 the SCH transaction duration, it is possible that a GPS session can timeout without a single GPS visit. Further, reduced data throughput and possible data application timeouts can occur while GPS tracking sessions are in progress.
There are three primary reasons for bad data throughput involving GPS and CDMA. 1) Tune-away from CDMA results in the loss of radio link protocol (RLP) data frames. The loss depends upon the amount of data exchanged and the time the mobile station spends on the new GPS frequency. The time spent on the GPS frequency is typically from 0.1 to 2 seconds (sec). 2) Tune-away results in loss of the Enhanced Supplemental Channel Assignment Message (ESCAM). Consequently, the mobile station will not receive data on the Supplemental Channel (SCH) even when the mobile station is tune to CDMA. 3) The subsequent tune-aways do not allow previously lost data to be recovered. This results in the loss of newly transmitted/re-transmitted data packets.
When a loss occurs of RLP data that cannot be recovered, the tune-aways result in frequent errors at the TCP layer. Complete RLP data frames can be lost. The extent of the loss of this information depends upon the amount of data sent and the time spent by the mobile station while it is tuned to other than the CDMA carrier frequency. In the case of tune-aways to the GPS carrier frequency, times of 0.1 to 2 seconds are typical.
The MSC signals the mobile station that it should prepare to receive a data burst by transmitting an Enhanced Supplemental Channel Assignment Message (ESCAM). In this message, the MSC indicates the number of channel codes, as well as the actual Walsh codes to be used for each channel.
During tune-aways, the ESCAM is lost. Without the ESCAM, the mobile station will not be able to decode data on an SCH, even when the mobile station is tuned to the CDMA carrier frequency. Subsequent tune-aways do not allow previously lost data to be recovered, and also result in the loss of new transmitted/retransmitted packets of data. Because there is a