NR) air interface and a service-based core. It offers faster broadband, low-latency services and very large numbers of IoT devices. Speeds depend heavily on the frequency band used.

802.11¶

The IEEE family of standards for wireless LANs, first published in 1997, which defines the physical and MAC layers that Wi-Fi products use. Amendments add speed and features, such as 802.11n, ac, ax and be.

802.3 Physical Layer¶

IEEE 802.3. The IEEE working group and family of standards that define wired Ethernet: the physical layers and the MAC. They cover speeds from 10 Mbit/s to hundreds of gigabits per second, over copper and fibre.

See also: Ethernet over twisted pair, MAC, CSMA.

S to Z and numbers¶

S¶

S-Tag¶

S-GW¶

SAAL¶

SaaS¶

SAE¶

SAFe¶

SBC¶

SC-FDMA¶

Scala¶

Scaled Agile¶

SCCP¶

Scenario¶

Schedule¶

Scheduler¶

SCM¶

Scratch¶

Scribe¶

Scrum¶

Scrum Master¶

Scrum of Scrums¶

SCTP¶

SDES¶

SDH¶

SDK¶

SDLC¶

SDMA¶

SDN¶

SDP¶

SDS¶

SDSL¶

Sed¶

Secure Element¶

SE¶

Semaphore¶

Semantic Versioning¶

Sequence Model¶

SF¶

SFTP¶

SGML¶

SGSN¶

SGW¶

Sh¶

SHA¶

Shell Scripting¶

Sidetone¶

Sign Up¶

SIM¶

SIMPLE¶

Simple Design¶

Simplex¶

Single Step Test¶

SIP¶

SIP Trunk¶

SISO¶

Skew¶

SLA¶

SLIP¶

SME¶

Smoke Test¶

Smoke testing¶

SMP¶

SMPF¶

SMSC¶

Snake Case¶

SNMP¶

SNTP¶

SOA¶

SOAP¶

Softswitch¶

Software Change Request¶

Software Configuration Management¶

Software Development Plan¶

Software Engineering Body of Knowledge¶

Software Engineering Lab¶

Software Engineering Process Group¶

Software Requirements Specification¶

SOLID Principles¶

SoGEA¶