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  Wireless Data    
       
  Overview
 
In modern times, we depend on e-mail and on the World Wide Web. We have access to these resources at home and at the offices, and we would like to use these resources while traveling. This is the goal that "wireless data" hopes to accomplish. In the following section, we will discuss various types of wireless data systems that run on top of cellular networks. Systems that run on other systems, such as ricochet, which runs on the pager network, will not be discussed.
 
Analog modems
 
Analog modems are the simplest type of wireless data. Analog modems work on any cellular system by simply encoding the digital data as audio signals. Analog cellular modems make the same squealing sounds as an ordinary desktop computer’s analog modem, but operate at far slower speeds. On an analog modem, these modems typically average between 4800 to 9600 bits per second. This is slower than the maximum possible rate because of the inefficiency of converting digital to analog. It should also be noted that digital cell systems do not support analog modems since their data compression would damage the analog modem signal, preventing it from being decoded by the receiving modem.
 
Packet Data (CDPD over IS-136 and AMPS networks)
 
By using some of the channels of the TDMA or FDMA (IS-136 and AMPS standards) network as large aggregated digital data lines, packetized digital data may be sent over this line. CDPD describes how every cell phone on the network may insert packets on this shared data channel without causing collisions, which would happen if two packets were sent at the same time by two different phones. CDPD is useful since it runs on top of old FDMA systems as well as newer TDMA systems and offers a 19.2 kilobits per second data rate. Using a TDMA system, the CDPD protocol can be scaled by adding more frequency channels.
 
GSM
 
GSM uses an intriguing method to send data. By simply sending computer data as it would send voice data, GSM allows every phone to be "data enabled." Since the GSM network is already a packet network of sorts because of its frequency hopping, it requires no additional hardware to support data. GSM allows data rates in multiples of 300 bits per second, up to 64 kilobits per second.
 
 
  Source: http://cellphones.about.com    

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