GSM (Global System for Mobile communications) is a cellular phone system standard popular outside the USA. You can find more info on GSM in general here

GSM includes a codec, often just referred to as the GSM when discussing codecs.

The original 'Full Rate' GSM speech codec is named RPE-LTP (Regular Pulse Excitation Long-Term Prediction). This codec uses the information from previous samples (this information does not change very quickly) in order to predict the current sample. The speech signal is divided into blocks of 20 ms. These blocks are then passed to the speech codec, which has a rate of 13 kbps, in order to obtain blocks of 260 bits.

Newer GSM systems use a couple of newer codecs however these are heavily patent encumbered:

• EFR (Enhanced Full Rate) uses ACELP (Algebraic Code Excited Linear Prediction)
• HR (Half Rate) uses CELP-VSELP (Code Excited Linear Prediction - Vector Sum Excited Linear Prediction)

• University of Berlin GSM Page
• Technical overview of GSM
• GSM Tutorial
• SMS Tutorial
• SMS Gateway

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Codecs