18 illustrated definitions. Start with a plain explanation, try an example, then follow the term into the field guide.
TERM 01
Bandwidth
In everyday networking language, the data-carrying capacity of a connection, usually expressed in bits per second. The precise meaning depends on the layer measured; useful application throughput can be lower.
FOR EXAMPLE
Illustrative: a 100-megabit-per-second connection can still have a long wait before the first response arrives.
Keep this distinction: Networking usage differs from radio-frequency bandwidth, which is a frequency range measured in hertz.
The constraint that limits the rate a connected process or service can sustain under the current conditions. Increasing capacity elsewhere may have little effect until that limiting constraint changes.
FOR EXAMPLE
Illustrative: faster train acceleration cannot by itself remove a station occupation limit that sets the achievable interval.
Keep this distinction: The busiest-looking component is not necessarily the constraint limiting the whole service.
An available inventory or margin that absorbs a temporary mismatch. Water storage can bridge unequal inflow and outflow, while spare timetable time can absorb extra occupation before a constraint becomes limiting.
FOR EXAMPLE
Illustrative: a 120-second planned train interval with a 90-second station constraint leaves 30 seconds of margin in the simplified model.
Keep this distinction: A buffer delays or absorbs an effect within limits; it does not remove the underlying constraint.
The amount or rate a system can accommodate under stated conditions. Its units and limits depend on the service: stored volume, electrical output, data transfer and train throughput describe different capacities.
FOR EXAMPLE
Illustrative: a reservoir holding 600 cubic metres and a pump delivering 100 cubic metres per hour describe two different capacities.
Keep this distinction: Capacity is an available limit; actual use or throughput may be lower.
A reliance on another resource, service or system to perform a function. The relationship can exist within one network or across sectors, and its effects depend on available alternatives and buffers.
FOR EXAMPLE
Illustrative: an electric pump depends on an available compatible electrical supply to move water.
Keep this distinction: A dependency can be one-way; interdependency describes mutual reliance.
The time a train spends stopped at a station for the passenger exchange and associated departure activities. Its effect on following trains depends on the available margin and other constraints.
FOR EXAMPLE
Illustrative: an extra 15 seconds at one stop can consume spare time without necessarily changing the sustained headway.
Keep this distinction: A longer dwell does not always reduce throughput; it must become a limiting constraint.
The number of cycles completed by an alternating electrical waveform each second, measured in hertz. Grid frequency is linked to the dynamic balance between electrical supply and demand.
FOR EXAMPLE
Illustrative: a 50-hertz alternating waveform completes 50 cycles in one second.
Keep this distinction: Electrical frequency is different from train service frequency, such as departures per hour.
A quantity that can be stored, transferred or converted between forms. Electrical use is often measured in kilowatt-hours, combining the rate of power use with the duration of operation.
FOR EXAMPLE
Illustrative: an ideal 1-kilowatt-hour store supplies a 1-kilowatt load for one hour if losses and other limits are ignored.
Keep this distinction: A kilowatt-hour measures energy; a kilowatt measures power.
The time interval between successive trains at the same reference point. A planned headway describes the intended spacing; the achievable interval depends on the railway's limiting operating conditions.
FOR EXAMPLE
Illustrative: a steady two-minute interval gives 30 trains passing that point in an hour.
Keep this distinction: Headway separates trains; dwell measures one train's time at a stop.
The tendency of a moving mass to resist a change in motion. In electricity systems, energy in synchronously connected rotating machinery can initially oppose rapid frequency changes after a power imbalance.
FOR EXAMPLE
Illustrative: a spinning machine briefly gives up some stored rotational energy as its speed falls after a disturbance.
Keep this distinction: Fast controlled response from an inverter is not automatically the same physical mechanism as rotating inertia.
The time taken for data to travel between specified points. A measurement may describe one-way delay or a round trip; propagation, transmission, processing and waiting in queues can contribute.
FOR EXAMPLE
Illustrative: a small request takes 40 milliseconds to make a round trip, even on a high-capacity link.
Keep this distinction: Latency is elapsed time; bandwidth describes a data-carrying rate.
A unit of data handled by a network protocol. An IP packet includes a header used for delivery and a payload; one application message may require many packets.
FOR EXAMPLE
Illustrative: sending a photograph involves many packets rather than one photograph-sized object moving intact through each router.
Keep this distinction: A packet is a delivery unit, not necessarily a complete message, page or file.
The rate at which energy is transferred or converted. Electrical power is commonly measured in watts or kilowatts; one watt represents one joule of energy transferred each second.
FOR EXAMPLE
Illustrative: a constant 2-kilowatt load operating for half an hour uses 1 kilowatt-hour of energy.
Keep this distinction: Power is a rate; energy is the accumulated amount transferred or stored.
Additional equipment or paths able to support a function if another becomes unavailable. Their value depends on usable capacity, operating arrangements and whether the same event can disable the alternatives together.
FOR EXAMPLE
Illustrative: two network routes share one exposed power source, so that source can remain a common point of failure.
Keep this distinction: Two lines on a diagram do not prove independent, functioning alternatives.
The ability to prepare for disruption, withstand its effects, adapt and recover. It depends on equipment, people, arrangements and supporting services, and can include maintaining a reduced but useful service.
FOR EXAMPLE
Illustrative: stored water maintains a limited supply while the utility's normal pumping support is unavailable.
Keep this distinction: Resilience does not mean immunity to failure or an unlimited backup supply.
A resource held for later use, such as water in a reservoir or energy in a battery. Usable inventory, delivery limits and replenishment determine what service that store can support.
FOR EXAMPLE
Illustrative: 600 cubic metres serving a fixed demand of 100 cubic metres per hour gives six hours of volume buffer with no inflow.
Keep this distinction: An inventory does not by itself establish deliverable pressure, power or service quality.
The volume of water passing a point during a stated time interval. Comparing inflow and outflow reveals whether a store is filling, emptying or holding a constant volume.
FOR EXAMPLE
Illustrative: inflow of 150 cubic metres per hour minus outflow of 100 adds 50 cubic metres per hour to storage.
Keep this distinction: Flow rate describes volume per time; pressure describes force per area.
The force exerted per unit area by water. In a distribution network, elevation, pumping and losses along the flow path help determine the pressure available at a particular location.
FOR EXAMPLE
Illustrative: a tank still contains water, but that fact alone does not establish adequate pressure at an uphill tap.
Keep this distinction: Pressure is not stored volume or flow rate, and it does not certify water quality.