The handoff procedure in CDPD is shown in Figure 9.5. The handoff initiation and decision in CDPD are as follows.
The handoff is mobile controlled. The M-ES always measures the signal strength of the reference channel [SAL99b]. An M-ES scans for alternative channels when its signal deteriorates. Because certain cells may have large shadowing effects within them, the operator can set a RSS1 scan value to determine when a M-ES should start scanning for alternative channels. An M-ES will ignore a drop in signal level if the RSS1 scan value is large enough or start scanning for alternative channels if it is small. This value is also useful (and should be made small) when the signal strength does not drop even when the M-ES has moved well into a neighbouring cell. When additional thresholds for RSS1 hysteresis, block error rate (BLER) and/or symbol error rate (SER) are reached, the M-ES will go through a list of channels of adjacent cells that the current BS is broadcasting and tune in to the one with the best signal strength. The M-ES informs the new BS that it has entered its cell.
The mobile serving function of the new MD-IS uses a redirect-request and redirect-confirm procedure with the mobile home function of M-ES. The mobile home function also informs the old serving MD-IS about the handoff and directs it through its MSF to redirect packets it may have received for the M-ES to the new serving MD-IS or flush them. Depending on the nature of handoff (interoperator or intraoperator), the delay of registration and traffic redirection will vary.