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10/5/2009
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Q
Quad cable
An audio multi-pair cable with copper conductors where groups of four conductors (a quad) are the basic building block. The central core of the cable comprises one or four quads with further quads being built up in layers around the central core.
Quality of Service applies to a wide range of parameters that determines how a user perceives a networks service, it includes the following factors:
- Availability.
- Errors, caused by noise, congestion or component failures
- Delay
- Jitter (variation on delay)
- Bandwidth or throughput
- Loss
Performance and QoS are closely related.
In the context of IP services in particular, it implies prioritising network resources against pre-determined classifications of traffic determined by class of service (CoS). The lowest classification is 'best effort' which is used by the public Internet. Session-based QoS is used to improve the delivery of a flow or application session to an end user or users and allows a particular application session to be distinguished fromother traffic being received by the same user. This is described in more detail in Session-based quality of service, BTTJ, Vol 22, No 2.
A quantised level, such as in Pulse Code Modulation, means that when an analogue signal is sampled the resulting value is allocated to a specific level. An analogue signal waveform can have an infinite number of levels but these are converted into a finite number of levels for subsequent digital coding. In converting the sampled level to a quantised level some rounding up or down will be involved such that when a large number of sequential samples are quantised, encoded, transmitted, decoded and restored the resulting signal will have steps in it whereas the original was of a smooth waveform. If the sampling rate is sufficiently high then the resulting waveform distortion, manifested as noise and known as quantisation noise or quantisation distortion, will not be significant.
A quantum unit of information. The basic element of quantum computing and communications. A qubit makes use of the quantum mechanical properties of atoms and can be multi-dimensional compared to a standard binary digit (bit) thus allowing different approaches to computing and communications. For instance, two related qubits will stay related even when separated by distance. The concept has been around for many decades (since the 1970s) but will probably one day produce radical new techniques.
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