originate at the GMSC, even though it is the visiting MSC that completes the connection to the PSTN. Plans that include nationwide long distance and/or nationwide roaming at no additional charge over "local" outgoing calls are popular. Mobile networks in Europe, Asia (except Hong Kong, Macau (Macao) and Singapore), Australia, and Argentina only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber with the exception of receiving a call while the subscriber is roaming as described below. However, callers typically pay a higher rate when calling mobile phones. Special prefixes are used to designate mobile numbers so that callers are aware they are calling a mobile phone and therefore will be charged a higher rate. From the caller's point of view, it does not matter where the mobile subscriber is, as the technical process of connecting the call is the same. If a subscriber is roaming on a different company's network, the subscriber, instead of the caller, may pay a surcharge for the connection time. International roaming calls are often quite expensive, and as a result some companies require subscribers to grant explicit permission to receive calls while roaming to certain countries. Speech encoding During a GSM call, speech is converted from analogue sound waves to digital data by the phone itself, and transmitted through the mobile phone network by digital means. (Though older parts of the fixed Public Switched Telephone Network may use analog transmission.) The digital algorithm used to encode speech signals is called a codec. The speech codecs used in GSM are called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR), Adaptive Multirate (AMR) and Wideband AMR also known as HD voice. All codecs except AMR operate with a fixed data rate and error correction level. Data transmission The GSM standard also provides separate facilities for transmitting digital data. This allows a mobile phone to act like any other computer on the Internet, sending and receiving data via the Internet Protocol. The mobile may also be connected to a desktop computer, laptop, or PDA, for use as a network interface (just like a modem or Ethernet card, but using one of the GSM data protocols described below instead of a PSTN-compatible audio channel or an Ethernet link to transmit data). Some GSM phones can also be controlled by a standardised Hayes AT command set through a serial cable or a wireless link (using