Frequency-shift keying

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Topics in Modulation techniques

Analog modulation

AM | FM | PM | QAM

Digital modulation

ASK | PSK | FSK | QAM | OFDM | MSK

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An example of binary FSK

Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency.

Continuous phase forms of FSK exist in which there is no phase discontinuity in the modulated signal. The example shown at right is of such a form.

Other names for FSK are frequency-shift modulation and frequency-shift signaling.

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent frequency-shift keying. In MSK the difference between the higher and lower frequency is identical to half the bit rate. As a result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.

[edit] Standards for use in Caller ID applications

ETSI FSK

The European Telecommunications -1 and -2, and ES 200 778-1 and -2, the latter replacing ETS 300 778-1 & -2) allows 3 physical transport layers (Bellcore, BT and CCA) combined with 2 data formats (MDMF & SDMF), plus the DTMF system and a no-ring mode for meter-reading and the like. It's more of a recognition that the different types exist than an attempt to define a single "standard".

Bellcore FSK

The Bellcore standard is used in the USA, Canada (but see below), Australia, China, Hong Kong and Singapore (who have their own standard?) It sends the data after the first ring tone and uses the 1200 baud Bell 202 tone modulation. The data may be sent in Single Data Message Format (SDMF) which includes the date, time and number or in Multiple Data Message Format (MDMF) which adds a NAME field. There's m454ore details at testmark.com.

BT (British Telecom) FSK

British Telecom developed their own standard, which wakes up the display with a line reversal, then sends the data as CCITT V23 modem tones in a format similar to MDMF. It is used by BT, wireless networks like the late Ionica, and some cable companies (Nynex?). Details are to be found in BT Supplier Information Notes (SINs) 227 and 242; another useful document is Designing Caller Identification Delivery Using XR-2211 for BT from the EXAR website.

CCA (Cable Communications Association) FSK

The Cable Communications Association in the UK developed their own standard which sends the data after a short first ring, as either Bell 202 or V23 tones. They developed a new standard rather than change some street 'boxes' (multiplexors) which couldn't cope with the BT standard, and Oftel let them get away with it. The UK cable industry use a variety of switches - most are Nortel DMS-100's, some are System X, System Y and Nokia DX220's, and some of these use the BT standard instead of the CCA one! The data format is similar to BT's, but the transport layer is more like Bellcore, so North American or European kit is more likely to detect it.