Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 15
3.2.9. CDPD Data Transmission Format
CDPD data is sent in Reed-Solomon blocks of 378 bits, each consisting of 63 six-bit symbols. Of
the 63 symbols, 47 carry user data or protocol, while the remaining 16 symbols provide parity.
Since the airlink is inherently vulnerable to interference and noise, the high number of parity bits
enables the protocol to recover blocks in which up to 7 of the 63 symbols have been damaged.
Full-duplex devices are permitted to chain multiple Reed-Solomon data blocks. The CDPD
recommended default value for the maximum number of blocks is 64. Many carriers use a lower
number—32 is common.
CDPD adds capacity to a sector in a cellular system by supporting several active CDPD data link
channels at one time (these are also known as channel streams—see section 4.1.2). The CDPD
network manages the control of these multiple data links such that users always maintain their
end-to-end data connection. It prevents the subscriber’s device from becoming confused about
which data link it is operating on. This control is transparent to the user.