Shortly after cellular first launched, third-party entrepreneurs
began offering modems for cellular data transmission. These
modems resemble the standard analog modems circa 1985.
They transmitted data at 300 baud, used a special cellular protocol
called MNP-10, and an AT command set. MNP-10 made
these modems different from regular modems because they
had to be fault-tolerant to cellular hand-offs and the in-band
signaling tones used in analog cellular. One such modem was
created by a Dallas, Texas company, Spectrum Cellular, that
actually consisted of a modem set or pair. The land or fixed side
modem was installed at the cellular switch. The mobile half
was connected to the mobile phone and computer. The connection
was circuit-switched, which meant that the phone was
connected as long as data was transmitted and the cellular traffic
channel was dedicated to that one user.
The next milestone for data over cellular was Cellular
Digital Packet Data (CDPD) in 1992. An industry consortium
of leading wireless communications companies set out to
develop the CDPD specification. Their design objectives were
to send digital data over the existing AMPS wireless infrastructure
without major changes to the AMPS infrastructure, with
reasonable performance (19.2 Kbps), high reliability, and security;
and to support seamless roaming. CDPD also had to coexist
with voice traffic. Because it was based on standard
Internet TCP/IP protocols, service providers that do not have
next-generation digital solutions for consumers today can still
use CDPD as an Internet connection. The CDPD market is
alive for the time being, but this may change as next-generation
networks become active.