Comfort Noise Is

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Comfort Noise is 'realistic' background acoustic noise that is inserted at a receiver during periods that there is no signal received.

Comfort noise is used in association with discontinious transmission (DTX).

DTX means that a transmitter is switched off during silent periods. Therefore, the background acoustic noise abruptly disappears

at the receiving end. This can be very annoying for the receiving party. The receiving party might even think that the line is dead

if the silent period is rather long. To overcome these problems 'comfort noise' is generated at the receiving end whenever the

transmission is switched off. The comfort noise is generated by a Comfort Noise Generator (CNG). To generate 'realistic' acoustic

background noise the DTX module sends Silence Insertion Descriptor (SID) frames periodically to the CNG module at the receiving

end.

If the comfort noise is well matched to that of the transmitted background acoustic noise during speech periods, the gaps between

speech periods can be filled in such a way that the receiving party does not notice the switching during the conversation. Since the

noise constantly changes, the comfort noise generator should be updated regularly.

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Voice Activity Detection (VAD) is a (software) module to detect if there is a silent period during a speech connection. This

knowledge is used to switch off the transmission during silent periods. This is called discontinuous transmission (DTX).

Voice activity detection must be very accurate. If it considers speech as noise, part of the conversation will get lost. If on the other

hand, it can not effectively detect a silent period, DTX will not be very effective.

If the transmission stops during silent periods, the background acoustic noise abruptly disappears at the receiving end. This can

be very annoying for the receiving party. To overcome this problem 'comfort noise' is generated at the receiving end whenever the

transmission is switched off. The comfort noise is generated by a Comfort Noise Generator (CNG). To generate 'realistic' acoustic

background noise the DTX module sends Silence Insertion Descriptor (SID) frames periodically to the CNG module at the receiving

end.

The Visitor Location Register(VLR) is a database in a mobile communications network associated to a Mobile Switching Centre

(MSC). The VLR contains the exact location of all mobile subscribers currently present in the service area of the MSC. This

information is necessary to route a call to the right base station. The database entry of the subscriber is deleted when the

subscriber leaves the service area.

The Visitor Location Register(VLR) is a database in a mobile communications network associated to a Mobile Switching Centre

(MSC). The VLR contains the exact location of all mobile subscribers currently present in the service area of the MSC. This


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information is necessary to route a call to the right base station. The database entry of the subscriber is deleted when the

subscriber leaves the service area.

An antenna focuses the radio waves in a certain direction. Usually, this is called the main direction. Because of that, in other

directions less energy wil be emitted. The gain of an antenna, in a given direction, is usually referenced to an (hypothetical)

isotropic antenna, which emits the radiation evenly strong in all directions. The antenna gain is the power in the strongest

direction divided by the power that would be transmitted by an isotropic antenna emitting the same total power. In this case the

antenna gain (Gi) is often specified in dBi, or decibels over isotropic.

Other reference antennas are also used, especially:

y gain relative to a half-wave dipole (Gd), when the reference antenna is a half-wave dipole antenna;y gain relative to a short vertical antenna (Gv), when the reference antenna is a l inear conductor, much shorter than one

quarter of the wavelength.

Spread spectrum

Spread spectrum is an technique used in radio transmission based on the concept that the narrowband signal is manipulated

(scrambled) prior to transmission in such a way that the signal occupies a much larger part of the RF spectrum then strictly

needed. This makes the signal more robust against interference and jamming.

The manipulation requires a pseudo random noise code which, in the original concept, was only known to the parties at each end

of the radio connection. Spread spectrum technology was invented in the 1940s, and has been used extensively since then for

military and other applications that require robustness and resistance to jamming or eavesdropping.

There are three different ways of spreading the signal:

y direct sequence,y frequency hopping,y time hopping,

The direct sequence spreadspectrum (DSSS) approach is based on multiplication of the original data signal with a much faster

pseudo random noise code, which is also called the spreading code. This results in a scrambled signal with a much wider

spectrum. DSSS significantly improves protection against interfering signals, especially narrowband interference. It also provides a

multiple access capability, when the several different spreading codes are being used simultaneously. The use of DSSS for multiple

access is called CDMA, and is used e.g. in the 3th generation mobile communications.

In case offrequency hopping spread spectrum (FHSS) the RF frequency of the narrowband transmission is quickly changed

within a certain range, according to a pseudo random noise code. Hence, a hopping pattern can be observed in the spectrum. In


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terms of spectral coexistence with other systems, FHSS is an avoidance technique, in other words if the hop coincides with

someone elses transmission on the same channel, the collision will take only the duration of the hop, which is typically in the

order of milliseconds or even less. Like DSSS, FHSS also provides a multiple access capability by using orthogonal hopping codes

for different (logical) communication channels. FHSS is for instance used by Bluetooth. Bluetooth hoppes 1600 times per second

between the 79 available channels.

In case oftime hopping a train of short duration pulses is transmitted which is derived from the narrowband information carrying

signal through scrambling with a pseudo random modulated impulse train. The short pulse duration generates the spread

spectrum profile. Time Hopping is used as a technique to generate a certain type of UWB signals.

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QoS

The Quality of Service (QoS) refers to the capability of a network to offer a service with a certain quality. The quality of a service

can be related to a number of different parameters. Much used parameters are:

y Availability of a link;y Number of bit errors;y Latency (delay in the network);y Jitter.

Which parameter is of importance depends on the service. For example, voice and video services require low latency but tolerate

some error rate. By contrast, generic data applications can not tolerate error, but latency is not critical.

The ITU has defined four different categories of services. Each with it's own quality of service associated to it:

y Conversational,o Temporal relation preserved,o Weak and rigorous delays,o Services: Telephony, videotelephony, real time video games;

y Streaming,o Temporal relation preserved,o Services: Multimedia services;

y Interactive,o Demand of response,o Data integrity preserved,


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o Services: Internet (web browsing);y Background,

o No restraints on delays,o Data integrity preserved,o Services: E-mail.

This classification is used for instance by 3GPP in the development ofUMTS.

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Comfort Noise Is