请高手解答GPRS的一个小问题！

stalamo

什么叫GPRS Real Time Difference？？那位大侠给解释一下！

还有：
The total number of system information messages of a certain type within a consistent set is indicated by a PSI or SI count parameter included in each message in the set. The position of a certain message instance within the consistent set of system information messages is indicated by a PSI or SI index parameter.

中consistent set和instance分别是什么意思？
谢谢！