| Doing data and fax over
the digital GSM cellular networks requires firstly, a GSM cellphone that
supports data and fax and a special piece of hardware called an adapter
(the "modem") which will act as an interface between the phone
and your PC and/or fax machine. These data/fax adapters usually the credit card size PCMCIA (PC Cards) which plug into a special PCMCIA slot in a notebook or PC. The GSM-only components of these data/fax interfaces are not actually modems as we traditionally understand them: the "modem" component you need to send and receive data actually resides at the network headquarters (also known as the Mobile Switching Centre, or MSC), using a special modem-like device called an IWU, or Inter Working Unit. Data is sent digitally from the PC
via the special hardware adapter, through the phone and then through the
air to the IWU. It is only when the digital data is received by the IWU that it is converted by the IWU to the analogue Frequency Shift Keys (FSK) tones characteristic of Analogue Modems, making it possible to connect via PSTN fixed-lines to an ordinary Analogue Modem. And if an ordinary fax machine or fax/modem sends data to your mobile phone, then the IWU converts the analogue tones received into the "digital data stream" suitable for transmission across the network to your GSM mobile. If one mobile unit sends fax or data to another
mobile, then there's no need for an analogue-to-digital conversion since the
entire "air interface" is already digital, as is required by the
GSM specification.
|
|
||||||||||||||
