Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 29
MAC also measures and maintains statistics about:
• The number of frames, blocks, and bursts transmitted, and how many succeeded
• How many frames had to be rescheduled and why
5.5.1. Details of MAC Transmission Access Management
The MAC protocol enables the CDPD carrier to set a limit on how many times an M-ES can
attempt to transmit the same data block, to prevent an overflow of stale data that a modem is
unable to send. The transmit and receive channels of the CDPD connection are also interlinked
using MAC to provide status information and permit many users to share them (see section 3.2.8).
The forward (receive) channel from the MDBS to the M-ES is always keyed with a signal carrier.
This constant signal supplies a busy/idle flag that indicates to the M-ES whether the reverse
(transmit) channel is ready for data. The busy/idle flag is set at the MDBS to indicate that the
reverse channel is either busy or available.
So, when a CDPD modem has data to send through the network:
• The modem checks the busy/idle flag on the forward channel to determine if the reverse
channel is idle—available for transmission.
• If the channel is available, the modem begins to send data.
• While sending on the reverse channel, the modem regularly checks the busy/idle and decode
status on the forward channel to determine if the transmission is proceeding as expected. In a
normal transmission the MDBS will see the incoming data from the modem and set the busy
flag on the reverse channel to prevent other devices from attempting to access the channel
while it is in use. In a transmission without unrecoverable errors, the MDBS also sets the
decode flag as positive to indicate that the blocks arrived intact.
• If the modem senses that the incoming busy/idle flag is busy and that the decode status is
positive for the data blocks sent so far, it will continue with its transmission. If either the flag
is idle or there is a decode failure, the modem must stop transmission immediately and try
again.
• When the last of the chained data blocks has been sent, the modem must wait until the final
decode status is received before it can terminate this specific transmission.
If it has more data to send, the modem cannot immediately start on the next set of blocks. The
MDBS uses the MAC protocol to specify a minimum waiting period to allow sharing of the
channel between multiple devices.
5.5.2. The Exponential Back-Off Process
When the modem is unable to get access to the reverse channel due to a data collision or similar
problem, it waits for a randomized period before retrying. The period is determined by a formula
based on MAC parameters provided by the MDBS.
The CDPD carrier sets these parameters individually for each MDBS, typically using values
recommended in the CDPD specification. After waiting a randomized time based on the formula,
the modem must reacquire the channel (which may be busy) before it can try again. If the re-
transmission is successful, the back-off counters are reset.
5.6. Radio Resource Management (RRM)
Radio Resource Management (RRM) refers to the process of managing CDPD channel
acquisition, channel hopping, cell transfer, and signal strength. The RRM process for CDPD
enables the limited available spectrum to serve many CDPD users.